F-box only and CUE proteins are crucial ubiquitination-associated components for conidiation and pathogenicity in the rice blast fungus, Magnaporthe oryzae

Ubiquitination, an important process in post-translational modification, regulates various mechanisms in eukaryotes including protein degradation and interaction, cell cycle, stress response, and pathogenicity. The Skp1/Cullin/F-box and the endoplasmic reticulum-associated degradation (ERAD) complexes, RING E3 ligase complexes, are involved in ubiquitin-mediated proteolysis and protein quality control. The F-box protein has FBXO (F-box only or others), FBXW (with WD40), and FBXL (with LRR) classes depending on which interaction domain is present on the C-terminus. The ubiquitin system component cue (CUE) protein is a key factor of ERAD. However, the biological roles of FBXO and CUE proteins are largely unknown in plant pathogenic fungi including Magnaporthe oryzae. To elucidate the roles of FBXO and CUE proteins in fungal development and pathogenicity, MoFBX15 and MoCUE1 were functionally characterized in M. oryzae. Two ubiquitination-associated genes were crucial for conidiation, alkaline stress tolerance, and pathogenicity in M. oryzae. In particular, MoCUE1 was important for ER stress response and localization and translocation of cytoplasmic effectors. Moreover, ubiquitination and SUMOylation levels were decreased and transcript levels of deSUMOylation-associated genes were increased in Delta Mofbx15 and Delta Mocue1. This study will provide not only comprehensive understanding of the role of ubiquitination but also new insights on crosstalk between ubiquitination and SUMOylation in rice blast fungus and other fungal pathogens.