Carotenoids and Photosynthesis - Regulation of Carotenoid Biosyntesis by Photoreceptors

Carotenoids are isoprenoid molecules of 40 carbons which are synthesized in a wide variety of photosynthetic (plants, algae) and non photosynthetic (some fungi and bacteria) organisms. So far, over 750 carotenoid structures are known, and these are divided into nonoxygenated molecules designated as carotenes and into oxygenated carotenoids referred to as xanthophylls. In photosynthetic organisms, carotenoids are synthesized in the plastids, such as chloroplasts. They are localized and accumulated in the thylakoid membranes of chloroplasts (Cunningham & Gantt, 1998), near the reaction center of photosystem II in the light harvesting complexes (LHC), along with other pigments such as chlorophyll a and b. Carotenoids act as accessory pigments in the LHC, where they absorb light in a broader range of the blue spectrum (400-500 nm) than chlorophyll. Carotenoids transfer the absorbed energy to chlorophyll a during photosynthesis (Britton, 1995). Carotenoids also protect plant cells from photo-oxidative damage as a result of their antioxidant characteristic giving by the conjugate bonds of the polyene chain (Britton, 1995; Britton et al., 1998). In this context carotenoids absorb the excess of energy from reactive oxygen species (ROS) and quench singlet oxygen produced from the chlorophyll triplet in the reaction center of photosystem II (Telfer, 2005). Carotenoids also protect the plant from photo-oxidative damage through thermal dissipation by means of the xanthophyll cycle (Baroli & Nigoyi, 2000). This process occurs when excessive light increases the thylakoid ΔpH, which activatates the enzyme violaxanthin de-epoxidase (VDE), converting violaxanthin to zeaxanthin. Zeaxanthin molecules and protons may change the conformation in the LHC, favoring the thermal dissipation. Carotenoids are also synthesized and accumulated in chromoplasts, plastids that accumulate pigments in flowers, fruits and storage roots. Carotenoids are stored in lipid bodies or in crystalline structures inside the chromoplasts where they are more stable because they are protected from light (Vishnevetsky et al., 1999). In addition, carotenoids are precursors for apocarotenoids such as the phytohormones abscisic acid (ABA) and stringolactones. ABA is involved in dormancy, development and differentiation of plant embryos, stomata open-closure and in tolerance to abiotic stress (Crozier et al., 2000). The stringolactones act as shoot branching inhibitor hormones. Also they are involved in plant signaling to both harmful (parasitic weeds) and beneficial (arbuscular mycorrhizal fungi) rhizosphere residents (Walter et al, 2010).