Development of a fast and efficient root transgenic system for exploring the function of RsMYB90 involved in the anthocyanin biosynthesis of radish

Radish (Raphanus sativus L.) is recalcitrant to regeneration and genetic transformation, which has severely limited its gene function verification and germplasm innovation. To overcome the limitation, we have developed a rapid and stable regeneration and transformation of hairy roots in radish to explore the potential function of the critical genes involved in the anthocyanin biosynthesis via the optimization of explant selection, bacteria solution concentration, acetosyrinone (AS) concentration and strain types. The single factor experimental designing method was applied and each treatment was conducted for three independent biological replicates. The hairy roots induction rate was more than 90% when using cotyledon with petiole or rootless seedling as explants, while only 53.33% hypocotyls could induce hairy roots. Phenotypic observation and molecular identification indicated that the highest positive transgenic rate of hairy roots could reach approximately 17.51%, when employed optimal optical density (OD600) and AS concentration at 1.0 and 300 μM, respectively, as well as the best strain of MSU440. Subsequently, RsMYB90 was found to play a positive role in the anthocyanin accumulation through the established hairy root transgenic system, and a number of anthocyanin-related genes, such as RsUFGT, RsCHS and RsF3H, were up-regulated by RNA-seq and RT-qPCR analysis. In addition, yeast one-hybrid (Y1H) and dual-luciferase assays (DLA) showed that the RsMYB90 could bind to the promoter of RsUFGT to activate its expression. Taken together, the stable A. rhizogenes-mediated transformation system provides an avenue for gene function assay, genetic engineering and study of secondary metabolisms in radish.