Cloning and expression analysis of phytoene desaturase and ζ-carotene desaturase genes in Carica papaya.

The fruit flesh color of papaya is an important nutritional quality trait and is due to the accumulation of carotenoid. To elucidate the carotenoid biosynthesis pathway in Carica papaya, the phytoene desaturase (PDS) and the ζ-carotene desaturase (ZDS) genes were isolated from papaya (named CpPDS and CpZDS) using the rapid amplification of cDNA ends (RACE) approach, and their expression levels were investigated in red- and yellow-fleshed papaya varieties. CpPDS contains a 1749 bp open reading frame coding for 583 amino acids, while CpZDS contains a 1716 bp open reading frame coding for 572 amino acids. The deduced CpPDS and CpZDS proteins contain a conserved dinucleotide-binding site at the N-terminus and a carotenoid-binding domain at the C-terminus. Papaya genome sequence analysis revealed that CpPDS and CpZDS are single copy; the CpPDS was mapped to papaya chromosome LG6, and the CpZDS was mapped to chromosome LG3. Quantitative PCR showed that both CpPDS and CpZDS were expressed in all tissues examined with the highest expression in maturing fruits, and that the expression of CpPDS and CpZDS were higher in red-fleshed fruits than in yellow-fleshed fruits. These results indicated that the differential accumulation of carotenoids in red- and yellow-fleshed papaya varieties might be partly explained by the transcriptional level of CpPDS and CpZDS.