Cloning and molecular characterization of R2R3-MYB and bHLH-MYC transcription factors from Citrus sinensis

Members of the MYB and MYC family regulate the biosynthesis of phenylpropanoids in several plant species. Two sequences, called CsMYB8 and CsMYC2, were identified from Citrus sinensis, and both the cDNA and the genomic clones were isolated and characterized from the flesh of common and blood oranges. Analysis by real-time polymerase chain reaction showed that the expression pattern of CsMYC2 is generally higher in rind than in flesh and in blood oranges than in common ones. In contrast, no significant difference in expression was observed for CsMYB8. The expression pattern of the structural genes chalcone synthase, anthocyanidin synthase, and UDP-glucose-flavonoid 3-O-glucosyltransferase, which code for three enzymes involved in the anthocyanin biosynthetic pathway, was also analyzed and correlated with CsMYC2, in flesh, rind, and leaf of the common and blood oranges, and in leaf of Citrus limon cultivars (characterized by anthocyanin absence or variable content). Surprisingly, CsMYC2 is highly expressed in the leaf and expression is correlated with UFGT expression in this organ. These results suggest that CsMYC2 is involved in the regulation of the flavonoid biosynthetic pathway in Citrus.