MoRad6-mediated ubiquitination pathways are essential for development and pathogenicity in Magnaporthe oryzae

The ubiquitin system modulates protein functions through targeting substrates for ubiquitination. Here, E2 conjugating enzyme MoRad6-related ubiquitination pathways are identified and analyzed in Magnaporthe oryzae, the causal agent of rice blast disease. Disruption of MoRad6 leads to severe defects in growth, sporulation, conidial germination, appressorium formation, and plant infection. To depict the functions of MoRad6, three putative ubiquitin ligases, MoRad18, MoBre1 and MoUbr1, are also characterized. Deletion of MoRad18 causes minor phenotypic changes, while MoBre1 is required for growth, conidiation and pathogenicity in M. oryzae. Defects in ΔMobre1 likely resulted from the reduction in di- and tri-methylation level of Histone 3 lysine 4 (H3K4). Notably, MoUbr1 is crucial for conidial adhesion and germination, possibly by degrading components of cAMP/PKA and mitogen-activated protein kinase (MAPK) Pmk1 signaling pathways via the N-end rule pathway. Germination failure of ΔMoubr1 conidia could be rescued by elevation of cAMP level or enhanced Pmk1 phosphorylation resulting from further deletion of MoIra1, the M. oryzae homolog of yeast Ira1/2. These reveal vital effects of cAMP/PKA and MAPK Pmk1 signaling on conidial germination in M. oryzae. Altogether, our results suggest that MoRad6-mediated ubiquitination pathways are essential for the infection-related development and pathogenicity of M. oryzae.