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The mitogen-activated protein kinase kinase kinase SsOS4 regulates vegetative growth and fungicide sensitivity in Sclerotinia sclerotiorum

Tao Li: Nanjing Agricultural University

<div>The osmotic signaling system, which consists of the upstream two-component regulatory system and the downstream HOG MAP kinase signal system, plays an essential role in regulating morphology, virulence, osmotic stress, and fungicide sensitivity in filamentous fungi. HOG system can be activated by phosphorylation through the upstream two-component regulatory system and regulates intracellular glycerol synthesis and cell adapts to the environmental stresses. Group-Ⅲ histidine kinase is believed to regulate the two-component signaling system. Dicarboximide and phenylpyrrole fungicides have a strong antifungal activity against many phytopathogenic fungi. Action target of the two classes of fungicides has been documented to be involved in the osmotic signaling system. But fungicidal mechanism and resistant mechanism have not been fully elucidated. In our previous study, we characterized group-Ⅲ histidine kinase Shk1 in <em>Sclerotinia sclerotiorum</em> and revealed that Shk1 was involved in the sensitivity to fludioxonil and osmotic stresses. In this study, we characterized the MAP kinase kinase kinase SsOS4 (SS1G_06598), a key kinase in HOG system in <em>S. sclerotiorum</em>. Compared with the wild type isolate, disruption of SsOS4 showed a significant reduction in vegetative growth, a decreased sensitivity to phenylpyrrole and an increased sensitivity to osmotic stress. The virulence of the mutant was also assessed on leaves of rapeseed, but no significant difference was detected in the size of lesions. Meanwhile, Western blotting test showed that Hog1 phosphorylation was present in the wild type strain but not in the SsOS4-deleted mutant. This indicated that SsOS4 regulated the phosphorylation transmitting to the downstream Hog1 in HOG system.</div>