Functional analyses of Arabidopsis MAPK gene families

In plants, mitogen-activated protein kinase (MAPK) cascades have been implicated in controlling intracellular signaling in developmental processes and in response to many external stimuli, including biotic and abiotic stresses. The hallmark of a MAPK cascade is the participation of three classes of protein kinases (MAPK, MAPKK and MAPKKK) that operate hierarchically to amplify the initial signal. Plant genomes appear to encode an exceptionally rich array of MAPK cascade proteins (at least 20 MAPK and 10 MAPKK homologues have been identified in Arabidopsis) but functional analysis of this extensive matrix is just beginning.;I have analyzed further the cell and tissue distribution of their expression during development and in response to many external stimuli through use of promoter::GUS (beta-glucuronidase) reporter plants. On the one hand, my results show that AtMKK6 and AtMPK13 promoters are specifically active at the primary root zones where lateral root primordia (LRP) are emerging. Auxin treatment further stimulates the promoter of these genes in emerging LRP, and those promoter activities are suppressed by NPA, an auxin transport inhibitor. The AtMPK12 promoter, on the other hand, is specifically active in stomatal guard cells and can be induced by high salt and osmotic stress treatments. I have also conducted a phenotypic analysis of AtMKK6, AtMPK13 and AtMPK12 loss-of-function mutant plants. Together, these results indicate diverse roles of MAPKK/MAPK genes. I show that AtMKK6 and AtMPK13 activities are both associated with the lateral root formation process, while AtMPK12 plays discrete roles during stomatal development.;To gain insight into the specificity/redundancy of MAPKs and MAPKKs, I have used RT-PCR to examine the expression profiles of each of the identified MAPKK and MAPK genes in Arabidopsis. Gene expression patterns have been examined in various tissues, at several developmental stages and following a series of stress treatments. The findings reveal distinct expression patterns of AtMKK6, AtMPK13 and AtMPK12 genes.