Carotenoids in Stress Tolerance: Current Understanding and Future Prospects.

Carotenoids are ubiquitous isoprenoid metabolites whose biological significance extends far beyond their classical roles as antioxidants and light-harvesting pigments. Across the tree of life, these molecules exhibit remarkable structural and functional diversity, enabling organisms to fine-tune photoprotection, redox homeostasis, membrane organization, and stress-responsive signaling. In this review, we provide a conceptual synthesis of carotenoid diversity and function across organisms, with emphasis on how carotenoid composition and accumulation are dynamically regulated under environmental and biotic stresses. We highlight stress-induced carotenoid plasticity, manifested as selective accumulation, rerouting of pathways, and the production of signaling apocarotenoids as a central adaptive strategy across photosynthetic organisms. Finally, we discuss emerging evidence and competing hypotheses regarding the mechanistic diversity of non-photochemical quenching and the evolving yet debated role of carotenoids in energy-dependent quenching. Together, this review emphasizes carotenoids as dynamic metabolic regulators that integrate many facets of photosynthetic performance, stress responses and signaling.