Exogenous Melatonin Alleviates Cadmium Toxicity in Wheat (Triticum turgidum L.) by Modulating Endogenous Nitric Oxide and Hydrogen Sulfide Metabolism

Melatonin (MEL), a versatile molecule, has garnered significant attention in current research due to its potential as a plant growth promoter, and a modulator of multiple developmental processes and stresses. The application of 1, 10 and 100 μM MEL as a pre-treatment of seeds prior to germination or as a treatment during germination process, and the protective effects of exogenous MEL were studied on seedlings of durum wheat (Triticum turgidum L.) in response to Cadmium (Cd) stress. The concentration of 1 μM for co-treatment and 10 μM for 12-hour pretreatment were the most effective in mitigating the detrimental effects of Cd on seedlings growth. At the metabolic level, MEL seems to interfere endogenously within the internal metabolic pathways involved in ROS scavenging and proteins protection against oxidative damages. MEL did not affect positively hydrogen sulfide (H₂S) metabolism, which was regulated by the decreased activities of L-cysteine desulfhydrase (LCD) and D-cysteine desulfhydrase (DCD). However, MEL interfered positively at NO metabolism via the stimulation of the endogenous generation of NO and the up-regulation of activities of NO related enzymes, such as nitrate reductase (NR) and nitrite reductase (NiR). Our work brings insight at the existence of endogenous crosstalk between NO and MEL, despite being applied exogenously, thereby promoting the role of MEL as a growth regulator. This allows to elucidate the mechanisms of MEL’s protective effects that likely contributes to the defence response of wheat to Cd stress.