Isolation and expression of the genes encoding the early carotenoid biosynthetic enzymes in the fruit peel of pummelo (Citrus grandis cv. Melomas) during maturation

Three different partial cDNA clones encoding the early carotenoid biosynthetic enzymes, pummelo phytoene synthase (PumPSY), pummelo phytoene desaturase (PumPDS) and pummelo lycopene-beta-cyclase (PumLYCb) were isolated from the peel (flavedo) of the local (Citrus grandis cv. Melomas) citrus fruit. Comparison of the deduced partial amino acid sequences of all three genes showed more than 90% identity with the Satsuma mandarin (Citrus unshiu Marc.), Citrus x Paradisi and Citrus sinensis. Expression analysis revealed a high level of the PumPSY transcript in the peel at the early stages of fruit development which declined gradually towards the third month of fruit development and then accumulated again during fruit maturation. In contrast, the PumLYCb transcript was present in lower levels than PumPSY in the peel at the early developmental stages and then increased slightly towards fruit maturation. On the other hand, the PumPDS transcript remained in very low amounts throughout the developing stages compared to PumPSY and PumLYCb. The expression patterns indicated non-coordinated regulation of the genes and the fluctuations were not in accordance with carotenoid accumulation and chlorophyll disappearance that leads to the peel colour change from green to orange as observed in the flavedos of Satsuma mandarin and Valencia oranges.