Abstract
The R2R3-MYB gene family participates in several plant physiological processes, especially the regulation of the biosynthesis of secondary metabolites. However, little is known about the functions of R2R3-MYB genes in Gynostemma pentaphyllum (G. pentaphyllum), a traditional Chinese medicinal herb that is an excellent source of gypenosides (a class of triterpenoid saponins) and flavonoids. In this study, a systematic genome-wide analysis of the R2R3-MYB gene family was performed using the recently sequenced G. pentaphyllum genome. In total, 87 R2R3-GpMYB genes were identified and subsequently divided into 32 subgroups based on phylogenetic analysis. The analysis was based on conserved exon–intron structures and motif compositions within the same subgroup. Collinearity analysis demonstrated that segmental duplication events were majorly responsible for the expansion of the R2R3-GpMYB gene family, and Ka/Ks analysis indicated that the majority of the duplicated R2R3-GpMYB genes underwent purifying selection. A combination of transcriptome analysis and quantitative reverse transcriptase-PCR (qRT-PCR) confirmed that Gynostemma pentaphyllum myeloblastosis 81 (GpMYB81) along with genes encoding gypenoside and flavonol biosynthetic enzymes exhibited similar expression patterns in different tissues and responses to methyl jasmonate (MeJA). Moreover, GpMYB81 could bind to the promoters of Gynostemma pentaphyllum farnesyl pyrophosphate synthase 1 (GpFPS1) and Gynostemma pentaphyllum chalcone synthase (GpCHS), the key structural genes of gypenoside and flavonol biosynthesis, respectively, and activate their expression. Altogether, this study highlights a novel transcriptional regulatory mechanism that suggests that GpMYB81 acts as a “dual-function” regulator of gypenoside and flavonol biosynthesis in G. pentaphyllum.
Highlights
The myeloblastosis (MYB) transcriptional regulators comprise one of the largest and important families in the plant kingdom (Riechmann et al, 2000)
86 R2R3-GpMYB genes were mapped to 11 chromosomes and renamed from GpMYB1 to GpMYB86 according to their location on the chromosomes
The results revealed that the promoters of Gynostemma pentaphyllum farnesyl pyrophosphate synthase 1 (GpFPS1) and Gynostemma pentaphyllum chalcone synthase (GpCHS) contained conserved MYB-recognition elements (MREs) or ACrich elements, suggesting that Gynostemma pentaphyllum myeloblastosis 81 (GpMYB81) might bind to the promoters of GpFPS1 and GpCHS
Summary
The myeloblastosis (MYB) transcriptional regulators comprise one of the largest and important families in the plant kingdom (Riechmann et al, 2000). A classical characteristic of MYB proteins is that they contain conserved MYB DNA-binding domain repeats in the N-terminus. The conserved MYB domain is composed of one to four continuous and non-redundant imperfect sequence repeats, designated as R1, R2, and R3 according to their similarity to the c-MYB protein. Each conserved MYB DNA-binding domain repeat is approximately 50 amino acids in length and encodes three α-helices (Dubos et al, 2010). Depending on the number of MYB repeats in the MYB domain, MYB transcription factors (TFs) can be subdivided into R2R3-MYB (2R-MYB), R1R2R3-MYB (3R-MYB), 4R-MYB (containing four R1/R2 repeats), and the MYB-related subfamily (containing a single or a partial MYB repeat) (Stracke et al, 2001). In the R2R3-MYB family, the last two α-helices of each MYB repeat form a helix–turn–helix (HTH) structure and the third α-helix of R2 and R3 repeats are essential for DNA binding, allowing direct contact and insertion into the DNA major groove (Gabrielsen et al, 1991; Jia et al, 2004)
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