Cell Wall Associated, Receptor Protein Kinases

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Cell walls are carbohydrate and protein structures that surround and separate plant cells.As plant cells expand in a regulated fashion they must necessarily modify and enlargetheir cell walls to permit the subsequent increase in volume. The cell wall itself mightinfluence this expansion process. Thus the cell has the potential to influence almost everyaspect of plant function simply because of its position and physical properties. For thisreason many have speculated on its role in a plant's development and response to theoutside world. Cell growth can occur in many dimensions, such as the polarizedexpansion of a pollen tube tip, the creation of elongated cells characteristic of manyvegetative tissues, the establishment of a meristem, or even the jigsaw-like arrangementof cells at the leaf surface. Currently the understanding of how cell wall architecture iscoordinated with programmed and hormone regulated cell expansion events duringdevelopment is not well understood in angiosperms.Recent studies have identified a number of proteins at the cell surface that coulddirectly regulate cell wall functions, and the Cell Wall Associated Kinases, or WAKs, areamong these. There are five cell wall associated kinases in Arabidopsis andrepresentatives in other angiosperm families. WAKs each have a cytoplasmicserine/threonine protein kinase domain, span the plasma membrane and extend a domaininto the cell wall. WAKs physically link the plasma membrane to the carbohydrate matrixand are unique in that they have the potential to directly signal cellular events throughtheir kinase domain.The WAK extracellular domain is variable between the five isoforms, and collectivelythe family is expressed in all organs. WAK1 and WAK2 are the most ubiquitously andabundantly expressed of the five tandemly arrayed genes, and their messages are presentin vegetative meristems, junctions of organ types, and areas of cell expansion. They arealso induced by pathogen infection and wounding. The WAK1 but not WAK2 cell walldomain binds to a glycine rich protein (GRP) of the cell wall in in vitro assays. WAK1and GRP can be co-immunoprecipitated from leaf or seedling extracts, and this WAK isphosphorylated. Recent experiments demonstrate that antisense WAK expression leadsto the loss of cell expansion, and preliminary results indicate that antisense GRPproduction causes an enhancement of cell expansion (and eventual cell death). Thus GRPmay be a negative regulator of WAK activity. A large amount of WAK is also covalentlylinked to pectin, and most of WAK that is bound to pectin is also phosphorylated. Thedata support a model where WAK1 is bound to GRP as a phosphorylated kinase, and alsobinds to pectin. How WAKs are involved in signaling from the pectin extracellular matrixin coordination with GRPs will be key to the understanding of the cell wall's role in cellgrowth.WAKs are poised to provide a direct signal from the cell wall to the cytoplasm of manycell types during the development of Arabidopsis. In order to characterize theassociations and roles of GRPs and WAKs in plants it will be necessary to generateadditional (partial) loss of function alleles of the two predominant WAKs and of GRP.These alleles can then be used to determine the relationship of WAK mediated cellexpansion to other known signaling pathways. WAK kinase substrates will be alsoidentified and used to assay the activation of WAK by wall components anddevelopmental cues, and eventually mutant alleles of these substrates will help in theanalysis of WAKs and cell elongation. The goal is to understand how WAKs inassociation with GRP and pectins regulate cytoplasmic components so that the cell wallis coordinated with processes of cell growth.