The role of melatonin in plant growth and metabolism, and its interplay with nitric oxide and auxin in plants under different types of abiotic stress

Abstract

Melatonin is a pleiotropic signaling molecule that reduces the adverse effects of abiotic stresses, and enhances the growth and physiological function of many plant species. Several recent studies have demonstrated the pivotal role of melatonin in plant functions, specifically its regulation of crop growth and yield. However, a comprehensive understanding of melatonin, which regulates crop growth and yield under abiotic stress conditions, is not yet available. This review focuses on the progress of research on the biosynthesis, distribution, and metabolism of melatonin, and its multiple complex functions in plants and its role in the mechanisms of metabolism regulation in plants grown under abiotic stresses. In this review, we focused on the pivotal role of melatonin in the enhancement of plant growth and regulation of crop yield, and elucidated its interactions with nitric oxide (NO) and auxin (IAA, indole-3-acetic acid) when plants are grown under various abiotic stresses. The present review revealed that the endogenousapplication of melatonin to plants, and its interactions with NO and IAA, enhanced plant growth and yield under various abiotic stresses. The interaction of melatonin with NO regulated plant morphophysiological and biochemical activities, mediated by the G protein-coupled receptor and synthesis genes. The interaction of melatonin with IAA enhanced plant growth and physiological function by increasing the levels of IAA, synthesis, and polar transport. Our aim was to provide a comprehensive review of the performance of melatonin under various abiotic stresses, and, therefore, further explicate the mechanisms that plant hormones use to regulate plant growth and yield under abiotic stresses.

Keywords: abiotic stresses; auxin; nitric oxide; phytomelatonin; plant growth and metabolism.

Figure 1

The regulatory role of biosynthetic melatonin under stress conditions. Tryptophan is the initial substrate of melatonin synthesis and is divided into four enzymatic steps catalyzed by six enzymes: tryptophan decarboxylase (TDC), tryptophan hydroxylase (TPH), tryptamine 5‐hydroxylase (T5H), serotonin N‐acetyltransferase (SNAT), N‐acetylserotonin methyltransferase (ASMT), and caffeic acid O‐methyl transferase (COMT). Serotonin is catalyzed via SNAT to form N-acetylserotonin, which is further methoxylated by ASMTs to form melatonin and acts as a growth regulator stress defense molecule. The T5H gene improves serotonin biosynthesis. TDC catalyzes decarboxylation of tryptophan, and it is considered the first step of melatonin biosynthesis. Suppression of the T5H gene increases the concentration of melatonin in rice plants and increases yield. Auxin produced naturally in plants is biosynthesized from tryptophan in four ways: indole-3-acetaldoxime (IAOx), indole-3-pyruvic acid (IPyA), indole-3-acetamide (IAM), and tryptamine (TAM).

Figure 2

The distribution and regulatory roles of an exogenous supply of melatonin in mitigating abiotic stresses are divided into three parts. (i) Melatonin distributed in the roots and seeds. Sodium chloride (NaCl) induces slow mobilization of enzymes and reduces the enzymes’ activity in the oily body of plants in seedlings and roots and, as a result, alters salt stress. The supply of exogenous melatonin enhances the level of plant antioxidants and enzyme activities. (ii) Salt stress reduces chlorophyll content, but exogenous melatonin improves chlorophyll content, chloroplast gene expression, and protein content, and, as a result, enhances photosynthesis activity and chlorophyll accumulation. In addition, it enhances plant Pn and CE and, as a result, inhibits chlorophyll degradation. The improvement of all these traits enhances plant growth and metabolism. (iii) Drought stress causes ROS in plants, which induces Ch degradation and Ch reduction. The application of endogenous melatonin during drought reduces ROS and O₂ ^– content and, as a result, increases chlorophyll content and plant growth physiological function. CE, carboxylation efficiency; Ch, chlorophyll; Pn, net photosynthetic rate; ROS, reactive oxygen species.

Figure 3

The interactive role of melatonin with nitric oxide (NO) in mitigating abiotic stress. The application of melatonin in tomato plants enhances NO content. NO further triggers antioxidant enzymes activity in plants, which shows resistance to abiotic stress and enhances root growth, iron deficiency, aging, and the expression of synthesis genes. In addition, G protein as a melatonin receptor enhances root growth, iron deficiency, aging, and the expression of synthesis genes mediates hydrogen peroxide (H₂O₂) signaling transduction that is involved in a melatonin-induced stomatal closure in Arabidopsis.

Figure 4

The interactive role of melatonin with auxin (IAA, indole-3-acetic acid) in mitigating the effects of drought stress. The application of melatonin increases IAA content in plants. Melatonin and IAA promote plant photosynthetic activity in the same way. When plants are under drought stress, melatonin interacts with IAA at the maturity stage, and increases plant growth and yield.