Zeaxanthin and Other Carotenoids: Roles in Abiotic Stress Defense with Implications for Biotic Defense.

Xanthophylls are carotenoids with diverse roles in stress protection across all taxa of life. This review highlights chloroplast-localized xanthophylls (with a focus on zeaxanthin) of plants by presenting an overview of the protective effects of xanthophylls as well as the role of carotenoids as precursors of multiple plant stress hormones. It also examines the roles of xanthophylls and stress hormones in signaling cascades between the chloroplast and nuclear genes that control plant growth, development, and stress defenses. This overview addresses the biosynthetic pathways of xanthophylls and carotenoid-derived plant stress hormones, functions of xanthophylls in photoprotection of photosynthesis, carotenoids as essential human micronutrients, and roles of xanthophylls in membrane integrity. Attention is given to the involvement of zeaxanthin in both abiotic and biotic defense as well as its impact on components of the biotic defense system with contrasting targets. Examples for the multiple principal loops of signaling cascades between the chloroplast and nucleus, which are based on chloroplast redox state and modulated by xanthophylls, are summarized. This review integrates the role of chloroplast carotenoids in controlling light-use efficiency and providing photoprotection with their system-wide regulatory effects as precursors of carotenoid-derived plant stress hormones and modulators of chloroplast redox state. A better understanding of these connections is needed to guide development of plant lines with improved resilience and productivity in complex, changing, and challenging environments.