Accumulation of β-citraurin and other carotenoids in relation to the expression of PSY1 and CCD4b1 in peel of twelve red-peeled citrus cultivars

Peel color in citrus is largely genetically controlled. Red peel is favored by customers and is important, therefore, for fruit external quality. In order to understand the factors affecting distribution of carotenoids and color intensity of red-peeled citrus, we compared carotenoid accumulation and gene expression in the peel of twelve fully ripe red-peeled citrus cultivars with different genetic backgrounds. The results showed that β-citraurin, a red C30 carotenoid, was the second or third most abundant colored carotenoid in the peel of all twelve red-peeled citrus, ranging from 9.4 % to 26.7 % of total carotenoids. Statistical analysis of carotenoid composition and content showed that of the main carotenoids β-citraurin had the highest correlation with red appearance of cultivars. In addition, β-cryptoxanthin, which accounted for 3.6 % to 16.4 % of the total carotenoids, were also significantly positively correlated with citrus color index (CCI). The expression of two key genes, phytoene synthase 1 (PSY1) and carotenoid cleavage dioxygenase 4b1 (CCD4b1), differed among cultivars with a range of 3.5- and 48.8-fold, respectively. Surprisingly, accumulation of total carotenoids and β-citraurin was not correlated with the expression of PSY1 and CCD4b1, respectively, implying that diverse factors influence high carotenoid accumulation in red-peeled citrus.