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Enhancement of potato defense mechanisms by BABA in multigenic
resistance potato cultivars. Systemic response against Phytophthora
and Fusarium.
Olivieri FP, Lobato MC, Gonzalez Altamiranda E, Wolski E A, Guevara MG, Daleo
GR and Andreu AB.
IIB. FCEyN. Universidad Nacional de Mar del Plata (7600) Argentina.
e- mail: [email protected]
Plants have developed mechanisms to successfully co-exist with the presence of
pathogenic organisms. Some interactions between plants and pathogens are based on
recognition of specific elicitor molecules from avirulent pathogen races (avr gene
products), which are described in the gene for gene resistance theory. Another type of
resistance, multigenic (horizontal) resistance, is a less (well)-studied phenomenon that
depends upon multiple genes in the plant host. All plants possess resistance mechanisms
which can be induced upon pre-treatment of plants with a variety of compounds. This
general phenomenon is known as systemic acquired resistance (SAR). At least in some
plants species, SAR depends on the timely accumulation of multiple gene products, such
as hydrolytic enzymes, peroxidases or other gene products related to plant defense. The
pre-treatment of plants with an inducing organism or chemical compound appears to
conduct the plant to mount an effective defense response upon subsequent encounter
with pathogens, converting what would have been a compatible interaction to an
incompatible one. Tuzum (2001) showed that in pathosystems (Lycopersicon spp.Alternaria solani) treated with BABA as elicitors, multigenic-resistant plants
constituvely express specific isozymes of hydrolytic enzymes that release cell wall
elicitors which, in turn, may activate other defense mechanisms. The aim of this work
was to characterize, in systems of potato multigenic resistance, the components of
biochemical mechanisms by which BABA (β-amino-butyric acid) increases resistance in
potato against phytopathogens by inducing a mechanism SAR.
Two potato cultivars with different level of horizontal resistance: the cultivar Bintje,
susceptible a P. infestans (Wastie, 1991; Huarte, 2002) and the cultivar Pampeana (MP
159.789/12 x Huinkul MAG) with a high grade of horizontal resistance, free of known
R genes Solanum demissum (Huarte et al., 1997) were analyzed in this work.
BABA applications and defense response observations were carried out as follow:
Three weeks after emergence, plants were sprayed with 2mM BABA, as chemical
inducer or with water as control treatment. Three or twenty days after this treatment,
leaves were cut and inoculated with P. infestans. At different times after inoculation,
symptoms were registered and treated leaves were processed for biochemical analysis.
Another set of treated plants were maintained in greenhouse until tuber maturation and
during this period, BABA was applied three more times at 20 days intervals. Recently
harvested tubers from treated or control plants were inoculated with P. infestans and, in
order to evaluate the specificity of systemic acquired response, other tubers were
inoculated with Fusarium solani f. sp. eumartii. Again, at different times after
inoculations, symptoms were recorded and tuber tissue was processed for the analysis of
defense components inductions.
The highest level of protection in leaves against P. infestans was observed after 3 days
of treatment with BABA, although this protection decreased in foliage at 20 days after
treatment in both cultivars. At this time the protective effect was more evident in
Pampeana than in Bintje. On the other hand, the levels of phytoalexins (antimicrobial
compounds) and phenols were increased only after 20 days in treated leaves from plants
of both cultivars. This increase was more than twice in each cultivar respect to the level
detected in infected but untreated plants. Callose content was also quantified.
Constitutively content was 40 % higher in Bintje than in Pampeana leaves but when
foliage was inoculated with P. infestans, only Pampeana leaves presented an increase
approximately of 25% in callose. The BABA treatment show a tendency to increase
callose level in both cultivars and contrary the further infection suppress this response.
The expression of two pathogenesis related proteins (PRPs), basic chitinases and
aspartic proteases was analyzed in leaves at 24 and 48 hs after inoculation with P.
infestans for both times of treatment with BABA (3 and 20 days). The accumulation of
PRPs was estimated by western-blot analysis. In cv. Pampeana, the expression of these
enzymes was increased in plants at 3 days of treatment; this induction decreased at 20
days. In cv. Bintje, the effect of BABA on the induction of these enzymes was less
evident than in Pampeana at 3 days but increased at 20 days after BABA treatment. For
the aspartic protease, the increase at this time was major than Pampeana cultivar. In
summary, at least for these PRPs, BABA application induces an accumulation of these
proteins and this response occurs later in the susceptible cultivar.
The systemic acquired resistance (SAR) induced by BABA treatment was verified in
harvested tubers coming from treated plants. BABA application in early stages of plant
development did not affect tuber harvest yield. Moreover, in our greenhouse conditions,
the number and weight of tuber recuperated from BABA treated plants increased respect
to control plants. In these tubers the protection was observed for P. infestans and also for
Fusarium eumartii. Tubers were inoculated with zoospores of P. infestans and
maintained in moister chamber during 7 days. F .eumartii was inoculated as disks of
mycelium introduced in tuber by wounding, and maintained for 10 days at 25 oC. At
different times tuber tissue surrounding treated area was processed for protein analysis.
For both cultivars, the expression level of aspartil proteases in tubers coming from
treated plant was major than in control tubers. On the other hand, previous results
demonstrated that the tissue coming from F. eumartii infected tubers present a high level
of serine protease activity, called FESP (Fusarium Extracellular Serine Protease) and
that the accumulation of this serine protease activity correlates with infection process
(Olivieri et al, 2004 ). In our experiments, FESP activity decrease approximately 40% in
infected tubers coming from BABA treated plants respect to infected tubers coming from
non-treated plants.
More than 3000 articles on induced pathogen resistance have been published since 1995.
However, many questions are still unanswered and required further investigation.
Stronger efforts are required to identify the compounds causing resistance, to quantify
these compounds and to correlate with the biologically detectable resistance to
characterize the induced stage in horizontal resistance. It is also necessary to evaluate the
durability of this defense response in tuber during storage and to know the optimal time
of BABA application to obtain a effective SAR.
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