Studies on the molecular mechanism of oscmotic stress-induced MAP kinase signalling in Arabidopsis

Research project P 14114 Salt Stress Signalling Heribert HIRT 24.01.2000 In many areas of the world, the salt content of the soil has become a major limiting factor in agriccultrual performance. As adaptation to salinity relies on expression of specific gene products, initial efforts concentrated on the identification of salt stress-induced genes. In contrast to the rapidly growing list of such genes, we understanding of how cells sense and transmit salt stress. In yeast and mammalian cells, salt stress is signaled through highly conserved MAPK (Mitogen-activated protein kinase) cascades. We have recently identified a specific MAPK pathway that is activated by salt stress in Medicago and Arabidopsis. In this work, a genetic investigation of the biological function of the Arabidopsis MAPK pathway will be performed. In addition, upstream and downstream elements of the MAPK pathway will be isolated by interaction and activation screening methods in yeast. The function of these components will be subsequently investigated by both genetic and biochemical means.

During the course of the research project "Salt stress signal transduction" the work was continued on the functional characterisation of SIMKK, an activator of the salt stress-activated SIMK, a member of the class of mitogen- activated protein kinases (MAPKs). SIMK does not only interact with SIMKK, but also with another MAPK Kinase, PRKK (Pathogen-Responsive MAPKK). Our studies revealed, that SIMKK as well as PRKK can SIMK, but salt stress is exclusively mediated through SIMKK whereas elicitors (pathogen factors) can be mediated via both MAPKKs. These data suggested a satisfying molecular model, how different stress signals can activate the same MAPK signaling pathway. In cooperation with the Austrian Research Center Seibersdorf (ARCS), a patent was formulated and further investigations were carried out on the role of SIMKK in conferring salt stress tolerance. For this purpose, Arabidopsis was transformed with an expression vector for overexpression of SIMKK. The resulting 19 individual lines were investigated for salt stress tolereance, but no single lines was obtained that showed higher tolerance than wild type plants. I however, some lines revelaed a phenotype that is known as the triple response, and which is normally induced by the gaseous phytohormone ethylene. The triple response phenotype was found to be correlated with a constitutive activation ot the SIMK protein kinase. Based upon this observation, we analyzed whether SIMKK and SIMK are part of the long searched ethylene MAPK pathway. In fact, we could prove by biochemical and genetic analysis that SIMKK and SIMK mediate ethylene signaling in plants. These results were accepted Ihr publication in the highly respected EMBO Journal and formed the basis for a recently accepted research program on ethylene signal transduction.