Botrytis cinerea causes pre- and postharvest diseases in more than 200 economically important crops. In this study, the roles of cell wall integrity (CWI)-related MAPK kinase BcMkk1in regulating B. cinerea virulence were investigated using genetic and biochemical approaches. Ma Zhonghua and Yin Yanni found that the MAPK kinase BcMkk1 positively regulates virulence via the CWI pathway. Unexpectedly, BcMkk1 also negatively regulates fungal virulence via restraining oxalic acid production, by impeding phosphorylation of the PAS kinase BcRim15 mediated by the kinase BcSch9. To our knowledge, this is the first report that a MAPK kinase can negatively modulate fungal virulence on host plants. These results provide novel insight into biological functions of a MAPK kinase in fungal pathogenesis.

A proposed model for the roles of BcMkk1 in virulence of B. cinerea.

BcMkk1 positively regulates pathogenicity mainly via the cell wall integrity (CWI) pathway, which controls vegetative growth, conidiation and responses to cell wall and oxidative stresses. On the other hand, BcMkk1 impedes phosphorylation of BcRim15 mediated by BcSch9, subsequently inhibits oxalic acid production and the activity of extracellular hydrolases. In addition, BcPro40 is a scaffold protein of the CWI pathway in B. cinerea. Dotted lines represent unidentified routes.