Melatonin Improves Drought Stress Tolerance of Tomato by Modulating Plant Growth, Root Architecture, Photosynthesis, and Antioxidant Defense System

Abstract

Tomato is an important vegetable that is highly sensitive to drought (DR) stress which impairs the development of tomato seedlings. Recently, melatonin (ME) has emerged as a nontoxic, regulatory biomolecule that regulates plant growth and enhances the DR tolerance mechanism in plants. The present study was conducted to examine the defensive role of ME in photosynthesis, root architecture, and the antioxidant enzymes’ activities of tomato seedlings subjected to DR stress. Our results indicated that DR stress strongly suppressed growth and biomass production, inhibited photosynthesis, negatively affected root morphology, and reduced photosynthetic pigments in tomato seedlings. Per contra, soluble sugars, proline, and ROS (reactive oxygen species) were suggested to be improved in seedlings under DR stress. Conversely, ME (100 µM) pretreatment improved the detrimental-effect of DR by restoring chlorophyll content, root architecture, gas exchange parameters and plant growth attributes compared with DR-group only. Moreover, ME supplementation also mitigated the antioxidant enzymes [APX (ascorbate peroxidase), CAT (catalase), DHAR (dehydroascorbate reductase), GST (glutathione S-transferase), GR (glutathione reductase), MDHAR (monodehydroascorbate reductase), POD (peroxidase), and SOD (superoxide dismutase)], non-enzymatic antioxidant [AsA (ascorbate), DHA (dehydroascorbic acid), GSH (glutathione), and GSSG, (oxidized glutathione)] activities, reduced oxidative damage [EL (electrolyte leakage), H2O2 (hydrogen peroxide), MDA (malondialdehyde), and O2•− (superoxide ion)] and osmoregulation (soluble sugars and proline) of tomato seedlings, by regulating gene expression for SOD, CAT, APX, GR, POD, GST, DHAR, and MDHAR. These findings determine that ME pretreatment could efficiently improve the seedlings growth, root characteristics, leaf photosynthesis and antioxidant machinery under DR stress and thereby increasing the seedlings’ adaptability to DR stress.

Seed Priming With Melatonin Promotes Seed Germination and Seedling Growth of Triticale hexaploide L. Under PEG-6000 Induced Drought Stress

“…Drought has a negative effect on photosynthesis in wheat ( Ahmad et al, 2018 ). Drought stress can also impair photosynthetic activities of plants, leading to severe growth retardation ( Altaf et al, 2022 ). A study reported that Pn of cotton decreased with the increase in drought intensity ( Deeba et al, 2012 ).…”

“…MT priming enhances ROS scavenging ability, and reduces MDA accumulation ( Altaf et al, 2022 ). The accumulation of MDA is positively correlated with the degree of cell membrane lipid peroxidation, reflecting the degree of damage to the cell membrane ( Imran et al, 2021 ).…”

Crosstalk between Melatonin and Reactive Oxygen Species in Plant Abiotic Stress Responses: An Update

“…Moreover, to reestablish the redox homeostasis, melatonin also enhanced the expression of genes encoding antioxidant enzymes (such as APX1 , APX2 , CAT1 , FSD1 , CuZnSOD , and MnSOD ), and improved the activities of APX, SOD, CAT, POD, Δ1-pyrroline-5-carboxylate synthesis (P5CS), as well as the levels of antioxidants (ASC, GSH, proline, and total soluble carbohydrates) [ 12 , 35 , 36 , 40 , 42 , 47 , 54 , 55 , 56 , 57 , 58 , 59 ] ( Table 1 ). Similarly, it was well established that melatonin boosted the activities of many antioxidant enzymes (including SOD, POD, APX, CAT, DHAR, GST, GR, MDHAR, and PPO) and the levels of antioxidants (including ASC, DHA, GSH, proline, flavonoid, carotenoid, and phenolic compounds), thus reducing ROS levels and improving tolerance to drought stress in plants, such as maize, tomato, citrus, soybean, Malus , or kiwifruit plants [ 14 , 39 , 60 , 61 , 62 , 63 , 64 ] ( Table 1 ). Melatonin also acted as a priming agent to improve Medicago sativa tolerance to drought stress via the nitro-oxidative homeostasis [ 65 ].…”

Exogenous Melatonin Improves Cold Tolerance of Strawberry (Fragaria × ananassa Duch.) through Modulation of DREB/CBF-COR Pathway and Antioxidant Defense System

“…Plants can protect themselves from the oxidative stress caused by cold stress by increasing their antioxidant capacity. However, if their antioxidant capacity is weak, applying several exogenous compounds with high-antioxidant properties could help them increase stress tolerance [36]. The use of plant growth regulators and biostimulators such as "melatonin" could be an efficient approach to achieve this objective.…”