Systemic role of melatonin in enhancing temperature stress tolerance in fenugreek: coordination of antioxidant defense, hormonal regulation, energy status, sulfur metabolism, and diosgenin pathway genes.

Background: Temperature stresses, encompassing heat and cold, significantly hinder the growth and productivity of fenugreek. Melatonin, known for its antioxidant and signaling properties, enhances plant stress tolerance, yet its specific role and mode of action in fenugreek under temperature stress remain largely uncharacterized. This study investigated the protective effects of exogenous melatonin application (0, 30, 60, and 90 ppm) on fenugreek plants exposed to temperature stress (10 °C and 42 °C).

Results: Melatonin, particularly at 60 ppm, substantially improved photosynthetic efficiency by increasing the levels of photosynthetic pigments. Additionally, it mitigated cellular damage by enhancing the activity of antioxidant enzymes, as evidenced by reduced electrolyte leakage and lower malondialdehyde levels. These changes, along with increased total protein content, collectively alleviated oxidative stress. Melatonin also elevated the concentrations of protective metabolites, such as proline and total phenols, and maintained relative water content. Through interaction with nitric oxide, hydrogen sulfide, and cysteine-producing pathways, melatonin minimized the accumulation of reactive oxygen species, thereby reducing damage to cellular structures and maintaining cellular redox potential. Hormonal balance was also improved, as indicated by increased levels of abscisic acid and auxin. In the context of energy metabolism, melatonin, through its interaction with HS, increased ATP content and cellular energy charge while reducing AMP levels. It also enhanced ATPase activity, thereby contributing to the maintenance of ATP homeostasis. Furthermore, melatonin up-regulated diosgenin biosynthesis genes (SQS, SEP, CAS, and SSR), associated with higher diosgenin content, particularly under heat stress.

Conclusions: Collectively, melatonin enhanced fenugreek's tolerance to temperature stress by modulating photosynthetic efficiency, antioxidant defense mechanisms, hormonal balance, sulfur metabolism, and energy homeostasis, highlighting its potential as an effective stress-mitigating agent. Overall, our findings indicated that melatonin functions not only as an antioxidant but also as a central regulator coordinating hormonal signaling, redox balance, and energy metabolism. Future studies incorporating metabolomic and transcriptomic analyses may further substantiate these findings, providing more comprehensive insights into the mechanistic role of melatonin in plant stress adaptation.