Carotenoid biosynthesis in plant storage organs: recent advances and prospects for improving plant food quality

Carotenoids are components of the photosynthetic machinery, intermediates in the biosynthesis of abscisic acid and other apocarotenoids and act as coloured pigments particularly in floral and fruit tissue. Humans benefit in a number of ways from dietary carotenoids present in green leaf tissue and many fruits, seeds, roots and tubers. Carotenoids with a beta-ring end group are required for the synthesis of vitamin A, and deficiency for this vitamin remains a major health problem in some parts of the world. Epidemiological studies suggest that carotenoids also have important roles in a range of diseases including age-related macular degradation and some cancers. The isoprenoid pathway is described, which leads to the carotenoids via the condensation of five-carbon isoprenoid units to form a 40-carbon chain in phytoene. The early part of the biosynthetic pathway involves crosstalk between plastidic and cytosolic components. Desaturation, isomerization, cyclization, hydroxylation and epoxidation of the 40-carbon phytoene gives the range of carotenoids found in plants. The nuclear-encoded enzymes for these stages are targeted to plastids, and in some cases, different members of a gene family are active in plastids in different tissue types. Studies on the transgenic manipulation of the pathway, natural and induced mutants, gene silencing and gene and quantitative trait locus mapping are increasingly unravelling the pathway and its control, giving opportunities for directed manipulation of the types and quantities of these nutritionally important compounds in crop tissues used in human and animal diets.