SH3P2-mediated autophagosomal targeting of the CCZ1-MON1-RABG3e module regulates autophagosome-vacuole fusion in Arabidopsis.

Macroautophagy/autophagy is a highly conserved pathway responsible for the bulk degradation of cytoplasmic material through the formation of a double-membrane structure known as the autophagosome. However, the precise mechanisms governing the transport of autophagosomes to the vacuole for degradation in plants remain largely elusive. There exists an ongoing debate about whether RAB7, a key regulatory protein, is involved in the plant autophagy pathway. In this study, we demonstrate that upon autophagy induction by BTH treatment, RABG3e, a member of the RAB7 family, exhibits a partial localization with late-stage autophagosomes in Arabidopsis root cells, and its dysfunction leads to the accumulation of enlarged multilayered autophagosomes and a significant reduction in autophagic flux. We also showed that RABG3e is recruited to autophagosomes by its guanine nucleotide exchange factor (GEF) complex, MON1-CCZ1, which is targeted through the interaction between CCZ1 and SH3P2 (SH3 DOMAIN-CONTAINING PROTEIN 2), a plant-specific autophagy regulator. Subsequently, RABG3e recruits downstream effectors such as VPS39, which in turn promotes the recruitment of soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNARE) proteins, including SYP21, VTI12, SYP51, and VAMP711, that are essential for the fusion process. Arabidopsis mutants with dysfunction in the autophagosome-vacuole fusion process exhibit accelerated senescence and increased sensitivity to nitrogen starvation. Collectively, our findings provide new insights into the regulation of autophagosome-vacuole fusion in Arabidopsis, highlighting the essential roles of SH3P2-dependent targeting of the CCZ1-MON1-RABG3e module to late-stage autophagosomes as well as RABG3e effectors and a unique SNARE complex.