Integrative analysis of metabolome and transcriptome reveals molecular regulatory mechanism of flavonoid biosynthesis in Cyclocarya paliurus under salt stress

Cyclocarya paliurus is a highly valued and multiple function tree species, and especially its leaves are enriched in diverse secondary metabolites with healthy function. Flavonoids as a kind of secondary metabolites are not only an important part of plant defense system against environmental stresses but also commercial pharmaceutical substances. To meet the leaf production for value-added product development, coastal saline would be a potential land resources for developing C. paliurus plantations, whereas there is limited knowledge on the regulatory mechanisms of flavonoid biosynthesis under salt stress in C. paliurus. Here, we conducted an integrated transcriptomics and metabolomics analysis of C. paliurus under different salt treatments in the short (T1, treatment lasted for 15 days) and long (T2, treatment lasted for 30 days) term. Generally, salt treatments led to increased total flavonoid content in C. paliurus leaves, which increased gradually along with salt stress concentration. For instance, after seedlings were salt-treated with LS (0.15 %), MS (0.30 %) and HS (0.45 %), the total flavonoid content was increased by 26.23 %, 57.54 %, and 74.87 %, respectively, compared with the control (0.00 %, m/v) at T1. Correspondingly, significant enrichment of differentially expressed genes and metabolites was observed in the flavonoid biosynthesis pathways. Weighted gene co-expression network analysis (WGCNA) identified several key genes regulating the responses to salt stress, such as genes encoding phenylalanine ammonia lyase (PAL), chalcone synthase (CHS) and flavonol synthase (FLS). In addition, fifteen transcription factors (TFs) were found to regulate flavonoid biosynthesis by activating or repressing the expression of multiple structural genes in C. paliurus leaves during salt stress. These findings provide insight into the salt stress associated transcriptional regulation, and would drive progress in genetic improvement and plantation development of C. paliurus.