Molecular mechanism of phenolic acid biosynthesis in callus of a Tibetan medicinal plant (Mirabilis himalaica) under UV-B treatment

Mirabilis himalaica is a traditional Tibetan medicinal plant in China. However, it has become a class I endangered medicinal plant. Therefore, the utilization of M. himalaica callus to propagate germplasm resources is of great significance. Numerous active compounds have been found in M. himalaica, most of which have been isolated from phenolic acids. In this study, UV-B radiation enhanced the accumulation of phenolic acids effectively in M. himalaica callus. The multi-omics profiles were used to reveal the co-expression regulated patterns of genes related to phenolic acid synthesis in UV-B treated M. himalaica callus. In this way, 8 medicinal metabolites were identified, including rosmarinic acid, sinapic acid, coniferin, salicylic acid, tyrosol, isochlorogenic acid A, isochlorogenic acid B and isochlorogenic acid C. The transcriptome data were divided into 26 modules based on similar expression pattern by using the weighted gene correlation network analysis (WGCNA). It revealed that MEturquoise module correlated with above eight target metabolites. Eventually, 11 structural genes and 31 transcription factors related to phenolic acid biosynthesis were screened. Subsequently, co-expression network was constructed between these genes, transcription factors and metabolites, among which PAL, CAD and CYP73A had strong co-expression relationship with above eight target metabolites. Transcription factors such as MYB, bHLH, bZIP, PLATZ, AP2/ERF-ERF73, WRKY33 and WRKY42 had strong co-expression relationship with eight target metabolites. Consequently, these findings indicated that UV-B treated callus is a cost-efficient alternative to wild M. himalaica, which is valuable for the use of this endangered plant.