Advances in studies on carotenoids in loquat fruit

Carotenoids are the second most abundant naturally occurring pigments and play key roles innumerous plant biological processes. Carotenoids have a β-ring end group and are the precursors of vita-min A that has fundamental role for animal nutrition, including humans, which cannot synthesize this vita-min. Although there are hundreds of carotenoid structures in nature world, it can be classified in two ma-jor classes: carotenes(hydrocarbons that can be cyclized at one or both ends of the molecule) and xantho-phylls(oxygenated derivatives of carotenes). Carotenoids are important components responsible for the co-louration of fruits. Loquat(Eriobotrya japonica Lindl.) belong to the Rosaceae family and carotenoids arethe major pigments in mature fruit. The plant originated in southeastern of China and has been commer-cially cultivated in China, Japan, Spain, Turkey and some other countries. Both content and compositionof carotenoids affect fruit pigmentation and nutritional value. The color of loquat fruits varies with culti-vars from yellow to orange red in the peel and white to orange in the flesh. Loquat cultivars are usuallysorted into white- and red-fleshed groups according to the flesh color. The pulp of red-fleshed loquat ap-pears red- orange because of abundant content of carotenoids while white- fleshed ones appears ivorywhite because of a low level carotenoid content. This paper reviewed the achievements of the studies oncarotenoids of loquat fruit. The composition and content of carotenoids of red- and white-fleshed loquatswere analysed, And the carotenoids content in the peel and flesh of red-fleshed cultivars are higher thanthose of white-fleshed ones. the total carotenoids content were lower in the flesh than in the peel in samevariety; β-Carotene and lutein were the major carotenoids in the peel, β–Carotene and β-cryptoxanthinwere the most abundant carotenoids in the flesh in some red-fleshed cultivars. The change rules duringfruit development and storage were studied. Carotenoid components in cultivars of Dahongpao and Qing-zhong that belong to two different loquat pulp types were determined during the fruit coloring process, Car-otene content, especially β-carotene content, was the major carotenoid component affecting pulp carot-enoid accumulation and color levels in cultivars of Dahongpao and Qingzhong. Loquat is a fruit with a lim-ited postharvest life. Fresh of loquat is highly perishable and low temperature storage is recommended toprevent deterioration. The concentration of caroteniods were effected by storage temperatures, storage timeand different storage techniques. In view of their important roles, carotenoid biosynthesis in higher plantshas been widely studied and the current knowledge on the genes and enzymes of the carotenoid biosynthet-ic pathway has been almost completely elucidated. Only a limited number of studies has addressed the ex-pression and function of the carotenoid biosynthetic genes of loquat. Four phytoene synthase(PSY) genes,Ej PSY1, Ej PSY2 A, Ej PSY2 B, and Ej PSY3, were profound analysed to understand their function, evolu-tion and structure in two loquat types of carotenoids biosynthetic pathway, and those genes have differentfunction and tissue-specific expression patterns. Ej PSY2 A, was identified in the genome of white-fleshedloquat cultivars and shown to explain the low levels of carotenoids accumulated in the flesh of white-fleshed loquat. The tissue-specific expression patterns of Ej PSY1 and Ej PSY2 B well explained how peeland leaf tissues can still accumulate carotenoids in white-fleshed cultivars, which have lost the functionalEj PSY2 A expressed in fruit flesh. It was speculated that Ej PSY3 is functionless in loquat; In higherplants, the complete pathway of carotenoid biosynthesis takes place within the plastid, Carotenoid accumu-lation has been shown to be related closely to the size and number of plastids and the level of gene expres-sion and enzyme activity. The formation of chromoplasts enhances carotenoid metabolic sink strength andcontrols carotenoid accumulation in plants. The different carotenoid composition patterns between thepeel and flesh in loquat might be related to the differences in chromoplast ultrastructure. The inability toform chromoplasts in the flesh of Baisha(white-fleshed loquat) is the mostly likely explanation for thelow carotenoid accumulation in Baisha(white- fleshed)compared with Luoyangqing(red- fleshed) flesh.The expression of plastid lipid-associated protein(PAP) was well correlated with chromoplast numbersand carotenoid accumulation of loquat fruit. In addition, the future research trend was recommended. Therelationship between the formation of chromoplasts and carotenoid accumulation in loquat requires furtherstudy. Regulatory mechanism of the loquat carotenoid biosynthesis pathway still remains to further research.