The MYB305 Transcription Factor Regulates Expression of Nectarin Genes in the Ornamental Tobacco Floral Nectary

We have isolated and characterized the cDNA encoding the ornamental tobacco (Nicotiana langsdorffii X N. sanderae) homolog of the antirrhinum (Antirrhinum majus) MYB305. This transcription factor was robustly expressed at Stage 12 of nectary development but was only weakly expressed in the earlier Stage 6 nectaries. The ornamental tobacco MYB305 contains a conserved R2R3 MYB DNA binding domain with 76 amino acids in the activation domain. A green fluorescent protein-MYB305 fusion localized to nucleus of tobacco protoplasts and yeast one-hybrid assays demonstrated that it functions as a transcription activator. A conserved 23-amino acid C-terminal domain is required to activate gene expression. The coding region of the myb305 cDNA was expressed in Escherichia coli as a glutathione S-transferase fusion protein and was purified to homogeneity. This protein shows binding to two consensus MYB binding sites on the ornamental tobacco Nectarin I (nec1) promoter as well as to the single site located on the Nectarin V (nec5) promoter. Deletions of either of the binding sites from the nec1 promoter significantly reduced expression in nectary tissues. Temporally, MYB305 expression precedes that of nec1 and nec5, as would be expected if the MYB305 factor regulates expression of the nec1 and nec5 genes. Ectopic expression of MYB305 in foliage was able to induce expression of both nec1 and nec5, as well as two flavonoid biosynthetic genes in the foliage. Finally, RNA interference knockdown of MYB305 resulted in reduced expression of both nectarins and flavonoid biosynthetic genes. We conclude that expression of MYB305 regulates expression of the major nectarin genes in the floral nectary.