Melatonin alleviates salt damage in tomato seedling: A root architecture system, photosynthetic capacity, ion homeostasis, and antioxidant enzymes analysis
Graphical abstract
Introduction
Global agricultural production gets limited by salinity in the soil. The potassium ion (K+) absorption gets inhibited due to the excessive presence of sodium ion (Na+) in the soil, which ultimately abates the growth and yield of a particular crop (Zhu, 2003). Reactive oxygen species (ROS) are formed in considerable amounts in plants under Salt stress (SS). An excessive amount of ROS damages cells of plants and imparts oxidative stress when plants' scavenging capacity is reached beyond a certain limit. Plants form an intricate antioxidant system in response to environmental stresses to regulate redox homeostasis, including SOD, CAT, APX and POD (Peroxidase), as well as other free radical scavengers (Liang et al., 2017). The physiology of plants gets disturbed by SS through various mechanisms, including the damage of cellular organelles in response to higher concentrations of Na+ ions which results in the inhibition of enzymatic activities and proteins formation and distorting the process of respiration and photosynthesis. It also causes an imbalance in nutrients which ultimately causes a decline in transport and uptake of nutrients from root to shoot. Besides, plants suffer from physiological drought as SS limits the water uptake and minimizes the osmotic potential of soil (Ruiz-Lozano et al., 2012).
Photosynthesis is the first physiological process affected by SS. It reduces leaf water potential, minimizes stomatal and mesophyll resistance, and increases gas flow, thus limiting photosynthesis activity (Flexas et al., 2004). The root system of plants gets most affected by SS and plant adapts accordingly to abate its damaging effects. Former researches have evinced that SS causes a reduction in length of roots of tomato plant, and abatement in dry matter of roots was combined with an increase in root/shoot ratio under SS (Lovelli et al., 2012).
In 1995, in higher plants, first ME was discovered by two research groups (Dubbels et al., 1995; Hattori et al., 1995). Till now, ME has been discovered in many plants species, including tomato (Wang et al., 2014), rice (Wang et al., 2009), and pepper (Korkmaz et al., 2014). Earlier, the significance of ME in regulating various biological processes in animals was known, but later its significance as an antioxidant in plants was also discovered, which notably protects plants from environmental stresses including SS (Altaf et al., 2020a), water stress (Liu et al., 2015a), heat stress (Zhang et al., 2017a), and heavy metal stress (Jahan et al., 2020). These studies have evinced the notable role of ME in enhancing tolerance in plants to several environmental stresses.
In the world, tomato is an important crop and it is the most-cultivated vegetable in the solanaceous family. Like other crops, tomato is greatly affected by SS and inhibits its physiological and biochemical processes (Benazzouk et al., 2018). Hence, the physiological and biochemical processes of tomato plants are necessary to understand to enhance SS tolerance by adapting various cultural practices. Therefore, to establish the role of ME in the tolerance of tomato plants to salinity, the present study was conducted to evaluate: (1) the role of ME on plant growth, root architecture, ion homeostasis and photosynthesis of tomato under SS (2) the function of ME in the antioxidant system under SS, and (3) the collected information for the provision of a comprehensive method for enhancing tomato growth under SS.
Section snippets
Tomato seeds and reagents
The seeds of tomato cultivar Shidun 197 were purchased from a seed store in Shouguang Minghao Seed Industry Co., Ltd. China. Melatonin (N-acetyl-5-methoxytryptamine) and NaCl were purchased from Hainan View Kr Biotech. Co., Ltd. (Xilong Scientific Co., Ltd) Haikou, Hainan, China.
Plant material, growth conditions, and treatments
To achieve the above-mentioned objectives, this study was carried out under controlled conditions, Hainan University, Haikou, Hainan, China, with a day/night temperature of 24/13 °C, relative humidity between 65 and
Exogenous ME' effects on plant growth
To evaluate the effects of exogenous ME application on SS in tomato cultivar, the nutrient solution with supplementary ME was applied to roots for 3 days before salt treatment. As shown in results (Fig. 1), a clear difference is visualized among tomato seedlings under SS, with and without ME pretreatment. Further, no difference was seen in seedlings without ME pretreatment (control) and seedlings treated with 100 µM ME under no stress condition.
As shown in (Fig. 2E), the plant height was
Discussion
Melatonin is a pleiotropic molecule acting as an amphiphilic agent; thus it passes quickly through the cell membrane into subcellular compartments. Being an innovative growth regulator, this molecule poses characteristics of environmental protection with remarkable outcomes. Melatonin is a natural agent (Siddiqui et al., 2019) broadly utilized for promoting growth and development of plants under SS (Zhou et al., 2016). Our results stipulated that growth and biomass production of tomato
Conclusion and future perspectives
As a well-known agent, ME has several essential roles in plants. In a current study found out that the growth of tomato seedlings was severely affected by NaCl supplied to tomato seedlings, while ME pretreatment of tomato seedlings significantly enhanced NaCl stress tolerance. Melatonin pretreatment ameliorated the growth and biomass production, improved the activity of enzymatic and non-enzymatic antioxidant enzymes, reduced H2O2, O2−•, EL, and MDA content, promote root architecture, enhanced
Contribution
Muhammad Ahsan Altaf: Conceptualization, Methodology, Formal analysis, Investigation, Writing - original draft, Software. Rabia Shahid: Writing - review & editing. Ming-Xun Ren:, Conceptualization, Visualization, Supervision, Funding acquisition, Project administration, Resources, Writing - review & editing. Muhammad Mohsin Altaf: Formal analysis. Latif Ullah Khan: Formal analysis. Sidra Shahid: Writing - review & editing. Mohammad Shah Jahan: Writing - review & editing.
Declaration of Competing Interest
All authors declare that they have no conflict of interest
Acknowledgement
We would like to thank Wang Zhiwei from the College of Horticulture and Landscape Architecture, Hainan University, Haikou for his kind guidance and laboratory equipment. This program was financially supported by the Innovative Team Program of Hainan Natural Science Foundation (2018CXTD334), National Natural Science Foundation of China (41871041).
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