A synthetic chlorophagy receptor promotes plant fitness by mediating chloroplast microautophagy

Abstract Chloroplasts are major photosynthetic and protein-containing organelles in green plants and algae. Unwanted chloroplast proteins and entire chloroplasts are cleared through various degradation pathways including autophagy. Nevertheless, canonical chlorophagy receptors remain unidentified, and whether and to what extent chlorophagy can be enhanced to benefect the plants remain unknown. Here we designed and validated a synthetic chloroplast autophagy receptor using biochemical, genetical, and imaging approaches. This synthetic receptor, LIR-SNT-BFP, was constructed by fusing a fragment containing the LC3-interacting region (LIR) of the selective autophagy receptor NBR1 and the N-terminal amphipathic α-helix of the chloroplast outer envelope protein SFR2. The fusion protein LIR-SNT-BFP coated the chloroplast and attracted ATG8a in planta. Upon induction, the synthetic receptor elicited vacuole-mediated microautophagy of entire chloroplasts independent of the ATG8 conjugation machinery proteins ATG5 or ATG7. Meanwhile, it induced chloroplast division; however the induced chlorophagy was independent of PDV2. Notably, moderate chlorophagy improves rosette growth, but excessive levels are detrimental. Furthermore, induced chlorophagy partially protects against herbicide-induced leaf chlorosis. This study demonstrates controlled chloroplast degradation using a synthetic chlorophagy receptor.