Chloroplast proteostasis: Coupling chlorophyll and carotenoid metabolism and chlorophyll-binding protein homeostasis in plants

Abstract Photosynthetic efficiency depends on the homeostatic accumulation of chlorophyll-binding proteins in photosystems and their light-harvesting antenna complexes within the thylakoid membrane. The assembly of these proteins requires stoichiometric amounts of chlorophylls and carotenoids, which not only drive light harvesting and photochemistry but also promote the folding, membrane insertion, and stability of chlorophyll-binding proteins. Because free pigments and unassembled apoproteins are potentially deleterious, optimal photosynthetic performance requires tight coordination between pigment metabolism and the biogenesis, repair, and turnover of chlorophyll-binding proteins. Emerging evidence indicates that chloroplast proteostasis, mediated by molecular chaperones, assembly factors, and proteolytic machinery, is the key post-translational mechanism linking these processes. This review summarizes how proteostasis networks coordinate chlorophyll and carotenoid metabolism with the biogenesis, assembly, and maintenance of chlorophyll-binding proteins, and highlights the auxiliary factors that couple pigment supply, protein quality control, and photosystem function. Collectively, these insights provide a foundation for future studies of photosynthetic acclimation and plastid development and may formulate effective strategies to improve photosynthesis and chloroplast performance in crops.