Download Strategies of attack and defence in woody plantPhytophthora

For. Path.
© 2014 Blackwell Verlag GmbH
doi: 10.1111/efp.12096
REVIEW ARTICLE
Strategies of attack and defence in woody plant–Phytophthora interactions
By W. Oßwald1,17, F. Fleischmann1, D. Rigling2, A. C. Coelho3, A. Cravador4, J. Diez5, R. J. Dalio1, M. Horta Jung4, H. Pfanz6,
C. Robin7, G. Sipos8, A. Solla9, T. Cech10, A. Chambery11, S. Diamandis12, E. Hansen13, T. Jung4,14, L. B. Orlikowski15,
J. Parke13, S. Prospero2 and S. Werres16
1
Technische Universit€at M€
unchen, Section Pathology of Woody Plants, Freising, Germany; 2WSL Swiss Federal Research Institute,
3
Birmensdorf, Switzerland; Centro de Investigac!~ao em Qu"ımica do Algarve (CIQA), University of Algarve, Faro, Portugal; 4Centre of
Genomics and Biotechnology, Institute for Biotechnology and Bioengineering, University of Algarve, Faro, Portugal; 5Sustainable Forest
Management Research Institute, University of Valladolid-INIA, Palencia, Spain; 6Applied Botany, University of Duisburg-Essen, Essen,
Germany; 7INRA, UMR 1202 BIOGECO, F-33402 Cestas, France; 8Institute of Silviculture and Forest Protection, University of West-Hungary,
Sopron, Hungary; 9Ingenier"ıa Forestal y del Medio Natural, Universidad de Extremadura, Plasencia, Spain;
10
Federal Research Centre for
11
Forests, Natural Hazards and Landscape, Vienna, Austria; Department of Life Sciences, Second University of Naples, Caserta, Italy;
12
Laboratory of Forest Pathology & Mycology, Forest Research Institute, Thessaloniki, Greece; 13Department of Botany and Plant Pathology,
Oregon State University, Corvallis, OR, USA;
Skierniewice, Poland;
14
Phytophthora Research and Consultancy, Brannenburg, Germany;
15
Institute of Horticulture,
16
Julius K€
uhn Institut – Federal Research Centre for Cultivated Plants, Institute for Plant Protection in Horticulture
and Forests, Braunschweig, Germany;
17
E-mail: [email protected] (for correspondence)
Summary
This review comprises both well-known and recently described Phytophthora species and concentrates on Phytophthora–woody plant interactions. First, comprehensive data on infection strategies are presented which were the basis for three models that explain invasion and
spread of Phytophthora pathogens in different woody host plants. The first model describes infection of roots, the second concentrates on
invasion of the trunk, and the last one summarizes infection and invasion of host plants via leaves. On the basis of morphological, physiological, biochemical and molecular data, scenarios are suggested which explain the sequences of reactions that occur in susceptible and tolerant plants following infections of roots or of stem bark. Particular emphasis is paid to the significance of Phytophthora elicitins for such
host–pathogen interactions. The overall goal is to shed light on the sequences of pathogenesis to better understand how Phytophthora
pathogens harm their host plants.
1 Introduction
The genus Phytophthora belongs to the phylum Oomycota, which only recently has been separated from the kingdom fungi
and now resides within the kingdom stramenopiles (Dick 2001). The genus has received considerable attention because
many Phytophthora species are important plant pathogens (Erwin and Ribeiro 1996). There are a number of well-known
Phytophthora species that cause destructive diseases in agricultural crops. In particular, Phytophthora infestans on potato
and P. sojae on soya bean have been intensively studied, and the results of these investigations have significantly contributed to our current knowledge of Phytophthora–plant interactions (for recent reviews see Schornack et al. 2009; Oliva et al.
2010; Vleeshouwers et al. 2011; Bozkurt et al. 2012; Jiang and Tyler 2012).
A remarkable feature of the genus Phytophthora is that many species have the ability to infect trees and shrubs either as
foliar, bark or root pathogens (Hansen et al. 2012). Many species such as P. cinnamomi, P. cryptogea, P. nicotianae, P. palmivora and P. ramorum have very large host ranges that include members of different plant families (Erwin and Ribeiro
1996; Hardham 2005; Cahill et al. 2008; Gr€
unwald et al. 2008). Other Phytophthora species such as P. alni, P. lateralis and
P. quercina infect only a few host species (Erwin and Ribeiro 1996; Jung et al. 1999; Brasier et al. 2004a). The number of
described Phytophthora species that are associated with woody plants has increased dramatically in the past decade
(Hansen et al. 2012; Martin et al. 2012). New species have been detected either because they were invasive causing severe
diseases on new non-coevolved host plants or because of intensive sampling campaigns, particularly in forest soils and
streams (Jung et al. 2013a). In addition, some morphologically defined Phytophthora species turned out to be complexes of
several cryptic species. For example, the species formerly known as P. citricola has recently been divided into eight distinct
species, P. plurivora (Jung and Burgess 2009), P. multivora (Scott et al. 2009), P. mengei (Hong et al. 2009), P. pini (Hong
et al. 2011), P. capensis (Bezuidenhout et al. 2010), P. elongata (Rea et al. 2010), P. acerina (Ginetti et al. 2013) and P. citricola sensu stricto. All of them are pathogenic to woody plants.
To cope with different hosts and host tissues, Phytophthora species have evolved sophisticated mechanisms to manipulate plant cells and cause infections. The mechanisms of infection are being revealed for the model species, P. infestans and
P. sojae (Jiang and Tyler 2012).
In general, invading pathogens have to face two defensive layers of plant immunity (Jones and Dangl 2006; H€
uckelhoven
2007). At first, the plant recognizes conserved microbial- or pathogen-associated molecular patterns (MAMPs or PAMPs) by
transmembrane pattern recognition receptors (PRRs). Sensing of PAMPs leads to PAMP-triggered immunity (PTI; Fig. 1a)
Received: 12.8.2013; accepted: 5.1.2014; editor: V. Andrea
http://wileyonlinelibrary.com/
2
W. Oßwald, F. Fleischmann, D. Rigling et al.
that can halt further colonization by inducing primary defence responses such as deposition of callose, cell wall remodelling, accumulation of defence-related proteins and phytoalexins (Jones and Dangl 2006; Van Loon et al. 2006; Zipfel 2009;
Oßwald et al. 2012). The species of Phytophthora and all true fungi, which can either suppress PTI or manipulate cellular
functions and structures of the host cells, secrete and deploy an array of variable effectors that promote effector-triggered
susceptibility (ETS) and enable parasitic infection (Jones and Dangl 2006; Kamoun 2006) (Fig. 1b,d).
Many Phytophthora species are hemibiotrophic pathogens with an initial biotrophic stage during early infection followed
by necrotrophic colonization of the host tissue. When invading host cells, biotrophic pathogens often develop haustoria,
intracellular structures for the resorption of nutrients which are surrounded by the extrahaustorial matrix. The latter is
formed between the pathogen’s and host’s specialized plasma membranes and constitutes the primary battlefield where all
potential effectors are secreted (Dodds et al. 2009; Leborgne-Castel et al. 2010). Effectors cover a broad range of molecules
with diverse biological functions (Tyler et al. 2006; Kamoun 2007). They can act either at the host–pathogen interface
(apoplastic effectors) or inside the host cell (cytoplasmic effectors) (Kamoun 2006). The Phytophthora apoplastic effectors
include cell wall-degrading enzymes, enzyme inhibitors, toxins and elicitins, unique 10-kD peptides (see Section 4)
(Hardham and Cahill 2010). The cytoplasmic effectors, besides their secretory signals, harbour an additional host-targeting
(HT) leader sequence that is required for translocation across the host plasma membrane. The HT signal of many Phytophthora effectors is composed of a highly conserved N-terminal RxLR and a dEER motif further downstream (Whisson et al.
2007; Dou et al. 2008). The second major group of suspected cytoplasmic effectors, so-called CRN or Crinkler effectors,
lacks the RxLR but have a KxLFLAK motif instead (Torto et al. 2003). While Crinklers are present in all investigated oomycete species, effectors with RxLRs seem to be specific to Phytophthora (Schornack et al. 2010; Bozkurt et al. 2012).
As an evolutionary result, specific effector proteins are monitored directly by resistance proteins (RPs) encoded by disease resistance or R genes. Thus, RPs, which are able to control their targets either on the outer surface or in the cytoplasm of the host cells, participate in the secondary defence response that results in effector-triggered immunity (ETI;
Fig. 1c,e) (Jones and Dangl 2006; Hein et al. 2009). ETI, often manifested as a localized hypersensitive response (HR) or
induced programmed cell death, is indeed a non-specific response that leads to an effective defence against multiple pathogens. The different modes of susceptibility and resistance developed during co-evolution are shown in Fig. 1.
This review focuses on the interactions between Phytophthora species and woody plants and covers (i) infection strategies and growth in host plants, (ii) morphological, physiological, biochemical and molecular mechanisms of infected plants,
(iii) elicitins, specific Phytophthora proteins and their impact for woody plant interactions.
2 Infection strategies and growth of Phytophthora pathogens in host plants
One has to elucidate their infection strategies and spread within host plants to answer the question, how Phytophthora
spp. harm woody plants. The present evaluation concentrates on fourteen important species of the genus Phytophthora
with the potential to invade at least 24 important woody hosts from seventeen different plant genera. At least five of the
(a)
(b)
(c)
(d)
(e)
Fig. 1. Scheme of the co-evolution between plants and pathogens. (a) Upon pathogen attack, pathogen-associated molecular patterns
(PAMPs) activate pattern-recognition receptors (PRRs) in the host, resulting in a downstream signalling cascade, usually through WRKY
transcription factors that leads to PAMP-triggered immunity (PTI). (b) Virulent pathogens evolved effectors that suppress PTI, resulting in
effector-triggered susceptibility (ETS). (c) In turn, plants have acquired resistance (R) proteins that recognize specific effectors, resulting in
a secondary immune response called effector-triggered immunity (ETI). (d) Under selection pressure, the pathogen alters its effectors to
avoid ETI, resulting in a new ETS phase. (e) As a consequence, by natural selection, new resistance (R) proteins can emerge and ETI can
be triggered again and immunity is restored. This process can be repeated indefinitely.
Phytophthora – woody plant interactions
3
Phytophthora species are able to infect more than three hosts, whereas for three of these Phytophthora species, colonization
has been reported only for a single host genus, so far.
During infection, Phytophthora spp. cause a variety of histological changes in woody hosts. The observed changes depend
on the pathogen species, the type of tissue colonized and the release of toxins able to cause local and distant disruption of
cells (Davison 2011). On the basis of the literature, three conceptual models were developed, describing the primary infection of roots (Fig. 2a), of the trunk (Fig. 2b) and of leaves (Fig. 2c).
2.1 Primary infection of roots
Most Phytophthora species are soilborne pathogens, and hence, the most direct way infecting host plants is via the fine root
system (Table 1, Fig. 2a). Infected coarse roots can be observed as well, but empirical evidence for a primary infection of
intact coarse roots by zoospores in the field is missing. Infection of coarse roots possibly takes place via ingrowth from fine
roots or via wound infection. Successful infection is usually initiated by biflagellate zoospores that are chemotactically
attracted to nearby roots (see Section 2.4) (Khew and Zentmyer 1973). The zoospores attach to sites on the root surface
that are favourable for subsequent penetration (e.g. grooves) and then encyst (Fig. 3a), secreting adhesive that glues them
to the root surface (Hardham 2001). The germ tube that emerges from the cyst swells and forms a penetration hypha
(Fig. 3b) before it invades the root epidermis (Oh and Hansen 2007). At first, hyphae grow intercellularly along the cell
walls (Fig. 3c). Subsequent intracellular growth of hyphae can be observed, too (Fig. 3d,e).
Extracellular enzymes of Phytophthora might be involved in the degradation of the host cell walls (Brummer et al. 2002)
including the pectin component of the middle lamella (Hardham 2001). The development of haustorium-like structures is
accompanied by the accumulation of electron-dense material between the hyphae and the plasmalemma of the host cell,
visible in transmission electron microscopy (Fig. 3f). The nature and origin of the electron-dense material is unknown,
although its pattern of radiation from the pathogen side of the wall suggests that it is a result of pathogen secretory activity (Hardham 2001). Other authors reporting on electron-dense material around the haustorium-like structures suggest that
this material probably contains phenolic compounds originating from the host (Jang and Tainter 1990; Brummer et al.
2002; Horta et al. 2010). Haustorium-like structures cannot be observed in all interactions (Oh and Hansen 2007; Jung et
al. 2013b), indicating that not all Phytophthora species are hemibiotrophic.
In susceptible hosts, hyphae invade the cortex, rapidly growing inside the pericycle, damaging the phloem and to a lesser
extent the xylem (Brummer et al. 2002; Oh and Hansen 2007; Horta et al. 2010; Portz et al. 2011) (Fig. 3g). In infected
cells, the plasmalemma often separates from the cell wall, indicating severe loss of cell turgor (Brummer et al. 2002). This
plasmolysis is accompanied with thickening of cell walls and endodermis destruction. Intensive growth of Phytophthora
hyphae can result in plasmolysis within the root cortex even in non-infected cells indicating a severe imbalance in osmoregulation of roots (Portz et al. 2011). In addition, cell wall degradation was observed in xylem vessels even distant from the
Phytophthora infestation (Portz et al. 2011; Fig. 3h).
When host tissue is severely damaged, the development of resting spores (chlamydospores or oospores) in infected tissue is often observed (Mircetich and Zentmyer 1966; Jung et al. 1996, 2013b; Parke et al. 2007; Riedel et al. 2008, 2012;
Crone et al. 2013). Under wet conditions, Phytophthora spp. form secondary sporangia on the root surface which release
zoospores causing multicyclic infections.
As described above, for some few host–Phytophthora interactions (Table 1, first part), growth of the pathogen is
restricted to the root system eventually causing severe root rot under conducive conditions. As a consequence, secondary
(a)
(b)
(c)
MR
W
LC
X
B
Fig. 2. Infestation models of Phytophthora: (a) Primary infection of roots; red arrows: primary infection of fine roots; blue arrows:
secondary growth into the bark and/or xylem of the trunk. (b) Primary infection of the trunk; red arrows: primary infection of trunk via
lenticels (LC) or wounds (W); blue arrows: secondary growth in the bark tissue (B) and via medullary rays (MR) into the xylem (X) of the
trunk. (c) Primary infection of leaves; red arrows; primary infection of leaves; blue arrows: secondary growth into petioles and twigs.
4
W. Oßwald, F. Fleischmann, D. Rigling et al.
Table 1. Summary of Phytophthora–woody plant pathosystems in which infection starts on roots.
Host
Phytophthora species
Infection of roots and growth exclusively in roots (red arrows, Fig. 2a)
Fagus sylvatica
Juglans regia
Persea americana
Quercus ilex; Q. suber
Quercus robur; Q. petrea
P.
P.
P.
P.
P.
cactorum; P. plurivora
cambivora
cinnamomi
cinnamomi
cambivora; P. plurivora; P. quercina
Infection of roots and growth from roots into the trunk (red and blue arrows, Fig. 2a)
Abies fraseri
P. cinnamomi
Agathis australis
P. ‘taxon Agathis’ (PTA)
Alnus glutinosa
P. alni
Castanea sativa, C. dentata
P. cambivora, P. cinnamomi
Chamecyparis lawsoniana
P. lateralis
Citrus spp.
P. citrophthora; P. nicotianae
Eucalyptus spp.
P. cinnamomi
Fagus sylvatica
P. cambivora; P. kernoviae; P. plurivora; P. pseudosyringae
Juglans regia
P. cambivora
Malus domestica
P. cactorum, P. plurivora
Persea americana
P. citricola
Quercus rubra; Q. robur
P. cinnamomi
Quercus spp.
P. kernoviae
Rhododendron spp.
P. kernoviae; P. ramorum
Taxus brevifolia
P. lateralis
symptoms such as leaf chlorosis, thinning and dieback of the crown develop (see Section 3 and Figs 6 and 9). In mature
trees, these symptoms usually become apparent years after the first infections of fine roots.
Subsequent growth of Phytophthora hyphae from roots into the trunk depends either on the host plants capacity to
restrict the pathogen to roots (Hardham 2005) or on environmental conditions weakening the host (Jung 2009). Those
host–pathogen interactions, where the growth of Phytophthora from roots into the collar and further upwards into the
trunk is observed, are summarized in Table 1 (second part). In such cases, Phytophthora advances in the cambium layer
and from there colonizes the neighbouring phloem and xylem tissue (Tippett et al. 1983; Davison et al. 1994; Giesbrecht et
al. 2011).
2.2 Primary infection of the trunk
Many Phytophthora species are able to directly infect the trunk of certain host species via lenticels, adventitious roots or
wounds and then invade and destroy cortex and phloem tissue (Fig. 2b). Well-known examples are summarized in Table 2.
For some of these Phytophthora spp., for example P. ramorum, inoculum was found in the soil, but the pathogen was
rarely detected in roots (Fichtner et al. 2007). In other cases, like P. alni on Alnus glutinosa and A. incana, environmental
and ontogenetic factors determine, whether infection starts at roots or at the collar. Alder trees along rivers are mainly
infected via lenticels and adventitious roots during temporary flooding events, while roots are the primary infection site of
nursery-grown planted alder saplings on non-flooded sites (Jung and Blaschke 2004). In a similar study, Hardy et al.
(1996) and O’Gara et al. (1997) found out that P. cinnamomi invaded Eucalyptus marginata and E. calophylla directly
through the trunk under conditions of temporary ponding and waterlogging, an event that often happens in revegetated
bauxite-mined areas. Besides the different primary infection site, there is no general difference in spread and symptom
development as compared to trunk infection starting from root tissue.
The successful development of an infection also relies on the physiological dependence between the host and the pathogen, which is influenced by phenological changes during an annual cycle (Biere and Honders 1996; Kennelly et al. 2005).
The hypothesis that synchronous pathogenicity and host development are necessary for infection was tested by Dodd et al.
(2008) for P. ramorum on Quercus agrifolia. The authors experimentally showed how the variation in cambial phenology
among trees correlates with variation in time of maximum lesion size among trees. Based on this result, they suggested
that an active cambium is necessary for successful infection. The infection rate is highest when cambial activity coincides
with maximum infectious sporulation of P. ramorum, which typically occurs in spring (Davidson et al. 2005). Trees with a
late onset of physiological activity in spring may thereby avoid infection. Similarly, Robin et al. (1994) detected significant
seasonal changes in the susceptibility of northern red oaks (Quercus rubra) to P. cinnamomi, which could not be entirely
explained by a climatic effect on the development of the pathogen. The susceptibility to P. cinnamomi varied among trees,
and this variation could partly be accounted for by differences in tree phenology.
Besides the phenotypic situation of the host, the phenotypic status of the pathogen seems to be of great importance for
a successful infection, too. A considerable phenotypic variation of pathogens including virulence was found within clonal
lineages in both P. cinnamomi (H€
uberli et al. 2001) and P. ramorum (H€
uberli and Garbelotto 2011). A recent study by Kasuga et al. (2012) indicated that phenotypic diversification in P. ramorum is associated with the host species from which the
pathogen was isolated from and may be triggered by derepression of transposable elements in the pathogen.
Phytophthora – woody plant interactions
(a)
(b)
5
(c)
H
CY
CY
EP
H
H
H
(d)
(e)
H
H
RC
EP
(f)
HA
H
XV
H
H
IS
EDM
2 µm
H
RC
(g)
S
HA
(h)
X
XV
H
P
XV
X
XP
S
Fig. 3. Light microscopic and transmission electron microscopic (Fig. 3b, insert of 3f and 3h) histopathological observations in roots upon
Phytophthora infection: (a) encysted zoospores in the vicinity of infected cells of the root cortex; (b) germinating cyst forming an
penetration hypha; (c) intercellular growth of hyphae in the root cortex; (d, e) intracellular growth of hyphae in the root cortex and in
xylem vessels, respectively; (f) haustorium-like structures formed in cells of the root cortex; (g) destruction of phloem tissue (arrows) and
intracellular growth of hyphae in xylem vessels; (h) destruction of cell walls of xylem vessels in the hypocotyl in the absence of
Phytophthora hyphae. All figures show roots of Fagus sylvatica artificially infected with zoospores of P. plurivora, except Fig. 3b (root of
Chamaecyparis lawsoniana infected with P. lateralis) and insert of Fig. 3f (Quercus robur infected with P. quercina). Figure 3a was originally
published by Oh and Hansen (2007) and Fig. 3d, e and h by Portz et al. (2011), insert of Fig. 3f by Brummer et al. (2002). CY: encysted
zoospore; EDM: electron-dense material; EP: epidermal cell; EX: external zone; IS: intercellular space; H: hypha; HA: haustorium; OS:
oospore; P: phloem; RC: root cortex; S: starch granule; X: xylem; XP: xylem parenchyma; XV: xylem vessel.
Table 2. Summary of Phytophthora–woody plant pathosystems of which infection starts on the trunk.
Host
Aesculus hippocastanum
Alnus glutinosa
Eucalyptus marginata
Fagus sylvatica
Notholithocarpus densiflorus
Quercus agrifolia
Phytophthora species
P.
P.
P.
P.
P.
P.
cactorum; P. plurivora
alni
cinnamomi
cambivora; P. plurivora
ramorum
ramorum
Infected cells of bark tissue are often shrunken, and the shape of the cells are altered (Pogoda and Werres 2004; Giesbrecht et al. 2011). The plasmodesmata appear to be plugged by a bright material in response to pathogen infection (Giesbrecht et al. 2011). The host responds to Phytophthora infection with tissue discoloration, cell collapse, callose, starch and
crystal depositions and the formation of a new periderm, which protects adjacent tissue against secondary infection (Pogoda and Werres 2004; Oh and Hansen 2007; Giesbrecht et al. 2011). The discoloration of infected cells could reflect the diffusion of polyphenols from parenchyma cells as part of the host defence (Tippett et al. 1983; Brown and Brasier 2007),
also partly explaining the dark colour of the bleeding sap, sometimes released by disrupted bark tissue. The occurrence of
these bleeding cankers is the most striking – however unspecific – visible symptom of Phytophthora trunk infection.
6
W. Oßwald, F. Fleischmann, D. Rigling et al.
Depending on the aggressiveness of the Phytophthora species to the respective host species and the environmental
conditions, the restriction of Phytophthora activity by the newly formed periderm may only be temporary and the pathogen
may be able to breakout and invade new phloem (Tippett et al. 1983). Depending on the host–Phytophthora combination,
the pathogen may move deep into phloem and xylem tissue (Davison et al. 1994; Brown and Brasier 2007; Davison 2011)
and finally – if existing – into the pith (Pogoda and Werres 2004; Giesbrecht et al. 2011). Xylem invasion is generally recognized as a precursor to xylem dysfunction, hydraulic failure and sudden tree death. Xylem infections may not be immediately apparent because there is either no or very limited discoloration of the sapwood. Although, the sapwood underlying
Phytophthora cankers is sometimes discoloured, there have been only few records of isolations specifically from the xylem
of living trees (Brown and Brasier 2007; Davison 2011). Cell-to-cell penetration is direct or through xylem pits. Further evidence of xylem invasion is given by Parke et al. (2007) and Collins et al. (2009), who showed that P. ramorum is able to
invade the sapwood, resulting in tyloses of xylem vessels (Fig. 4) and hence reduced hydraulic conductivity (see Section 3).
It is currently under discussion whether there is any growth from the inner xylem back into the phloem and bark tissue
which could explain the occurrence of serial aerial bleeding cankers along the stems which are separated from each other
by healthy xylem and phloem tissues (Jung et al. 2005; Brown and Brasier 2007; Parke et al. 2007; Jung 2009). However,
empirical evidence for the re-invasion of phloem from xylem is lacking.
2.3 Primary infection of leaves
Some Phytophthora pathogens such as P. ilicis, P. palmivora, P. pinifolia, P. kernoviae and P. ramorum are known to infect
leaves and sometimes also fruits of certain host species from where they might grow into petioles and twigs, causing
severe defoliations and dieback of twigs (Erwin and Ribeiro 1996; Rizzo et al. 2005; Duran et al. 2008; Fig. 2c; Table 3).
(a)
H
H
(b)
CS
(c)
CS
CS
(d)
(e)
T
T
Fig. 4. Scanning electron microscopy of sapwood of Notholithocarpus densiflorus (tanoak) naturally infected with Phytophthora ramorum.
Hyphae (a) and chlamydospores (b, c) of P. ramorum in as well as tyloses (d, e) in xylem vessel (originally published by Parke et al.
2007). CS: chlamydospore; H: hypha; T: tylose.
Phytophthora – woody plant interactions
7
Table 3. Summary of Phytophthora–woody plant pathosystems in which infection starts on leaves.
Host
Phytophthora species
Chamaecyparis lawsoniana
Larix spp.
Notholithocarpus densiflorus
Pinus radiata
Rhododendron spp.
Umbellularia californica
P. lateralis
P. ramorum
P. ramorum
P. pinifolia
P. ramorum
P. ramorum
These pathogens cause leaf spots and/or blights and petiole necrosis on mature woody hosts. While the infection of
leaves itself has a less severe impact on plants, because of their ability to regenerate their foliage, severe damage and even
death of mature trees can be observed, especially if the infestation spreads from leaves to twigs and even stems, as
described for P. ramorum on Notolithocarus densiflorus (Gr€
unwald et al. 2008). Intense sporulation of Phytophthora species
on leaves facilitates infection of branches and stem bark tissue on the same host as shown for P. ramorum on Larix kaempferi and P. pinifolia on Pinus radiata, (Duran et al. 2008; Brasier and Webber 2010; Webber and Brasier 2012) or on other
host species growing in close vicinity as demonstrated for P. ramorum, in case of the leaf host Umbellularia californica and
the canker host N. densiflorus in California (Anacker et al. 2008). In the UK, Rhododendron ponticum and L. kaempferi are
the most important leaf hosts of P. ramorum and P. kernoviae (only R. ponticum) which enable massive production and
spread of sporangia onto nearby stems of Fagus sylvatica (Brasier et al. 2004b; Webber and Brasier 2012). Infection of
Chamaecyparis lawsoniana with P. lateralis usually starts in the roots, but in some cases, especially in atlantic parts of
Europe, aerial infection of the foliage was observed (Robin et al. 2011; Green et al. 2013). In Taiwan, P. lateralis was also
found infecting the foliage of the endemic Chamaecyparis obtusa (Webber et al. 2012). Recently, Nechwatal et al. (2011)
reported on the infection of twigs of lower branches of European beech by P. plurivora and P. cambivora. However, these
usually soilborne pathogens did not colonize leaves, but infected not fully lignified twigs of the current year, causing dieback. Histopathological changes in leaves and petioles of infected woody hosts deserve further research.
2.4 Attraction of zoospores
As mentioned above, the infection of Phytophthora spp. occurs via zoospores. Recognition of the host plant by soilborne
Phytophthora pathogens is essential for root infection (Tyler 2002). Root-secreted chemicals are known to mediate many
interactions in the rhizosphere (Badri and Vivanco 2009). When roots of seedlings are placed into zoospore suspensions in
vitro, zoospores accumulate at specific sites, typically at the zone of root elongation and wound sites (Zentmyer 1961).
These are locations of maximum nutrient release and least host resistance. Zoospores of many Phytophthora and Pythium
species are attracted in vitro by different amino acids such as glutamate, aspartate, asparagine and glutamine (Deacon and
Donaldson 1993; Cahill and Hardham 1994). Combinations of amino acids are often more attractive than single compounds.
This non-specific attraction is typical for Phytophthora species showing a wide host range. However, species with a
restricted host range exhibit much more specificity regarding their attraction towards root exudates. Cameron and Carlile
(1978) reported that isovaleraldehyde and valeraldehyde produced by Theobroma cacao roots are potent chemoattractants
for P. palmivora zoospores down to concentrations of 1 lM. They further concluded from their data that these attractants
act through a specific receptor (Cameron and Carlile 1981).
The best investigated example of specificity is the attraction of zoospores of P. sojae to daidzein and genistein (Fig. 5).
These isoflavones are found in soybean seeds and are released by roots into the rhizosphere. Morris and Ward (1992)
proved that only zoospores of P. sojae were attracted to concentrations of these compounds down to 0.1 nM. However, substantial levels of variation were recorded in the attraction of zoospores of different P. sojae genotypes (Tyler et al. 1996).
Besides zoospore attraction, hyphal germ tubes of P. sojae were also shown to respond to soybean isoflavones (Morris et al.
1998). A comparison of various phenolic compounds revealed that those carrying the phenolic 4′- and 7-hydroxyl groups
on the isoflavone structure were the most active compounds in attracting zoospores (Tyler et al. 1996). The signal transduction pathways involved for the responses of P. sojae to daidzein and genistein are largely unknown. Hua et al. (2008)
reported that a single-copy gene in P. sojae, named PsGPA1, which codes for a G-protein a-subunit was involved in chemotaxis to soybean isoflavones. All PsGPA1-silenced P. sojae mutants were severely affected in their chemotaxis to soybean
isoflavones. Additionally, zoospore encystment and cyst germination were also altered. These PsGPA1-silenced mutants were
no longer able to infect soybean. These results clearly show that chemotaxis of Phytophthora zoospores by root exudates
plays a key role in host recognition as well as infection and may have an influence on the host range.
3 Physiological, biochemical and molecular reactions of infected host plants
3.1 Susceptible Phytophthora–woody plant interactions during root infection
Fleischmann et al. (2004) investigated the effect of P. plurivora (under its previous name P. citricola; Jung and Burgess
2009) and P. cambivora on biomass and nutrient contents of beech saplings growing in natural soil. Both root pathogens
reduced significantly the total biomass of beech as compared to controls, and finally, most of the plants died at the end of
8
W. Oßwald, F. Fleischmann, D. Rigling et al.
HO
(a)
HO
O
O
4',7-dihydroxy-isoflavone
OH
(b)
O
OH
O
OH
4',5,7-trihydroxy-isoflavone
Fig. 5. Chemical structures of (a) daidzein and (b) genistein. The hydroxyl groups in 4′ and 7 position, which are responsible for zoospore
attraction, are bold.
the experiment eleven months after inoculation. Fine root lengths as well as the number of root tips of all infected plants
were reduced between 30 and 50%. In addition, P. plurivora and P. cambivora infection resulted in a significant reduction
in nitrogen concentration of leaves, whereas this nutrient was slightly increased in fine and coarse roots of infected beech.
This inhibition of nitrogen allocation from roots into leaves of beech saplings infected with P. plurivora was confirmed by
15
N-labelling studies (F. Fleischmann, unpublished data). Vice versa carbohydrate allocation from leaves into roots was
blocked by root infection. Similar results were also reported by Labanauskas et al. (1976) for avocado seedlings infected
with P. cinnamomi.
Many studies have shown that after root infection, Phytophthora spp. grow inter- and intracellularly in host tissue, thus
causing severe structural changes (see Section 2.1) (Cahill and Mccomb 1992; Oh and Hansen 2007; Portz et al. 2011;
Corcobado et al. 2013). A common consequence of root destruction by Phytophthora spp. is a decreased water absorption
capacity of infected plants. This was shown indirectly by measuring the soil water content in the rhizosphere of infected
plants as compared to non-infected controls (Sterne et al. 1978; Whiley et al. 1986; Maurel et al. 2001b). This decreased
ability to absorb and conduct water can cause death of infected trees by itself, especially when water is located in deep soil
layers (Shea et al. 1982) or when root infection is close to the bole (Davison and Tay 1995). Portz et al. (2011) proved that
inhibition of water consumption of beech seedlings infected with P. plurivora (under its previous name P. citricola) was
positively correlated with the amount of zoospores used in the experiment. Dawson and Weste (1984) described a failure
in root water transport for the susceptible Eucalyptus sieberi, when infected with P. cinnamomi. Surprisingly, the major
reduction in hydraulic conductivity was measured within the first 2 weeks after infection, although the pathogen had colonized only 8–12% of the total root system. These results showed that the infection by Phytophthora spp of a susceptible
host could trigger a generalized dysfunction in plant water relations, which could be mediated by hormonal changes. It was
suggested that a change in the balance between cytokinins and abscisic acid (ABA) could cause the change in water relations in E. marginata trees (Cahill et al. 1986). The authors showed that the concentrations of the Z-type and IP-type cytokinins were reduced in the xylem sap of the susceptible E. marginata after P. cinnamomi infection prior to reduction in
root hydraulic conductivity. Cahill et al. (1986) furthermore concluded that ABA concentration was increased in roots
infected by P. cinnamomi. ABA is known to play a crucial role in the regulation of stomatal conductance of plants suffering
drought stress (Morgan 1984; Davies and Zhang 1991). A common reaction observed in several Phytophthora pathosystems
studied under different conditions was a decrease in stomatal conductance and transpiration occurring at the early stages
of infection. In Castanea sativa saplings grown in split-root systems, decrease in stomatal conductance was correlated with
the proportion of roots infected by P. cinnamomi (Maurel et al. 2001a). The observed concentrations of ABA in the xylem
sap of infected chestnut plants suffering drought stress confirmed that a root-to-shoot hormonal signal could trigger stomata closure after Phytophthora root infection (Maurel et al. 2004). In oaks, the response to infection by P. cinnamomi varied according to their root susceptibility. In pedunculate, cork and red oaks, which displayed moderate to low
susceptibility, only slight changes to infection were observed regarding water relations and above-ground biomass (Maurel
et al. 2001b; Robin et al. 2001). Similar results were obtained for hybrid chestnuts (clone CA125, C. crenata 9 C. sativa
hybrid) known to be tolerant to ink disease. However, in holm oaks infected with P. cinnamomi, which lost about 67% of
their root system, stomatal conductance and water potential were strongly reduced, whereas water use efficiency was
higher as compared to not inoculated trees. This indicates that water relations were more strongly impaired than photosynthesis. Consistently with the observation of naturally infected holm oaks (Gallego et al. 1999), the aerial biomass was also
reduced for these inoculated saplings. Fleischmann et al. (2005) studied the effects of P. plurivora on physiological parameters of infected beech seedlings. They showed that net photosynthesis rates decreased about 2 days after inoculation as
compared to controls, whereas electron quantum yield of photosystem II, leaf water potential and total water consumption
were only slightly impaired until 6 days after infection. These data are in good agreement with results of Crombie and Tippett (1990), who showed that leaf water potentials and stomatal conductance of E. marginata trees infected by P. cinnamomi
in the field were significantly lower as compared to apparently healthy trees. Similar results were also reported by Sterne
et al. (1978) and Whiley et al. (1986), who compared field transpiration values and stomatal conductance of P. cinnamomiinfected avocado trees with uninfected control plants.
Portz et al. (2011) showed in a split root experiment that invertases must have been involved in the local and systemic
conversion of sucrose in roots of beech seedlings infected with P. plurivora. In this study, the glucose and fructose contents
of infected as well as of not infected roots of the same seedlings were significantly increased at the expense of the total
Phytophthora – woody plant interactions
9
sucrose concentration. This supply of energy-rich compounds is of great importance for the pathogen during its biotrophic
growth within the host. Moreover, the authors proved that the 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase gene
was transiently expressed in leaves of infected seedlings in parallel with the first peak of a biphasic ethylene outburst.
Additionally, they measured a systemic upregulation of aquaporin transcripts mainly in leaves of seedlings which were
infected at their roots with P. plurivora. Manter et al. (2007) showed for the first time that purified P. ramorum elicitins
(see Section 4) caused an ethylene increase in leaves of the three host taxa Rhododendron macrophyllum, Lithocarpus
densiflorus and U. californica. The investigations of Fleischmann et al. (2004) furthermore showed that the concentrations
of a-tocopherol and xanthophyll cycle pigments were increased in plants infected by P. plurivora and P. cambivora, possibly
indicating that several reactive oxygen species (ROS) might be formed in leaves during infection.
Schlink (2010) analysed the host–pathogen interaction between P. plurivora and F. sylvatica with an oligo-microarray
approach to analyse the expression profiles of important defence genes characteristic for the hemibiotrophic host–pathogen
infection. The author demonstrated that there was a significant downregulation for salicylic acid-responsive genes, mainly
PR proteins, in roots of infected as compared to control beech saplings during the biotrophic growth of P. plurivora. She
concluded that the root pathogen was able to escape recognition by the host and to suppress the host’s defence gene
expression, therefore successfully colonizing host tissue. Similar results were described by Cahill and Mccomb (1992) who
compared changes in phenylalanine ammonium lyase (PAL) activity, lignin and phenolic synthesis of roots of the susceptible E. marginata infected with P. cinnamomi and control plants. There was no change in PAL activities for lignin and phenolic synthesis in infected roots of the susceptible host as compared to non-infected E. marginata saplings. These results
again show, at the protein level, that similar to P. plurivora, P. cinnamomi might be able to avoid recognition by the susceptible host and in consequence prevent the induction of defence.
Many investigations showed that ontogeny of plants influences their response towards Phytophthora pathogens; in most
cases, susceptibility decreased with age and physiological responses were delayed. For instance, 1-year-old beech plants
infected with P. plurivora did not show a significant decrease in net assimilation and transpiration until bud break in the
second year of infection. Water use efficiency (WUE) data clearly indicated again that infected plants suffered from severe
drought (Fleischmann et al. 2002).
To understand symptom development of Phytophthora diseases, the involvement of toxic metabolites in pathogenesis has
been proposed. The first report was by Wolf (1933), who suggested that proteins released by P. parasitica are involved in
black shank disease of tobacco. Woodward et al. (1980) and Keen et al. (1975) reported that ß-glucans of Phytophthora cell
walls were able to cause wilt in their host plants. Capasso et al. (1997) discussed a novel phytotoxic peptide produced by
P. nicotianae, called phytophorin, to be involved in symptom development.
Besides bacteria and fungi, Phytophthora pathogens are also known to release necrosis and ethylene-inducing peptide 1
(Nep1)-like proteins (NLPs) that trigger leaf necrosis and also induce defence responses in various plants (Dong et al.
2012). The authors have shown that an intact heptapeptide (Gly-His-Arg-His-Asp-Trp-Glu) motif was important for necrosis-inducing activity. They further proved that most of the NLP genes were expressed during cyst germination of P. sojae
and early infection stages. Ottmann et al. (2009) also concluded from their data that NLPs of P. parasitica contribute to
host infection by plasma membrane destruction and cytolysis and therefore described them as virulence factors. Interestingly, the NLPs are also known to induce defence responses in host plants such as parsley or Arabidopsis, possibly due to
their membrane-disrupting activity (Fellbrich et al. 2002; K€
ufner et al. 2009). Several corresponding genes of these cytolytic toxins (NLPs) were identified in the genome of P. sojae and P. ramorum (Tyler et al. 2006). Unfortunately, up to now,
none of these peptides were identified or described during interactions of Phytophthora species and woody plants. However, it can be assumed that similar proteins exist and function similarly during susceptible woody plant–Phytophthora
interactions.
The main physiological, biochemical and molecular reactions occurring after root infection of susceptible Phytophthora–
host interactions are summarized in Fig. 6.
3.2 Susceptible Phytophthora–woody plant interactions during trunk infection
Understanding how trunk infection affects tree physiology requires knowledge of the spread of Phytophthora in the stem
tissue. In Table 2, we have summarized woody plant–Phytophthora interactions, from which it is known that the pathogen
mainly infects the trunk and grows within parts of the phloem and the xylem.
Parke et al. (2007) showed that P. ramorum colonized the phloem as well as the xylem of tanoak (N. densiflorus). Hyphae
were observed in xylem vessels, ray parenchyma and fibre tracheids. During lesion development, a reduction in midday sap
flux and specific hydraulic conductivity was recorded, which was caused by tylosis development in infected sapwood tissue.
Brown and Brasier (2007) were also successful in isolating different Phytophthora species such as P. ramorum, P. kernoviae,
P. cambivora, P. cinnamomi and P. citricola from discoloured xylem beneath phloem lesions and concluded that this colonization must lead to a local xylem dysfunction.
Mombour (2009) studied the impact of alder Phytophthora (P. alni subsp. alni) on stem and twig metabolism (corticular
photosynthetic metabolism) of infected alder trees in the field. Additionally, bark chlorophyll fluorescence in alder was
explored in controlled climate chambers 10 weeks after basal stem inoculation. The presence of the pathogen induced a
sharp reduction in maximum (Fv/Fm) and effective (DF/Fm′) quantum yield of photosystem II within the visually detectable stem lesions. Observations of the axial as well as radial spread of the pathogen revealed that near the point of inoculation and in the whole centre of the tongue-shaped stem lesion, Fv/Fm and DF/Fm′ of the cortex chlorenchyma decreased
to almost zero, indicating tissue necrosis. Low values of Fv/Fm and DF/Fm′ were also found in presymptomatic regions
10
W. Oßwald, F. Fleischmann, D. Rigling et al.
Chlorosis and wilting of leaves
Above ground
10
10
Ethylene
Photosynthesis
ACC-oxidase
Stomatal
aperture
8
ACC-synthase
Toxins ?
(elicitins?)
Zoospores
Penetration
1
Elicitins
6
Leaf water potential
(more
negative)
7
Below ground
Thinning of the canopy
10
5
Bio-/necrotrophic growth
Elicitins
and other
effectors
Root destruction
Plant defence
Uptake of
water/nutrients
2
3
4
ABA
Cytokinins
9
Fig. 6. Effects of root infection and growth of Phytophthora on physiological, biochemical and molecular reactions of susceptible host
plants. 1: Release of elicitins into the rhizosphere, facilitating penetration; 2: Downregulation of defence genes, thus facilitating growth of
the pathogen; 3: Destruction of roots during growth and impairment of water and nutrient uptake; 4: Increase in abscisic acid (ABA) in
roots; 5: Decrease in leaf water potential (more negative); 6: Closure of stomata, and in consequence, decrease in photosynthesis;
7: Possible release of toxins and effector molecules into the host tissue during biotrophic growth of the pathogen and transport into the
canopy via xylem sap flow; 8: Upregulation of genes of the ethylene pathway and release of the phytohormone by leaves; 9: Decrease in
cytokinin content in roots during the necrotrophic growth of the pathogen; 10: Chlorosis and wilting of leaves in the canopy in
consequence of a changed water and hormonal status of the host plant caused by Phytophthora root infection. (red lines and text: growth
of Phytophthora; blue lines and text: effects of Phytophthora metabolites; green lines and text: effects and metabolites of the host plant;
large arrows (red, blue and green): stimulation of the corresponding factor; blocked lines (blue and green): inhibition of the corresponding
factor; black arrows: increase/upregulation or decrease/downregulation of metabolites/genes).
beyond the visibly stem lesion, proofing the use of fluorescence to detect the hidden damage of Phytophthora long before
macroscopic symptoms appear.
In contrast, substantial photosynthetic activity was found in non-invaded parts of inoculated and control trees. Thus, corticular photosynthesis stayed unaffected in these stem parts supporting stem carbon balance (Pfanz et al. 2002; Wittmann
and Pfanz 2008). Additional chlorophyll fluorescence measurements in the field further illustrated that stem infection with
P. alni subsp. alni and its effect on the bark tissue is characterized by very quick temporal changes, due to a rapid destruction of the photosynthetic apparatus by the pathogen. Within 4 months, bark infection spread over an area of 900 cm2.
Clemenz et al. (2008) investigated physiological and biochemical responses of three-year-old A. glutinosa plants to stembase inoculation with the same pathogen. One main result was the finding that leaf starch concentration of infected plants
was significantly higher than in control plants possibly indicating that the destruction of bark tissue by P. alni subsp. alni
impaired phloem transport from leaves to roots. The authors discussed that leaf stomatal closure of infected plants was
more likely related to inhibition of photosynthesis due to leaf starch accumulation, than to impaired uptake and conduction
of water. In consequence, leaf water potentials increased (became less negative) as compared to control plants. This conclusion is supported by the finding that rates of photosynthesis decreased with the extent of phloem tissue destruction (girdling) by the pathogen at the stem base (Fig. 7). The results of Clemenz et al. (2008) are in good agreement with Krapp
and Stitt (1995) who measured a decline in photosynthesis, a downregulation of the rbcS gene (small subunit of Rubisco)
and a twofold to fivefold accumulation of carbohydrates in leaves after cold-girdling of the petioles of spinach leaves.
Furthermore, Clemenz et al. (2008) measured no differences in predawn leaf water potential between inoculated and
control plants. Thus, both water-absorbing fine roots as well as the xylem appeared to have been functioning adequately.
This was the opposite of the findings of Sterne et al. (1978), Dawson and Weste (1984) and Fleischmann et al. (2005),
who showed that due to primary root infection and destruction, plants suffered severe water stress. Thus, minimum water
potential values became more negative, and in consequence, plants closed their stomata and photosynthesis decreased.
Manter et al. (2007) also reported measurements on photosynthesis of R. macrophyllum infected with P. ramorum at the
stem base. They showed that the first decline in CO2 uptake rates in visibly asymptomatic leaves was prior to stomatal closure. The authors concluded that the broader decline in photosynthesis after the development of severe stem lesions might
be due to water stress caused by loss in water transport capacity. Unfortunately, they did not measure leaf water potential
values in parallel with photosynthesis parameters. Therefore, their data cannot be compared with those of Clemenz et al.
(2008).
The main physiological and biochemical reactions reported on P. alni subsp. alni or P. ramorum and their susceptible
hosts after trunk infection are summarized in Fig. 8.
Phytophthora – woody plant interactions
16
11
r 2 = 0.60
Net CO2 uptake rate
Amax [µmol m–2 seconds–1]
14
12
10
8
6
4
2
0
30%
40%
50%
60%
70%
80%
90%
100%
Extent of cortex destruction (girdling)
[% of stem circumference]
Fig. 7. Correlation of net CO2 uptake rates (Amax) with the extent of girdling symptoms at the stem base of 2-year-old Alnus glutinosa
saplings after artificial inoculation with Phytophthora alni. subsp. alni. Closed circles: inoculation on one side of the stem; open circles:
inoculation on two opposite stem sides; values represent means ! standard error (Clemenz et al. 2008).
Chlorosis and wilt of leaves
14
14
7
Leaf
nutrients
13
Stomatal
aperture
8
12
11
Leaf water potential
(more
positive)
Zoospores
Below ground
Photosynthesis
1
(more
negative)
6
Leaf
carbohydrates
(accumulation)
10
Sap wood
conductance
Bio-/nectotrophic
Penetration
growth
in bark tissue
Tylosis of
xylem vessels
9
2
Growth
into xylem
vessels
Destruction
of phloem
Uptake of
nutrients
Fine root growth
4
(root degeneration)
Root symbionts
Phloem transport
Above ground
5
Dieback of the canopy
14
3
(girdling)
Root
carbohydrates
Fig. 8. Effects of trunk infection and growth of Phytophthora ramorum and P. alni subsp. alni on host physiology. 1: Trunk infection and
growth within the bark tissue; 2: Partial destruction of phloem tissue by the pathogen; 3: Inhibition of carbohydrate transport from leaves
into roots via phloem; 4: Reduced support of roots and root symbionts with carbohydrates, thus degeneration of fine roots and by this
impairment of water and nutrient uptake; 5: Decrease in leaf nutrients; 6: Inhibition of photosynthesis due to carbohydrate accumulation
in leaves, resulting in stomatal closure (7); 8: Leaf water potential increases; 9: Additional growth of the pathogen within the xylem and
occlusion of vessels; 10: Impairment of xylem hydraulic conductivity; leaf water potential becomes more negative (11), stomatal closure
(12) and decrease in photosynthesis (13); 14: Chlorosis and wilting of leaves in the canopy in consequence of a changed water and
nutrient status of the host plant caused by Phytophthora trunk infection. (red lines and text: growth of Phytophthora; green lines and text:
effects and metabolites of the host plant; large arrows (red and green): stimulation of the corresponding factor; blocked green lines:
inhibition of the corresponding factor; black arrows: increase or decrease in metabolites/physiological parameters).
3.3 Resistant Phytophthora–woody plant interactions
Much less information is available for tolerant or even resistant woody plant–Phytophthora interactions than is known for
herbaceous plants.
Important information was obtained by Cahill and Mccomb (1992), who investigated the significance of the secondary
metabolism for the susceptible and resistant interaction between Eucalyptus saplings and P. cinnamomi. They demonstrated
that inoculation of roots with P. cinnamomi induced the activity of PAL and the concentration of phenolic compounds in
root segments of the resistant species Corymbia calophylla but not in the susceptible E. marginata. In particular, the lignin
12
W. Oßwald, F. Fleischmann, D. Rigling et al.
(a)
(b)
L
H
I
Pa
Pl
Fig. 9. (a) Transmission electron microscopy of a papilla formed in parenchyma cell of Quercus robur root in close vicinity to a hypha of
Phytophthora quercina (M. Brummer and W. Oßwald, unpublished data). (b) and lignituber (b) Parenchymous fine root cells of the
resistant Eucalyptus megacarpa in Western Australia producing lignitubers around invading coralloid hyphae of Phytophthora cinnamomi
(Fig. 9a: M. Brummer, unpublished; Fig. 9b was originally published by Jung et al. 2013b). H: hypha; I: intercellular space; L: lignituber; Pa:
papilla; Pl: plasmalemma.
contents of inoculated roots of E. marginata were increased up to 53% as compared to controls when inoculated with
P. cinnamomi (see Fig. 11). However, the lignin concentrations of inoculated roots of E. marginata were unchanged. Results
of thin-layer chromatography indicated the induction of new compounds in roots of infected E. calophylla saplings. The
treatment of roots with the PAL inhibitor amino-oxyacetate transformed resistant saplings into susceptible ones and inhibited the induction of lignin and the accumulation of phenolic compounds. Thus, induction of the secondary metabolism by
P. cinnamomi in Corymbia calophylla was casually linked with resistance against the root pathogen. The same defence
mechanisms were also found in clonally propagated E. marginata seedlings resistant to P. cinnamomi (Cahill et al. 1993).
The apposition of callose layers onto inner cell walls (=formation of papillae) is a general response of most woody plants
to prevent penetration of cell walls by pathogens (Aist 1976; Heath 1980). The rates of formation and frequency vary with
the degree of host resistance and the environmental conditions (Cahill and Weste 1983; Cahill et al. 1989). In resistant
plant species as well as in susceptible hosts under environmental conditions that are disfavouring the pathogen invading
intracellular hyphae of P. cinnamomi are often encapsulated by callose layers deposited by the host onto a host-derived
membrane that is continuous with the plasmalemma of the invaded root cell (Cahill et al. 1989; Jung et al. 2013b). In resistant species such lignitubers may criss-cross and completely fill invaded root cells (Jung et al. 2013b; Fig. 9b). Papilla and
lignituber formation were also visible in the interactions between Quercus robur and P. quercina (Jung et al. 1996; Brummer et al. 2002) (Fig. 9a) and between C. lawsoniana and P. lateralis (Oh and Hansen 2007).
Ockels et al. (2007) compared the amount of different phenolic compounds of phloem tissue of coast live oaks (Q. agrifolia) which were artificially or naturally infected with P. ramorum with that of apparently healthy tissue. In most cases,
infected tissue showed much higher concentrations for gallic acid, which inhibited colony growth of P. ramorum and other
Phytophthora species significantly. On the basis of their results, the authors concluded that changes in phloem chemistry
might be related to resistant phenotypes of Q. agrifolia observed throughout California forests. In this context, Del Rio et al.
(2003) figured out that phenolic compounds such as catechin and tyrosol might be responsible for the induction of resistance in olive plants against Phytophthora sp. after Brotomax treatment.
Boava et al. (2011b) compared the expression of defence-related genes in Poncirus trifoliata and Citrus sunki susceptible
and resistant, respectively, towards the hemibiotrophic pathogen P. nicotianae. They clearly showed that gene expression
for pathogenesis-related genes, such as PR1 (function unknown), PR2 (b-1,3-endoglucanase), PR3 (chitinase class I), PR5
(Thaumatin-like protein) as well as chalcone synthase (CHS), PAL, lipoxygenases (LOX) and peroxidases (POD), was significantly higher in the resistant as compared to the susceptible interaction. Thus, the authors concluded that these proteins
might be involved in the resistant interaction between P. trifoliata and P. nicotianae.
For Quercus suber, it is supposed that the infection by P. cinnamomi has a mixed profile of compatible and incompatible
interactions. In affected areas of Portugal and Spain, cork oak and holm oak trees can be found ranging from slow decline
(tolerant interaction) to sudden death (highly susceptible interaction). These symptoms are shown in Fig. 10 in comparison
with a healthy not affected tree.
Several molecular approaches have been applied to study the Q. suber–P. cinnamomi pathosystem in order to disclose
pathogenesis-related genes and to figure out how the regulation of these genes could be connected with disease expression.
Coelho et al. (2011) cloned and characterized seven Q. suber genes in this pathosystem coding for phenylalanine ammonia
lyase (QsPAL), cinnamyl alcohol dehydrogenase2 (QsCAD2), CAD1 (QsCAD1), NBS-LRR resistance protein (QsRPc),
RelA/SpoT protein (QsRSH), disulphide isomerase (QsPDI) and a cationic peroxidase (QsPOX1). Because the expression of
the last five genes was increased 24 h post-infection, the authors concluded that they are potentially involved in the
defence of Q. suber to P. cinnamomi. With the molecular data obtained, a hypothetical model was conceived that illustrates
the initial events of the interaction between Q. suber and P. cinnamomi and other woody plant–Phytophthora interactions
(Fig. 11). For a tolerant interaction, it is proposed that effector molecules like aldehyde aromatic compounds, similar to the
eutypine toxin, released by P. cinnamomi during early infection, can be reduced and inactivated to alcohols by a Q. suber
cinnamyl alcohol dehydrogenase 1 (QsCAD1) (Coelho et al. 2006). In the case of the sudden death phenotype, trees might
contain inactive QsCAD1 proteins, as a result of polymorphic forms of the coding gene.
Phytophthora – woody plant interactions
(a)
(b)
13
(c)
Fig. 10. Cork oak (Quercus suber) infected with Phytophthora cinnamomi. Healthy tree (a), tree showing slow decline (b) and tree showing
sudden death (c) (for more information see: (http://w3.ualg.pt/~acoelho/Thecorkoakquercussuber.wmv).
Effector(s)
Phenylalanine
Cell wall
4-hydroxycinnamate
Plasma
membrane
4
(p)ppGpp
(TF)
ROS
QsPDI
S
5
Phenols
S
S
S
P
C
TF
R3
R2
2
TM
ATP + GTP Qs
NBS
RSH
8
4-hydroxycinnamlyaldehydes
LRR
H2O2
QsRPc
9
4-hydroxycinnamyl
alcohols
CAD2
PAL
7
Qs POX
O
H
R1
H2O
Suberin
Lignin
Effector
O
H
R1
CC/
TIR
R2
R3
QsCAD1
1
3
CH2OH
S
S
TF
S
R1
R2
R3
S
6
Regulation of defense related genes
Cytoplasm
Nucleus
Fig. 11. Hypothetical molecular mechanism model for woody plants expressing defence mechanisms towards Phytophthora pathogens
(modified according to Coelho et al. 2011). 1: Deactivation of virulence factors of the pathogen by the QsCAD1 (Quercus suber cinnamyl
alcohol dehydrogenase 1) protein. 2: Specific interaction between Phytophthora effector molecules and the LRR region of the QsRPc protein
(Q. suber resistance protein to Phytophthora cinnamomi) as well as signal transduction via NBS and coiled-coil (CC)/TIR domains 3.
Activation of the MAPK cascade and release of transcription factors 4:Synthesis and release of the transcription factor (p)ppGpp from ATP
and GTP catalysed by the QsRSH (Q. suber RelA/SpoT homologue) protein. 5: Activation of transcription factors and other proteins, by
oxidation or reduction and/or isomerization of cysteine residues, catalysed by QsPDI (Q. suber disulphide isomerase), in response to ROS.
6: Regulation of defence-related genes by different transcription factors resulting from reactions 2 to 5. 7/8: Activation of phenylalanine
ammonium lyase (PAL) and CAD2 (cinnamyl alcohol dehydrogenase2) involved in the synthesis of phenolic compounds, precursors of
lignin and suberin (shown for Eucalyptus and Citrus). 9: H2O2-dependent polymerization of phenols forming lignin and suberin catalysed
by the peroxidase QsPOX, (Q. suber peroxidase). Abbreviations: TF (transcription factor); TIR (Toll-interleukin-1receptor); ROS (reactive
oxygen species); (p)ppGpp (guanosine tetra and pentaphosphate); TM (transmembrane domain); CC (coiled-coil domain); NBS (Nucleotidebinding site); LRR (Leucine-rich repeat); MAPK (mitogen-activated protein kinases); ATP (adenosine triphosphate); ADP (adenosine
diphosphate); GTP (guanosine triphosphate), QsCAD1 (Q. suber cinnamyl alcohol dehydrogenase 1) protein, QsRPc protein (Q. suber
resistance protein to P. cinnamomi), QsRSH (Q. suber RelA/SpoT homologue), QsPDI (Q. suber disulphide isomerase), PAL (phenylalanine
ammonia lyase), CAD2 (cinnamyl alcohol dehydrogenase2), CW = cell wall, Pl = plasmalemma, Cyt = cytoplasm.
In tolerant hosts, additional effector molecules secreted by the oomycete might be recognized by Q. suber pattern recognition receptors (see also Fig. 1), a group of proteins, known to play a significant role in pathogen recognition and in
downstream plant defence responses (Ku 2004; Matsushima and Miyashita 2012). In response to recognition, MAP kinase
14
W. Oßwald, F. Fleischmann, D. Rigling et al.
cascades will be activated, resulting in the release of transcription factors which upregulate defence genes and finally trigger cellular defence pathways. More recently, the transcriptome analysis of Q. suber confirmed the existence of differentially
expressed R genes in this pathosystem that encode to putative intracellular receptors with coiled-coil (CC) or Toll/
interleukin-1 (TIR) domains (A. C. Coelho, unpublished data). The release of further transcription factors such as (p)ppGpp
catalysed by the QsRSH (Q. suber RelA/SpoT homologue) protein or those formed by the oxidation or reduction and/or
isomerization of cysteine residues via the Q. suber disulphide isomerase (QsPDI protein) is discussed (Fig. 11). The polymerization of phenols into the matrix of lignin and suberin catalysed by the peroxidase QsPOX in a H2O2-dependent process
is known to be a significant defence response in woody plants (Fig. 11). It is well known for many herbaceous plants,
infected by different pathogens, that hydrogen peroxide is generated by a plasmalemma-bound NADPH oxidase that
releases the oxidant into the cell wall (Marino et al. 2012; O’Brien et al. 2012).
Another extensive gene expression study was carried out by Boava et al. (2011a) in Citrus to evaluate the transcriptional
changes in response to P. parasitica. Their microarray data suggested that in resistant Citrus genotypes, defence was specifically correlated with the upregulation of the R genes ‘TIR-NBS-LRR’ and ‘RPS4’. The ‘Toll-interleukin-1 receptor’ (TIR)
domain of both genes is known to be responsible for cytoplasmic signalling in animals (Hammond-Kosack and Jones 1997)
(see also Fig. 11). These NBS-LRR RPs have been found in a wide range of plants after the attack of biotrophic or hemibiotrophic pathogens, such as fungi, oomycetes, bacteria and viruses (Jones and Dangl 2006).
The alternative 454 pyrosequencing methodology on the GS FLX Titanium platform was applied to unravel the molecular
mechanisms of interaction between Persea americana and P. cinnamomi (Mahomed and van den Berg 2011). The avocado
root transcriptome was classified according to Gene Ontology terms and includes genes involved in cellular processes and
defence mechanisms highlighting genes such as metallothionein, thaumatin, pathogenesis-related (PR-10) psemI, mlo and
profilin as genes that were differentially regulated in response to P. cinnamomi.
Unfortunately, the limitation of such approaches is associated with the difficulty in identifying and characterizing genes
related to pathogenesis, when there are no genome data as a reference.
4 Elicitins – specific Phytophthora proteins and their impact on woody plant interactions
Elicitins (ELI) belong to the group of apoplastic effectors, holoproteins that are produced by Phytophthora and closely
related Pythium species and are absent from any other organism studied so far. All elicitins have in common a totally conserved domain composed of six cysteine residues that form three distinct disulphide bridges (Fefeu et al. 1997; Ponchet et
al. 1999). Other common features are a significant abundance of a few amino acids, such as serine and threonine residues
(ca. 30% of the protein), lack of tryptophan, histidine and arginine residues and a signal peptide that is post-translationally
removed.
According to Kamoun et al. (1997a), Baillieul et al. (2003) and Qutob et al. (2003), the elicitins were divided into three
classes (Table 4). Canonical elicitins (class 1) (ELI-1) have a highly conserved 98-amino acid domain and have been classified as a- (class-I A) or b- (class-I B) according to their isoelectric point (pI), acidic for a (4–5) or basic for b (7.5–8.5),
determined by their Lys content (2–4 in a, 6 in b). Besides the pI, they differ in their necrotic capacity that is related to
the nature of the amino acid at position 13, a hydrophilic residue in b-elicitins (more necrotic), usually a lysine (Pernollet
et al. 1993; Odonohue et al. 1995; Pleskova et al. 2011), and a hydrophobic residue in a-elicitins, a valine.
Tertiary structures of cryptogein (Boissy et al. 1996; Fefeu et al. 1997) and b-cinnamomin (b-CIN, Rodrigues et al. 2006)
were solved showing globular proteins with six a-helices, a short two-stranded b-sheet and a large Ω-loop. This fold is
strengthened by three disulphide bonds. The hairpin b-sheet and the Ω-loop form a beak-like motif, which borders a large
hydrophobic cavity that may accommodate a sterol molecule (Mikes et al. 1997, 1998; Vauthrin et al. 1999; Lascombe et al.
2002; Rodrigues et al. 2006). These data suggest an intrinsic function of the elicitins as extracellular carriers of sterols and
other lipids supposed to be essential to Phytophthora which cannot synthesize sterols.
Less frequent are class II elicitins with a short C-terminal tail (103–104 amino acid-long ORFs), often hyperacidic (HAE,
pI = 3.5) (for a revision see Ponchet et al. 1999).
Several new elicitin-like (ELL) sequences (class III) with diverse shorter or longer elicitin domains that are more diverse
at the sequence level than the conserved domains in ELIs were found in P. infestans, P. sojae, P. brassicae and P. ramorum
(Table 4).
Except for the elicitins of class I and II, ELL peptides possess C-terminal domains of variable length, many of which have
a high threonine, serine or proline content suggesting an association with the cell wall. In addition, some of these ELLs
have a predicted glycosylphosphatidylinositol site suggesting anchoring of the C-terminal domain to the cell membrane
(Jiang et al. 2006).
Table 4. Classification and characteristics of elicitin proteins.
Class
Class
Class
Class
Class
I: subclass IA (a)
I: subclass IB (b)
II
III (Elicitin-like; ELL)
pI, isoelectric point.
Amino acids (active protein)
Characteristics
pI
98
98
98+ some few on carboxy terminal
98+ many on carboxy terminal
Position13: valine (lipophilic)
Position13: lysine (hydrophilic)
Acidic
Basic
Highly acidic
Acidic
Phytophthora – woody plant interactions
15
The phylogenetic tree of 128 ELIs and ELLs from P. sojae, P. brassicae, P. infestans and P. ramorum published by Jiang
et al. (2006) consists of 17 ELI and ELL clades. Clade 1 comprises peptides grouped here as class I and II elicitins. Some
Phytophthora elicitins together with their characteristic feature are summarized in Table 5.
Overall, eli and ell genes are expressed at different levels (expression of eli genes seem to be higher than expression of
ell genes (Jiang et al. 2006)) and during different life cycle stages. Elicitins of class I and II are strongly represented in
mycelium, mating cultures (e.g. P. infestans) and infected plant tissue (e.g. P. sojae). In contrast, ELLs are primarily found in
zoospores of both P. infestans and P. sojae.
In P. cinnamomi, canonical b-cin appears more abundantly expressed in the early stages of infection, and its expression
decreases as colonization progresses in infected Q. suber roots (Horta et al. 2008). The a-cinnamomin gene (a-cin) behaves
in an opposite way. The b-hae and a-hae genes are also expressed but at a much lower rate, the former absent in the early
stages and the latter decreasing its expression with time. Like a-cin, other canonical a-ELIs genes appear to increase their
Table 5. Summary of Phytophthora elicitins.
Phytophthora spp.
P. alni
P. cambivora
P. cactorum
P. capsici
P. cinnamomi
P. citrophthora
P. cryptogea
P. drechsleri
P. fragariae
P. infestans
P. hibernalis
P. megasperma
P. nicotianae
P. palmivora
P. parasitica
P. plurivora (formerly P. citricola)
P. quercina
P. ramorum
P. syringae
P. sojae
pI, isoelectric point.
Elicitin
AE1.1
AE1.2
AE2
BE1
BE2
HAE1
AE1.1
AE2
CAC-A (cacto)
Capsicein (a)
CAP-Pa28
Cinnamomins
a-CIN
b-CIN
HAE1-cin
HAE2-cin
Citro
Cryptogeins
Cry-a
Cry-b
HAE1-cry
HAE2-cry
Dre-a
Dre-b
AE1.1
AE2
INF1
INF2a
INF2b
INF4
INF5
INF6
hib1 = syringicin
hib2
hib3
a-megaspermin
b-megaspermin
ϒ-megaspermin
172 = parasiticein
Palmivorein
PARA1 (Parasiticein)
310 = Parasiticein
Citricolin = plurivorin
Quercinin
(ram-alpha 1 and ram-alpha 2)
+ 3 elicitins
Syringicin
SOJA, SOJB=
Sojein1, Cojein2
Class
pI
I-A (acidic)
I-A (acidic)
I-A (acidic)
I-B (basic)
I-B (basic)
II Highly acidic
I-A (acidic)
I-A (acidic)
I-A (acidic)
I-A (acidic)
I-A (acidic)
4.99
4.99
4.99
8.22
8.22
3.95
4.99
4.99
4
3.5
4.23
I-A (acidic)
I-B (basic)
II Highly acidic
II Highly acidic
I-A (acidic)
4.4
8.9
3.38
3.54
3.5
I-A (acidic)
3.6
I-B (basic)
9.8
II Highly acidic 3.88
II Highly acidic 3.34
I-A (acidic)
4.6
I-B (basic)
8.96
I-A (acidic)
4.99
I-A (acidic)
4.99
I-A (acidic)
4.22
III
3.37
III
3.73
I-B
9.97
III
4.09
III
3.34
I-A (acidic)
I-A (acidic)
I-A (acidic)
I-A (acidic)
4
I-B (basic)
8.36
III
3.8
I-A (acidic)
<4
I-A (acidic)
4.0 ! 0.2
I-A (acidic)
4.22
I-A (acidic)
4.7
I-A (acidic)
4.2
I-A (acidic)
3.6
3.9
I-B (basic)
8.3
Class I elicitins ?
I-A (acidic)
I-A (acidic)
4.31
"3, "5
4.0; 6.16
References
Ioos et al. (2007)
Ioos et al. (2007)
Pernollet et al. (1993)
Pernollet et al. (1993)
Kim et al. (2010)
Pernollet et al. (1993)
Duclos et al. (1998)
Pernollet et al. (1993)
Pernollet et al. (1993)
Panabieres et al. (1995)
Ioos et al. (2007)
Pernollet et al. (1993)
Baillieul et al. (2003)
Ioos et al. (2007)
Baillieul et al. (2003)
Capasso et al. (2008)
Baillieul et al. (2003)
Capasso et al. (1999)
Churngchow and Rattarasarn (2000)
Kamoun et al. (1993)
Mouton-Perronnet et al. (1995)
Fleischmann et al. (2005)
Heiser et al. (1999)
Manter et al. (2010)
Capasso et al. (2001)
Yousef et al. (2009)
Baillieul et al. (2003)
16
W. Oßwald, F. Fleischmann, D. Rigling et al.
rate of expression as tissue colonization progresses. This was shown for the host/pathogen systems: F. sylvatica/P. plurivora (Fleischmann et al. 2005), tomato/P. infestans (Huitema et al. 2005), potato/P. infestans (Kamoun et al., 1997b),
tobacco/P. parasitica (Colas et al. 2001). In Q. robur roots infected with P. quercina, production of b-quercinin increases in
concert with the pathogen biomass (Brummer et al. 2002). The meaning of these differentiated behaviours of ELIs in the
biology of Phytophthora is still not fully understood. However, a reasonable picture emerges from recent work showing that
the silencing of the b-cin gene drastically reduced the colonization of Q. suber seedling roots by a mutant P. cinnamomi
strain, suggesting that b-cin plays a key role in the invasion process, possibly acting as an aggressiveness factor (Horta et
al. 2010). Recently, Dalio et al. (2011) showed that the acidic elicitin citricolin of P. plurivora was essential for the infection
of beech roots by the pathogen. A specific antibody raised against this proteinous effector molecule converted the highly
susceptible interaction into a resistant one. Using laser scanning microscopy, the authors proved that, due to the specific
interaction between the elicitin and its antibody, the effector got stuck to the root surface and was not found in the apoplastic space of root tissue as usual; in consequence, the pathogen was no longer able to colonize roots. In addition, in the
presence of the anticitricolin antibody, specific defence genes, such as PR-proteins and WRKY, were upregulated if compared to beech saplings treated with the pathogen in the absence of the antibody. These results suggest that the acidic elicitin of P. plurivora acts in a dual way in the susceptible interaction between beech and the pathogen; first, it is casually
linked with the penetration of the pathogen by still unknown mechanisms, and second, it suppresses important defence
genes of the host plant, finally causing ETS. All these results are in good agreement with data of Manter et al. (2010) who
found a positive relation between the amount of elicitin production by P. ramorum and its virulence. Summarizing these
findings, it can be concluded that some elicitins are associated either directly or indirectly with infection of susceptible
host–pathogen interactions. The question rises whether in these cases elicitins act as suppressors of PTI and which compounds of PTI are impaired (Fig. 1).
Other reports showed that a-elicitins do not appear to be directly involved in the early steps of colonization as their
expression is repressed at this stage. Their function seems to be related to later stages of infection, in particular sporulation and/or pathogen survival under saprophytic conditions. As the elicitin family diversified prior to speciation (Jiang et al.
2006), it is reasonable to admit that each elicitin group has its own distinct set of functions. The differential expression of
each type of elicitin, with highly variable isoelectric points (pI), may be an adaptive response of the pathogen to regulate
interactions within different surrounding environments.
When ELIs were discovered, they were found to cause a HR in tobacco and hence considered as elicitor proteins (Ricci
et al. 1989). Since then, the involvement of elicitins in the stimulus of defence responses has been frequently debated. In
tobacco, they were shown to cause extended leaf necrosis as well as systemic acquired resistance (SAR) to normally virulent pathogens (Ricci 1997). The HR induced in tobacco suggests that ELIs play a major role in the basic resistance of this
plant to Phytophthora spp. (Kamoun et al. 1994; Keller et al. 1999). Several studies showed that with the exception of Nicotiana, most plant species lack the capacity to respond to elicitins. In addition, some but not all cultivars of two Brassicaceae
species, radish and turnip developed necrosis (Kamoun et al. 1993). It was concluded that elicitins might be genus-specific
elicitors within Solanaceae and cultivar specific within Brassicaceae. More recently, oligandrin, an ELL secreted by Pythium
oligandrum was shown to induce systemic resistance to Fusarium crown and root rot in tomato plants (Benhamou et al.
2001) and defence reactions against Botrytis cinerea when applied to the roots of grapevine (Mohamed et al. 2007).
Resistance responses of Q. suber roots against P. cinnamomi induced by the ELI-1s cryptogein, capsaicin (Medeira et al.
2012b) and a-cinnamomin (Maia et al. 2008) and of C. sativa induced by the same peptide (Medeira et al. 2012a) were
observed, too. More recently, it was shown that a- and b-cinnamomin significantly reduced the pathogen biomass in pretreated Q. suber roots which confirms the role of elicitins in promoting the protection of oak root cells against P. cinnamomi
(G. Ebadzad and A. Cravador, unpublished data). Hence, species from the Fagaceae family also respond to ELIs suggesting
that these act as elicitors more generally than previously thought. Recently, Manter et al. (2007) showed that the two acidic
elicitins of P. ramorum, the causal agent of sudden oak death in California and Oregon, induced a hypersensitive-like
response in the incompatible host Nicotiana tabacum SR1 and in three compatible hosts, R. macrophyllum, N. densiflorus
and U. californica. Additionally, the P. ramorum elicitins caused a significant decline in chlorophyll fluorescence of leaves of
all four host plants which was correlated with a simultaneous H+ uptake and an ethylene outburst (see Fig. 6). Dutsadee
and Nunta (2008) purified an elicitin from P. palmivora, a pathogen of Hevea brasiliensis which induced defence responses
such as scopoletin induction in cell culture tissue and resistance in the rubber plant. The apparent contradicting behaviour
of elicitins, promoting virulence and functioning as avirulent determinants by eliciting defence reactions, is an example of
the dual role of pathogen-derived proteins. Therefore, the neutral term, effector, was coined to describe pathogen molecules
that have dual and conflicting functions depending on the genotype of the host and a variety of other variables (Kamoun
2007).
5 Conclusion and outlook
Phytophthora spp. have evolved different strategies to infect and invade trees. The most common way to colonize woody
host plants is (i) via roots, causing damage exclusively to this part of the tree or moving up into the trunk destroying considerable parts of the bark and sometimes girdling and killing the tree. (ii) Some Phytophthora species are able to aerially
infect stem bark with sporangia or zoospores but are seldom found in the xylem tissue. Finally, (iii) a small number of
species can infect through the leaves or needles causing severe necrosis and crown defoliation. Many Phytophthora species
are adapted to colonize different hosts in different ways.
Phytophthora – woody plant interactions
17
Various structural, physiological and biochemical reactions occur in woody plants infected by Phytophthora pathogens
Many studies have shown that the common reactions after root or bark infection are a gradual reduction in water uptake,
the fast closure of stomata and in consequence a decrease in photosynthesis. The visible symptoms often are chlorosis and
wilting of leaves, a thinning of the crown and eventually the death of the infected tree. Different hormonal imbalances are
considered to be involved in this process.
Despite considerable scientific progress in recent years, the molecular mechanisms involved in susceptible or resistant
woody plant–Phytophthora interactions are still poorly understood. Recent studies have shown that susceptible interactions
are characterized by a broad downregulation of defence-related genes in the host during pathogen invasion. Similar to herbaceous plants, the formation of secondary compounds seems to be of great importance for defence reactions in woody
plants, too. Thus, key enzymes of the secondary metabolism, such as PAL, CHS and POD, are significantly activated in the
resistant interaction. Additionally, genes for pathogenesis-related (PR) proteins are upregulated in resistant as compared to
susceptible plants. However, the molecular mechanisms underlying these interactions are still widely unknown. Compared
to herbaceous plant–Phytophthora interactions, where elicitins induce several defence reactions and are assumed to act as
avirulence factors, these small peptides are essential for the invasion of woody plant tissue by different Phytophthora species.
Further research is needed to elucidate the significance and the modes of action of these unique peptides in susceptible
and resistant woody plant–Phytophthora interactions. In future, research has to focus on characterising the root exudates of
woody plants in terms of composition and their role in the attraction of Phytophthora zoospores as a main precondition for
a successful infection. Furthermore, investigations of the Phytophthora secretome on the release of effectors into the rhizosphere will shed light into the very early mechanisms of host–pathogen interactions particularly in the case of soilborne pathogens. Very exciting will be the analysis of the Phytophthora genome data, which are currently generated by the
‘Phytophthora Genus Sequencing Consortium’ (Center for Genome Research and Biocomputing, Oregon State University), in
terms of host range, aggressiveness and the arsenal of effector and suppressor molecules.
Acknowledgements
This manuscript was prepared within Working Group 2 of the European COST Action FP0801: Established and Emerging Phytophthora:
Increasing Threats to Woodland and Forest Ecosystems in Europe (http://www.cost.eu/domains_actions/fps/Actions/FP0801). We thank
the COST office and the European Council for giving us the opportunity for stimulating discussions on Phytophthora: host–pathogen interactions.
References
Aist, J. R., 1976: Papillae and related wound plugs of plant cells. Annu. Rev. Phytopathol. 14, 145–163.
Anacker, B. L.; Rank, N. E.; Huberli, D.; Garbelotto, M.; Gordon, S.; Harnik, T.; Whitkus, R.; Meentemeyer, R., 2008: Susceptibility to Phytophthora ramorum in a key infectious host: landscape variation in host genotype, host phenotype, and environmental factors. New Phytol.
177, 756–766.
Badri, D. V.; Vivanco, J. M., 2009: Regulation and function of root exudates. Plant, Cell Environ. 32, 666–681.
Baillieul, F.; De Ruffray, P.; Kauffmann, S., 2003: Molecular cloning and biological activity of alpha-, beta-, and gamma-megaspermin, three
elicitins secreted by Phytophthora megasperma H2O. Plant Physiol. 131, 155–166.
Benhamou, N.; Belanger, R. R.; Rey, P.; Tirilly, Y., 2001: Oligandrin, the elicitin-like protein produced by the mycoparasite Pythium oligandrum, induces systemic resistance to Fusarium crown and root rot in tomato plants. Plant Physiol. Biochem. 39, 681–696.
Bezuidenhout, C. M.; Denman, S.; Kirk, S. A.; Botha, W. J.; Mostert, L.; McLeod, A., 2010: Phytophthora taxa associated with cultivated Agathosma, with emphasis on the P. citricola complex and P. capensis sp. nov. Persoonia 25, 32–49.
Biere, A.; Honders, S. J., 1996: Impact of flowering phenology of Silene alba and S. dioca on susceptibility to fungal infection and seed
predation. Oikos 77, 467–480.
Boava, L. P.; Cristofani-Yaly, M.; Mafra, V. S.; Kubo, K.; Kishi, L. T.; Takita, M. A.; Ribeiro-Alves, M.; Machado, M. A., 2011a: Global gene
expression of Poncirus trifoliate, Citrus sunki and their hybrids under infection of Phytophthora parasitica. BMC Genomics 12, 39.
Boava, L. P.; Cristofani-Yaly, M.; Stuart, R. M.; Machado, M. A., 2011b: Expression of defense-related genes in response to mechanical wounding and Phytophthora parasitica infection in Poncirus trifoliata and Citrus sunki. Physiol. Mol. Plant Pathol. 76, 119–125.
Boissy, G.; Delafortelle, E.; Kahn, R.; Huet, J. C.; Bricogne, G.; Pernollet, J. C.; Brunie, S., 1996: Crystal structure of a fungal elicitor secreted
by Phytophthora cryptogea, a member of a novel class of plant necrotic proteins. Structure 4, 1429–1439.
Bozkurt, T. O.; Schornack, S.; Banfield, M. J.; Kamoun, S., 2012: Oomycetes, effectors, and all that jazz. Curr. Opin. Plant Biol. 15, 483–492.
Brasier, C.; Webber, J., 2010: Plant pathology sudden larch death. Nature 466, 824–825.
Brasier, C. M.; Kirk, S. A.; Delcan, J.; Cooke, D. E. L.; Jung, T.; Man in’t Veld, W. A., 2004a: Phytophthora alni sp. nov. and its variants: designation of emerging heteroploid hybrid pathogens spreading on Alnus trees. Mycol. Res. 108, 1172–1184.
Brasier, C. M.; Denman, S.; Brown, A.; Webber, J. F., 2004b: Sudden oak death (Phytophthora ramorum) discovered on trees in Europe.
Mycol. Res. 108, 1107–1110.
Brown, A. V.; Brasier, C. M., 2007: Colonization of tree xylem by Phytophthora ramorum, P. kernoviae and other Phytophthora species. Plant.
Pathol. 56, 227–241.
Brummer, M.; Arend, M.; Fromm, J.; Schlenzig, A.; Oßwald, W. F., 2002: Ultrastructural changes and immunocytochemical localization of the
elicitin quercinin in Quercus robur L. roots infected with Phytophthora quercina. Physiol. Mol. Plant Pathol. 61, 109–120.
Cahill, D. M.; Hardham, A. R., 1994: Exploitation of zoospore taxis in the development of a novel dipstick immunoassay for the specific
detection of Phytophthora cinnamomi. Phytopathology 84, 193–200.
Cahill, D. M.; Mccomb, J. A., 1992: A Comparison of changes in phenylalanine ammonia-lyase activity, lignin and phenolic synthesis in the
roots of Eucalyptus calophylla (field resistant) and E. marginata (susceptible) when infected with Phytophthora cinnamomi. Physiol. Mol.
Plant Pathol. 40, 315–332.
18
W. Oßwald, F. Fleischmann, D. Rigling et al.
Cahill, D. M.; Weste, G., 1983: Formation of callose deposits as a response to infection with Phytophthora cinnamomi. Trans. Br. Mycol. Soc.
80, 23–29.
Cahill, D. M.; Weste, G. M.; Grant, B. R., 1986: Changes in cytokinin concentrations in xylem extrudate following infection of Eucalyptus
marginata Donn-Ex-Sm with Phytophthora cinnamomi Rands. Plant Physiol. 81, 1103–1109.
Cahill, D. M.; Legge, N.; Grant, B.; Weste, G., 1989: Cellular and histological changes induced by Phytophthora cinnamomi in a group of plant
species ranging from fully susceptible to fully resistant. Phytopathology 79, 417–424.
Cahill, D. M.; Bennett, I. J.; McComb, J. A., 1993: Mechanisms of Resistance to Phytophthora cinnamomi in clonal, micropropagated Eucalyptus marginata. Plant. Pathol. 42, 865–872.
Cahill, D. M.; Rookes, J. E.; Wilson, B. A.; Gibson, L.; McDougall, K. L., 2008: Phytophthora cinnamomi and Australia’s biodiversity: impacts,
predictions and progress towards control. Aust. J. Bot. 56, 279–310.
Cameron, J. N.; Carlile, M. J., 1978: Fatty-acids, aldehydes and alcohols as attractants for zoospores of Phytophthora palmivora. Nature 271,
448–449.
Cameron, J. N.; Carlile, M. J., 1981: Binding of isovaleraldehyde, an attractant, to zoospores of the fungus Phytophthora palmivora in relation
to zoospore chemotaxis. J. Cell Sci. 49, 273–281.
Capasso, R.; Cristinzio, G.; Evidente, A.; Visca, C.; Parente, A., 1997: Phytophorin, a phytotoxic peptide, and its phytotoxic aggregates from
Phytophthora nicotianae. Phytopathol. Mediterr. 36, 67–73.
Capasso, R.; Cristinzio, G.; Evidente, A.; Visca, C.; Ferranti, P.; Blanco, F. D.; Parente, A., 1999: Elicitin 172 from an isolate of Phytophthora
nicotianae pathogenic to tomato. Phytochemistry 50, 703–709.
Capasso, R.; Cristinzio, G.; Di Maro, A.; Ferranti, P.; Parente, A., 2001: Syringicin, a new alpha-elicitin from an isolate of Phytophthora syringae, pathogenic to citrus fruit. Phytochemistry 58, 257–262.
Capasso, R.; Di Maro, A.; Cristinzio, G.; De Martino, A.; Chambery, A.; Daniele, A.; Sannin, F.; Testa, A.; Parente, A., 2008: Isolation, characterization and structure-elicitor activity relationships of hibernalin and its two oxidized forms from Phytophthora hibernalis Carne 1925.
J. Biochem. 143, 131–141.
Churngchow, N.; Rattarasarn, M., 2000: The elicitin secreted by Phytophthora palmivora, a rubber tree pathogen. Phytochemistry 54, 33–38.
Clemenz, C.; Fleischmann, F.; Haberle, K. H.; Matyssek, R.; Oßwald, W., 2008: Photosynthetic and leaf water potential responses of Alnus glutinosa saplings to stem-base inoculation with Phytophthora alni subsp alni. Tree Physiol. 28, 1703–1711.
Coelho, A. C.; Horta, H.; Neves, D.; Cravador, A., 2006: Involvement of a cinnamyl alcohol dehydrogenase of Quercus suber in the defence
response to infection by Phytophthora cinnamomi. Physiol. Mol. Plant Pathol. 69, 62–72.
Coelho, A. C.; Horta, M.; Ebadzad, G.; Cravador, A., 2011: Quercus suber – Phytophthora cinnamomi interaction: a hypothetical molecular
mechanism model. N. Z. J. For. Sci. 41, S143–S157.
Colas, V.; Conrod, S.; Venard, P.; Keller, H.; Ricci, P.; Panabieres, F., 2001: Elicitin genes expressed in vitro by certain tobacco isolates of
Phytophthora parasitica are down regulated during compatible interactions. Mol. Plant Microbe Interact. 14, 326–335.
Collins, B. R.; Parke, J. L.; Lachenbruch, B.; Hansen, E. M., 2009: The effects of Phytophthora ramorum infection on hydraulic conductivity
and tylosis formation in tanoak sapwood. Can. J. For. Res. 39, 1766–1776.
Corcobado, T.; Cubera, E.; Moreno, G.; Solla, A., 2013: Quercus ilex forests are influenced by annual variations in water table, soil water deficit and fine root loss caused by Phytophthora cinnamomi. Agric. For. Meteorol. 169, 92–99.
Crombie, D. S.; Tippett, J. T., 1990: A comparison of water relations, visual symptoms, and changes in stem girth for evaluating impact of
Phytophthora cinnamomi dieback on Eucalyptus marginata. Can. J. For. Res. 20, 233–240.
Crone, M.; Mccomb, J. A.; O’brien, P. A.; Hardy, G. E. St. J., 2013: Survival of Phytophthora cinnamomi as oospores, stromata, and thick-walled
chlamydospores in roots of symptomatic and asymptomatic annual and herbaceous perennial plant species. Fungal Biol. 117, 112–123.
Dalio, R. J.; Fleischmann, F.; Oßwald, W., 2011: Localization of Phytophthora plurivora effector protein citricolin in Fagus sylvatica roots by
light and fluorescence laser scanning microscopy. Phytopathology 101, S40.
Davidson, J. M.; Wickland, A. C.; Patterson, H. A.; Falk, K. R.; Rizzo, D. M., 2005: Transmission of Phytophthora ramorum in mixed-evergreen
forest in California. Phytopathology 95, 587–596.
Davies, W. J.; Zhang, J. H., 1991: Root signals and the regulation of growth and development of plants in drying soil. Annu. Rev. Plant Physiol. Plant Mol. Biol. 42, 55–76.
Davison, E. M., 2011: How do Phytophthora spp. de Bary kill trees? N. Z. J. For. Sci. 41, 25–37.
Davison, E. M.; Tay, F. C. S., 1995: Damage to surface roots of Eucalyptus marginata trees at sites infested with Phytophthora cinnamomi.
Aust. J. Bot. 43, 527–536.
Davison, E. M.; Stukely, M. J. C.; Crane, C. E.; Tay, F. C. S., 1994: Invasion of phloem and xylem of woody stems and roots of Eucalyptus
marginata and Pinus radiata by Phytophthora cinnamomi. Phytopathology 84, 335–340.
Dawson, P.; Weste, G., 1984: Impact of root infection by Phytophthora cinnamomi on the water relations of two Eucalyptus species that
differ in susceptibility. Phytopathology 74, 486–490.
Deacon, J. W.; Donaldson, S. P., 1993: Molecular recognition in the homing responses of zoosporic fungi, with special reference to Pythium
and Phytophthora. Mycol. Res. 97, 1153–1171.
Del Rio, J. A.; Baidez, A. G.; Botia, J. M.; Ortuno, A., 2003: Enhancement of phenolic compounds in olive plants (Olea europaea L.) and their
influence on resistance against Phytophthora sp. Food Chem. 83, 75–78.
Dick, M. W., 2001: Straminipilous Fungi: Systematics of the Peronosporomycetes Including Accounts of the Marine Straminipilous Protists,
the Plasmodiophorids and Similar Organisms. Dordrecht/Boston/London: Kluwer Academic Publishers, 670 pp.
Dodd, R. S.; H€
uberli, D.; Mayer, W.; Harnik, T. Y.; Afzal-Rafii, Z.; Garbelotto, M., 2008: Evidence for the role of synchronicity between host
phenology and pathogen activity in the distribution of sudden oak death canker disease. New Phytol. 179, 505–514.
Dodds, P. N.; Rafiqi, M.; Gan, P. H. P.; Hardham, A. R.; Jones, D. A.; Ellis, J. G., 2009: Effectors of biotrophic fungi and oomycetes: pathogenicity factors and triggers of host resistance. New Phytol. 183, 993–999.
Dong, S. M.; Kong, G. H.; Qutob, D.; Yu, X. L.; Tang, J. L.; Kang, J. X.; Dai, T. T.; Wang, H.; Gijzen, M.; Wang, Y. C., 2012: The NLP toxin family
in Phytophthora sojae includes rapidly evolving groups that lack necrosis-inducing activity. Mol. Plant Microbe Interact. 25, 896–909.
Dou, D.; Kale, S. D.; Wang, X.; Chen, Y.; Wang, Q.; Wang, X.; Jiang, R. H. Y.; Arredondo, F. D.; Anderson, R. G.; Thakur, P. B.; McDowell, J. M.;
Wang, Y.; Tyler, B. M., 2008: Conserved C-terminal motifs required for avirulence and suppression of cell death by Phytophthora sojae
effector Avr1b. Plant Cell 20, 1118–1133.
Duclos, J.; Fauconnier, A.; Coelho, A. C.; Bollen, A.; Cravador, A.; Godfroid, E., 1998: Identification of an elicitin gene cluster in Phytophthora
cinnamomi. DNA Seq. 9, 231–237.
Duran, A.; Gryzenhout, M.; Slippers, B.; Ahumada, R.; Rotella, A.; Flores, F.; Wingfield, B. D.; Wingfield, M. J., 2008: Phytophthora pinifolia sp.
nov. associated with a serious needle disease of Pinus radiata in Chile. Plant. Pathol. 57, 715–727.
Phytophthora – woody plant interactions
19
Dutsadee, C.; Nunta, C., 2008: Induction of peroxidase, scopoletin, phenolic compounds and resistance in Hevea brasiliensis by elicitin and a
novel protein elicitor purified from Phytophthora palmivora. Physiol. Mol. Plant Pathol. 72, 179–187.
Erwin, D. C.; Ribeiro, O. K., 1996: Phytophthora Diseases Worldwide. St. Paul, MN: APS Press, 562 pp.
Fefeu, S.; Bouaziz, S.; Huet, J. C.; Pernollet, J. C.; Guittet, E., 1997: Three-dimensional solution structure of beta cryptogein, a beta elicitin
secreted by a phytopathogenic fungus Phytophthora cryptogea. Protein Sci. 6, 2279–2284.
Fellbrich, G.; Romanski, A.; Varet, A.; Blume, B.; Brunner, F.; Engelhardt, S.; Felix, G.; Kemmerling, B.; Krzymowska, M.; Nurnberger, T., 2002:
NPP1, a Phytophthora-associated trigger of plant defense in parsley and Arabidopsis. Plant J. 32, 375–390.
Fichtner, E. J.; Lynch, S. C.; Rizzo, D. M., 2007: Detection, distribution, sporulation, and survival of Phytophthora ramorum in a california
redwood-tanoak forest soil. Phytopathology 97, 1366–1375.
Fleischmann, F.; Schneider, D.; Matyssek, R.; Oßwald, W. F., 2002: Investigations on net CO2 assimilation, transpiration and root growth of
Fagus sylvatica infested with four different Phytophthora species. Plant Biol. 4, 144–152.
Fleischmann, F.; G€
ottlein, A.; Rodenkirchen, H.; L€
utz, C.; Oßwald, W., 2004: Biomass, nutrient and pigment content of beech (Fagus sylvatica)
saplings infected with Phytophthora citricola, P. cambivora, P. pseudosyringae and P. undulata. For. Pathol. 34, 79–92.
Fleischmann, F.; Koehl, J.; Portz, R.; Beltrame, A. B.; Oßwald, W., 2005: Physiological change of Fagus sylvatica seedlings infected with
Phytophthora citricola and the contribution of its elicitin “Citricolin” to pathogenesis. Plant Biol. 7, 650–658.
Gallego, F. J.; De Algaba, A. P.; Fernandez-Escobar, R., 1999: Etiology of oak decline in Spain. Eur. J. For. Pathol. 29, 17–27.
Giesbrecht, M. B.; Hansen, E. M.; Mitin, P., 2011: Histology of Phytophthora ramorum in Notholithocarpus densiflorus bark tissues. N. Z. J.
For. Sci. 41, 89–100.
Ginetti, B.; Moricca, S.; Squires, J. N.; Cooke, D. E. L.; Ragazzi, A.; Jung, T., 2013: Phytophthora acerina sp. nov., a new species causing bleeding cankers and dieback of Acer pseudoplatanus trees in planted forests in Northern Italy. Plant. Pathol. DOI: 10.1111/ppa.12153.
Green, S.; Brasier, C. M.; Schlenzig, A.; Mccracken, A.; Macaskill, G. A.; Wilson, M.; Webber, J. F., 2013: The destructive invasive pathogen
Phytophthora lateralis found on Chamaecyparis lawsoniana across the UK. For. Pathol. 43, 19–28.
Gr€
unwald, N. J.; Goss, E. M.; Press, C. M., 2008: Phytophthora ramorum: a pathogen with a remarkably wide host range causing sudden oak
death on oaks and ramorum blight on woody ornamentals. Mol. Plant Pathol. 9, 729–740.
Hammond-Kosack, K. E.; Jones, J. D. G., 1997: Plant disease resistance genes. Annu. Rev. Plant Physiol. Plant Mol. Biol. 48, 575–607.
Hansen, E. M.; Reeser, P. W.; Sutton, W., 2012: Phytophthora beyond agriculture. Annu. Rev. Phytopathol. 50, 359–378.
Hardham, A. R., 2001: The cell biology behind Phytophthora pathogenicity. Australas. Plant Pathol. 30, 91–98.
Hardham, A. R., 2005: Phytophthora cinnamomi. Mol. Plant Pathol. 6, 589–604.
Hardham, A. R.; Cahill, D. M., 2010: The role of oomycete effectors in plant-pathogen interactions. Funct. Plant Biol. 37, 919–925.
Hardy, G. E. St. J.; Colquhoun, I. J.; Nielsen, P., 1996: The early development of disease caused by Phytophthora cinnamomi in Eucalyptus
marginata and Eucalyptus calophylla growing in rehabilitated bauxite mined areas. Plant. Pathol. 45, 944–954.
Heath, M. C., 1980: Reactions of non-suscepts to fungal pathogens. Annu. Rev. Phytopathol. 18, 211–236.
Hein, I.; Gilroy, E. M.; Armstrong, M. R.; Birch, P. R. J., 2009: The zig-zag-zig in oomycete-plant interactions. Mol. Plant Pathol. 10, 547–562.
Heiser, I.; Fromm, J.; Giefing, M.; Koehl, J.; Jung, T.; Osswald, W., 1999: Investigations on the action of Phytophthora quercina, P-citricola and
P-gonapodyides toxins on tobacco plants. Plant Physiol. Biochem. 37, 73–81.
Hong, C. X.; Gallegly, M. E.; Browne, G. T.; Bhat, R. G.; Richardson, P. A.; Kong, P., 2009: The avocado subgroup of Phytophthora citricola constitutes a distinct species, Phytophthora mengei sp. nov. Mycologia 101, 833–840.
Hong, C.; Gallegly, M. E.; Richardson, P. A.; Kong, P., 2011: Phytophthora pini Leonian resurrected to distinct species status. Mycologia 103,
351–360.
Horta, M.; Sousa, N.; Coelho, A. C.; Neves, D.; Cravador, A., 2008: In vitro and in vivo quantification of elicitin expression in Phytophthora
cinnamomi. Physiol. Mol. Plant Pathol. 73, 48–57.
Horta, M.; Caetano, P.; Medeira, C.; Maia, I.; Cravador, A., 2010: Involvement of the beta-cinnamomin elicitin in infection and colonisation of
cork oak roots by Phytophthora cinnamomi. Eur. J. Plant Pathol. 127, 427–436.
Hua, C. L.; Wang, Y. L.; Zheng, X. B.; Dou, D. L.; Zhang, Z. G.; Govers, F.; Wang, Y. C., 2008: A Phytophthora sojae G-protein alpha subunit is
involved in chemotaxis to soybean isoflavones. Eukaryot. Cell 7, 2133–2140.
H€
uberli, D.; Garbelotto, M., 2011: Phytophthora ramorum is a generalist plant pathogen with differences in virulence between isolates from
infectious and dead-end hosts. For. Pathol. 42, 8–13.
H€
uberli, D.; Tommerup, I. C.; Dobrowolski, M. P.; Calver, M. C.; Hardy, G. E. St. J., 2001: Phenotypic variation in a clonal lineage of two
Phytophthora cinnamomi populations from Western Australia. Mycol. Res. 105, 1053–1064.
H€
uckelhoven, R., 2007: Cell wall associated mechanisms of disease resistance and susceptibility. Annu. Rev. Phytopathol. 45, 101–127.
Huitema, E.; Vleeshouwers, V. G. A. A.; Cakir, C.; Kamoun, S.; Govers, F., 2005: Differences in intensity and specificity of hypersensitive
response induction in Nicotiana spp. by INF1, INF2A, and INF2B of Phytophthora infestans. Mol. Plant Microbe Interact. 18, 183–193.
Ioos, R.; Panabieres, F.; Industri, B.; Andrieux, A.; Frey, P., 2007: Distribution and expression of elicitin genes in the interspecific hybrid
oomycete Phytophthora alni. Appl. Environ. Microbiol. 73, 5587–5597.
Jang, J. C.; Tainter, F. H., 1990: Cellular responses of pine callus to infection by Phytophthora cinnamomi. Phytopathology 80, 1347–1352.
Jiang, R. H. Y.; Tyler, B. M., 2012: Mechanisms and evolution of virulence in oomycetes. Annu. Rev. Phytopathol. 50, 295–318.
Jiang, R. H. Y.; Tyler, B. M.; Whisson, S. C.; Hardham, A. R.; Govers, F., 2006: Ancient origin of elicitin gene clusters in Phytophthora genomes.
Mol. Biol. Evol. 23, 338–351.
Jones, J. D. G.; Dangl, J. L., 2006: The plant immune system. Nature 444, 323–329.
Jung, T., 2009: Beech decline in Central Europe driven by the interaction between Phytophthora infections and climatic extremes. For.
Pathol. 39, 73–94.
Jung, T.; Blaschke, M., 2004: Phytophthora root and collar rot of alders in Bavaria: distribution, modes of spread and possible management
strategies. Plant. Pathol. 53, 197–208.
Jung, T.; Burgess, T. I., 2009: Re-evaluation of Phytophthora citricola isolates from multiple woody hosts in Europe and North America
reveals a new species, Phytophthora plurivora sp. nov. Persoonia 22, 95–110.
Jung, T.; Blaschke, H.; Neumann, P., 1996: Isolation, identification and pathogenicity of Phytophthora species from declining oak stands. Eur.
J. For. Pathol. 26, 253–272.
Jung, T.; Cooke, D. E. L.; Blaschke, H.; Duncan, J. M.; Osswald, W., 1999: Phytophthora quercina sp. nov., causing root rot of European oaks.
Mycol. Res. 103, 785–798.
Jung, T.; Hudler, G.; Jensen-Tracy, S.; Griffiths, H.; Fleischmann, F.; Oßwald, W., 2005: Involvement of Phytophthora species in the decline of
European beech in Europe and the USA. Mycologist 19, 159–166.
Jung, T.; Vettraino, A. M.; Cech, T. L.; Vannini, A., 2013a: The impact of invasive Phytophthora species on European forests. In: Phytophthora:
A Global Perspective. Ed. by Lamour, K. Wallingford, UK: CABI, pp. 146–158.
20
W. Oßwald, F. Fleischmann, D. Rigling et al.
Jung, T.; Colquhoun, I. J.; Hardy, G. E. St. J., 2013b: New insights into the survival strategy of the invasive soilborne pathogen Phytophthora
cinnamomi in different natural ecosystems in Western Australia. For. Pathol. 43, 266–288.
Kamoun, S., 2006: A catalogue of the effector secretome of plant pathogenic oomycetes. Annu. Rev. Phytopathol. 44, 41–60.
Kamoun, S., 2007: Groovy times: filamentous pathogen effectors revealed. Curr. Opin. Plant Biol. 10, 358–365.
Kamoun, S.; Young, M.; Glascock, C. B.; Tyler, B. M., 1993: Extracellular protein elicitors from Phytophthora: host-specificity and induction of
resistance to bacterial and fungal pathogens. Mol. Plant Microbe Interact. 6, 15–25.
Kamoun, S.; Young, M.; Foerster, H.; Coffey, M. D.; Tyler, B. M., 1994: Potential role of elicitins in the interaction between Phytophthora
species and tobacco. Appl. Environ. Microbiol. 60, 1593–1598.
Kamoun, S.; Lindqvist, H.; Govers, F., 1997a: A novel class of elicitin-like genes from Phytophthora infestans. Mol. Plant Microbe Interact.
10, 1028–1030.
Kamoun, S.; Vanwest, P.; Dejong, A. J.; Degroot, K. E.; Vleeshouwers, V. G. a. A.; Govers, F., 1997b: A gene encoding a protein elicitor of
Phytophthora infestans is down-regulated during infection of potato. Mol. Plant-Microbe Interact. 10, 13–20.
Kasuga, T., Kozanitas, M., Bui, M., H€
uberli, D., Rizzo, D. M., Garbelotto, M., 2012: Phenotypic diversification is associated with host-induced
transposon derepression in the sudden oak death pathogen Phytophthora ramorum. PLoS ONE 7, e34728.
Keen, N. T.; Wang, M. C.; Bartnickigarcia, S.; Zentmyer, G. A., 1975: Phytotoxicity of mycolaminarans-beta-1,3-glucans from Phytophthora
spp. Physiol. Plant Pathol. 7, 91–97.
Keller, H.; Pamboukdjian, N.; Ponchet, M.; Poupet, A.; Delon, R.; Verrier, J. L.; Roby, D.; Ricci, P., 1999: Pathogen-induced elicitin production
in transgenic tobacco generates a hypersensitive response and nonspecific disease resistance. Plant Cell 11, 223–235.
Kennelly, M. M.; Gadoury, D. M.; Wilcox, W. F.; Magarey, P. A.; Seem, R. C., 2005: Seasonal development of ontogenetic resistance to downy
mildew in grape berries and rachises. Phytopathology 95, 1445–1452.
Khew, K. L.; Zentmyer, G. A., 1973: Chemotactic response of zoospores of five species of Phytophthora. Phytopathology 63, 1511–1517.
Kim, Y. T.; Oh, J.; Kim, K. H.; Uhm, J. Y.; Lee, B. M., 2010: Isolation and characterization of NgRLK1, a receptor-like kinase of Nicotiana glutinosa that interacts with the elicitin of Phytophthora capsici. Mol. Biol. Rep. 37, 717–727.
Krapp, A.; Stitt, M., 1995: An evaluation of direct and indirect mechanisms for the “sind-regulation” of photosynthesis in spinach: changes
in gas exchange, carbohydrates, metabolites, enzyme activities and steady-state transcript levels after cold-girdling source leaves”. Planta
195, 313–323.
Ku, T., 2004: Leucine-rich repeat receptor kinases in plants: structure, function, and signal transduction pathways. Int. Rev. Cytol. 234,
1–46.
K€
ufner, I.; Ottmann, C.; Oecking, C.; N€
urnberger, T., 2009: Cytolytic toxins as triggers of plant immune response. Plant Signal Behav. 4,
977–979.
Labanauskas, C. K.; Stolzy, L. H.; Zentmyer, G. A., 1976: Effect of root infection by Phytophthora cinnamomi on nutrient-uptake and translocation by avocado seedlings. Soil Sci. 122, 292–296.
Lascombe, M. B.; Ponchet, M.; Venard, P.; Milat, M. L.; Blein, J. P.; Prange, T., 2002: The 1.45 angstrom resolution structure of the cryptogein-cholesterol complex: a close-up view of a sterol carrier protein (SCP) active site. Acta Crystallogr. D Biol. Crystallogr. 58,
1442–1447.
Leborgne-Castel, N.; Adam, T.; Bouhidel, K., 2010: Endocytosis in plant-microbe interactions. Protoplasma 247, 177–193.
Mahomed, W.; van den Berg, N., 2011: EST sequencing and gene expression profiling of defence-related genes from Persea americana
infected with Phytophthora cinnamomi. BMC Plant Biol. 11, 167.
Maia, I.; Medeira, C.; Melo, E.; Cravador, A., 2008: Quercus suber infected by Phytophthora cinnamomi. Effects at cellular level of cinnamomin
on roots, stem and leaves. Microsc. Microanal. 14, 146–147.
Manter, D. K.; Kelsey, R. G.; Karchesy, J. J., 2007: Photosynthetic declines in Phytophthora ramorum infected plants develop prior to water
stress and in response to exogenous application of elicitins. Phytopathology 97, 850–856.
Manter, D. K.; Kolodny, E. H.; Hansen, E. M.; Parke, J. L., 2010: Virulence, sporulation, and elicitin production in three clonal lineages of
Phytophthora ramorum. Physiol. Mol. Plant Pathol. 74, 317–322.
Marino, D.; Danand, C.; Puppo, A.; Pauly, N., 2012: A burst of plant NADPH oxidases. Trends Plant Sci. 17, 9–15.
Martin, F. N.; Abed, Z. G.; Baldi, Y.; Ivors, K., 2012: Identification and detection of Phytophthora: reviewing our progress, identifying our
needs. Plant Dis. 96, 1080–1103.
Matsushima, N.; Miyashita, H., 2012: Leucine-rich repeat (LRR) domains containing intervening motifs in plants. Biomolecules 2, 288–311.
Maurel, M.; Robin, C.; Capdevielle, X.; Loustau, D.; Desprez-Loustau, M. L., 2001a: Effects of variable root damage caused by Phytophthora
cinnamomi on water relations of chestnut saplings. Ann. For. Sci. 58, 639–651.
Maurel, M.; Robin, C.; Capron, G.; Desprez-Loustau, M. L., 2001b: Effects of root damage associated with Phytophthora cinnamomi on water
relations, biomass accumulation, mineral nutrition and vulnerability to water deficit of five oak and chestnut species. For. Pathol. 31,
353–369.
Maurel, M.; Robin, C.; Simonneau, T.; Loustau, D.; Dreyer, E.; Desprez-Loustau, M. L., 2004: Stomatal conductance and root-to-shoot signalling in chestnut saplings exposed to Phytophthora cinnamomi or partial soil drying. Funct. Plant Biol. 31, 41–51.
Medeira, C.; Maia, I.; Ribeiro, C.; Candeias, I.; Melo, E.; Sousa, N.; Cravador, A., 2012a: Alpha cinnamomin elicit a defence response against
Phytophthora cinnamomi in Castanea sativa. Acta Hort. 940, 315–320.
Medeira, C.; Quartin, V.; Maia, I.; Diniz, I.; Matos, M. C.; Semedo, J. N.; Scotti-Campos, P.; Ramalho, J. C.; Pais, I. P.; Ramos, P.; Melo, E.; Leitao,
A. E.; Cravador, A., 2012b: Cryptogein and capsicein promote defence responses in Quercus suber against Phytophthora cinnamomi infection. Eur. J. Plant Pathol. 134, 145–159.
Mikes, V.; Milat, M. L.; Ponchet, M.; Ricci, P.; Blein, J. P., 1997: The fungal elicitor cryptogein is a sterol carrier protein. FEBS Lett. 416,
190–192.
Mikes, V.; Milat, M. L.; Ponchet, M.; Panabieres, F.; Ricci, P.; Blein, J. P., 1998: Elicitins, proteinaceous elicitors of plant defense, are a new
class of sterol carrier proteins. Biochem. Biophys. Res. Commun. 245, 133–139.
Mircetich, S. M.; Zentmyer, G. A., 1966: Production of oospores and chlamydospores of Phytophthora cinnamomi in roots and soil. Phytopathology 56, 1076–1078.
Mohamed, N.; Lherminier, J.; Farmer, M. J.; Fromentin, J.; Beno, N.; Houot, V.; Milat, M. L.; Blein, J. P., 2007: Defense responses in grapevine
leaves against Botrytis cinerea induced by application of a Pythium oligandrum strain or its elicitin, oligandrin, to roots. Phytopathology
97, 611–620.
€
Mombour, J., 2009: Okophysiologische
und phytopathologische Untersuchungen an Phytophthora alni – infizierten Schwarzerlen (Alnus glutinosa [L.] Gaertn.) unter besonderer Ber€
ucksichtigung der Ausbreitungsdynamik des Pathogens im Rindengewebe. Essen: Universit€at
Duisburg-Essen, 183 pp.
Morgan, J. M., 1984: Osmoregulation and water stress in higher plants. Annu. Rev. Plant Physiol. 35, 299–319.
Phytophthora – woody plant interactions
21
Morris, P. F.; Ward, E. W. B., 1992: Chemoattraction of zoospores of the soybean pathogen, Phytophthora sojae, by Isoflavones. Physiol. Mol.
Plant Pathol. 40, 17–22.
Morris, P. F.; Bone, E.; Tyler, B. M., 1998: Chemotropic and contact responses of Phytophthora sojae hyphae to soybean isoflavonoids and
artificial substrates. Plant Physiol. 117, 1171–1178.
Mouton-Perronnet, F.; Bruneteau, M.; Denoroy, L.; Bouliteau, P.; Ricci, P.; Bonnet, P.; Michel, G., 1995: Elicitin produced by an isolate of
Phytophthora parasitica pathogenic to tobacco. Phytochemistry 38, 41–44.
Nechwatal, J.; Hahn, J.; Schonborn, A.; Schmitz, G., 2011: A twig blight of understorey European beech (Fagus sylvatica) caused by soilborne
Phytophthora spp. For. Pathol. 41, 493–500.
O’Brien, J. A.; Daudi, A.; Butt, V. S.; Bolwell, G. P., 2012: Reactive oxygen species and their role in plant defence and cell wall metabolism.
Planta 236, 765–779.
Ockels, F. S.; Eyles, A.; Mcpherson, B. A.; Wood, D. L.; Bonello, P., 2007: Phenolic chemistry of coast live oak response to Phytophthora ramorum infection. J. Chem. Ecol. 33, 1721–1732.
Odonohue, M. J.; Gousseau, H.; Huet, J. C.; Tepfer, D.; Pernollet, J. C., 1995: Chemical synthesis, expression and mutagenesis of a gene encoding beta-cryptogein, an elicitin produced by Phytophthora cryptogea. Plant Mol. Biol. 27, 577–586.
O’Gara, E.; McComb, J. A.; Colquhoun, L. J.; Hardy, G. E. St. J., 1997: The infection of non-wounded and wounded periderm tissue at the
lower stem of Eucalyptus marginata by zoospores of Phytophthora cinnamomi, in a rehabilitated bauxite mine. Australasian Plant Pathol.
26, 135–141.
Oh, E.; Hansen, E. A., 2007: Histopathology of infection and colonization of susceptible and resistant port-orford-cedar by Phytophthora
lateralis. Phytopathology 97, 684–693.
Oliva, R.; Win, J.; Raffaele, S.; Boutemy, L.; Bozkurt, T. O.; Chaparro-Garcia, A.; Segretin, M. E.; Stam, R.; Schornack, S.; Cano, L. M.; Van
Damme, M.; Huitema, E.; Thines, M.; Banfield, M. J.; Kamoun, S., 2010: Recent developments in effector biology of filamentous plant
pathogens. Cell. Microbiol. 12, 705–715.
Oßwald, W.; Fleischmann, F.; Treutter, D., 2012: Host-parasite interactions and trade-offs between growth-and defence-related metabolism
under changing environments. In: Growth and Defence in Plants. Ed. by Matyssek, R.; Schnyder, H.; Oßwald, W.; Ernst, D.; Munch, J. C.;
Pretzsch, H. Berlin, Heidelberg: Springer-Verlag, pp. 53–83.
Ottmann, C.; Luberacki, B.; Kufner, I.; Koch, W.; Brunner, F.; Weyand, M.; Mattinen, L.; Pirhonen, M.; Anderluh, G.; Seitz, H. U.; Nurnberger,
T.; Oecking, C., 2009: A common toxin fold mediates microbial attack and plant defense. Proc. Natl Acad. Sci. USA 106, 10359–10364.
Panabieres, F.; Marais, A.; Leberre, J. Y.; Penot, I.; Fournier, D.; Ricci, P., 1995: Characterization of a gene cluster of Phytophthora cryptogea
which codes for elicitins, proteins inducing a hypersensitive-like response in tobacco. Mol. Plant Microbe Interact. 8, 996–1003.
Parke, J. L.; Oh, E.; Voelker, S.; Hansen, E. M.; Buckles, G.; Lachenbruch, B., 2007: Phytophthora ramorum colonizes tanoak xylem and is associated with reduced stem water transport. Phytopathology 97, 1558–1567.
Pernollet, J. C.; Sallantin, M.; Salletourne, M.; Huet, J. C., 1993: Elicitin isoforms from 7 Phytophthora species – comparison of their physicochemical properties and toxicity to tobacco and other plant species. Physiol. Mol. Plant Pathol. 42, 53–67.
Pfanz, H.; Aschan, B.; Langenfeld-Heyser, R.; Wittmann, C.; Loose, M., 2002: Ecology and ecophysiology of tree stem photosynthesis: corticular and wood photosynthesis. Naturwissenschaften 89, 147–162.
Pleskova, V.; Kasparovsky, T.; Oboril, M.; Ptackova, N.; Chaloupkova, R.; Ladislav, D.; Damborsky, J.; Lochman, J., 2011: Elicitin-membrane
interaction is driven by a positive charge on the protein surface: role of Lys13 residue in lipids loading and resistance induction. Plant
Physiol. Biochem. 49, 321–328.
Pogoda, F.; Werres, S., 2004: Histological studies of Phytophthora ramorum in Rhododendron twigs. Can. J. Bot. 82, 1481–1489.
Ponchet, M.; Panabieres, F.; Milat, M. L.; Mikes, V.; Montillet, J. L.; Suty, L.; Triantaphylides, C.; Tirilly, Y.; Blein, J. P., 1999: Are elicitins cryptograms in plant-oomycete communications? Cell. Mol. Life Sci. 56, 1020–1047.
Portz, R. L.; Fleischmann, F.; Koehl, J.; Fromm, J.; Ernst, D.; Pascholati, S. F.; Oßwald, W. F., 2011: Histological, physiological and molecular
investigations of Fagus sylvatica seedlings infected with Phytophthora citricola. For. Pathol. 41, 202–211.
Qutob, D.; Huitema, E.; Gijzen, M.; Kamoun, S., 2003: Variation in structure and activity among elicitins from Phytophthora sojae. Mol. Plant
Pathol. 4, 119–124.
Rea, A.; Jung, T.; Burgess, T. I.; Stukely, M. J. C.; Hardy, G. E. St. J., 2010: Phytophthora elongata sp. nov. a novel pathogen from the Eucalyptus marginata forest of Western Australia. Australas. Plant Pathol. 39, 477–491.
Ricci, P., 1997: Induction of the hypersensitive response and systemic acquired resistance by fungal protein: the case of elicitins. Plant
Microbe Interact. 3, 53–75.
Ricci, P.; Bonnet, P.; Huet, J. C.; Sallantin, M.; Beauvais-Cante, F.; Bruneteau, M.; Billard, V.; Michel, G.; Pernollet, J. C., 1989: Structure and
activity of proteins from pathogenic fungi Phytophthora eliciting necrosis and acquired resistance in tobacco. Eur. J. Biochem. 183,
555–563.
Riedel, M.; Wagner, S.; G€
otz, M.; Belbahri, L.; Lefort, F.; Werres, S., 2008: Studies of tissue colonization in Rhododendron by Phytophthora
ramorum. Proc. Sudden Oak Death 3rd Science Symposium, Santa Rosa, California; March, 5–9, 2007, U.S. Department of Agriculture,
Forest Service, Pacific Southwest Research Station, 485–487.
Riedel, M.; Werres, S.; Elliott, E.; McKeever, K.; Shamoun, S. F., 2012: Histopathological investigations of the infection process and propagule
development of Phytophthora ramorum on rhododendron leaves. Forest Phytophthoras 2, DOI: 10.5399/osu/fp.2.1.3036.
Rizzo, D. M.; Garbelotto, M.; Hansen, E. A., 2005: Phytophthora ramorum: integrative research and management of an emerging pathogen in
California and Oregon forests. Annu. Rev. Phytopathol. 43, 309–335.
Robin, C.; Dupuis, F.; Desprez-Loustau, M. L., 1994: Seasonal changes in Northern red oak susceptibility to Phytophthora cinnamomi. Plant
Dis. 78, 369–373.
Robin, C.; Capron, G.; Desprez-Loustau, M. L., 2001: Root infection by Phytophthora cinnamomi in seedlings of three oak species. Plant.
Pathol. 50, 708–716.
Robin, C.; Piou, D.; Feau, N.; Douzon, G.; Schenck, N.; Hansen, E. M., 2011: Root and aerial infections of Chamaecyparis lawsoniana by Phytophthora lateralis: a new threat for European countries. For. Pathol. 41, 417–424.
Rodrigues, M. L.; Archer, M.; Martel, P.; Miranda, S.; Thomaz, M.; Enguita, F. J.; Baptista, R. P.; Melo, E. P.; Sousa, N.; Cravador, A.; Carrondo,
M. A., 2006: Crystal structures of the free and sterol-bound forms of beta-cinnamomin. BBA-Proteins. Proteom. 1764, 110–121.
Schlink, K., 2010: Down-regulation of defense genes and resource allocation into infected roots as factors for compatibility between Fagus
sylvatica and Phytophthora citricola. Funct. Integr. Genomics 10, 253–264.
Schornack, S.; Huitema, E.; Cano, L. M.; Bozkurt, T. O.; Oliva, R.; Van Damme, M.; Schwizer, S.; Raffaele, S.; Chaparro-Garcia, A.; Farrer, R.;
Segretin, M. E.; Bos, J.; Haas, B. J.; Zody, M. C.; Nusbaum, C.; Win, J.; Thines, M.; Kamoun, S., 2009: Ten things to know about oomycete
effectors. Mol. Plant Pathol. 10, 795–803.
22
W. Oßwald, F. Fleischmann, D. Rigling et al.
Schornack, S.; Van Damme, M.; Bozkurt, T. O.; Cano, L. M.; Smoker, M.; Thines, M.; Gaulin, E.; Kamoun, S.; Huitema, E., 2010: Ancient class of
translocated oomycete effectors targets the host nucleus. Proc. Natl Acad. Sci. USA 107, 17421–17426.
Scott, P. M.; Burgess, T. I.; Barber, P. A.; Shearer, B. L.; Stukely, M. J. C.; Hardy, G. E. St. J.; Jung, T., 2009: Phytophthora multivora sp nov., a
new species recovered from declining Eucalyptus, Banksia, Agonis and other plant species in Western Australia. Persoonia 22, 1–13.
Shea, R. R.; Shearer, B.; Tippett, J., 1982: Recovery of Phytophthora cinnamomi Rands from vertical roots of Jarrah (Eucalyptus marginata
Sm). Australas. Plant Pathol. 11, 25–28.
Sterne, R. E.; Kaufmann, M. R.; Zentmyer, G. A., 1978: Effect of Phytophthora root-rot on water relations of avocado - interpretation with a
water transport model. Phytopathology 68, 595–602.
Tippett, J. T.; Shea, S. R.; Hill, T. C.; Shearer, B. L., 1983: Development of lesions caused by Phytophthora cinnamomi in the secondary
phloem of Eucalyptus marginata. Aust. J. Bot. 31, 197–210.
Torto, T. A.; Li, S.; Styer, A.; Huitema, E.; Testa, A.; Gow, N. A. R.; van West, P.; Kamoun, S., 2003: EST mining and functional expression
assays identify extracellular effector proteins from the plant pathogen Phytophthora. Genome Res. 13, 1675–1685.
Tyler, B. M., 2002: Molecular basis of recognition between Phytophthora pathogens and their hosts. Annu. Rev. Phytopathol. 40, 137–167.
Tyler, B. M.; Wu, M. H.; Wang, J. M.; Cheung, W.; Morris, P. F., 1996: Chemotactic preferences and strain variation in the response of
Phytophthora sojae zoospores to host isoflavones. Appl. Environ. Microbiol. 62, 2811–2817.
Tyler, B. M.; Tripathy, S.; Zhang, X. M.; Dehal, P.; Jiang, R. H. Y.; Aerts, A.; Arredondo, F. D.; Baxter, L.; Bensasson, D.; Beynon, J. L.; Chapman,
J.; Damasceno, C. M. B.; Dorrance, A. E.; Dou, D. L.; Dickerman, A. W.; Dubchak, I. L.; Garbelotto, M.; Gijzen, M.; Gordon, S. G.; Govers, F.;
Grunwald, N. J.; Huang, W.; Ivors, K. L.; Jones, R. W.; Kamoun, S.; Krampis, K.; Lamour, K. H.; Lee, M. K.; Mcdonald, W. H.; Medina, M.;
Meijer, H. J. G.; Nordberg, E. K.; Maclean, D. J.; Ospina-Giraldo, M. D.; Morris, P. F.; Phuntumart, V.; Putnam, N. H.; Rash, S.; Rose, J. K. C.;
Sakihama, Y.; Salamov, A. A.; Savidor, A.; Scheuring, C. F.; Smith, B. M.; Sobral, B. W. S.; Terry, A.; Torto-Alalibo, T. A.; Win, J.; Xu, Z. Y.;
Zhang, H. B.; Grigoriev, I. V.; Rokhsar, D. S.; Boore, J. L., 2006: Phytophthora genome sequences uncover evolutionary origins and mechanisms of pathogenesis. Science 313, 1261–1266.
Van Loon, L. C.; Rep, M.; Pieterse, C. M. J., 2006: Significance of inducible defense-related proteins in infected plants. Annu. Rev. Phytopathol. 44, 135–162.
Vauthrin, S.; Mikes, V.; Milat, M. L.; Ponchet, M.; Maume, B.; Osman, H.; Blein, J. P., 1999: Elicitins trap and transfer sterols from micelles,
liposomes and plant plasma membranes. BBA-Proteins. Proteom, 1419, 335–342.
Vleeshouwers, V. G. a. A.; Raffaele, S.; Vossen, J. H.; Champouret, N.; Oliva, R.; Segretin, M. E.; Rietman, H.; Cano, L. M.; Lokossou, A.; Kessel,
G.; Pel, M. A.; Kamoun, S., 2011: Understanding and exploiting late blight resistance in the age of effectors. Annu. Rev. Phytopathol. 49,
507–531.
Webber, J.; Brasier, C. M., 2012: Landscape-scale epidemic of Phytophthora ramorum on larch in the UK. Sixth Meeting of the International
Union of Forest Research Organizations IUFRO Working Party 7-02-09 Phytophthora in Forests and Natural Ecosystems, C"
ordoba
(Spain) 9th–14th September 2012.
Webber, J. F.; Vettraino, A. M.; Chang, T. T.; Bellgard, S. E.; Brasier, C. M.; Vannini, A., 2012: Isolation of Phytophthora lateralis from Chamaecyparis foliage in Taiwan. For. Pathol. 42, 136–143.
Whiley, A. W.; Pegg, K. G.; Saranah, J. B.; Forsberg, L. I., 1986: The control of Phytophthora root-rot of avocado with fungicides and the effect
of this disease on water relations, yield and ring neck. Aust. J. Exp. Agric. 26, 249–253.
Whisson, S. C.; Boevink, P. C.; Moleleki, L.; Avrova, A. O.; Morales, J. G.; Gilroy, E. M.; Armstrong, M. R.; Grouffaud, S.; van West, P.; Chapman,
S.; Hein, I.; Toth, I. K.; Pritchard, L.; Birch, P. R. J., 2007: A translocation signal for delivery of oomycete effector proteins into host plant
cells. Nature 450, 115–118.
Wittmann, C.; Pfanz, H., 2008: General trait relationships in stems: a study on the performance and interrelationships of several functional
and structural parameters involved in corticular photosynthesis. Physiol. Plant. 134, 636–648.
Wolf, F. T., 1933: The pathology of tobacco black shank. Phytopathology 23, 605–612.
Woodward, J. R.; Keane, P. J.; Stone, B. A., 1980: Structures and properties of wilt-inducing polysaccharides from Phytophthora species.
Physiol. Plant Pathol. 16, 439–454.
Yousef, L. F.; Yousef, A. F.; Mymryk, J. S.; Dick, W. A.; Dick, R. P., 2009: Stigmasterol and cholesterol regulate the expression of elicitin genes
in Phytophthora sojae. J. Chem. Ecol. 35, 824–832.
Zentmyer, G. A., 1961: Chemotaxis of zoospores for root exudates. Science 133, 1595.
Zipfel, C., 2009: Early molecular events in PAMP-triggered immunity. Curr. Opin. Plant Biol. 12, 414–420.