Spearmint R2R3-MYB transcription factor MsMYB negatively regulates monoterpene production and suppresses the expression of geranyl diphosphate synthase large subunit (MsGPPS.LSU)

Many aromatic plants, such as spearmint, produce valuable essential oils in specialized structures called peltate glandular trichomes (PGTs). Understanding the regulatory mechanisms behind the production of these important secondary metabolites will help design new approaches to engineer them. Here, we identified a PGT-specific R2R3-MYB gene, MsMYB, from comparative RNA-Seq data of spearmint and functionally characterized it. Analysis of MsMYB-RNAi transgenic lines showed increased levels of monoterpenes, and MsMYB-overexpressing lines exhibited decreased levels of monoterpenes. These results suggest that MsMYB is a novel negative regulator of monoterpene biosynthesis. Ectopic expression of MsMYB, in sweet basil and tobacco, perturbed sesquiterpene-and diterpene-derived metabolite production. In addition, we found that MsMYB binds to cis-elements of MsGPPS. LSU and suppresses its expression. Phylogenetic analysis placed MsMYB in subgroup 7 of R2R3-MYBs whose members govern phenylpropanoid pathway and are regulated by miR858. Analysis of transgenic lines showed that MsMYB is more specific to terpene biosynthesis as it did not affect metabolites derived from phenylpropanoid pathway. Further, our results indicate that MsMYB is probably not regulated by miR858, like other members of subgroup 7.