Isolation and characterization of a cDNA clone corresponding to the Rt locus of Petunia hybrida

We have isolated, via differential screening of a Petunia hybrida petal cDNA library, a cDNA clone that corresponds to the Rt locus which controls the conversion of anthocyanidin-3-glucosides to anthocyanidin-3-rutinosides by the UDP rhamnose: anthocyanidin-3-glucoside rhamnosyltransferase (3RT). The cDNA encodes a 469 amino acid long polypeptide with regions of similarity to the UDP glucose: flavonoid glucosyltransferases (3GT) from barley and maize. Some sequence similarity was also observed with non-plant glycosyltransferases. Two aberrant transcripts are present in most of the rt/rt petunia lines examined. Excision of a transposon from an unstable Rt locus of one petunia line (Tr38) is associated with a change in transcript size back to wild-type. The Rt transcript is most abundant in petals from flowers at an early stage of development and levels decline as the flower matures. Transcripts are also detected in the style and anthers but not in leaf, stem, root, petiole, ovary or sepals. Incubation of leaves in glucose under high light conditions induces the expression of the Rt gene as well as other flavonoid pathway genes. In situ hybridization revealed that the Rt transcript predominantly accumulates in the epidermal cells of the petal, the site of anthocyanin accumulation.