Regulation of exogenous sugars on the biosynthesis of key secondary metabolites in Cyclocarya paliurus

The biosynthesis and accumulation of secondary metabolites play a vital role in determining the quality of medicinal plants, with carbohydrate metabolism often influencing secondary metabolism. To understand the potential regulatory mechanism, exogenous sugars (sucrose, glucose/fructose) were applied to the leaves of Cyclocarya paliurus, a highly valued and multiple function tree species. The results showed that exogenous sugars enhanced the accumulation of soluble sugar and starch while increasing the enzyme activity related to carbohydrate metabolism. In addition, the plant height was increased by a mixture of exogenous mixed sugars, the addition of sucrose promoted the net photosynthetic rate, while all types of exogenous sugars facilitated the accumulation of flavonoids and terpenoids. Based on weighted gene co-expression network analysis (WGCNA), two key gene modules and four candidate transcription factors (TFs) related to carbohydrate metabolism and secondary metabolite biosynthesis were identified. A correlation analysis between transcriptome and metabolome data showed that exogenous sugar up-regulated the expression of key structural genes in the flavonoid and terpenoid biosynthetic pathway. The expression levels of the four candidate TFs, TIFY 10A, WRKY 7, EIL 3 and RF2a, were induced by exogenous sugar and were strongly correlated with the key structural genes, which enhanced the synthesis of specific secondary metabolites and some plant hormone signal pathways. Our results provide a comprehensive understanding of key factors in the quality formation of medicinal plants and a potential approach to improve the quality.