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.
Highlights
Botany Department, Faculty of Science, Suez Canal University, Ismailia 41522, Egypt; Department of Agronomy, Faculty of Agriculture, Sher-e-Bangla Agricultural University, Dhaka 1207, Tel.: +86-020-3288-5970 (S.A.)
In the last few years, the role of melatonin as a multifunctional regulator of plant status under drought conditions, including (i) anatomical and (ii) physiological aspects, have been progressively studied and, notably, reached more than 42 studies within seven years (Table 1). (i) The anatomical changes are induced by melatonin within the different plant organs, including less cell membrane damage [63], more intact grana lamella of the chloroplast [45], alleviation of chloroplast ultrastructural damage and preservation of its system [33,61], safeguarding of the mitochondrial structure [67], maintenance of cell expansion [35], better leaf thickness, spongy tissue, and stomata size [33,35], cuticle formation [74], and wax accumulation [39]. (ii) By increasing drought severity, melatonin, which is biosynthesized in mitochondria and chloroplasts [75,76], exhibits more defense strategies
The question that still needs to be answered is whether the crosstalk of melatonin, nitric oxide, and hydrogen sulfide under water stress is similar to the situation under other environmental stresses or if they have a unique interaction in each situation
Summary
Despite the importance of melatonin in mitigating the harmful effects of abiotic stresses, the research on melatonin-induced waterlogging tolerance has only recently started to emerge (Table 2). The authors indicated that melatonin has a great ability to eliminate ROS, alleviate oxidative damage, resist waterlogging, and, revert losses in yield and quality [117]. After this ground-breaking work, Zheng et al [118] elucidated that melatonin is an effective phytohormone to protect apple plants under waterlogging stress. Zhang et al [119] investigated the impact of melatonin pretreatment on alfalfa under waterlogging stress and indicated that melatonin could alleviate the stress damage and enhance plant growth, chlorophyll content, and PSII efficiency. Melatonin treatment increased polyamine (putrescine, spermidine, and spermine) levels and decreased ethylene under stress, which are controlled via changes in gene expression [119]
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