Question

Carotenoids give their charactristuc orange colour in 400 -500 nm region because

Answer 1

relationship between carotenoid accumulation and the expression of carotenoid biosynthetic genes during fruit maturation was investigated in three citrus varieties, Satsuma mandarin (Citrus unshiuMarc.), Valencia orange (Citrus sinensis Osbeck), and Lisbon lemon (Citrus limon Burm.f.). We cloned the cDNAs for phytoene synthase (CitPSY), phytoene desaturase (CitPDS), ζ-carotene (car) desaturase (CitZDS), carotenoid isomerase (CitCRTISO), lycopene β-cyclase (CitLCYb), β-ring hydroxylase (CitHYb), zeaxanthin (zea) epoxidase (CitZEP), and lycopene ε-cyclase (CitLCYe) from Satsuma mandarin, which shared high identities in nucleotide sequences with Valencia orange, Lisbon lemon, and other plant species. With the transition of peel color from green to orange, the change from β,ε-carotenoid (α-car and lutein) accumulation to β,β-carotenoid (β-car, β-cryptoxanthin, zea, and violaxanthin) accumulation was observed in the flavedos of Satsuma mandarin and Valencia orange, accompanying the disappearance of CitLCYe transcripts and the increase in CitLCYbtranscripts. Even in green fruit, high levels of β,ε-carotenoids and CitLCYetranscripts were not observed in the juice sacs. As fruit maturation progressed in Satsuma mandarin and Valencia orange, a simultaneous increase in the expression of genes (CitPSY, CitPDS, CitZDS, CitLCYb, CitHYb, and CitZEP) led to massive β,β-xanthophyll (β-cryptoxanthin, zea, and violaxanthin) accumulation in both the flavedo and juice sacs. The gene expression of CitCRTISOwas kept low or decreased in the flavedo during massive β,β-xanthophyll accumulation. In the flavedo of Lisbon lemon and Satsuma mandarin, massive accumulation of phytoene was observed with a decrease in the transcript level for CitPDS. Thus, the carotenoid accumulation during citrus fruit maturation was highly regulated by the coordination of the expression among carotenoid biosynthetic genes. In this paper, the mechanism leading to diversity in β,β-xanthophyll compositions between Satsuma mandarin and Valencia orange was also discussed on the basis of the substrate specificity of β-ring hydroxylase and the balance of expression between upstream synthesis genes (CitPSY, CitPDS, CitZDS, and CitLCYb) and downstream synthesis genes (CitHYband CitZEP).

Carotenoids are essential components of the photosynthetic apparatus in plants, algae, and cyanobacteria, in which they protect against photooxidative damage and contribute to light harvesting for photosynthesis (Goodwin, 1980). In higher plants, the bright yellow, orange, and red colors provided by carotenoids accumulate in the chromoplasts of flowers and fruits. In these tissues, plants exploit carotenoids as colorants to attract pollinators and agents of seed dispersal. In addition, epoxy-carotenoids, violaxanthin, and neoxanthin are precursors for plant hormone abscisic acid (Rock and Zeevaart, 1991). Some carotenoids serve as precursors for vitamin A, which is essential to human and animal diets, and as antioxidants, which play a role in reducing the risk of certain forms of cancer