Download A cellular backline: specialization of host membranes for defence

Journal of Experimental Botany, Vol. 66, No. 6 pp. 1565–1571, 2015
doi:10.1093/jxb/erv021 Advance Access publication 25 February 2015
Review Paper
A cellular backline: specialization of host membranes for
defence
Christine Faulkner*
John Innes Centre, Norwich Research Park, Colney Lane, Norwich NR4 7UH, UK
* To whom correspondence should be addressed. E-mail: [email protected]
Received 3 November 2014; Revised 11 December 2014; Accepted 19 December 2014
Abstract
In plant–pathogen interactions, the host plasma membrane serves as a defence front for pathogens that invade from
the extracellular environment. As such, the lipid bilayer acts as a scaffold that targets and delivers defence responses
to the site of attack. During pathogen infection, numerous changes in plasma membrane composition, organization, and structure occur. There is increasing evidence that this facilitates the execution of a variety of responses,
highlighting the regulatory role membranes play in cellular responses. Membrane microdomains such as lipid rafts
are hypothesized to create signalling platforms for receptor signalling in response to pathogen perception and for
callose synthesis. Further, the genesis of pathogen-associated structures such as papillae and the extra-haustorial
membrane necessitates polarization of membranes and membrane trafficking pathways. Unlocking the mechanisms
by which this occurs will enable greater understanding of how targeted defences, some of which result in resistance,
are executed. This review will survey some of the changes that occur in host membranes during pathogen attack and
how these are associated with the generation of defence responses.
Key words: Callose, haustoria, lipid raft, papillae, pathogen, plasma membrane.
Introduction
Most plant pathogens infect tissues via an extracellular route.
Thus, the cell wall and plasma membrane act as key defence
fronts during an attempted pathogen invasion. Pattern recognition receptors (PRRs) that detect the presence of a
pathogen threat via pathogen-associated molecular patterns
(PAMPs) are anchored in the membrane and trigger signalling cascades that alter both the intracellular and extracellular environment, in some contexts by the transfer of signals
and/or molecules across the lipid bilayer. Host membranes
polarize in response to cellular penetration by filamentous
pathogens, targeting defence machinery to the site of attack.
They also remodel to accommodate subsequent invasive
structures such as haustoria. There is evidence that the host
plasma membrane changes in both composition and organization in order to mediate these interactions and to execute
defence responses. This appears to have an association with
dynamic membrane microdomains (possibly lipid rafts) that
might recruit relevant proteins and thus activate receptor
signalling and downstream responses. On a larger scale, the
membranes that surround penetration events and haustoria
are different in proteinaceous and lipid composition relative
to the rest of the plasma membrane. Thus, it is clear that host
cell membranes function as specific and dynamic scaffolds for
the execution of many defence responses.
Membrane-associated pathogen
perception and signalling
The recognition of pathogens by PRRs and their downstream
activation of defence is one of the best explored elements of
defence responses. PRRs are membrane-located receptors
© The Author 2015. Published by Oxford University Press on behalf of the Society for Experimental Biology. All rights reserved.
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1566 | Faulkner
that, in exposing receptor domains at the extracellular face of
the membrane, are able to bind PAMPs and thus recognize a
pathogen threat (Macho and Zipfel, 2014). PRRs are receptor
kinases or receptor proteins that span, or are tethered to, the
plasma membrane. Consequently, PRR signalling activity,
which occurs when a PRR binds its cognate PAMP ligand,
is anchored in the membrane in which it resides (Figure 1).
PRR activation triggers downstream signalling via several
different pathways. Amongst these are at least three membrane-associated responses: the production of reactive oxygen species (ROS) by NADPH oxidases; the influx of calcium
ions from the cell wall into the cytoplasm; and the production
of callose by membrane-resident callose synthases (see below).
The production of ROS (known as the oxidative burst) and
the influx of Ca2+ ions from the apoplast are interdependent processes (Segonzac et al., 2011). The influx of Ca2+ from
the apoplast is mediated by the balance of activity of membrane calcium channels and transporters (Frei dit Frey et al.,
2012), while the initial ROS burst is mediated by the NADPH
oxidase RESPIRATORY BURST OXIDASE D (RBOHD;
Nuhse et al., 2007; Zhang et al., 2007), which, being located
in the plasma membrane, produces ROS on the extracellular
face of the membrane. It is thought that the production of
ROS in the apoplast contributes to cell wall strengthening via
cross-linking of glycoproteins (Bradley et al., 1992).
The PRR FLAGELLIN SENSING2 (FLS2) binds
flagellin and mediates the initiation of a suite of antibacterial defences including ROS burst, Ca2+ influx, and callose
deposition. Flagellin binding occurs while FLS2 is resident
in the plasma membrane, triggering accumulation of FLS2
in detergent-resistant membrane microdomains (Keinath
et al., 2010). Following activation, and presumably any
lateral rearrangement of FLS2 within the membrane, the
receptor is internalized and enters the endocytic membrane
trafficking pathway (Beck et al., 2012). It is unclear what
FLS2 functions are associated with this change in localization, but this active rearrangement of FLS2-containing
membranes must contribute to defence signalling, even in
the simplest context in which internalization removes bound
receptor from the plasma membrane to allow for newly
synthesized receptor to take its place. FLS2 also mediates
flagellin-triggered redistribution of the ABC transporter
PENETRATION3 (PEN3; Underwood and Somerville,
2013). Following treatment of plant tissue with the flagellin
derivative flg22, PEN3 localization shifts from uniform distribution in the plasma membrane to focal accumulations in
what are assumed to be sites of detection (Underwood and
Somerville, 2013). This rearrangement is actin dependent,
but not dependent upon membrane secretion, suggesting
the possibility of lateral rearrangement within the existing
membrane. FLS2 signalling is dependent on the co-receptor
BRI1-ASSOCIATED KINASE1 (BAK1) (Chinchilla et al.,
2007), but, while PEN3 accumulation in response to flg22
is dependent on FLS2, it is not dependent on BAK1. Thus,
it seems likely that PEN3 accumulation still requires FLS2
binding of flagellin, but that lateral membrane reorganization occurs via an unknown co-receptor or an entirely independent mechanism.
Callose synthesis as a membraneanchored response
The deposition of callose in the apoplast is triggered in a
number of different contexts during plant–pathogen interactions. Callose is a β-1,3-glucan and has been hypothesized to
fortify cell walls and tissues against an invading pathogen.
Callose synthases (or glucan synthases) are located in membranes and synthesize callose towards the extracellular face
to deposit it in the cell wall. Thus, while the callose deposition
is detectable and measurable as an extracellular defence, the
response is associated with the activity of proteins that are
located in the plasma membrane.
The perception of PAMPs such as chitin and flagellin
causes what appears to be random deposition of callose in the
apoplast. However, there are also examples of more targeted
activity of callose synthases in response to pathogens. For
example, papillae that form in response to attempted cellular
penetration by filamentous pathogens are filled with callose,
as are encasements that surround the haustoria of both fungal and oomycte pathogens (Figure 1). In these cases, callose
synthesis is active at specific membrane domains. Similarly,
callose is deposited at plasmodesmata in response to bacterial
infection (Lee et al., 2011; Wang et al., 2013), again exemplifying targeted activity of callose synthases at a specialized
membrane domain during infection.
Callose deposition is well documented during papilla formation. Papillae are cell wall appositions that form at the site
of attempted penetration of a filamentous pathogen such as
the powdery mildews. While it was originally hypothesized
that callose forms a crucial part of a physical or chemical barrier to penetration, recent data have cast some doubt on the
simplicity of this model. According to the hypothesis that callose deposition increases structural resistance to penetration,
it was reasonable to expect the mutants for the GLUCAN
SYNTHASE-LIKE5 (GSL5; also known as POWDERY
MILDEW RESISTANT4, PMR4) gene, that do not deposit
callose in papillae during penetration, would show increased
susceptibility to an adapted powdery mildew. However, such
mutants exhibit increased resistance in this context (Jacobs
et al., 2003; Nishimura et al., 2003; Ellinger et al., 2013).
While increased resistance was co-incident with increased salicylic acid responses, the penetration success of the adapted
pathogen Golovinomyces cichoracearum on pmr4 mutants was
comparable with that observed in wild-type plants (Ellinger
et al., 2013), indicating that callose deposition does not significantly contribute to any defensive function of papillae and
that altered resistance in this interaction was the consequence
of responses downstream of callose deposition in papillae. By
contrast, the deposition of callose in papillae in response to
attempted penetration by the non-adapted powdery mildew
Blumeria graminis f. sp. hordei (Bgh) does contribute significantly to the defensive function of the papilla. Ellinger et al.
(2013) observed a small but significant increase in penetration
success of Bgh in pmr4 mutants, exhibiting 20% success compared with the 8% success rate observed on wild-type plants.
While these data show that callose deposition does positively
contribute to penetration resistance, the small nature of the
Membranes in defence | 1567
change indicates that even in this context callose deposition is
a minor component.
Studies in which callose deposition was enhanced by
overexpression of PMR4 suggest that despite the minor
role played by callose in an endogenous context, callose
can indeed act as a barrier to pathogen penetration. Plants
which express 35S::PMR4-GFP exhibit complete penetration resistance to both adapted and non-adapted powdery
mildews (Ellinger et al., 2013; Eggert et al., 2014). These lines
also enabled high resolution imaging of callose deposition at
papillae and identified that while callose is deposited primarily at the membrane face of the cell wall, as might be expected
from the membrane location of callose synthases, the callose
polymers can penetrate the cellulose matrix. In PMR4 overexpression lines, callose polymers completely penetrate the
cell wall and form a layer of callose at the external surface
of the wall (Eggert et al., 2014), leading to the hypothesis
that callose deposition protects the cell wall from hydrolysing enzymes produced by fungal pathogens. Indeed, cellulose
digestion of inoculated leaves induced degradation of the cellulose–callose network at papillae which was not observed in
35S::PMR4-GFP plants. In wild-type plants, cellulase had a
greater effect on callose than on cellulose at the papilla, suggesting that callose may act as a sacrificial protectant against
cellulose hydrolysis.
The observation that callose deposition is targeted to specific sites, such as at the papillae and haustoria, supports
the notion that callose plays some role in physical or chemical defence against pathogen invasion. Further it requires a
mechanism by which callose synthases are activated in, or
targeted to, specific domains within the membrane. Vesicle
trafficking machinery has been implicated in deposition
of callose at the papilla in barley and Arabidopsis. ARF
GTPases recruit coat proteins to membrane domains and
mediate vesicle budding; in barley, the GTPase ARFA1b/1c
was found to be required for callose deposition at the papilla
(Bohlenius et al., 2010), but not for formation of the papilla
structure. ARFA1b/1c also contributes to penetration resistance of Bgh and functions in the same membrane trafficking
pathway as the REQUIRED FOR MLO RESISTANCE2
(ROR2)/PENETRATION1 (PEN1) syntaxin (Bohlenius
et al., 2010). The co-localization of ARFA1b/1c with ARA7
led the authors to hypothesize that callose synthesis might
occur in multivesicular bodies (MVBs) prior to trafficking of
the membranes and cargoes to the penetration site (Figure 1).
However, ARA7 is also located in early endosomes (Ueda
et al., 2004; Beck et al., 2012) and it was reported that callose
could not be detected in MVBs, despite their proximity to
the penetration site (An et al., 2006), suggesting that further
information is required to determine if callose is made prior
to targeted deposition at the papilla in this system.
Further implicating membrane trafficking pathways in
targeted callose deposition, recent work on the Arabidopsis–
Golovinomyces interaction identified that a Rab GTPase,
RabA4c, interacts directly with PMR4 and that this interaction might recruit PMR4 to the penetration site (Ellinger et al.
2014a). Callose synthesis appeared to be linked to the functional state and location of RabA4c rather than changes in
its expression in response to infection, suggesting that membrane trafficking and protein interactions control the activity
and location of callose synthesis in response to pathogen penetration. Another scenario which has implicated membrane
composition in callose deposition is the production of lipase
by the pathogen Fusarium graminarum. Production of the
lipase reduces callose deposition during pathogen infection
of spikelets, and this was hypothesized to occur via lipasemediated production of free fatty acids that possibly disrupts
the lipid composition of the membrane and consequently
reduces callose synthase activity (Ellinger et al., 2014b). Thus
it seems not only that membranes act as a scaffold for callose
synthesis but that membrane trafficking and composition
define the activity of callose synthases.
Callose deposition is also highly localized in the encasement of pathogen haustoria. Haustorial encasement can
occur in both fungal and oomycete interactions, such as
Hyaloperonospora arabidopsidis (Hpa)–Arabidopsis and
Uromyces fabae–wheat. Like at the papilla, pmr4 mutants
show reduced callose deposition at the Hpa encasement but
increased resistance (Vogel and Somerville, 2000), suggesting
that callose plays a minor role in defence. However, recent data
provide evidence that the deposition of callose in the encasement positively regulates defence. The PLASMODESMATA
LOCATED PROTEIN1 (PDLP1) localizes to the extra-haustorial membrane (EHM) prior to development of the encasement and regulates callose deposition in the encasement as it
develops (Caillaud et al., 2014). Like PMR4, overexpression
of PDLP1 enhances callose deposition, which in this interaction is co-incident with enhanced membrane convolution
surrounding the haustorium. Significantly, overexpression
of PDLP1 increases resistance of Arabidopsis to Hpa, and
pdp1,2,3 mutants show both increased susceptibility and
reduced callose deposition in the encasement. This demonstrates that callose deposition does contribute significantly to
defence in this interaction.
The body of data relating to the role of callose deposition
during defence provides what appears to be conflicting data
regarding the significance of the response. However, it is clear
that in some contexts callose deposition positively contributes to defence responses (Ellinger et al., 2013; Caillaud et al.,
2014). It is possible that the simple hypothesis that callose
deposition fortifies the apoplast against invasion is in part
correct, but that the ultimate contribution that this makes to
defence is dependent on spatio-temporal factors and crosstalk with other defence responses.
Membrane specialization via protein
composition
Haustoria are assumed to act as sites of molecular exchange
between host and pathogen, and therefore many of these processes must be localized at the EHM (Figure 1). While the
EHM is continuous with the plasma membrane of the host
cell, plasma membrane-resident proteins are not always present in the EHM, indicating that there is specialization of
the membrane (Roberts et al., 1993; Koh et al., 2005; Wang
1568 | Faulkner
Fig. 1. Membrane specialization during (A) PRR signalling, (B) papillae formation, and (C) haustorial accommodation. Lipid rafts that are enriched in
sphingolipids and sterols, and contain remorins, have been implicated in the definition of membrane domains that facilitate receptor signalling (A) and
callose synthesis (B, C). (A) The PRR FLS2 binds bacterial flagellin and this induces complex formation with the co-receptor BAK1. This triggers a
calcium ion influx, the generation of reactive oxygen species by RBOHD, and callose deposition, as well as multiple non-membrane-anchored responses.
(B) Papillae are cell wall appositions that form in response to attempted penetration by filamentous pathogens, here illustrated by what is known for
powdery mildew pathogens. The final composition of papillae-associated membranes has been speculated to result from VAMP721/722-associated
membrane trafficking and deposition of materials, such as lipids and callose synthases, at the papillae-associated membrane. Multivesicular bodies have
been proposed to target PEN1 to papilla as PEN1 has been observed in membranes in the papillary matrix. It is also possible that lateral redistribution
of some proteins such as the ABC transporter PEN3 results in accumulation beneath the papilla. (C) The source of membrane for the genesis of the
extrahaustorial membrane (EHM) is unknown, but VAMP721/722 vesicles have been implicated in the trafficking of RPW8 and PDLP1 to the EHM.
The lipid raft protein remorin has also been associated with the EHM, suggesting that it might be further specialized into microdomains. Membranes
associated with the encasement are sites of callose synthesis.
et al., 2009; Lu et al., 2012). Electron micrographs have also
determined that the EHM is different in appearance (Chou,
1970; Knauf et al., 1989; Mims et al., 2002) from the plasma
membrane. In several interactions the EHM appears convoluted or invaginated, which increases the membrane surface
in proximity to the pathogen (Chou, 1970; Mims et al., 2002;
Micali et al., 2011). It is assumed that pathogen effectors are
delivered across this membrane, as are nutrients from host to
pathogen, and thus it is conceivable that the increased surface of the membrane is manipulated by, and to the benefit
of, the pathogen. However, as defence responses are spatially
anchored in the membrane, it is also likely that this membrane
convolution enhances the capacity for defence responses to be
targeted at the site of attack. The identification of PDLP1 at
the EHM and its association with callose deposition indicates
that membrane convolutions in the EHM are positively correlated with increased callose deposition (Caillaud et al., 2014),
suggesting that this increased membrane scaffold enhances at
least one defence response. The trade-off between host and
pathogen processes can only be unravelled as molecular players, such as pathogen effectors of known translocation mechanism and activity or host EHM-associated proteins with a
known mechanism of defence activity, are identified.
With respect to the protein composition of the EHM, Lu
et al. (2012) observed that in the Hpa–Arabidopsis interaction the calcium ATPase AUTOINHIBITED CA2+-ATPASE
(ACA8) and the aquaporin PLASMA MEMBRANE
INTRINSIC PROTEIN1;4 (PIP1;4) were excluded from the
EHM but maintained normal association with the plasma
membrane during infection. The authors noted that this
specificity indicated that the mechanism by which proteins
were targeted to the EHM was unlikely to involve diffusion
of proteins from the plasma membrane, or the simple redirection of general membrane trafficking to the plasma membrane. In this study the EHM did contain plasma membrane
proteins such as the PRR FLS2 and the syntaxin PEN1, and
proteins that mark endomembrane compartments indicated
that these accumulate around the haustoria. This suggests
that membrane trafficking plays a role in the genesis of the
EHM, and possibly in the mechanism by which this membrane is specified.
Independent studies have implicated the R-SNARE
VESICLE ASSOCIATED MEMBRANE PROTEIN
(VAMP) 721/722 in membrane trafficking to the EHM
and papillae. These vesicle-associated proteins are necessary to target the resistance protein RPW8 to the haustoria
of Golovinomyces orontii (Kim et al., 2014), and have been
identified in PDLP1-containing membranes (Caillaud et al.,
2014), suggesting that both proteins use the same membrane
trafficking pathway to target haustoria of unrelated pathogens. Similarly, VAMP721/722 were found to be required
for resistance to penetration of non-adapted Erysiphe
Membranes in defence | 1569
pisi on Arabidopsis and to form a complex with PEN1
and the t-SNARE SOLUBLE N-ETHYLMALEIMIDESENSITIVE FACTOR ADAPTOR PROTEIN33 (SNAP33;
Kwon et al., 2008). This implicates the same membrane trafficking pathway in polarization of membranes at papillae. It
is likely that characterization of this pathway will identify
factors that define the specificity of the membrane at the
host–pathogen interface, leading to dissection of the mechanisms by which the host exploits this membrane as a targeted
site of defence responses.
PEN1 is present in the plasma membrane when the cell is
unchallenged by a pathogen. Following attempted penetration, PEN1 accumulates at membranes beneath papillae triggered by powdery mildew pathogens (Collins et al., 2003;
Meyer et al., 2009). This accumulation has been associated
with membranes in the extracellular papillary matrix (Meyer
et al., 2009; Nielsen et al., 2012) rather than continuous with
the plasma membrane, raising the possibility that PEN1 is
secreted by exosomes. Two studies have independently tested
the accumulation of PEN1 following pharmacological treatment that disrupts elements of the acto-myosin system.
While both cytochalasin E (which disrupts actin filaments)
and 2,3-butanedione monoxime (BDM; a myosin inhibitor)
induce a subtle change in the distribution of PEN1 beneath
papillae to a more diffuse and slightly punctate pattern, an
accumulation still occurs (Underwood and Somerville, 2013;
Yang et al., 2014). This was interpreted differently in each
study as a dependence (Yang et al., 2014) and an independence (Underwood and Somerville, 2013) of PEN1 targeting
on the actin cytoskeleton. Underwood and Somerville (2013)
showed that PEN1 accumulation is dependent upon secretion, but the subtle change in PEN1 arrangement beneath
the papillae may point to a two-step mechanism for focal
accumulation. The ABC transporter PEN3 is located in
the plasma membrane and also accumulates at membranes
beneath papillae following a pathogen challenge (Stein et al.,
2006). Unlike PEN1, PEN3 accumulation depends upon the
actin cytoskeleton but not on secretion (Underwood and
Somerville, 2013), suggesting that targeting of different proteins to the site of pathogen invasion employs different membrane trafficking and/or re-organization mechanisms.
Membrane specialization via lipid
composition
Lipids within the plasma membrane are not homogenous and
the spatial concentration of different types of lipids within
the membrane can define clusters of functional specialization
within the membrane. This is of particular significance to
receptor signalling in animal cells, with activation or formation of receptor complexes dependent upon the lipid environment (Simons and Gerl, 2010). Lipid rafts are characterized
by an increased concentration of sphingolipids and sterols,
and are often correlated with detergent-resistant membrane
(DRM) extracts. As evidence of the role of lipid composition
in the polarization of membranes, sphingolipids were found
to be essential for the polarization of both PIN1 and AUX1,
and sterols for polarization of PIN2, in Arabidopsis mutants
for the respective lipid synthesis pathways (Men et al., 2008;
Markham et al., 2011). Further, the Fusarium toxin fumonisin, which prevents ceramide synthase activity, also disrupted
the polarity of PIN proteins without disrupting endocytic
pathways (Aubert et al., 2011).
As the polarization and specialization of membranes is also
observed in the genesis of papillae-associated membranes
and the EHM, it is likely that lipid composition also contributes to this phenomenon. Indeed, both of these membrane
subdomains have been associated with lipid rafts. Papillae
formed by attempted penetration of the Bgh pathogen on the
non-host Arabidopsis were stained with the fluorochrome filipin, which is sequestered by sterols, suggesting that the host
membrane at the penetration site is enriched by sterols and
thus shares characteristics with lipid rafts (Bhat et al., 2005).
Remorins are plant-specific proteins that are located in steroland sphingolipid-enriched membranes (Raffaele et al., 2009).
During infection of Nicotiana benthamiana by the oomycte
Phytophthora infestans, transient expression of the potato
remorin StRem1.3 showed localization to the EHM (Bozkurt
et al., 2014), similarly implicating lipid rafts in the specialization of defence-associated membranes.
Further supporting the hypothesis that both papillae-associated membranes and the EHM contain lipid rafts, several
studies have identified that DRMs in different plant species have β-1,3-glucan synthase activity and are capable of
producing callose (Him et al., 2001; Bessueille et al., 2009;
Cifuentes et al., 2010; Srivastava et al., 2013). This activity correlates with the identification of callose synthases in
DRMs (Srivastava et al., 2013) and indirectly suggests the
possibility that the papillae-associated membrane and the
EHM have characteristics of lipid rafts as they are sites of
callose synthesis. The observation that PDLP1 localizes to
both the plasmodesmal plasma membrane and the EHM in
the Arabidopsis–Hpa interaction (Caillaud et al., 2014), and
influences callose deposition in both locations, suggests that
callose deposition utilizes the same regulatory mechanisms in
both membranes. Remorins have also been observed to localize to plasmodesmal membranes (Raffaele et al., 2009; Gui
et al., 2014), raising the possibility that callose deposition in
plasmodesmata, and thus possibly in the EHM, has an association with lipid rafts (Gui et al., 2014).
Close examination of the localization of both RPW8 and
PDLP1 to the EHM surrounding powdery mildew (Berkey
et al., 2012) and downy mildew (Hpa; Caillaud et al., 2014)
haustoria, respectively, identifies uneven distribution of the
proteins. Similarly, StRem1.3 localization to the EHM of
Phytophthora haustoria is not homogeneous (Bozkurt et al.,
2014) and does not overlap with the localization of another
EHM-resident protein, SYNAPTOTAGMIN1. Each of
these uneven distributions occurs in haustoria from different
pathogens, but identify membrane domains within the EHM.
Again, this is an indirect and speculative association between
possible lipid rafts and defence-associated membranes, but
examination of all markers in a single interaction would
determine if these microdomains are indeed lipid rafts (ie. colocalize with remorin).
1570 | Faulkner
Membrane microdomains have also been implicated in PRR
activation and signalling. Several components of PRR signalling have been identified in DRMs following elicitation with
pathogen-derived molecules, including RBOHD (Mongrand
et al., 2004; Noirot et al., 2014) and the PRR FLS2 (Keinath
et al., 2010). It is possible that these microdomains form signalling platforms and recruit necessary components, but
further characterization of the spatio-temporal dynamics of
protein–protein interactions during defence activation, as well
as protein–lipid interactions are needed to understand fully
how lateral membrane reorganization contributes to defence.
Conclusions
Host membranes act as scaffolds and signalling platforms
for defence responses in several contexts. In anchoring PRRmediated pathogen perception, they act as one of the earliest defence fronts and may depend upon membrane domains
for activity. Dynamic reorganization of membranes not only
accommodates invasive structures but targets defences such
as callose deposition to specific subcellular sites. The mechanisms by which membranes facilitate and/or regulate this
activity will elucidate not only how a single cell responds to
pathogen attack but also how cellular polarization can occur
in fully expanded cells. It is clear that while lipid composition
is a poorly explored facet of plant cell biology, it has unconsidered regulatory roles in cellular responses. Further analysis
of the composition and function of membrane domains will
detail how membranes respond to pathogens, and orchestrate
and target specific responses.
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