Polyamine and nitrogen metabolism regulation by melatonin and salicylic acid combined treatment as a repressor for salt toxicity in wheat (Triticum aestivum L.) plants

Melatonin (MT) and salicylic acid (SA) are known to improve plant tolerance to environmental stresses; however, no reports have evaluated the effect of their combined treatment on plants under both normal and stressful conditions. For the first time, the present study aimed to investigate MT and SA’s potential role in regulating polyamine and nitrogen metabolism in wheat plants subjected to salty soils, and counterbalancing oxidative damage induced by salt stress. Moreover, this research aimed to elucidate a possible link between melatonin, salicylic acid, nitrogen and polyamine levels. Wheat ( Triticum aestivum L. cv. Sids 14) plants were grown under non-saline or saline conditions (6.0 and 12.0 dS m –1 ) and were foliar sprayed with 70 µM MT and/or 75 mg l −1 SA. The injury impacts of salt stress on wheat growth and production were significantly alleviated by exogenous MT and/or SA treatments. This was evidenced by increasing polyamines content through accelerating the metabolic flow from the precursor amino acids arginine and methionine to polyamines, decreasing the polyamines degradation, and enhancing the polyamines biosynthesis. Furthermore, MT and/or SA are involved in promoting nitrogen metabolism through up-regulating the activities of N uptake and metabolism related enzymes (nitrate reductase, nitrite reductase, glutamine synthetase, glutamate synthase) as well as enhancing the content of nitrogen, nitrate and protein in salt-stressed plants. The mitigation was also detected in reduced lipid peroxidation, hydrogen peroxide content, carbonyl content, and protease activity in stressed treated plants. Notably, the best response was registered for the combined treatment of MT and SA. Therefore, the co-application of MT and SA is a promising way for ameliorating salt toxicity in sustainable agricultural systems.