Upregulation of the MYB2 Transcription Factor is Associated with Increased Accumulation of Anthocyanin in the Leaves of Dendrobium bigibbum

Orchids with colorful leaves and flowers have significant ornamental value. Here, we used gamma-irradiation-based mutagenesis to produce a Dendrobium bigibbum mutant that developed purple instead of the normal green leaves. RNA sequencing of the mutant plant identified 2513 differentially expressed genes, including 1870 up- and 706 downregulated genes. The purple leaf color of mutant leaves was associated with increased expression of genes that encoded key biosynthetic enzymes in the anthocyanin biosynthetic pathway. In addition, the mutant leaves also showed increased expression of several families of transcription factors including the MYB2 gene. Transient overexpression of D. biggibum MYB2 in Nicotiana benthamiana was associated with increased expression of endogenous anthocyanin biosynthesis genes. Interestingly, transient overexpression of orthologous MYB2 genes from other orchids did not upregulate expression of endogenous anthocyanin biosynthesis genes. Together, these results suggest that the purple coloration of D. biggibum leaves is at least associated with increased expression of the MYB2 gene, and the MYB2 orthologs from orchids likely function differently, regardless of their high level of similarity.