Autophagy is a well-defined catabolic mechanism whereby cytoplasmic materials are engulfed into a structure termed the autophagosome. In plants, little is known about the underlying mechanism of autophagosome formation. In this study, we report that SH3 DOMAIN-CONTAINING PROTEIN2 (SH3P2), a Bin-Amphiphysin-Rvs domain–containing protein, translocates to the phagophore assembly site/preautophagosome structure (PAS) upon autophagy induction and actively participates in the membrane deformation process. Using the SH3P2–green fluorescent protein fusion as a reporter, we found that the PAS develops from a cup-shaped isolation membranes or endoplasmic reticulum–derived omegasome-like structures. Using an inducible RNA interference (RNAi) approach, we show that RNAi knockdown of SH3P2 is developmentally lethal and significantly suppresses autophagosome formation. An in vitro membrane/lipid binding assay demonstrates that SH3P2 is a membrane-associated protein that binds to phosphatidylinositol 3-phosphate. SH3P2 may facilitate membrane expansion or maturation in coordination with the phosphatidylinositol 3-kinase (PI3K) complex during autophagy, as SH3P2 promotes PI3K foci formation, while PI3K inhibitor treatment inhibits SH3P2 from translocating to autophagosomes. Further interaction analysis shows that SH3P2 associates with the PI3K complex and interacts with ATG8s in Arabidopsis thaliana, whereby SH3P2 may mediate autophagy. Thus, our study has identified SH3P2 as a novel regulator of autophagy and provided a conserved model for autophagosome biogenesis in Arabidopsis.

The TRAF family proteins MUSE14/TRAF1a and MUSE13/TRAF1b regulate the dynamics of autophagy by facilitating both the proteolysis and stabilization of AUTOPHAGY

Environmental stress activates autophagy and leads to autophagosome formation at the endoplasmic reticulum.

Chloroplasts provide energy for all plants by producing sugar during photosynthesis. To adapt to various environmental and developmental cues, plants have developed specific strategies to control chloroplast homeostasis in plant cells, including chloroplast degradation during leaf senescence and the transition of chloroplasts into other types of plastids during the day-night cycle. In recent years, autophagy has emerged as an essential mechanism for selective degradation of chloroplast materials (also known as chlorophagy) into the vacuole. Different types of membrane structures have been shown to be involved in the delivery of distinct chloroplast contents. However, the underlying mechanisms for the formation of these chlorophagy-related structures and the selectivity of chloroplast proteins remain poorly understood. Here we provide a current overview on chlorophagy and discuss the possible chloroplast receptors and upstream signals in this process.