OsATG1 and OsATG8 exhibit autophagy-independent functions to oppositely regulate ROP GTPase-mediated plant immunity in rice.

Abstract

ROP GTPases regulate various cellular processes, including plant immunity. While ROP GTPase activation has been reported during plant immune responses, the mechanisms underlying the dynamic deactivation of ROP GTPases remain unclear. In this study, we identified the autophagy kinase OsATG1 as a key regulator that interacts with and phosphorylates SPIN6, a plant-specific ROP GTPase-activating protein (RhoGAP), which in turn deactivates the ROP GTPase OsRac1. OsATG1-mediated multi-site phosphorylation is necessary for the GAP activity of SPIN6 to hydrolyze OsRac1-GTP, and overexpression of a phosphomimic form of SPIN6 attenuates rice immunity. We showed that two isoforms of OsATG1, OsATG1a and OsATG1b, operate redundantly in rice immunity to the fungal pathogen Magnaporthe oryzae. Double mutants of OsATG1a and OsATG1b exhibit stronger resistance as well as developmental defects and complete sterility. Moreover, OsATG1 interacts with OsATG8. Phenotypic analyses of OsATG8 transgenic plants reveal that OsATG8 positively regulates rice immunity, but OsATG8 activates immunity partially independent of its function in autophagy, because overexpressing the lipidation-defective OsATG8^G117A or accumulating non-lipidated OsATG8 in the osatg7 mutant enhances rice disease resistance. Mechanistically, OsATG8 promotes OsATG1 turnover, whereas OsATG8^G117A is sufficient to deplete OsATG1, leading to SPIN6 dissociation and degradation. As autophagy is essential in nutrient recycling, we found that nutrient limitations induce OsATG8 expression and rice immunity while suppressing SPIN6. However, SPIN6 phosphorylation inhibits this nutrient-limitation-induced immunity. Taken together, our results suggest that OsATG1 and OsATG8 possess autophagy-independent functions to transform nutrient limitation into immunity via plant-specific ROP GTPase signaling.