Insights into the mechanism underlying UV-B induced flavonoid metabolism in callus of a Tibetan medicinal plant Mirabilis himalaica.

Mirabilis himalaica is an important Tibetan medicinal plant in China. However, it has become a rare and class I endangered Tibetan medicine plant. Therefore, the use of callus to propagate germplasm resources is of great significance. We found that the flavonoid content of M. himalaica callus increased continuously with the extension of UV-B treatment. Multi-omics profiles were used to reveal the co-expression patterns of gene networks of flavonoid metabolism in M. himalaica callus during UV-B radiation. Results showed that five medicinal metabolics, including geranin, eriodictyol, astragalin, isoquercetin, pyrotechnic acid, and one anthocyanin malvide-3-O-glucoside were identified. The transcriptome data were divided into 46 modules according to the expression pattern by WGCNA (weighted gene co-expression network analysis), of which the module Turquoise had the strongest correlation with six target metabolites. We found that seven structural genes and twenty-five transcription factors were related to the metabolism of flavonoid synthesis, among which the structural genes CHI, C4H and UGT79B6 had strong co-expression relationships with the 6 target metabolites. WRKY42, WRKY7, bHLH128 and other transcription factors had strong co-expression relationships with multiple structural genes. Consequently, these findings suggest callus grown under UV-B treatment could be an effective alternative medical resource of M. himalaica, which is valuable for conservation and usage of this wild and endangered plant.