Effect of Exogenous Spermidine on Osmotic Adjustment, Antioxidant Enzymes Activity, and Gene Expression of Gladiolus gandavensis Seedlings Under Salt Stress

Salt stress is the main factor limiting plant growth. Researches on physiological and biochemical mechanisms of plant responses to salt stress are of great significance in exploring and improving plant salinity tolerance. Gladiolus gandavensis is an ornamental plant with beautiful flowers and good adversity adaptability that is native to the southeastern coast of China. The purpose of this study was to identify the salt tolerance level of G. gandavensis and explore the mechanism by which exogenous spermidine (Spd) adjusts the growth of G. gandavensis under different NaCl conditions. A pot experiment was conducted to examine the chlorophyll content, photosynthetic parameters, proline, reactive oxygen species (ROS) concentrations, antioxidant activities, and relative expression of the chlorophyll a/b response protein, CAT1, POD, MnSODM, P5CS, BADH genes and bZIP, DREB transcription factors in G. gandavensis seedlings under 0.3%, 0.6%, and 0.9% NaCl conditions, with and without 0.1 mmol·L−1 Spd. G. gandavensis seedlings exhibited reduced chlorophyll content; a decreased net photosynthetic rate (Pn); and increased relative electric conductivity (REC), malondialdehyde (MDA), proline and soluble protein contents, antioxidant activities, and relative expression of the CAT1, POD, MnSODM, P5CS, and BADH genes under 0.3%, 0.6%, and 0.9% NaCl conditions. And Spd spraying under 0.3% and 0.6% salt treatment reduced the decrease in chla content by approximately 55.2% and 23.4%, while increased Pn, proline content and the expression of CAT1, POD, MnSODM, P5CS, and BADH genes. Exogenously applied Spd effectively alleviated the damage caused by salt stress (0.3% and 0.6%) by upregulating the REC, proline content, gas exchange, antioxidant enzyme activity, and expression of CAT1, POD, MnSODM, P5CS, and BADH genes in G. gandavensis seedlings. However, when the seedlings were grown under 0.9% NaCl, no significant differences were found in the physiological and molecular responses between Spd-treated and non-Spd-treated plants. Therefore, the present study suggests that exogenous Spd can efficiently counteract the adverse effect of low (0.3%) and moderate (0.6%) salt stress on G. gandavensis seedlings.