Melatonin-Induced Water Stress Tolerance in Plants: Recent Advances

Abstract

Water stress (drought and waterlogging) is severe abiotic stress to plant growth and development. Melatonin, a bioactive plant hormone, has been widely tested in drought situations in diverse plant species, while few studies on the role of melatonin in waterlogging stress conditions have been published. In the current review, we analyze the biostimulatory functions of melatonin on plants under both drought and waterlogging stresses. Melatonin controls the levels of reactive oxygen and nitrogen species and positively changes the molecular defense to improve plant tolerance against water stress. Moreover, the crosstalk of melatonin and other phytohormones is a key element of plant survival under drought stress, while this relationship needs further investigation under waterlogging stress. In this review, we draw the complete story of water stress on both sides-drought and waterlogging-through discussing the previous critical studies under both conditions. Moreover, we suggest several research directions, especially for waterlogging, which remains a big and vague piece of the melatonin and water stress puzzle.

Keywords: abiotic stress; antioxidants; drought; melatonin; phytohormones; stress signaling; water stress; waterlogging.

Figure 1

A schematic model explaining the mechanism underlying the melatonin-mediated drought stress response. At the cellular level, a stress signal from the cell membrane is received by the nucleus, which starts to activate the melatonin biosynthesis pathway from its precursor, tryptophan, in mitochondria and chloroplasts by upregulating the melatonin-biosynthesis genes. Melatonin sends its feedback on such stress to the nucleus to activate omics regulation. Consequently, the genes encoding the proteins related to plant anatomical, physiological, and biochemical responses are regulated directly and/or indirectly via a simultaneous defense network. The omics-mediated responses include photosynthesis, biosynthesis, enzymatic and nonenzymatic antioxidants, photoprotection, cell membrane stability, ROS and oxidative damage, osmoprotection, water status, and leaf senescence, in addition to the anatomical changes, which lead to drought tolerance. Consequently, the whole plant status is enhanced, including growth and development, flowering, yield, quality, and survival rate, while the toxic substances are decreased.

Figure 2

A schematic model explaining the effect of melatonin on other phytohormones under drought stress: Under drought, melatonin enhances the levels of brassinosteroids (BRs), cytokinins (CKs), gibberellins (GAs), and jasmonates (JAs) and decreases the abscisic acid (ABA) level and auxins. Eth, ethylene; ABA, abscisic acid; BRs, brassinosteroids; CKs, cytokinins; SA, salicylic acid; GAs, gibberellins; JA, jasmonic acid; SLs, strigolactones. Red connectors, not studied; green connectors, reduced; black connectors, enhanced; blue connectors, nonsignificant effect. ↑, upregulated; ↓, downregulated.

Figure 3

Schematic model explaining the protective mechanisms of melatonin in waterlogging tolerance. The solid arrows indicate stimulation, while the dashes indicate inhibition. ▲ and ▼ shapes indicate enhanced or decreased levels, respectively. Waterlogging induces ethylene, melatonin (2- to 5-fold), polyamines (PAs), and reactive oxygen species (ROS). Melatonin is also induced in response to ROS generation and exogenous melatonin. Melatonin stimulates PA biosynthesis, photosynthesis, and membrane stability, while it inhibits ethylene biosynthesis, growth reduction, leaf senescence, ROS, and oxidative damage. Excessive ROS causes oxidative damage leading to anaerobic respiration, which is scavenged by antioxidant enzymes. Additionally, growth reduction and leaf senescence are increased by ethylene, while they are decreased by PAs. Moreover, photosynthesis and membrane stability is enhanced by PAs, while they are reduced by ethylene induction and oxidative damage. The role of melatonin in waterlogging tolerance still needs further study. This figure is a combination of the two published mechanisms of Zheng et al. [118] and Zhang et al. [119], with some modifications.