The putative Gγ subunit gene MGG1 is required for conidiation, appressorium formation, mating and pathogenicity in Magnaporthe oryzae

Heterotrimeric G-proteins play key roles in the transduction of extracellular signals to various downstream effectors in eukaryotes. In our previous study, a T-DNA insertional mutant A1-412, in which the promoter of a putative G gamma subunit gene MGG1 was disrupted, was impaired in asexual/sexual sporulation, appressorium formation, and pathogenicity in Magnaporthe oryzae. Here the roles of MGG1 in regulating fungal development and plant infection were further investigated and verified using a gene deletion strategy. Targeted gene deletion mutants of MGG1 exhibited similar phenotypes to those of A1-412. The Delta mgg1 mutants were unable to differentiate appressorium on hydrophobic surfaces and nonpathogenic to susceptible hosts. The defects of the Delta mgg1 mutants in appressorium formation were partially restored by adding exogenous cAMP or IBMX (a phosphodiesterase inhibitor), although the induced appressoria were still nonfunctional. Expressing Mgg1-GFP fusion protein in an Delta mgg1 mutant could complement all phenotypes of the mutant, and bright GFP fluorescence was observed at the periphery of fungal cells, indicating that Mgg1 mainly localizes to plasma membrane. Quantitative RT-PCR analysis revealed that deletion of MGG1 resulted in a significant reduction in mRNA levels of the genes encoding G alpha (MagA, MagB, and MagC), G beta (Mgb1), and adenylate cyclase (Mac1). Moreover, intracellular cAMP accumulation was significantly reduced in Delta mgg1 mutants compared to that in the wild-type strain. Taken together, our results suggested that G gamma subunit Mgg1 might act upstream of cAMP signaling pathway and play critical roles in regulation of conidiation, appressorium formation, mating, and plant infection in M. oryzae.