Interaction of the Arabidopsis receptor protein kinase Wak1 with a glycine-rich protein, AtGRP-3.

The Arabidopsis wall-associated receptor kinase, Wak1, is a member of the Wak family (Wak1-5) that links the plasma membrane to the extracellular matrix. By the yeast two-hybrid screen, we found that a glycine-rich extracellular protein, AtGRP-3, binds to the extracellular domain of Wak1. Further in vitro binding studies indicated that AtGRP-3 is the only isoform among the six tested AtGRPs that specifically interacts with Waks, and the cysteine-rich carboxyl terminus of AtGRP-3 is essential for its binding to Wak1. We also show that Wak1 and AtGRP-3 form a complex with a molecular size of approximately 500 kDa in vivo in conjunction with the kinase-associated protein phosphatase, KAPP, that has been shown to interact with a number of plant receptor-like kinases. Binding of AtGRP-3 to Wak1 is shown to be crucial for the integrity of the complex. Wak1 and AtGRP-3 are both induced by salicylic acid treatment. Moreover, exogenously added AtGRP-3 up-regulates the expression of Wak1, AtGRP-3, and PR-1 (for pathogenesis-related) in protoplasts. Taken together, our data suggest that AtGRP-3 regulates Wak1 function through binding to the cell wall domain of Wak1 and that the interaction of Wak1 with AtGRP-3 occurs in a pathogenesis-related process in planta.

Plant glycine-rich proteins: a family or just proteins with a common motif?

Twelve years ago a set of glycine-rich proteins (GRP) of plants were characterized and since then a wealth of new GRPs have been identified. The highly specific but diverse expression pattern of grp genes, taken together with the distinct sub-cellular localisation of some GRP groups, clearly indicate that these proteins are implicated in several independent physiological processes. Notwithstanding the absence of a clear definition of the role of GRPs in plant cells, studies conducted with these proteins have provided new and interesting insights on the molecular and cell biology of plants. Complex regulated promoters and distinct mechanisms of gene expression regulation have been demonstrated. New protein targeting pathways, as well as the exportation of GRPs from different cell types have been discovered. These data show that GRPs can be useful as markers and/or models to understand distinct aspects of plant biology. In this review, the structural and functional features of this family of plant proteins will be summarised. Special emphasis will be given to the gene expression regulation of GRPs isolated from different plant species, as it can help to unravel their possible biological functions.

Arabidopsis thaliana class IV chitinase is early induced during the interaction with Xanthomonas campestris.

Endochitinases are widely distributed among higher plants, including a number of important crop species. They are generally considered to be involved in plant defence against potential pathogens. We have cloned a class IV chitinase gene (AtchitIV) from Arabidopsis thaliana. Southern blot analysis allowed the detection of two cross-hybridising genes in the A. thaliana genome. AtchitIV transcripts are detected in seedpods, but not in roots, inflorescence stems, leaves and flowers of healthy plants. The transcripts accumulated very rapidly in leaves after inoculation with Xanthomonas campestris. Maximum mRNA accumulation was reached one hour after infection and decreased to very low levels 72 hours after induction. This result suggests an involvement of AtchitIV in the initial events of the hypersensitive reaction. Nevertheless, A. thaliana plants transformed with the gus gene under the control of a class IV chitinase bean promoter, showed GUS activity in seed embryos. These data, together with the constitutive expression of the endogenous gene in the seedpods, points to additional physiological roles for this protein.