Plant Signaling & Behavior
ISSN: (Print) 1559-2324 (Online) Journal homepage: https://www.tandfonline.com/loi/kpsb20
The state of cell wall pectin monitored by wall
associated kinases: A model
Bruce D Kohorn
To cite this article: Bruce D Kohorn (2015) The state of cell wall pectin monitored by
wall associated kinases: A model, Plant Signaling & Behavior, 10:7, e1035854, DOI:
10.1080/15592324.2015.1035854
To link to this article: https://doi.org/10.1080/15592324.2015.1035854
Published online: 07 Aug 2015.
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ARTICLE ADDENDUM
Plant Signaling & Behavior 10:7, e1035854; July 2015; © 2015 Taylor & Francis Group, LLC
The state of cell wall pectin monitored by wall associated kinases:
A model
Bruce D Kohorn*
Department of Biology; Bowdoin College; Brunswick, ME USA
T
he Wall Associated Kinases (WAKs)
bind to both cross-linked polymers
of pectin in the plant cell wall, but have a
higher affinity for smaller fragmented
pectins that are generated upon pathogen
attack or wounding. WAKs are required
for cell expansion during normal seedling
development and this involves pectin
binding and a signal transduction pathway involving MPK3 and invertase
induction. Alternatively WAKs bind
pathogen generated pectin fragments to
activate a distinct MPK6 dependent
stress response. Evidence is provided for
a model for how newly generated pectin
fragments compete for longer pectins to
alter the WAK dependent responses.
Keywords: Arabidopsis, cell wall, plant
defense, receptors, signaling
*Correspondence to: Bruce D Kohorn; Email:
bkohorn@bowdoin.edu
Submitted: 03/23/2015
Accepted: 03/24/2015
http://dx.doi.org/10.1080/15592324.2015.1035854
www.tandfonline.com
The cell walls of angiosperms are composed of a complex arrangement of cellulose, hemicellulose and pectin. The
pectins can be selectively and locally modified to be cross-linked into a structural
network that can have dramatic effects on
cell enlargement,1-3 but numerous pathogens and mechanical disruptions fragment
this pectin network, leading often to a
plant stress response.4-6 The Wall Associated Kinases (WAKs) are receptor kinases
that bind pectin in the cell wall, and span
the plasma membrane to place a serine/
threonine kinase in the cytoplasm.7-16 A
number of studies have shown that WAKs
are required for cell expansion during
development, but also mediate a pectin
fragment induced stress response.12,16,17
How these receptors can be involved in 2
distinct responses is not well understood,
but the key perhaps lies in that WAKs
bind to long polymers of pectin crosslinked in the cell wall of unchallenged
plants, but also to pectin fragments or
Plant Signaling & Behavior
oligogalacturonides (OGs) generated by
wounding or pathogens as they
invade.12,18 What has been missing is a
model for how WAKs might distinguish
the 2 pectin states so as to trigger expansion versus a stress response. We proposed
a model where newly generated OGs compete with native pectin for WAKs, and
activate alternate signal transduction
pathways.13
Background
Plant cell walls arise through a complex,
developmentally regulated coordination of
synthesis, turnover, and interactions
between protein and carbohydrates.12
Screens for mutants in developmental processes have not surprisingly then revealed
numerous alleles of cell wall biosynthesis
genes, and conversely mutations in cell wall
function have identified alleles in genes normally associated with a variety of metabolic
and developmental pathways.21 These
genes include receptor kinases such as
THE1,FER, HERK, ANX, and RLP44 and
have been termed cell wall sensors.19-26 Of
the “wall sensors” only WAKs, also receptor
kinases, are known to bind to a cell wall
component, pectin.
Pectin is first made as methyl esterified
a1–4 D-galacturonic acid in the golgi,
secreted,1,3,27 and then modified and
cross-linked in the extracellular space.
Localized activity of Pectin Methylesterases (PME) expose an oxygen to bind calcium that mediates a crosslinking. It is
thought that regulation of the location
and extent of PME activity can influence
wall structure and directionality of loosening thereby influencing cell growth.24
Because WAKs bind to pectin they long
e1035854-1
different co-receptors to distinguish the
pectin, and these receptor complexes have
different downstream partners. Analysis of
the components of the 2 pathways should
help to answer this next question.
Disclosure of Potential Conflicts of Interest
No potential conflicts of interest were
disclosed.
References
Figure 1. (A) Model for Pectin and OG (pectin fragment) Activation of 2 Responses through WAK.
Orange lines represent cell wall pectin, cross-linked or fragmented by pathogens or wounding. OGs
outcompete longer polymers for activation of a WAK (red boxes) dependent stress response. WAKs
are predicted to associate with co-receptors (green or black) to mediate different responses. In the
absence of pathogen, WAKs monitor cell wall pectin and are required for cell expansion. See text
for details.
have been candidates for the pectin and
thus wall sensors for both expansion and
pathogen disturbances. WAKs are bound
to native pectins in plants10,12,13,16 but in
vitro binding assays demonstrate that
WAKs have a higher binding of deesterified over esterified pectins,9,28,29 and
have a preference (competition assays) for
short OGs of degree of polymerization.9-15
Pathogens tend to target de-esterified pectins. During seedling growth, WAKs are
required for cell expansion and have been
shown to be involved in the pectin activation of MPK3 and a vacuolar invertase
that can increase turgor driven expansion.9,10,12 But WAKs are also required for
a response to pathogen, are necessary for
the OG stress response14 and bind and
trigger a response to OGs in a transient
assay.17 This stress response appears to
have a distinct signaling pathway and
includes a ROS burst, MPK6 activity, and
the EDS1 and PAD4 dependent activation
of numerous genes, including the
ca. 1000 fold induction of a downstream
target gene FADlox which serves as a
robust indicator.13,14
A Model
We recently showed that a dominant
WAK2 allele WAK2cTAP whose encoded
e1035854-2
protein requires a functional pectin binding domain and an active kinase induces a
constitutive stress response.13,14 But
importantly, this WAK allele is suppressed
by a null allele of a pectin methyl esterase,
pme3.13 This provides genetic evidence
that WAKs are sensing the de-esterified
form of pectin, consistent with the higher
affinity in vitro of WAKs for de-esterified
over esterified pectin. But we also found
that the pme3/pme3 mutant plant is more
responsive to OGs than WT plants. One
explanation is that since WAK is bound
less tightly to esterified pectin in the
mutant, then more is available to receive
incoming OGs. Collectively the data
are consistent with a model (Fig. 1) where
OGs are competing with native pectin for
WAKs, and this provides a mechanism for
WAKs to distinguish pectins, and activate
alternate pathways.
Future Questions
The question now remains as to how
the 2 different types of pectins can trigger
one receptor to activate different paths. It
is possible that part of the mechanism lies
in the heterogeneity of the WAK family,
as there are 5 WAKs tightly clustered in a
30 KB Arabidopsis locus.30 But it is also
possible that WAKs associate with
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