LAUSR

BACKGROUND Flower pigment and shape are determined by the coordinated expression of a set of structural genes during flower development. R2R3-MYB transcription factors are known regulators of structural gene expression. The current study focused on two members of this large family of transcription factors that were predicted to have roles in pigment biosynthesis and organ shape development in orchids. METHODS Phylogenetic analysis was used to identify candidate Dendrobium catenatum R2R3-MYB (DcaMYB) sequences associated with pigment and cell shape development. Gene silencing of candidate DhMYBs in Dendrobium hybrid by direct application of dsRNA to developing flowers was followed by observation of gene expression level and flower phenotypes. Silencing of the structural gene chalcone synthase was used as a comparative control. KEY RESULTS Ten candidate flower-associated DcaMYB were identified. Flowers treated with dsRNA of DhMYB22 and DhMYB60 sequences were less pigmented with relatively low expression of anthocyanin biosynthetic genes (F3'H and DFR), lower total anthocyanin concentration and markedly lower levels of cyanidin-3-glucoside and cyanidin-3-rutinoside. Petals of DhMYB22 treated flowers and sepals of DhMYB60 treated flowers showed the greatest colour difference relative to the same organs in untreated flowers. DhMYB22 treated flowers had relatively narrow and constricted lips, while DhMYB60 treated flowers had narrow and constricted sepals. No significant difference in shape was observed for DhCHS treated or untreated flowers. CONCLUSIONS Our results demonstrate that DhMYB22 and DhMYB60 regulate pigment intensity and floral organ shape in Dendrobium. This is a first report of MYB regulation of floral organ shape in orchids.