Melatonin in Micro-Tom Tomato: Improved Drought Tolerance via the Regulation of the Photosynthetic Apparatus, Membrane Stability, Osmoprotectants, and Root System

Abstract

Environmental variations caused by global climate change significantly affect plant yield and productivity. Because water scarcity is one of the most significant risks to agriculture's future, improving the performance of plants to cope with water stress is critical. Our research scrutinized the impact of melatonin application on the photosynthetic machinery, photosynthetic physiology, root system, osmoprotectant accumulation, and oxidative stress in tomato plants during drought. The results showed that melatonin-treated tomato plants had remarkably higher water levels, gas exchange activities, root system morphological parameters (average diameter, root activity, root forks, projected area, root crossings, root volume, root surface area, root length, root tips, and root numbers), osmoprotectant (proline, trehalose, fructose, sucrose, and GB) accumulation, and transcript levels of the photosynthetic genes SlPsb28, SlPetF, SlPsbP, SlPsbQ, SlPetE, and SlPsbW. In addition, melatonin effectively maintained the plants' photosynthetic physiology. Moreover, melatonin treatment maintained the soluble protein content and antioxidant capacity during drought. Melatonin application also resulted in membrane stability, evidenced by less electrolyte leakage and lower H₂O₂, MDA, and O₂^- levels in the drought-stress environment. Additionally, melatonin application enhanced the antioxidant defense enzymes and antioxidant-stress-resistance-related gene (SlCAT1, SlAPX, SlGR, SlDHAR, SlPOD, and SOD) transcript levels in plants. These outcomes imply that the impacts of melatonin treatment on improving drought resistance could be ascribed to the mitigation of photosynthetic function inhibition, the enhancement of the water status, and the alleviation of oxidative stress in tomato plants. Our study findings reveal new and incredible aspects of the response of melatonin-treated tomato plants to drought stress and provide a list of candidate targets for increasing plant tolerance to the drought-stress environment.

Keywords: abiotic stress; antioxidant system; climate changes; drought; melatonin; oxidative stress; photosynthesis; reactive oxygen species; tomato.

Figure 1

The physiological parameters relative water content (A) and electrolyte leakage (B) in tomato plant leaves under normal or 10-day drought-stress conditions with or without melatonin application. C: control; C+Mel: control with 100 µM melatonin pretreatment; D: drought, 10 days of withholding water; D+Mel: drought with 100 µM melatonin pretreatment. The values are the average of six replicates ± S.D. (n = 6). Significant differences among different treatments in the experiment were determined by LSD 0.05 test and are indicated by different letters.

Figure 2

Impacts of melatonin application on the oxidative stress response in tomato plants. (A) ROS fluorescence and (B) H₂O₂, (C) O₂⁻, and (D) MDA levels in tomato leaves after 10 days of drought stress. The green spots display the distribution of ROS. Bars, 50 µm. Data values are the means ± S.D. (n = 6). Significant differences among different treatments in the experiment were determined by LSD 0.05 test and are indicated by different letters.

Figure 3

Melatonin application effect on tomato plants’ photosynthetic parameters: (A) photosynthetic rate, (B) stomatal conductance, (C) transpiration rate, and (D) CO₂ assimilation rate after 10 days of drought stress. The values are the average of six replicates ± S.D. (n = 6). Significant differences among different treatments in the experiment were determined by LSD 0.05 test and are indicated by different letters.

Figure 4

Chlorophyll fluorescence parameters in tomato plants after 10 days of drought stress. Fluorescence images of maximum PSII yield (Fv/Fm) (A), maximum PSII yield (Fv/Fm) values (B), electron transport rate (ETR) (C), and non-photochemical quenching (NPQ) (D). Scale bars represent 100 µm in the fluorescence images of maximum PSII yield. The values presented above are the average of six replicates ± S.D. (n = 6). Significant differences among different treatments in the experiment were determined by LSD 0.05 test and are indicated by different letters.

Figure 5

The effective efficiency of PSII (Fq/Fm) in tomato plants after 10 days of drought stress. Fluorescence images of (Fq/Fm) (A) and the values of (Fq/Fm) (B). Scale bars represent 100 µm in the effective efficiency of PSII. The values presented above are the average of six replicates ± S.D. (n = 6). Significant differences among different treatments in the experiment were determined by LSD 0.05 test and are indicated by different letters.

Figure 6

Effects of melatonin on the expression levels of photosynthetic-machinery-associated genes in tomato plants after drought stress. (A) SlPetE, (B) SlPetF, (C) SlPsbP, (D) SlPsbQ, (E) SlPsbW, and (F) SlPsb28 in melatonin-supplemented and non-supplemented plants under control or stress conditions. The values presented above are the average of six replicates ± S.D. (n = 6). Significant differences among different treatments in the experiment were determined by LSD 0.05 test and are indicated by different letters.

Figure 7

Changes in melatonin content (A), soluble proteins (B), PPO (C), and antioxidant capacity (D) in tomato leaves after melatonin treatment under 10 days of drought stress. The values presented above are the average of six replicates ± S.D. (n = 6). Significant differences among different treatments in the experiment were determined by LSD 0.05 test and are indicated by different letters.

Figure 8

Effects of melatonin application on antioxidant defense enzyme activities. Catalase (CAT), ascorbate peroxidase (APX), glutathione reductase (GR), dehydroascorbate reductase (DHAR), peroxidase (POD), and superoxide dismutase (SOD) activities with or without melatonin in tomato leaves after 10 days of drought stress. The blue color illustrates lower values, and the green color illustrates higher values of antioxidant enzymes in the heat map. Significant differences among different treatments in the experiment were determined by LSD 0.05 test and are indicated by different letters.

Figure 9

The expression levels of antioxidant-defense-related genes in tomato plants after 10 days of drought-stress treatment. SlCAT1 (A), SlAPX (B), SlGR (C), SlDHAR1 (D), SlPOD (E), and SOD (F) gene expression levels in tomato leaves with or without melatonin after 10 days of drought stress. The values presented above are the average of six replicates ± S.D. (n = 6). Significant differences among different treatments in the experiment were determined by LSD 0.05 test and are indicated by different letters.

Figure 10

The activities of important osmoprotectants in tomato plants. Osmoprotectants (proline, trehalose, fructose, sucrose, starch, and GB) in melatonin-supplemented or non-supplemented tomato leaves after 10 days of drought stress. The pink color illustrates lower values, and the brown color illustrates higher values of osmoprotectants in the heat map. The values presented above are the average of six replicates ± S.D. (n = 6).

Figure 11

The expression levels of osmoprotectant genes in tomato plants. SlP5CS (A), SlP5CR (B), SlBADH (C), SlSPS (D), SlSUS3 (E), and SlT6PS (F) in tomato leaves with or without melatonin after 10 days of drought stress. The values presented above are the average of six replicates ± S.D. (n = 6). Significant differences among different treatments in the experiment were determined by LSD 0.05 test and are indicated by different letters.

Figure 12

The enzymatic activity of osmoprotectants in tomato plants. SlP5CS, SlP5CS, SlBADH, SlSPS, SlSUS3, and SlT6PS were measured in melatonin-supplemented or non-supplemented tomato leaves after 10 days of drought stress. The magenta color illustrates lower values, and the yellow color illustrates higher values of osmoprotectants in the heat map. The values presented above are the average of six replicates ± S.D. (n = 6).