Carotenoids are isoprenoid metabolites synthesized de novo in all photosynthetic organisms. Carotenoids are essential for plants with diverse functions in photosynthesis, photoprotection, pigmentation, phytohormone synthesis, and signaling. They are also critically important for humans as precursors of vitamin A synthesis and as dietary antioxidants. The vital roles of carotenoids to plants and humans have prompted significant progress toward our understanding of carotenoid metabolism and regulation. New regulators and novel roles of carotenoid metabolites are continuously revealed. This review focuses on current status of carotenoid metabolism and highlights recent advances in comprehension of the intrinsic and multi-dimensional regulation of carotenoid accumulation. We also discuss the functional evolution of carotenoids, the agricultural and horticultural application, and some key areas for future research.

Carotenoids are a group of widely distributed natural pigments. They give many horticultural plants the bright red, orange, and yellow colors, as well as the aroma and flavor. Carotenoids enhance the health value and represent an essential quality trait of horticultural products. Significant efforts have been made to correlate specific carotenoid production with pathway gene expression. Some transcription factors that directly regulate transcription of the pathway genes have been identified. Horticultural crops have evolved with complicated and multifaceted regulatory mechanisms to generate the enormous diversity in carotenoid content and composition. However, the diverse and complex control of carotenoid accumulation is still not well understood. In this review, we depict carotenoid accumulation pathways and highlight the recent progress in the regulatory control of carotenoid accumulation in horticultural plants. Because of the critical roles of chromoplasts for carotenoid hyperproduction, we evaluate chromoplast ultrastructures and carotenoid sequestrations. A perspective on carotenoid research in horticultural crops is provided.