Mining MYB transcription factors from the genomes of orchids (Phalaenopsis and Dendrobium) and characterization of an orchid R2R3-MYB gene involved in water-soluble polysaccharide biosynthesis

Members of the MYB superfamily act as regulators in a wide range of biological processes in plants. Despite this, the MYB superfamily from the Orchidaceae has not been identified, and MYB genes related to bioactive water-soluble polysaccharide (WSP) biosynthesis are relatively unknown. In this study, we identified 159 and 165 MYB genes from two orchids, Phalaenopsis equestris and Dendrobium officinale, respectively. The MYB proteins were classified into four MYB classes in both orchids: MYB-related (MYBR), R2R3-MYB, 3R-MYB and atypical MYB proteins. The MYBR proteins in both orchids were classified into five subfamilies and 12 genes were strongly up-regulated in response to cold stress in D. officinale. The R2R3-MYB proteins were both divided into 31 clades in P. equestris and D. officinale. Among these clades, nine contained MYB TFs related to secondary cell wall biosynthesis or testa mucilage biosynthesis in Arabidopsis thaliana. In D. officinale, 10 candidate genes showed an expression pattern corresponding to changes in the WSP content. Overexpression of one of these candidate genes (DoMYB75) in A. thaliana increased seed WSP content by about 14%. This study provides information about MYB genes in two orchids that will further help to understand the transcriptional regulation of WSP biosynthesis in these orchids as well as other plant species.