UvSMEK1, a Suppressor of MEK Null, Regulates Pathogenicity, Conidiation and Conidial Germination in Rice False Smut Fungus Ustilaginoidea virens

Abstract

Rice false smut, which is caused by Ustilaginoidea virens, is an emerging disease of rice spikelets in rice-growing areas worldwide. However, the infection mechanism of U. virens on rice spikelets is still unclear. Here, we characterized a suppressor of mitogen-activated protein kinase kinase or ERK kinase (MEK) null (UvSMEK1) in U. virens that is conserved among filamentous fungi. Compared with wild type U. virens strain P-1, UvSMEK1 deletion mutants were defective in pathogenicity and conidial germination. In addition, conidiation of UvSMEK1 deletion mutants was significantly reduced on yeast extract tryptone (YT) plates, but increased in YT broth compared with the wild type. Compared with UvSMEK1 expression level during the vegetative mycelia and conidiation stages, UvSMEK1 dramatically increased during infection of rice florets. Surprisingly, the UvSMEK1 deletion mutants exhibited higher tolerance to H2O2 and NaCl. In summary, presented evidence suggested that UvSMEK1 positively regulated pathogenicity, conidial germination and conidiation in YT broth, and negatively regulated conidiation on YT medium and tolerance to oxidative and osmotic stresses. The results enhance our understanding of the regulatory mechanism of pathogenicity of U. virens, and present a potential molecular target for blocking rice infection by U. virens.