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Mitogen-activated protein kinase (MAPK/MPK) cascades are key signaling modules that regulate plant immunity. ENHANCED DISEASE RESISTANCE1 (EDR1) encodes a Raf-like MAPK kinase kinase (MAPKKK) that negatively regulates plant defense in Arabidopsis (Arabidopsis thaliana). The enhanced resistance of edr1 requires MAPK KINASE4 (MKK4), MKK5, and MPK3. Although the edr1 mutant displays higher MPK3/6 activation, the mechanism by which plants increase MAPK cascade activation remains elusive. Our previous study showed that MAPKKK5 is phosphorylated at the Ser-90 residue in edr1 mutants. In this study, we demonstrated that the enhanced disease resistance of edr1 required MAPKKK5. Phospho-dead MAPKKK5S90A partially impaired the resistance of edr1, and the expression of phospho-mimetic MAPKKK5S90D in mapkkk5-2 resulted in enhanced resistance to the powdery mildew Golovinomyces cichoracearum strain UCSC1 and the bacterial pathogen Pseudomonas syringae pv. tomato (Pto) strain DC3000. Thus, Ser-90 phosphorylation in MAPKKK5 appears to play a crucial role in disease resistance. However, MAPKKK5-triggered cell death was not suppressed by EDR1. Furthermore, activated MPK3 phosphorylated the N terminus of MAPKKK5, and Ser-90 was one of the phosphorylated sites. Ser-90 phosphorylation increased MAPKKK5 stability, and EDR1 might negatively regulate MAPK cascade activation by suppressing the MPK3-mediated feedback regulation of MAPKKK5. Taken together, these results indicate that MPK3 phosphorylates MAPKKK5 to enhance MAPK cascade activation and disease resistance in edr1 mutants.
Wang et al. (Mon,) studied this question.