Design and validation of a synthetic chlorophagy receptor.

Chloroplasts are photosynthetic, protein-rich organelles unique to green plants and algae. In more complex plants, each mesophyll cell may contain up to 100 chloroplasts, each 5 - 10 µm in length, and 50% of plant proteins are stored in chloroplasts as RUBISCO. Plant cells clear parts of or entire chloroplasts by piecemeal autophagy, microautophagy, and other vesicular pathways to maintain homeostasis and to recycle nutrients in developmental and stress conditions. However, canonical, membrane-bound chlorophagy receptors remain unidentified, and whether chlorophagy can be induced to enhance plant fitness remains unclear. In a recent study, we designed and validated a synthetic chlorophagy receptor, LIR-SNT-BFP. This receptor, upon dexamethasone (DEX) induction, recruits ATG8 to chloroplasts and promotes chloroplast microautophagy independent of ATG5 and ATG7, and induces chloroplast fission. Expression of this synthetic receptor helps protect chloroplasts and seedlings from herbicide damage. Moderate induction of this synthetic receptor promotes rosette growth, while excessive induction inhibits growth and reduces chlorophyll levels. There is a clear upper limit to the benefits of inducing autophagy in plants. This synthetic chlorophagy receptor may help identify endogenous receptors and key players in microautophagy, and can be a powerful new tool for studies investigating chloroplast dynamics and nutrient remobilization.