The cell wall-associated kinases, WAKs, as pectin receptors

Abstract

The wall-associated kinases, WAKs, are encoded by five highly similar genes clustered in a 30-kb locus in Arabidopsis. These receptor-like proteins contain a cytoplasmic serine threonine kinase, a transmembrane domain, and a less conserved region that is bound to the cell wall and contains a series of epidermal growth factor repeats. Evidence is emerging that WAKs serve as pectin receptors, for both short oligogalacturonic acid fragments generated during pathogen exposure or wounding, and for longer pectins resident in native cell walls. This ability to bind and respond to several types of pectins correlates with a demonstrated role for WAKs in both the pathogen response and cell expansion during plant development.

A receptor-like protein mediates the response to pectin modification by activating brassinosteroid signaling

“…It is conceivable that abundance or modification of cell wall polymers is monitored directly; for example, by physical interaction with the extracellular domain of RLP44, perhaps in a way similar to that of the pectin-binding wall-associated kinases (4,5). Although there is as of yet no evidence supporting this hypothesis, it is noteworthy that at least one LRR protein, the polygalacturonase inhibitor protein, can interact with de-methylesterified pectin in the wall (37).…”

“…Whereas cell wall integrity signaling has been extensively studied in yeast (3), in plants this process remains poorly understood (2). A small number of putative cell wall sensors have been described, including the pectinbinding wall-associated kinases (4,5) and the receptor-like kinase THESEUS (6); however, very little is known about the signaling pathways involved. Here, we further investigated the previously identified function of brassinosteroid (BR) hormone signaling in maintaining cell wall integrity.…”

The maize disease resistance gene Htn1 against northern corn leaf blight encodes a wall-associated receptor-like kinase

“…Wallassociated non-RD kinases might therefore represent a novel class of quantitative immune receptors in monocots. Wall-associated kinases are the only known proteins that can physically link the cell wall to the plasma membrane, which enables them to perceive changes to the cell wall structure (10,35). WAKs can therefore serve as DAMP receptors that recognize changes to the cell wall during pathogen entry.…”

Requirement for Pectin Methyl Esterase and Preference for Fragmented over Native Pectins for Wall-associated Kinase-activated, EDS1/PAD4-dependent Stress Response in Arabidopsis

“…The expression of all five WAKs, clustered on a 30-kb locus on chromosome 1 (16,24), overlaps such that most tissues have some combination of these pectin receptors. Due to this overlapping expression and tight linkage, it has been hard to distinguish their respective contributions to pectin sensing (18). In vitro, WAK1 and WAK2 bind to long pectin chains reflective of a native pectin form (homogalacturonan) but have a preference for short OGs of degree of polymerization 9 -15 (17,25,26).…”