LAUSR

The roles of rho-family guanosine triphosphatases (GTPases) of plants (ROPs) in modulating plant growth and development have been well-characterized. However, little is known about the roles of ROP signaling pathways in regulating plant autophagy and autophagosome formation. Here we identify a unique ROP signaling mechanism, which switches developmental to autophagic transition under stress conditions in the model plant Arabidopsis. The protein ROP8 loss-of-function mutants showed stress-induced hypersensitive phenotypes and compromised autophagic flux. Similar to other ROPs in the ROP/RAC family, ROP8 exhibits both plasma membrane and cytosolic punctate patterns. Upon autophagic induction, active ROP8 puncta colocalize with autophagosomal markers and are degraded inside the vacuole. In human cells, RalB, an RAS subfamily GTPase, engages its effector Exo84 for autophagosome assembly. Nevertheless, a RalB counterpart is missing in the plant lineage. Intriguingly, we have discovered that plant ROP8 promotes autophagy via its downstream effector Sec5. Live cell super-resolution imaging showed that ROP8 and Sec5 reside on phagophores for autophagosome formation. Our findings highlight a previously unappreciated role of an ROP8-Sec5 signaling axis in autophagy promotion. This expands our understandings that plants utilize versatile ROP signaling networks to coordinate developmental and autophagic responses depending on environmental changes.