Melatonin alleviates NaCl-induced damage by regulating ionic homeostasis, antioxidant system, redox homeostasis, and expression of steviol glycosides-related biosynthetic genes in in vitro cultured Stevia rebaudiana Bertoni

Melatonin has been shown to play a variety of roles in regulating plant growth and response to various abiotic stresses, but its main function remains unclear in response to NaCl toxicity. The aim of the current study was to evaluate the effect of melatonin (0, 5, 10, and 20 μM) on growth, photosynthetic pigments, compatible solutes, ionic homeostasis, enzymatic and non-enzymatic antioxidants, the content of steviol glycosides, and expression of kaurenoic acid hydroxylase (KAH) and uridine diphosphate glycosyltransferase (UGTs) genes in stevia under 25 mM NaCl in in vitro culture. The results showed that in the presence of 25 mM NaCl, melatonin at 5 and 10 μM improved growth parameters, photosynthetic pigments, proline, and soluble sugar contents and reduced electrolyte leakage in stevia leaves. Interestingly, melatonin (5 and 10 μM) reduced the accumulation of malondialdehyde and the level of ROS by improving the redox state of the ascorbate-glutathione cycle and the activity of antioxidant enzymes. Additionally, melatonin decreased the accumulation of Cl and Na and improved the accumulation of K, N, Ca, and P, thus maintaining the ionic balance in leaves under NaCl toxicity. Melatonin increased the accumulation of rebaudioside A and stevioside in the leaves under salinity by upregulating the expression level of KAH and UGTs genes. However, high concentrations of melatonin (20 μM) exacerbated the negative effects of salinity and further reduced stevia growth under salinity. Therefore, the current study indicates the protective role of melatonin in the appropriate concentration in stevia against NaCl toxicity in in vitro conditions.