A bacterial effector that suppresses activation of specific members of MAMP-activated MAPKs.
J. LEE (1), L. Eschen-Lippold (1), D. Scheel (1) (1) Leibniz Institute of Plant Biochemistry, Germany

Mitogen-activated protein kinase (MAPK) cascades are key signaling pathways in eukaryotic immunity. In the model plant Arabidopsis thaliana, there are two main branches of microbe-associated molecular pattern (MAMP)-induced MAPK cascades, one leading to MPK3/MPK6, and the other to MPK4/MPK11. To enable infection, pathogens often target MAPK elements to subvert host cellular signaling. An example is the Pseudomonas syringae HopAI1 effector, a phosphothreonine lyase, which irreversibly modifies the MAPK activation loop, thus preventing their phosphorylation and (re)activation. However, such effectors will indiscriminately inactivate all MAPKs, which may not be the best strategy for pathogens since some MAPKs are negative regulators of defence responses. We report another P. syringae effector that blocks the MAMP-induced MPK4/MPK11 activation but not the MPK3/MPK6 pathway. Since plants with dexamethasone-inducible expression of this effector are more susceptible to virulent P. syringae, this represents a virulence function of the effector, and implies that MPK4/MPK11 also have positive defence signalling functions. The suppressive effect is dependent on the protease activity of this effector but it does not degrade MPK4/MPK11 directly. To elucidate the molecular mechanism, we cloned several putative homologs from different bacteria (both pathogenic and non-pathogenic origins) and screened them for MPK4/MPK11 suppression activity as well as the other known activities of this effector. We will present these new data and our current understanding of how this effector inhibits MPK4/MPK11 activation. In conclusion, our work shows a mechanism of pathogens to specifically manipulate subsets of MAPKs to presumably fine-tune defense signaling.

Abstract Number: P9-276
Session Type: Poster