MAP Kinase in Plant Disease Resistance Signaling Pathway

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Protein phosphorylation and dephosphorylation play important roles in plant disease resistance at multiple steps from the perception of invading pathogens to the activation of plant defense responses. WIPK, a tobacco mitogen-activated protein kinase (MAPK) is activated during resistance responses initiated by either non-host specific elicitors or 'gene-for-gene' interactions. Furthermore, induction of WIPK mRNA and protein occur systemically and correlates with the establishment of systemic acquired resistance (SAR). Extensive recent research in yeast and animals demonstrated that MAPK cascades are major pathways that transduce extracellular stimuli, including various stresses, into cellular responses. In contrast to all other characterized MAPKs that pre-exist in cells and require only phosphorylation activation, WIPK activation requires both post-translational phosphorylation and a preceding gene transcription and de novo synthesis of WIPK protein. Very interestingly, the activation of WIPK gene is mediated by H2O2, another important signaling molecule in plant defense responses.The goal of this project is to functionally define the role(s) of WIPK in both local and systemic resistance. A combination of molecular, biochemical and genetic approaches will be utilized to address this goal. To elucidate the function of WIPK, the upstream kinase of WIPK, WIPK kinase (WIPKK), that phosphorylates and activates WIPK, will be identified. In vivo WIPK activity will then be manipulated by overexpressing a constitutively active WIPKK mutant. The phenotypic change of such transgenic plants in response to pathogens or elicitors should provide the first functional definition of WIPK. To compliment the "gain-of-function" study, "loss-of-function" transgenic lines with suppressed WIPK or WIPKK expression and/or activation will be attempted by using anti-sense, co-suppression and overexpression of dominant negative mutant kinase. In addition, orthologs of WIPK and WIPKK in Arabidopsis will be identified which will facilitate the isolation of knock-out plants from T-DNA insertional mutant pools. One practical extension of this work is that the identification of an important regulatory component in the plant defense signaling pathway may lead to the engineering of crop plants with enhanced disease resistance. The development of such a strategy, which is based on the enhancement of the plant's own defenses, is important for sustaining agricultural production and improving the quality of our environment. Insights into the WIPK cascade should also provide a basis for (1) studying the regulation of plant MAPKs involved in responses to other stresses (e.g. cold, salt, and drought) or phytohormones (e.g. ethylene, auxin and ABA), (2) comparing stress-activated MAPKs in plants, yeast and mammals, (3) identifying unique aspects in the regulation and function of plant MAPKs. Equally important, understanding the role of WIPK, a regulatory component down-stream of H2O2, will greatly extend our knowledge about the signaling role of the oxidative burst in both local resistance and establishment of SAR.