Effect of Low-Temperature Stress on Secondary Metabolism of Astragalus membranaceus Bge. var. mongolicus Hsiao

Plant secondary metabolism is profoundly influenced by environmental stress. Low-temperature stress can induce the production of NO and ROS. The root of Astragalus membranaceus Bge. var. mongolicus Hsiao is commonly used as a medicinal herb in Asia. This study aimed to regulate the secondary metabolism and improve the quality of cultivated A. membranaceus var. mongolicus by subjecting it to different temperatures (5, 20, and 5/20°C). The activities and gene expressions of enzymes catalyzing the synthesis of secondary metabolites, including PAL, CHS, HMGR, SQS, were determined. Nitric oxide synthase (NOS) activity and the contents of NO, superoxide radical ( $${\text{O}}_{2}^{{\bullet - }}$$ ), hydrogen peroxide (H2O2), and malondialdehyde (MDA) and five secondary metabolites were measured. Low-temperature stress of 5°C significantly increased NOS activities and NO contents, which led to a considerable surge in the levels of $${\text{O}}_{2}^{{\bullet - }}$$ , H2O2, and MDA. Furthermore, the activities of SOD, CAT, and POD, increased rapidly and peaked on the 2nd and 3rd days. The gene expressions and activities of PAL, CHS, HMGR, and SQS were also markedly enhanced, which led to a substantial increase in all secondary metabolites. In the 5°C treatment group, calyculin-7-glucoside, ononin, calycosin, formononetin, and astragaloside IV were increased by 45.6, 36.6, 97.9, 123.4, and 74.9%, respectively, compared with the 20°C treatment group. Thus, low-temperature stress can significantly improve the secondary metabolism of A. membranaceus var. mongolicus, enhance the accumulation of medicinal chemical components, and augment the quality of the herb.