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.

褪黑素已显示出在调节植物生长和对各种非生物胁迫的响应中起多种作用，但其主要功能在对NaCl毒性的响应中仍不清楚。本研究的目的是评估褪黑激素（0、5、10和20μM）对生长，光合色素，相容性溶质，离子稳态，酶促和非酶促抗氧化剂，甜菊醇糖苷的含量以及25 mM NaCl下的甜菊叶中牛尿酸羟化酶（KAH）和尿苷二磷酸糖基转移酶（UGTs）基因的体外表达文化。结果表明，在25 mM NaCl存在下，褪黑素5和10μM可以改善甜叶菊叶片的生长参数，光合色素，脯氨酸和可溶性糖含量，并减少电解质泄漏。有趣的是，褪黑激素（5和10μM）通过改善抗坏血酸-谷胱甘肽循环的氧化还原状态和抗氧化酶的活性来减少丙二醛的积累和ROS的水平。此外，褪黑素减少了Cl和Na的积累并改善了K，N，Ca和P的积累，因此在NaCl毒性下保持了叶片中的离子平衡。褪黑素通过上调KAH和UGTs的表达水平来增加盐度下叶片中莱鲍迪苷A和甜菊苷的积累基因。但是，高浓度的褪黑激素（20μM）加剧了盐度的负面影响，并进一步降低了盐度下甜菊的生长。因此，目前的研究表明，在体外条件下，适当浓度的褪黑激素对甜菊糖具有抗NaCl毒性的保护作用。