Blue light induces biosynthesis of flavonoids in Epimedium sagittatum (Sieb.et Zucc.) Maxim. leaves, a study on a light-demanding medicinal shade herb

Light is one of the most important environmental factors in plant life activities, and it plays an important role in regulating the synthesis of plant secondary metabolites. As a light-demanding shade plant, Epimedium sagittatum has a typical secondary metabolic process under different light irradiations. However, few studies have investigated the changes in flavonoid contents in E. sagittatum under different light irradiations. In this study, the effects of different light irradiations, namely red light, yellow light, and blue light, on plant physiological characteristics and saikosaponin secondary metabolism are investigated in one-year-old E. sagittatum leaves collected after 10, 20, and 30 days of treatment. The average leaf area significantly increased by 55.11 %, 45.15 %, and 42.09 %, respectively (p < 0.05), after 10 days, 20 days, and 30 days of treatment with red light. The change in the average area of the E. sagittatum leaves was not significant after blue and yellow light treatment (p > 0.05). Red light, yellow light, and blue light significantly increased the chlorophyll content (SPAD value) of the E. sagittatum leaves (p < 0.05). Although red light treatment could increase the leaf area, it had no positive effect on the accumulation of flavonoids. Yellow light treatment significantly increased the epimedin C and icariin contents by 42.54 % and 15.02 %, respectively, to 108.24 ± 0.50 mg g−1 and 28.54 ± 0.14 mg g−1 at 30 days after treatment (p < 0.05). However, it was not conducive to the accumulation of epimedin A and epimedin B. The blue light treatment significantly increased the epimedin C, icariin and icariside Ⅱ contents by 29.42 %, 28.45 %, and 66.12 %, respectively, to 98.27 ± 0.44 mg g−1, 31.87 ± 0.51 mg g−1 and 2.85 ± 0.02 mg g−1 at 30 days after treatment (p < 0.05). However, it was not conducive to the accumulation of epimedin A and epimedin B. The effects of red light on the gene expression of the flavonoid synthesis pathway was limited, and it does not significantly improve its transcription level. In contrast, the expression of flavonoid synthesis related genes (4CL3, CHS2 and ANS) was significantly upregulated by the yellow light and the blue light (p < 0.05), especially the positive effect of the blue light. Blue light also significantly upregulated the expression levels of 4CL1, CHI1, UF3GT, and UF7GT (p < 0.05). Therefore, blue light has a more significant effect than yellow light on the regulation of icariin and icariside Ⅱ synthesis. The widely untargeted metabolome analysis further confirmed the positive effects of blue light on the synthesis of flavonoids. After filtering, 2191 high-quality metabolites were used to screen the differentially accumulated metabolites (DAMs). A total of 38 metabolites involved in flavonoid biosynthesis were identified, of which 17 flavonoids were significantly accumulated under the blue light treatment. We identified that icariin accumulated under the blue light treatment by the untargeted metabolome. Our study indicates that light conditions can be improved by blue light or with blue film covering to facilitate the accumulation of icariin during the ecological cultivation of E. sagittatum. Our study improves our understanding on shade plant responses to light, and helps to obtain more flavonoids by changing the light conditions during cultivation, which is highly important in the ecological cultivation of Chinese medicinal plants.