Co-chaperoning of chlorophyll and carotenoid biosynthesis by ORANGE family proteins in plants.

Chlorophylls and carotenoids are essential photosynthetic pigments. Plants spatiotemporally coordinate the needs of chlorophylls and carotenoids for optimal photosynthesis and fitness in response to diverse environmental and developmental cues. However, how the biosynthesis pathways of these two pigments are coordinated, particularly at posttranslational level to allow rapid control, remains largely unknown. Here, we report that the highly conserved ORANGE (OR) family proteins coordinate both pathways via posttranslationally mediating the first committed enzyme in each pathway. We demonstrate that OR family proteins physically interact with magnesium chelatase subunit I (CHLI) in chlorophyll biosynthesis pathway in addition to phytoene synthase (PSY) in carotenoid biosynthesis pathway and concurrently stabilize CHLI and PSY enzymes. We show that loss of OR genes hinders both chlorophyll and carotenoid biosynthesis, limits light-harvesting complex assembly, and impairs thylakoid grana stacking in chloroplasts. Overexpression of OR safeguards photosynthetic pigment biosynthesis and enhances thermotolerance in both Arabidopsis and tomato plants. Our findings establish a novel mechanism by which plants coordinate chlorophyll and carotenoid biosynthesis and provide a potential genetic target to generate climate-resilient crops.