High osmolarity glycerol (HOG) signalling in Magnaporthe oryzae: Identification of MoYPD1 and its role in osmoregulation, fungicide action, and pathogenicity.

This study comprises a first functional analysis of an YPD1-homologue in filamentous phytopathogenic fungi and its role in the HOG signalling pathway. We generated a gene deletion mutant of the gene MoYPD1 in Magnaporthe oryzae and characterized the resulting mutant strain. We have shown that MoYpd1p is a component of the phosphorelay system acting in the HOG pathway due to its Y2H protein interaction with the HKs MoHik1p and MoSln1p as well as with the response regulator MoSsk1p. Fungicidal activity of fludioxonil was reported to be based on the inhibition of MoHik1p resulting in hyperactivation of the HOG signalling pathway and lethality. Western analysis proved that both, osmotic stress and fludioxonil application resulted in the phosphorylation of the MoHog1p in a MoYpd1p-dependent manner. We therefore consider MoYpd1p to be essential for the regulation capability of the HOG pathway and the fungicide action of fludioxonil, but dispensible for viability. The results indicate that MoYpd1p functions as signal transfer protein between MoSln1p, MoHik1p, and MoSsk1p. Manipulations of the HOG signalling pathway affects the infection-related morphogenesis in M. oryzae, since the mutant strain ΔMoypd1 has a white and fluffy phenotype on complete media, is not able to form spores in various conditions and fails to colonize rice plants.