ORANGE: a tale of two pigments and two organelles

The ORANGE (OR) protein family exemplifies evolutionary innovation in plants, acting as dual-localized regulators coordinating nuclear transcription and plastidial metabolism. Discovered in the cauliflower or mutant, OR serves as a master regulator of carotenoid biosynthesis and chromoplast biogenesis. Its modular structure includes N-terminal targeting sequences (chloroplast transit peptide and nuclear localization signal), central transmembrane domains, and C-terminal cysteine-rich zinc finger domain facilitating protein interactions and holdase chaperone activity. Dual localization enables inter-organelle coordination. In plastids, OR stabilizes phytoene synthase for carotenoid production, promotes chromoplast differentiation, and inhibits division. In the nucleus, it interacts with TCP transcription factors to regulate chloroplast biogenesis during photomorphogenesis and endoreduplication. Phylogenetic analysis traces OR evolution via gene duplications in early land plants like mosses, yielding diversified OR/OR-Like subfamilies adapted to chromoplast specialization. Agriculturally, "golden SNP" variants enable CRISPR-based carotenoid enhancement in crops like tomato, potato, rice, and sorghum, bypassing transgenic limitations. Beyond nutrition, OR confers stress tolerance, influences flowering, and aids fruit development. Unresolved aspects, including targeting mechanisms, mutation/splicing variations, and species-specific differences, merit further investigation. Integrating genome editing with mechanistic insights offers opportunities for climate-smart, nutrient-rich crops addressing malnutrition, food security, and environmental adaptation.