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

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... ins (at least 20 MAPK and 10 MAPKK homologies have been identified in Arabidopsis) but functional analysis of this extensive matrix is just beginning. 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 AtMPKll genes. 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