Environmental impacts on carotenoid metabolism in leaves

Leaves are primary plant organs harbouring chloroplasts that enable the sensing of changes in environmental conditions and contribute to maintaining cellular homeostasis. Carotenoids, carotenoid-derived phytohormones and apocarotenoid metabolites facilitate photosynthesis, photoprotection and stress acclimation in leaves. The thylakoids in the chloroplasts from the photosynthetically functional leaves accumulate a specific composition of lutein, β-carotene, violaxanthin, and neoxanthin that act as accessory pigments to chlorophyll and maintain optimal photosynthetic performance. The developmental state of leaves and environmental conditions can affect carotenoid metabolism and, hence, chloroplast functions in leaves. The etioplasts in the cotyledons from the germinating seedlings abundantly accumulate lutein. The light exposure of cotyledons triggers ultrastructural development of chloroplast accumulating all primary foliar carotenoids and chlorophylls. The accumulation of zeaxanthin and antheraxanthin during intense light and high temperature maintains the functional integrity of thylakoid membranes and chloroplasts. Carotenoids are continuously synthesised and degraded in leaves, yet their composition remains tightly maintained. The disintegration of chloroplast, such as during the leaf senescence, ceases the carotenoid biosynthesis and triggers carotenoid degradation. This review sheds light on the fundamental aspects of carotenoid metabolism. The effects of altered light regimes, carbon dioxide (CO 2 ), temperature, drought, and soil nutrient levels are discussed in the context of carotenoid metabolism in leaves. The outstanding questions on how environmental perturbations can alter carotenoid metabolism and chloroplast development in leaves are highlighted. This review provides further insights on how carotenoid regulatory networks could be engineered to enrich carotenoids in the leaves of food and horticultural crops.