Exploring the synergistic effects of melatonin and salicylic acid in enhancing drought stress tolerance in tomato plants through fine-tuning oxidative-nitrosative processes and methylglyoxal metabolism

Enhancement of drought stress (DS) tolerance in plants has been reported with the separate applications of melatonin (MT) and salicylic acid (SA). However, the synergistic effects of their combined application and the exact mechanisms through which they enhance drought tolerance are still not fully understood. The study explored the joint effects of MT and SA on DS tolerance and their implications for oxide-nitrosative processes and methylglyoxal metabolism in tomato plants. To impose DS in tomato plants, they were treated with a 6% polyethylene glycol solution (PEG-6000) in a half-strength nutrient solution. The plants underwent a treatment consisting of individual or combined spraying with 0.5 mM SA and/or 0.1 mM MT every two days for 14 days to relieve the after effects of the PEG-induced stress. The combined MT and SA treatment improved plant drought tolerance, resulting in significant increases in shoot (75%) and root (204%) dry weights. Other improvements included a 141% increase in total chlorophyll content, a 61% increase in Fv/Fm, a 39% increase in Ψl, a 50% increase in RWC, and a 76% increase in proline content compared to DS alone. It also increased non-enzymatic antioxidants (ascorbate and glutathione) and decreased oxidized glutathione under DS. The combined treatment also led to decreased levels of hydrogen peroxide, carbonyl (-CO), and thiol (-SH) groups, indicating the activation of the oxide-nitrosative system. Additionally, the treatment with MT and SA together increased the activity of l-cysteine desulfhydrase (L-CD), resulting in the production of hydrogen sulfide (H2S) and nitric oxide (NO), while decreasing the activity of S-nitroso glutathione reductase, which may have accelerated S-nitrosylation. The synthesis of methylglyoxal (MG) was also reduced by the combined treatment, possibly through the upregulation of the glyoxalase system. Overall, our study presents evidence that the combination of MT and SA treatment can enhance tomato plant tolerance to DS by regulating the oxidative-nitrosative processes and methylglyoxal metabolism.