New advances in autophagy in plants: Regulation, selectivity and function

Abstract

Autophagy is a major and conserved pathway for delivering unwanted proteins or damaged organelles to the vacuole for degradation and recycling. In plants, it functions as a housekeeping process to maintain cellular homeostasis under normal conditions and is induced by stress and senescence; it thus plays important roles in development, stress tolerance and metabolism. Autophagy can both execute bulk degradation and be highly selective in targeting cargos under specific environmental conditions or during certain developmental processes. Here, we review recent research on autophagy in plants, and discuss new insights into its core mechanism, regulation, selectivity and physiological roles. Potential future directions are also highlighted.

Keywords: Autophagy; Plants; Regulation; Selective degradation; Stress responses; TOR.

Figure 1

Summary of the induction, process and functions of autophagy in plants. Autophagy can be induced by various developmental or environmental factors as indicated. Upon induction, a cup-shaped double-membrane phagophore forms around the cargo. The membrane elongates and closes to form a double-membrane vesicle, the autophagosome. The red dots on the autophagosome membrane represent ATG8-PE adducts. The outer membrane of the autophagosome fuses with the vacuole, and its contents are broken down by vacuolar hydrolases. Autophagy functions in a range of quality control, developmental and stress response pathways.

Figure 2

Schematic diagram of pathways for selective autophagy in plants. Five types of organelle-specific autophagy have been identified: degradation of entire chloroplasts and Rubisco-containing bodies (RCB) (chlorophagy), degradation of ER (reticulophagy), degradation of mitochondria (mitophagy), degradation of peroxisomes (pexophagy) and degradation of proteasomes (proteaphagy). ATI bodies participate in both the degradation of plastids and ER. Proteaphagy is mediated by the ubiquitin-binding receptor RPN10. Degradation of ribosomes (ribophagy) is not fully elucidated in plants, but autophagy is involved in rRNA turnover. Key genes for each type of selective autophagy are listed.