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.

Keywords: WAKs; cell expansion; cell wall; pathogen response; pectin; receptor kinase.

FIGURE 1

A model for pectin–WAK regulation of growth and the stress response. Cartoon of pectin in the cell wall (long brown lines) activating WAK in the plasma membrane, and subsequent activation of MPK3. MPK6 is either repressed or not activated, and downstream gene activation leads to cell growth. In the presence of OGs, fragmented pectins, generated by pathogens or wounding, WAKs now activate MPK6 as well as MPK3, and the subsequent change in gene expression leads to a stress response. MPK6 and MPK3 are activated by many other pathways, as indicated by black arrows not originating from WAK.