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

Abstract

ROP GTPases regulate various cellular pathways, including plant immunity. Although the activation of ROP GTPases has been reported during immunity, the mechanism for dynamic deactivation of ROP GTPases remains unclear. Here, we identified the autophagy kinase OsATG1 as a key regulator that interacts with and phosphorylates RhoGAP SPIN6, which deactivates ROP GTPase OsRac1. OsATG1-mediated multi-site phosphorylation is required for SPIN6 GAP activity to hydrolyze OsRac1-GTP, and overexpression of a phosphomimic form of SPIN6 attenuates rice immunity. We further showed that two isoforms of OsATG1, OsATG1a and OsATG1b, function redundantly in rice immunity to the fungal pathogen Magnaporthe oryzae. Double mutants of OsATG1a and OsATG1b exhibit stronger resistance phenotypes, as well as developmental defects and complete sterility. To validate the association between OsATG1-mediated immunity and autophagy, we found that OsATG1 interacts with OsATG8. Phenotyping analyses of OsATG8 transgenic plants reveal that OsATG8 positively regulates rice immunity. Interestingly, OsATG8 activates immunity partially independent of its role in autophagy, as overexpressing the lipidation-defective OsATG8^G117A or accumulating non-lipidated OsATG8 in the osatg7 mutant also enhances rice disease resistance. Mechanistically, OsATG8 promotes OsATG1 turnover, while OsATG8^G117A is sufficient to competitively deplete OsATG1, leading to SPIN6 dissociation and degradation. As autophagy is important in nutrient recycling, we also found that nutrient limitations induce OsATG8 expression and rice immunity while suppressing SPIN6. However, SPIN6 phosphorylation blocks this nutrient limitation-induced immunity. Together, OsATG1 and OsATG8 exhibit autophagy-independent functions to convert nutrient limitation into immunity via plant-specific ROP GTPase signaling.