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Establishing the biological role of MPK20 in primary cell wall formation in Arabidopsis Cheng, Siyu


Plants, as sessile organisms, need to grow while withstanding many environmental challenges. To give this capability, various cellular signal transduction systems are necessary for sensing changes in the environment and transmitting these signals to other components of the cell. One of the major mechanisms used for such signal transmission consists of mitogen-activated protein kinase (MAPK) cascades. MAPK cascades form an important signal transduction system in all eukaryotic organisms. In Arabidopsis thaliana, 20 MAPKs (MPKs) have been found, which belong to two sub-types: the TEY sub-type (MPK Groups A, B, and C) and the TDY sub-type (MPK Group D). Very little is known about the biological roles of Group D MPKs, but the expression of one group D MPK, MPK20, is correlated across many microarray experiments with expression of primary cell wall cellulose synthase (CesA) genes – CesA1, 3, and 6 [1]. This suggests that MPK20 may be involved in primary cell wall synthesis or associated biochemical processes. According to data in the MIND 0.5 database (Membrane-protein Interaction Network Database 0.5), MPK20 interacts with AtPIN1. As auxin is important for cell wall loosening and cell expansion, such an association with an auxin transporter could indicate that MPK20 plays a role in auxin signaling pathways, especially those that affect cell walls. In this study, the biological function of MPK20 has been explored by reverse genetics and phenotype assays. The mpk20 knock out (KO) lines display shorter hypocotyls and shorter primary roots compared with wild type (WT). However, these growth differences are small, although statistically significant. As MPK18, MPK19 and MPK20 are close homologues, double and triple KO mutants were built and analyzed. The loss-of-function mutants do not display any detectable auxin-related phenotype, nor do they differ from WT plants with respect to responses to other plant hormones tested except in their response to exogenous GA application. Attempts were made to purify a recombinant protein of MPK20 but these were unsuccessful. To explain the results of these experiments, models of MPK20 biological function, and of its interactions with the MPK20 paralogues, MPK18 and MPK19, have been proposed.

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