Drought is a major environmental stress that limits growth and development of plants.Strigolactone, a kind of carotenoid-derived sesquiterpenoid hormone, plays an important role in resisting abiotic stress for plants. In this study, alfalfa (Medicago Sativa L.) WL-712 was sprayed with GR24 (a synthetic strigolactone analog) at the concentration of 0.1 μM for 3 days under drought stress for 1 week. The results showed that GR24 treatment could increase the contents of soluble protein and soluble sugar and decrease the content of malondialdehyde, which negatively affected by drought stress in alfalfa.In addition, GR24 treatment significantly enhanced superoxide dismutase and the activities of peroxidase and catalase in alfalfa. The JIP test analysis also indicated that GR24 could alleviate photosynthesis negatively affected by drought stress. The expressions of genes related to photosynthesis were consistent with the changes of physiological indexes in alfalfa. This study indicated that GR24 could alleviate the adverse effects of drought stress on physiology and photosystem II function in alfalfa leaves.

The rac-GR24, an artificial analog of strigolactone, is known for its roles in inhibiting branches, and previous studies have reported that it has a certain mechanism to relieve abiotic stress, but the underlying metabolic mechanisms of mitigation for drought-induced remain unclear. Therefore, the objectives of this study were to identify associated metabolic pathways that are regulated by rac-GR24 in alfalfa (Medicago sativa L.) and to determine the metabolic mechanisms of rac-GR24 that are involved in drought-induced root exudate. The alfalfa seedling WL-712 was treated with 5% PEG to simulate drought, and rac-GR24 at a concentration of 0.1 µM was sprayed. After three days of treatment, root secretions within 24 h were collected. Osmotic adjustment substances and antioxidant enzyme activities were measured as physiological indicators, while LS/MS was performed to identify metabolites regulated by rac-GR24 of root exudate under drought. The results demonstrated that rac-GR24 treatment could alleviate the negative effects from drought-induced on alfalfa root, as manifested by increased osmotic adjustment substance content, cell membrane stability, and antioxidant enzyme activities. Among the 14 differential metabolites, five metabolites were uniquely downregulated in plants in rac-GR24 treatment. In addition, rac-GR24 could relieve drought-induced adverse effects on alfalfa through metabolic reprogramming in the pathways of the TCA cycle, pentose phosphate, tyrosine metabolism, and the purine pathway. This study indicated that rac-GR24 could improve the drought resistance of alfalfa by influencing the components of root exudates.