Differential expression of three key anthocyanin biosynthetic genes in a color-changing flower, Viola cornuta cv. Yesterday, Today and Tomorrow

Abstract

The natural floral color change in Viola cornuta cv. Yesterday, Today and Tomorrow (YTT) provides an opportunity to examine the expression of genes along the anthocyanin biosynthetic pathway in response to known environmental triggers during ontogeny. We have cloned and sequenced gene fragments from three anthocyanin biosynthetic genes, chalcone synthase ( CHS), dihydroflavonol 4-reductase ( DFR) and anthocyanidin synthase ( ANS). Using quantitative reverse-transcriptase-polymerase chain reaction (RT-PCR) we have determined that CHS and DFR are highly expressed in Stage I white YTT flowers and undergo only moderate increases in expression (25 and 56%, respectively) during the transition to Stage II lavender and Stage III purple flowers. In contrast, ANS is expressed at low levels in Stage I flowers and undergoes a dramatic increase (302%) in expression over ontogeny. Interestingly, expression of all three genes is significantly lower in the absence of two environmental triggers, pollination and light, both of which are necessary for floral color change in V. cornuta. Our results demonstrate the presence of at least two regulated steps in V. cornuta anthocyanin biosynthesis: an early step influenced by pollination and light induces expression of earlier genes ( CHS and DFR) in the anthocyanin biosynthetic pathway, and is necessary but insufficient for floral color change, followed by a second step that affects floral color change by inducing expression of later genes ( ANS). We have thus identified important environmental cues and developed molecular tools that establish V. cornuta as a new model system for the study of the regulation of natural floral color change.