The state of cell wall pectin monitored by wall associated kinases: A model

Abstract

The 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.

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 cross-linked in the cell wall of unchallenged plants, but also to pectin fragments or 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 α1–4 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 have been candidates for the pectin and thus wall sensors for both expansion and pathogen disturbances. WAKs are bound to native pectins in plants^10,12,13,16 but in vitro binding assays demonstrate that WAKs have a higher binding of de-esterified 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 WAK2^cTAP whose encoded 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.

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.

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 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.