Melatonin Alleviates Drought Stress by a Non-Enzymatic and Enzymatic Antioxidative System in Kiwifruit Seedlings

Crosstalk between ethylene and melatonin activates isoflavone biosynthesis and antioxidant systems to produce high-quality soybean sprouts

Isoflavone, which are mainly found in soybeans, are a secondary metabolite with a variety of physiological functions. In recent years, increasing the isoflavone content of soybeans has received widespread attention. Although ethephon treatment significantly increased isoflavone content in soybean sprouts, it also had a certain inhibitory effect on the growth of sprouts. Melatonin (MT), as a new type of plant hormone, not only alleviated the damage caused by abiotic stress to plants, but also promoted the synthesis of secondary metabolites. In this study, we aimed to elucidate the mechanism of exogenous MT in regulating the growth and development, and the metabolism of isoflavone in soybean sprouts under ethephon treatment. The results indicated that MT alleviated the adverse effects of ethephon treatment on soybean sprouts by increasing the activities of superoxide dismutase, peroxidase, catalase, and the expression of their corresponding genes, as well as decreased the content of malondialdehyde and hydrogen peroxide. In addition, MT further increased the isoflavone content by up-regulating the expression level of isoflavone synthesis genes and increased the activities of phenylalanine ammonia-lyase and cinnamic acid 4-hydroxylase under ethephon treatment. This study provided technical support and reference value for the production of high-quality soybean sprouts to a certain extent.

Exogenous melatonin alleviates sodium chloride stress and increases vegetative growth in Lonicera japonica seedlings via gene regulation

Melatonin (Mt) functions as a growth regulator and multifunctional signaling molecule in plants, thereby playing a crucial role in promoting growth and orchestrating protective responses to various abiotic stresses. However, the mechanism whereby exogenous Mt protects Lonicera japonica Thunb. (L. japonica) against salt stress has not been fully elucidated. Therefore, this study aimed to elucidate how exogenous Mt alleviates sodium chloride (NaCl) stress in L. japonica seedlings. Salt-sensitive L. japonica seedlings were treated with an aqueous solution containing 150 mM of NaCl and aqueous solutions containing various concentrations of Mt. The results revealed that treatment of NaCl-stressed L. japonica seedlings with a 60 µM aqueous solution of Mt significantly enhanced vegetative plant growth by scavenging reactive oxygen species and thus reducing oxidative stress. The latter was evidenced by decreases in electrical conductivity and malondialdehyde (MDA) concentrations. Moreover, Mt treatment led to increases in the NaCl-stressed L. japonica seedlings' total chlorophyll content, soluble sugar content, and flavonoid content, demonstrating that Mt treatment improved the seedlings' tolerance of NaCl stress. This was also indicated by the NaCl-stressed L. japonica seedlings exhibiting marked increases in the activities of antioxidant enzymes (superoxide dismutase, peroxidase, catalase, and ascorbate peroxidase) and in photosynthetic functions. Furthermore, Mt treatment of NaCl-stressed L. japonica seedlings increased their expression of phenylalanine ammonia-lyase 1 (PAL1), phenylalanine ammonia-lyase 2 (PAL2), calcium-dependent protein kinase (CPK), cinnamyl alcohol dehydrogenase (CAD), flavanol synthase (FLS), and chalcone synthase (CHS). In conclusion, our results demonstrate that treatment of L. japonica seedlings with a 60 µM aqueous solution of Mt significantly ameliorated the detrimental effects of NaCl stress in the seedlings. Therefore, such treatment has substantial potential for use in safeguarding medicinal plant crops against severe salinity.

Enterobacter ludwigii b3 in the rhizosphere of wild rice assists cultivated rice in mitigating drought stress by direct and indirect methods

Drought is the primary factor limiting rice production in ecosystems. Wild rice rhizosphere bacteria possess the potential to assist in the stress resistance of cultivated rice. This study examines the impact of wild rice rhizosphere bacteria on cultivated rice under drought conditions. From the rhizosphere soil of wild rice, 20 potential drought-resistant strains were isolated. Subsequent to the screening, the most effective strain b3, was identified as Enterobacter ludwigii. Pot experiments were conducted on the cultivated Changbai 9 rice. It was found that inoculation with the E. ludwigii b3 strain improved the drought resistance of the rice, promotion of rice growth (shoot height increased by 13.47 %), increased chlorophyll content (chlorophyll a, chlorophyll b and carotenoid increased by 168.74 %, 130.68 % and 87.89 %), improved antioxidant system (content of glutathione was increased by 60.35 %), and accumulation of osmotic regulation substances (soluble sugar and soluble protein increased by 70.36 % and 142.03 %). Furthermore, E. ludwigii b3 had a transformative effect on the rhizosphere bacterial community of cultivated rice, increasing its abundance and diversity while simultaneously recruiting beneficial rhizosphere bacteria, resulting in a more complex community. Additionally, E. ludwigii b3 acted directly and indirectly on cultivated rice through its metabolites (organic acids, amino acids, flavonoids and other substances), which helped alleviate drought stress. In conclusion, the E. ludwigii b3 shows promise as a drought-resistant strain and has the potential to improve the growth and productivity of cultivated rice in arid agricultural ecosystems. This study represents the first investigation of E. ludwigii in the rhizosphere of wild rice under drought conditions on cultivated rice.

Synergistic effects of melatonin and 24-epibrassinolide on chickpea water deficit tolerance

BACKGROUND

Water deficiency stress reduces yield in grain legumes, primarily due to a decrease in the pods number. Melatonin (ML) and 24-epibrassinolide (EBL) are recognized for their hormone-like properties that improve plant tolerance to abiotic stresses. This study aimed to assess the impact of different concentrations of ML (0, 100, and 200 µM) and EBL (0, 3, and 6 µM) on the growth, biochemical, and physiological characteristics of chickpea plants under water-stressed conditions.

RESULTS

The study's findings indicated that under water-stressed conditions, a decrease in seed (30%) and pod numbers (31%), 100-seed weight (17%), total chlorophyll content (46%), stomatal conductance (33%), as well as an increase in H2O2 (62%), malondialdehyde content (40%), and electrolyte leakage index (40%), resulted in a 40% reduction in chickpea plants grain yield. Our findings confirmed that under water-stressed conditions, seed oil, seed oil yield, and seed protein yield dropped by 20%, 55%, and 36%, respectively. The concurrent exogenous application of ML and EBL significantly reduces oxidative stress, plasma membrane damage, and reactive oxygen species (ROS) content. This treatment also leads to increased yield and its components, higher pigment content, enhanced oil and protein yield, and improved enzymatic and non-enzymatic antioxidant activities such as catalase, superoxide dismutase, polyphenol oxidase, ascorbate peroxidase, guaiacol peroxidase, flavonoid, and carotenoid. Furthermore, it promotes the accumulation of osmoprotectants such as proline, total soluble protein, and sugars.

CONCLUSIONS

Our study found that ML and EBL act synergistically to regulate plant growth, photosynthesis, osmoprotectants accumulation, antioxidant defense systems, and maintain ROS homeostasis, thereby mitigating the adverse effects of water deficit conditions. ML and EBL are key regulatory network components in stressful conditions, with significant potential for future research and practical applications. The regulation metabolic pathways of ML and EBL in water-stressed remains unknown. As a result, future research should aim to elucidate the molecular mechanisms by employing genome editing, RNA sequencing, microarray, transcriptomic, proteomic, and metabolomic analyses to identify the mechanisms involved in plant responses to exogenous ML and EBL under water deficit conditions. Furthermore, the economical applications of synthetic ML and EBL could be an interesting strategy for improving plant tolerance.

Melatonin alleviates UV-B stress and enhances phenolic biosynthesis in rosemary (Rosmarinus officinalis) callus

Although used in in vitro culture to boost secondary metabolite production, UV-B radiation can seriously affect plant growth if not properly dosed. Rosemary callus can be used as an important source of effective ingredients in the food and medicine industry. To balance the positive and negative effects of UV-B on rosmary callus, this study investigated the effects of melatonin on rosemary callus under UV-B radiation. The results showed that melatonin improved rosemary callus growth, with fresh weight and dry weight increased by 15.81% and 8.30%, respectively. The addition of 100 μM melatonin increased antioxidant enzyme activity and NO content in rosemary callus. At the same time, melatonin also significantly reduced membrane lipid damage and H2O2 accumulation in rosemary callus under UV-B stress, with malondialdehyde (MDA) and H2O2 contents reduced by 13.03% and 14.55%, respectively. In addition, melatonin increased the total phenol and rosmarinic acid contents in rosemary callus by 19% and 54%, respectively. Melatonin significantly improved the antioxidant activity of the extracts from rosemary callus. These results suggest that exogenous melatonin can alleviate the adverse effects of UV-B stress on rosemary callus by promoting NO accumulation while further enhancing phenolic accumulation and biological activity.

Differential responses of two fenugreek (Trigonella foenum-graecum L.) landraces pretreated with melatonin to prolonged drought stress and subsequent recovery

BACKGROUND

Drought impairs growth, disturbs photosynthesis, and induces senescence in plants, which results in crop productivity reduction and ultimately jeopardizes human food security. The objective of this study was to determine major parameters associated with drought tolerance and recovery ability of fenugreek (Trigonella foenum-graecum L.), by examining differential biochemical and phenological responses and underlying enzyme activities as well as melatonin roles during drought stress and re-watering for two contrasting landraces. Moreover, the relative expression of three key genes involved in the biosynthesis pathway of diosgenin, including SQS, CAS, and BG, was investigated.

RESULTS

Depending on the conditions, drought stress enhanced the activity of antioxidant enzymes and the osmoregulating compounds, non-enzymatic antioxidants, hydrogen peroxide content, and lipid peroxidation levels in most cases. Severe drought stress accelerated flowering time in Shushtar landrace (SHR) but had no significant effects on Varamin (VR). Pretreatment with melatonin delayed flowering time in SHR and caused high drought resistance in this landrace. Furthermore, melatonin significantly enhanced drought adaptability in VR by improving plant recovery ability.

DISCUSSION

Based on our results plants' responses to drought stress and melatonin pretreatment were completely landrace-specific. Drought stress caused an increase in the relative expression of CAS gene and ultimately the accumulation of steroidal saponins in SHR. Melatonin compensated for the decrease in biomass production due to drought stress and finally increased steroidal saponins performance in SHR. Our study showed that melatonin can improve drought stress and recovery in fenugreek, but different factors such as genotype, melatonin concentration, and plant age should be considered.

Melatonin modulates the tolerance of plants to water stress: morphological response of the molecular mechanism

Water stress (drought and waterlogging) leads to an imbalance in plant water distribution, disrupts cell homeostasis, and severely inhibits plant growth. Melatonin is a growth hormone that plants synthesise and has been shown to resist adversity in many plants. This review discusses the biosynthesis and metabolism of melatonin, as well as the changes in plant morphology and physiological mechanisms caused by the molecular defence process. Melatonin induces the expression of related genes in the process of plant photosynthesis under stress and protects the structural integrity of chloroplasts. Exogenous melatonin can maintain the dynamic balance of root ion exchange under waterlogging stress. Melatonin can repair mitochondria and alleviate damage caused by reactive oxygen species and reactive nitrogen species; and has a wide range of uses in the regulation of stress-specific genes and the activation of antioxidant enzyme genes. Melatonin improves the stability of membrane lipids in plant cells and maintains osmotic balance by regulating water channels. There is crosstalk between melatonin and other hormones, which jointly improve the ability of the root system to absorb water and breathe and promote plant growth. Briefly, as a multifunctional molecule, melatonin improves the tolerance of plants under water stress and promotes plant growth and development.

Pure biochemicals and nanomaterials as next generation biostimulants for sustainable agriculture under abiotic stress - recent advances and future scope

Sustainable agriculture faces major challenges under abiotic stress conditions owing to extensive application of chemical fertilizers which pollute water, soil and atmosphere. Biostimulants (BSs), comprising of naturally derived complex mixtures of uncharacterized biomolecules, pure biochemicals and nanomaterials, enhance nutrient use efficiency (NUE) and trigger crop's natural defense mechanisms. While it is difficult to specify the metabolic effects of uncharacterized natural mixtures (seaweed extract, protein hydrolyzates, etc.), exogenous application of pure biochemicals and nanomaterials offers an edge as BSs since their physiological roles and mechanisms of action are decipherable. Foliar application or seed treatment of some amino acids, polyamines and biopolymers (chitosan, lipochitin oligosaccharides and thuricin 17) enable plants to overcome drought and salinity stress via activation of mechanisms for reactive oxygen species (ROS) scavenging, osmolyte regulation and chlorophyll accumulation. Interaction of nitric oxide (NO) with some vitamins and melatonin exhibits potential significance as BSs for mitigating stress by ROS scavenging and maintenance of intracellular ionic balance and membrane integrity. Near future is likely to see wide applications of nanoparticles (NPs) and nanomaterials (NMs) as BSs in view of their biphasic mode of action (bio-physical activation of membrane receptors followed by gradual release of BS into the plant cells).

Melatonin enhances drought tolerance by affecting jasmonic acid and lignin biosynthesis in wheat (Triticum aestivum L.)

Drought severely affects the yield of wheat (Triticum aestivum L.), which is mainly grown in arid and semi-arid regions. Melatonin plays an important role in various types of stress resistance in plants, including drought resistance. However, the molecular mechanism through which melatonin affects drought tolerance remains largely unknown. In this study, we revealed that melatonin (100 μM) significantly improved drought resistance during the maturation stage of Chinese Spring, Shi4185, and Hanxuan10 varieties, but not Chang6878. Further physiological, transcriptomic, and proteomic data analysis at the wheat seedling stage revealed that melatonin increased jasmonic acid (JA) content, upregulating the expression of JA genes (LOX1.5 and LOX2.1) and two transcription factors (HY5 and MYB86) under drought conditions. It also upregulated genes related to lignin biosynthesis (4CL2, P5CS1, and CCR2) as well as starch and sucrose metabolism (PME53 and SUS4). Additionally, melatonin alleviated photosynthetic and cell membrane damage caused by drought stress through maintaining low levels of hydrogen peroxide. The current results elucidate melatonin-regulated pathways in wheat and provide evidence for using melatonin as a potential biostimulant to improve wheat drought resistance under field conditions in the future.

Exogenous melatonin enhances low-temperature stress of jute seedlings through modulation of photosynthesis and antioxidant potential

Developing new varieties of natural fibers that can grow throughout the year is very crucial to replace and avoid the bad effect of synthetic fiber. As a result of its beneficial role in protecting plants from abiotic stressors, melatonin (N-acetyl-5-methoxytryptamine) has gained recognition as a novel plant growth regulator. This study aimed to investigates the role of exogenous melatonin (200 μM) on two varieties of Corchorous olitorius and Corchorous capsularis in response to low-temperature stress (8 °C) for different periods of treatment (0, 24, 36, and 48 h) based on biochemical properties, and antioxidant system. The results demonstrated that exogenous melatonin had inhibitory effects of low-temperature stress on seedlings at different period of treatment when compared to non-melatonin treated seedlings, potentially improved photosynthetic apparatus (total chlorophyll up to 29.93 and 33.37%; total carotenoid up to 29.93 and 19.05%; anthocyanin up to 40.47 and 31.94% in M33 and Y49, respectively), reduced oxidative damage (MDA up to 53.59 and 44.28%; H2O2 up to 41.04 and 16.88% in M33 and Y49, respectively) by boosting the antioxidant enzymes (SOD up to 12.75 and 4.65%; POD up to 39.08 and 81.39%; total phenolic up to 43.38 and 56.48% in M33 and Y49, respectively) reduced electrolyte leakage (EL) up to 15.37 and 13.64% in M33 and Y49, respectively) and increased osmoregulation (soluble sugars up to 25.86 and 25.86%; proline up to 105.19 and 172.07%; FAA up to 48.50 and 30.06% in M33 and Y49, respectively) content. Thus, this study showed that exogenous melatonin effectively mitigated the low-temperature-induced oxidative in C. olitorius and C. capsularis seedlings by regulating the antioxidant system and improving the low-temperature resistance.

Melatonin enhances biomass, phenolic accumulation, and bioactivities of rosemary (Rosmarinus officinalis) in vitro shoots under UV-B stress

Melatonin is a stress-related hormone that plays a critical role in triggering the plant defence system and regulating secondary metabolism when plants are exposed to stress. To explore the potential roles of melatonin in response to Ultraviolet-B (UV-B) radiation, we examined the effects of exogenous melatonin on rosemary in vitro shoots under UV-B stress. The application of melatonin (50 μM) alleviated the adverse effects of UV-B stress on the biomass, photosynthetic pigment contents, and membrane lipids of the rosemary in vitro shoots. Melatonin significantly increased superoxide dismutase (1.15.1.1, SOD), peroxidase (1.11.1.7, POD), and catalase (1.11.1.6, CAT) activities by 62%, 99%, and 53%, respectively. The contents of total phenols, rosmarinic acid, and carnosic acid increased under UV-B stress, and they further increased by the melatonin treatment by 41%, 68%, and 67%, respectively, compared with the control group. Under UV-B stress, the increased total phenol content in melatonin-pretreated plants could be attributed to the activation of phenylalanine ammonia-lyase (4.3.1.5, PAL) and tyrosine aminotransferase (2.6.1.5, TAT). In addition, melatonin enhanced the antioxidant and antibacterial activities of the rosemary in vitro shoots under UV-B stress. These results suggest that melatonin can alleviate the damage caused by UV-B stress and also enhance the secondary metabolism and bioactivity of rosemary in vitro shoots.

Alleviation of cadmium-induced photoinhibition and oxidative stress by melatonin in Chlamydomonas reinhardtii

As one of the most threatening challenges to the natural environment and human health, cadmium (Cd) pollution has seriously impacted natural organisms. Green algae, such as Chlamydomonas reinhardtii (C. reinhardtii), can provide a safer, lower cost, and more effective ecological approach to the treatment of heavy metal ions in wastewater due to their sorption properties. However, heavy metal ions affect C. reinhardtii when adsorbed. Melatonin is able to protect the plant body from damage when the plant is under biotic/abiotic stress. Therefore, we investigated the effects of melatonin on the cell morphology, chlorophyll content, chlorophyll fluorescence parameters, enzymatic activity of the antioxidant system, gene expression, and the ascorbic acid (AsA)-glutathione (GSH) cycle of C. reinhardtii under the stress of Cd (13 mg/L). Our results indicated that Cd significantly induced photoinhibition and overaccumulation of reactive oxygen species (ROS). By application with the concentration of 1.0 μM melatonin, the algal solute of C. reinhardtii under the Cd stress gradually regained its green color, the cell morphology became intact, and the photosynthetic electron transport function was retained. However, in the melatonin-silenced strain, there was a significant decrease in all of the above indicators. In addition, the use of exogenous melatonin or the expression of endogenous melatonin genes could enhance the intracellular enzyme activities of catalase (CAT), peroxidase (POD), superoxide dismutase (SOD), ascorbate peroxidase (APX), and glutathione reductase (GR). It also upregulated the expression of active enzyme genes such as SOD1, CAT1, FSD1, GSH1, GPX5, and GSHR1. These results indicate that the presence of melatonin effectively protects the activity of photosynthetic system II in C. reinhardtii, enhances antioxidant activity, upregulates gene expression in the AsA-GSH cycle, and reduces the level of ROS, thereby alleviating the damage caused by Cd toxicity.

Melatonin: Current status and future perspectives in horticultural plants

Global warming in this century increases incidences of various abiotic stresses, restricting plant growth and productivity and posing a severe threat to global food production and security. Different phytohormones are produced by plants to mitigate the adverse effects of these stresses. One such phytohormone is melatonin (MEL), which, being a potential bio-stimulator, helps to govern a wide array of functions in horticultural crops. Recent advancements have determined the role of MEL in plants' responses to abiotic stresses. MEL enhances physiological functions such as seed germination, growth and development, seedling growth, root system architecture, and photosynthetic efficiency. The potential function of MEL in stressful environments is to regulate the enzymatic and non-enzymatic antioxidant activity, thus playing a role in the substantial scavenging of reactive oxygen species (ROS). Additionally, MEL, as a plant growth regulator and bio-stimulator, aids in promoting plant tolerance to abiotic stress, mainly through improvements in nutrient uptake, osmolyte production, and cellular membrane stability. This review, therefore, focuses on the possible functions of MEL in the induction of different abiotic stresses in horticultural crops. Therefore, this review would help readers learn more about MEL in altered environments and provide new suggestions on how this knowledge could be used to develop stress tolerance.

Exogenous melatonin enhances Cd stress tolerance in Platycladus orientalis seedlings by improving mineral nutrient uptake and oxidative stress

The development of agriculture and industry has led to a gradual increase in the levels of cadmium (Cd) in the soil, which, due to its high mobility in soil, makes Cd deposition in plants a serious threat to the health of animals and humans. The important role of melatonin (MT) in regulating plant growth and adaptation to environmental stress has become a pertinent research topic, but the mechanisms of action of MT in Cd-stressed Platycladus orientalis seedlings are unclear. Here, we investigated the mitigation mechanism of exogenous MT application on P. orientalis seedlings under Cd stress. Cd stress significantly inhibited the growth of P. orientalis seedlings by disrupting photosynthetic pigments, mineral balance, osmotic balance, and oxidative balance. In contrast, the application of exogenous MT significantly increased the growth parameters of P. orientalis seedlings, reduced Cd accumulation and transfer in the seedlings, increased the content of iron, manganese, zinc, copper, chlorophyll, soluble protein, soluble sugar, and proline, reduced the content of glutathione, increased the activities of superoxide dismutase and peroxidase, and significantly enhanced the expression of antioxidant-related genes (POD, GST, and APX). It also effectively reduced the content of hydrogen peroxide and malondialdehyde to inhibit the production of reactive oxygen species, thus alleviating Cd-induced oxidative stress. In addition, MT significantly upregulated the expression of the ethanol dehydrogenase (ADH) gene, which is effective in removing the acetaldehyde produced by anaerobic respiration in seedlings under stress, thereby reducing the toxic effects on P. orientalis. The results showed that exogenous MT enhanced the tolerance of P. orientalis seedlings to Cd stress by regulating photosynthesis, mineral balance, osmotic balance, and the antioxidant system and that the optimal concentration of MT was 200 μmol·L^-1.

Exogenous application of melatonin to mitigate drought stress-induced oxidative damage in Phoebe sheareri seedlings

Background

Drought stress is a major prevalent environmental factor impairing growth. Melatonin mitigates the impacts of drought stress on plants. However, melatonin's role in Phoebe sheareri (Hemsl.) Gamble (P. sheareri) is unknown. We aimed to reveal the protective effects of melatonin on P. sheareri seedlings under drought conditions.

Methods

Melatonin was sprayed under drought or normal water conditions. The parameters, including growth, physiological factors, and phytohormones of P. sheareri, were examined.

Results

Compared to the normal control group, drought stress inhibited the growth of seedlings and significantly reduced the content of carotenoids, SOD, POD, APX, PPO, CAT, GR, and soluble sugars, and increased the contents of MDA, O₂ ^•-, proline, soluble proteins, ABA, and JA-Me in P. sheareri seedlings. However, melatonin treatment significantly reversed the adverse drought-induced responses and promoted the P. sheareri seedling's growth. Moreover, the heatmap and principal component analysis suggested a high similarity in the behavior patterns of the six measured antioxidant enzymes in P. sheareri seedlings.

Conclusion

Our study reported for the first time that melatonin has a protective role in P. sheareri seedlings under drought-stress conditions. This role is related to ROS scavenging, activation of antioxidant enzymes, and crosstalk of phytohormones. This study provided a theoretical basis for improving the ability of P. sheareri adapted to arid environments.

Exogenous treatment with melatonin enhances waterlogging tolerance of kiwifruit plants

Waterlogging stress has an enormous negative impact on the kiwifruit yield and quality. The protective role of exogenous melatonin on water stress has been widely studied, especially in drought stress. However, the research on melatonin-induced waterlogging tolerance is scarce. Here, we found that treatment with exogenous melatonin could effectively alleviate the damage on kiwifruit plants in response to waterlogging treatment. This was accompanied by higher antioxidant activity and lower ROS accumulation in kiwifruit roots during stress period. The detection of changes in amino acid levels of kiwifruit roots during waterlogging stress showed a possible interaction between melatonin and amino acid metabolism, which promoted the tolerance of kiwifruit plants to waterlogging. The higher levels of GABA and Pro in the roots of melatonin-treated kiwifruit plants partly contributed to their improved waterlogging tolerance. In addition, some plant hormones were also involved in the melatonin-mediated waterlogging tolerance, such as the enhancement of ACC accumulation. This study discussed the melatonin-mediated water stress tolerance of plants from the perspective of amino acid metabolism for the first time.

Melatonin and arbuscular mycorrhizal fungi synergistically improve drought toleration in kiwifruit seedlings by increasing mycorrhizal colonization and nutrient uptake

Kiwifruit is a vine fruit tree that is vulnerable to water deficiency due to its shallow root system and large leaves. Although mycorrhizal inoculation and melatonin application has been proved to improve plants drought tolerance, their interaction effects are still unclear. In this study, arbuscular mycorrhizal (AM) fungi incubation and melatonin (MT) irrigation were applied to kiwifruit seedlings alone or in combination to investigate their effect on drought tolerance. The results revealed that AM had more effect on promoting root biomass, water use efficiency, and uptake of nitrogen, phosphorus and iron. While MT was more effective in promoting shoot biomass and antioxidant enzyme activities to remove reactive oxygen species accumulation. Moreover, MT supplementary significantly increased the AM colonization, spore density and hyphal length density in roots. Therefore, combined application of AM fungi and MT had additive effects on improvement biomass accumulation, increasing chlorophyll content, photosynthetic efficiency, catalase activity, and decreasing malondialdehyde accumulation under drought stress, thus promoting plant growth and alleviating the drought damage to plant. These results provide guidance for AM and MT combined application to improve abiotic resistance in plants.

Effect of Melatonin on Fruit Quality via Decay Inhibition and Enhancement of Antioxidative Enzyme Activities and Genes Expression of Two Mango Cultivars during Cold Storage

The postharvest deterioration of mango fruits is a critical issue limiting mango storage and preservation due to its climacteric nature. This study evaluated the storage behavior of two mango cultivars and their response to exogenous melatonin (MT, 1000 μmol L⁻¹) treatment in attenuating fruit decay and enhancing fruits’ physiological and metabolic processes and gene relative expression subjected to cold storage. MT treatment in both mango cultivars significantly delayed weight loss, firmness, respiration rate, and decay incidence. However, MT did not influence the TSS, TA, and TSS:TA ratio regardless of the cultivar. Moreover, MT inhibited the decrease in total phenol and flavonoid content and AsA content while delaying the increase in the MDA content of mango during storage in both cultivars. In addition, MT dramatically inhibited the enzyme activity of PPO. In contrast, an increase in the activities of antioxidant enzymes (SOD and APX) and PAL and their genes’ relative expression was noticed in MT-treated fruits versus control in both cultivars. However, MT treatment was cultivar dependent in most parameters under study. These results demonstrated that MT treatment could be an essential postharvest treatment in minimizing decay, maintaining fruit quality, and extending mango fruits’ postharvest shelf life by enhancing the physiological and metabolic processes during cold storage.

Phytomelatonin: an unexpected molecule with amazing performances in plants

Phytomelatonin, a multifunctional molecule that has been found to be present in all plants examined to date, has an important role in plants as a modulatory agent (a biostimulator) that improves plant tolerance to both biotic and abiotic stress. We present a review of phytomelatonin that considers its roles in plant metabolism and in particular its interactions with plant hormone network. In the primary metabolism of plants, melatonin improves the rate and efficiency of photosynthesis, as well related factors such as stomatal conductance, intercellular CO2, and Rubisco activity. It has also been shown to down-regulate some senescence transcription factors. Melatonin up-regulates many enzyme transcripts related to carbohydrates (including sucrose and starch), amino acids, and lipid metabolism, optimizing N, P, and S uptake. With respect to the secondary metabolism, clear increases in polyphenol, glucosinolate, terpenoid, and alkaloid contents have been described in numerous melatonin-treated plants. Generally, the most important genes of these secondary biosynthesis pathways have been found to be up-regulated by melatonin. The great regulatory capacity of melatonin is a result of its control of the redox and plant hormone networks. Melatonin acts as a plant master regulator, up-/down-regulating different plant hormone levels and signalling, and is a key player in redox homeostasis. It has the capacity to counteract diverse critical situations such as pathogen infections and abiotic stresses, and provide plants with varying degrees of tolerance. We propose possible future applications of melatonin for crop improvement and post-harvest product preservation.

Effects of melatonin on growth and antioxidant capacity of naked oat (Avena nuda L) seedlings under lead stress

Melatonin (MT) plays an important role in plant response to abiotic stress. In recent years, lead (Pb) pollution has seriously affected the living environment of plants. In this study, we applied two different concentrations of MT to naked oat seedlings under Pb stress to explore the effect of MT on naked oat seedlings under Pb pollution. The results showed that Pb stress seriously inhibited the growth and development of naked oat seedlings, which was alleviated by MT. MT could increase the soluble protein content and decrease the proline content of naked oat seedlings to maintain the osmotic balance of naked oat seedlings. The application of MT could accelerate the removal of reactive oxygen species (ROS) and improve the activities of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT), so as to maintain the redox balance in naked oat seedlings. Exogenous melatonin could significantly increase the chlorophyll content of naked oat seedlings under Pb treatment, so as to improve the photosynthesis efficiency of naked oat seedlings. MT could also remarkably up regulate the expression of the genes of LOX, POX and Asmap1, and affect the expression of transcription factors NAC and WRKY1. It might regulate the expression of downstream genes through MAPKs pathways and TFs to improve the Pb tolerance of naked oat seedlings. These results proved that MT could significantly promote the growth and development of naked oats seedlings under Pb stress, which is expected to be applied in agricultural production practice.

Physiological and Comparative Transcriptome Analysis Reveals the Mechanism by Which Exogenous 24-Epibrassinolide Application Enhances Drought Resistance in Potato (Solanum tuberosum L.)

Drought stress is a key factor limiting the growth and tuber yield of potatoes (Solanum tuberosum L.). Brassinosteroids (BRs) have been shown to alleviate drought stress in several plant species; however, little is known about the physiological and molecular mechanisms by which BRs enhance drought resistance in potatoes. Here, we characterized changes in the physiology and transcriptome of the tetraploid potato variety ‘Xuanshu-2′ in response to drought stress after 24-epibrassinolide (EBR) pretreatment. The abscisic acid (ABA) content, photosynthetic capacity, and the activities of antioxidant enzymes were increased; the intercellular CO2 concentration, relative conductivity, reactive oxygen species, malondialdehyde, proline, and soluble sugar content were decreased after EBR pretreatment compared with plants under drought stress. Transcriptome analysis revealed 1330 differently expressed genes (DEGs) involved in the response to drought stress after EBR pretreatment. DEGs were enriched in plant hormone signal transduction, starch and sucrose metabolism, circadian rhythm, flavonoid biosynthesis, and carotenoid biosynthesis. DEGs associated with the BR signaling and biosynthesis pathways, as well as ABA metabolic pathways were identified. Our findings provide new insights into the mechanisms by which BRs enhance the drought resistance of potatoes.

Carex rigescens caffeic acid O-methyltransferase gene CrCOMT confer melatonin-mediated drought tolerance in transgenic tobacco

Melatonin is an important, multifunctional protective agent against a variety of abiotic and biotic stressors in plants. Caffeic acid O-methyltransferase (COMT) catalyzes the last step of melatonin synthesis in plants and reportedly participates in the regulation of stress response and tolerance. However, few studies have reported its function in melatonin-mediated drought resistance. In this study, CrCOMT was identified and was strongly induced by drought stress in Carex rigescens. CrCOMT overexpression in transgenic tobacco increased tolerance to drought stress with high levels of seed germination, relative water content, and survival rates. CrCOMT overexpression in tobacco improved membrane stability, and plants exhibited lower relative electrolytic leakage and malondialdehyde content, as well as higher photochemical efficiency than the wildtype (WT) under drought stress. The transgenic plants also had higher levels of proline accumulation and antioxidant enzyme activity, which decreased oxidative stress damage due to reactive oxygen species (ROS) hyperaccumulation under drought stress. The transcription of drought stress response and ROS scavenging genes was significantly higher in the CrCOMT overexpression plants than in the WT plants. In addition, CrCOMT transgenic tobacco plants exhibited higher melatonin content under drought stress conditions. Exogenous melatonin was applied to C. rigescens under drought stress to confirm the function of melatonin in mediating drought tolerance; the relative water content and proline content were higher, and the relative electrolytic leakage was lower in melatonin-treated C. rigescens than in the untreated plants. In summary, these results show that CrCOMT plays a positive role in plant drought stress tolerance by regulating endogenous melatonin content.

Comparative transcriptomics reveals new insights into melatonin-enhanced drought tolerance in naked oat seedlings

The growth and development of naked oat (Avena nuda L.) seedlings, a grain recognized as nutritious and healthy, is limited by drought. Melatonin plays a positive role in plants under drought stress. However, its function is unclear in naked oats. This study demonstrated that melatonin enhances drought stress tolerance in oat seedlings. Melatonin application alleviated the declining growth parameters of two naked oat varieties, Huazao No.2 (H2) and Jizhangyou No.15 (J15), under drought stress by increasing the chlorophyll content and photosynthetic rate of leaves. Melatonin pretreatment induced differential gene expression in H2 and J15 under drought stress. Subsequently, the differential gene expression responses to melatonin in the two varieties were further analyzed. The key drought response transcription factors and the regulatory effect of melatonin on drought-related transcription factors were assessed, focusing on genes encoding proteins in the ABA signal transduction pathway, including PYL, PP2C, ABF, SNRK2, and IAA. Taken together, this study provides new insights into the effect and underlying mechanism of melatonin in alleviating drought stress in naked oat seedlings.

Crosstalk between Melatonin and Reactive Oxygen Species in Plant Abiotic Stress Responses: An Update

Melatonin acts as a multifunctional molecule that takes part in various physiological processes, especially in the protection against abiotic stresses, such as salinity, drought, heat, cold, heavy metals, etc. These stresses typically elicit reactive oxygen species (ROS) accumulation. Excessive ROS induce oxidative stress and decrease crop growth and productivity. Significant advances in melatonin initiate a complex antioxidant system that modulates ROS homeostasis in plants. Numerous evidences further reveal that melatonin often cooperates with other signaling molecules, such as ROS, nitric oxide (NO), and hydrogen sulfide (H₂S). The interaction among melatonin, NO, H₂S, and ROS orchestrates the responses to abiotic stresses via signaling networks, thus conferring the plant tolerance. In this review, we summarize the roles of melatonin in establishing redox homeostasis through the antioxidant system and the current progress of complex interactions among melatonin, NO, H₂S, and ROS in higher plant responses to abiotic stresses. We further highlight the vital role of respiratory burst oxidase homologs (RBOHs) during these processes. The complicated integration that occurs between ROS and melatonin in plants is also discussed.

Exogenous Melatonin Protects Lime Plants from Drought Stress-Induced Damage by Maintaining Cell Membrane Structure, Detoxifying ROS and Regulating Antioxidant Systems

Lime is an important commercial product in tropical and subtropical regions, where drought stress is becoming one of the most severe environmental challenges in the agricultural sector. Melatonin is an antioxidant molecule that helps plants regulate their development and respond to a variety of stresses. In this research, the effects of exogenous melatonin treatments were evaluated at different concentrations (0, 50, 100, and 150 μM) on biochemical aspects and gene expression in two species of lime plants (“Mexican lime” and “Persian lime”) under normal (100% field capacity (FC)) and drought stress conditions (75% and 40% FC). The experiments were factorial and based on a completely randomized design (CRD) with four replicates. Drought stress caused electrolyte leakage (EL) as well as accumulations of hydrogen peroxide (H2O2) and malondialdehyde (MDA), indicating the occurrence of damage to cellular membranes. In contrast, the melatonin pretreatment at various concentrations reduced the levels of EL, H2O2 and MDA while mitigating the negative effects of drought stress on the two lime species. The application of melatonin (100-μM) significantly increased the level of proline content and activity of antioxidant enzymes in plants under drought stress compared to control plants. According to real-time PCR analysis, drought stress and melatonin treatment enhanced the expression of genes involved in ROS scavenging, proline biosynthesis, and cell redox regulation in both species, as compared to their respective controls. According to these findings, melatonin is able to detoxify ROS and regulate antioxidant systems, thereby protecting lime plants from drought stress-induced damages.

In-Vivo Biophoton Emission, Physiological and Oxidative Responses of Biostimulant-Treated Winter Wheat (Triticum eastivum L.) as Seed Priming Possibility, for Heat Stress Alleviation

High temperature induces oxidative processes in wheat, the alleviation of which is promising using biostimulants. Priming has been used for enhancing stress tolerance of seedlings. However, the usage of biostimulants for priming is an unexplored area under either normal or stress conditions. Therefore, the aim of our study was to evaluate the heat stress alleviation capability of differentially applied biostimulant treatments on wheat seedlings. The investigation included stress parameters (fresh/dry weight ratio, chlorophyll content estimation, antioxidant capacity and lipid oxidation) combined with biophoton emission measurement, since with this latter non-invasive technique, it is possible to measure and elucidate in vivo stress conditions in real-time using lipid oxidation-related photon emissions. We confirmed that a single biostimulant pretreatment increased antioxidant capacity and decreased biophoton release and lipid oxidation, indicating the reduction of the harmful effects of heat stress. Therefore, biophoton emission proved to be suitable for detecting and imaging the effects of heat stress on wheat seedlings for the first time. Two-way analysis of variance (ANOVA) revealed that biostimulant (p = 4.01 × 10⁻⁷) treatments, temperature (p = 9.07 × 10⁻⁸), and the interaction of the two factors (p = 2.07 × 10⁻⁵) had a significant effect on the overall count per second values of biophoton emission, predicting more efficient biostimulant utilization practices, even for seed priming purposes.

Genome-Wide Identification of MYB Transcription Factors and Screening of Members Involved in Stress Response in Actinidia

MYB transcription factors (TFs) play an active role in plant responses to abiotic stresses, but they have not been systematically studied in kiwifruit (Actinidia chinensis). In this study, 181 AcMYB TFs were identified from the kiwifruit genome, unevenly distributed on 29 chromosomes. The high proportion (97.53%) of segmental duplication events (Ka/Ks values less than 1) indicated that AcMYB TFs underwent strong purification selection during evolution. According to the conservative structure, 91 AcR2R3-MYB TFs could be divided into 34 subgroups. A combination of transcriptomic data under drought and high temperature from four AcMYB TFs (AcMYB2, AcMYB60, AcMYB61 and AcMYB102) was screened out in response to stress and involvement in the phenylpropanoid pathway. They were highly correlated with the expression of genes related to lignin biosynthesis. qRT-PCR analysis showed that they were highly correlated with the expression of genes related to lignin biosynthesis in different tissues or under stress, which was consistent with the results of lignin fluorescence detection. The above results laid a foundation for further clarifying the role of MYB in stress.

Melatonin Improves Drought Stress Tolerance of Tomato by Modulation Plant Growth, Root Architecture, Photosynthesis, and Antioxidant Defense System

Tomato is an important vegetable that is highly sensitive to drought (DR) stress which impairs the development of tomato seedlings. Recently, melatonin (ME) has emerged as a nontoxic, regulatory biomolecule that regulates plant growth and enhances the DR tolerance mechanism in plants. The present study was conducted to examine the defensive role of ME in photosynthesis, root architecture, and the antioxidant enzymes’ activities of tomato seedlings subjected to DR stress. Our results indicated that DR stress strongly suppressed growth and biomass production, inhibited photosynthesis, negatively affected root morphology, and reduced photosynthetic pigments in tomato seedlings. Per contra, soluble sugars, proline, and ROS (reactive oxygen species) were suggested to be improved in seedlings under DR stress. Conversely, ME (100 µM) pretreatment improved the detrimental-effect of DR by restoring chlorophyll content, root architecture, gas exchange parameters and plant growth attributes compared with DR-group only. Moreover, ME supplementation also mitigated the antioxidant enzymes [APX (ascorbate peroxidase), CAT (catalase), DHAR (dehydroascorbate reductase), GST (glutathione S-transferase), GR (glutathione reductase), MDHAR (monodehydroascorbate reductase), POD (peroxidase), and SOD (superoxide dismutase)], non-enzymatic antioxidant [AsA (ascorbate), DHA (dehydroascorbic acid), GSH (glutathione), and GSSG, (oxidized glutathione)] activities, reduced oxidative damage [EL (electrolyte leakage), H₂O₂ (hydrogen peroxide), MDA (malondialdehyde), and O₂^•− (superoxide ion)] and osmoregulation (soluble sugars and proline) of tomato seedlings, by regulating gene expression for SOD, CAT, APX, GR, POD, GST, DHAR, and MDHAR. These findings determine that ME pretreatment could efficiently improve the seedlings growth, root characteristics, leaf photosynthesis and antioxidant machinery under DR stress and thereby increasing the seedlings’ adaptability to DR stress.

Salicylic acid pre-treatment modulates Pb2+-induced DNA damage vis-à-vis oxidative stress in Allium cepa roots

The current study investigated the putative role of salicylic acid (SA) in modulating Pb²⁺-induced DNA and oxidative damage in Allium cepa roots. Pb²⁺ exposure enhanced free radical generation and reduced DNA integrity and antioxidant machinery after 24 h; however, SA pre-treatment (for 24 h) ameliorated Pb²⁺ toxicity. Pb²⁺ exposure led to an increase in malondialdehyde (MDA) and hydrogen peroxide (H₂O₂) accumulation and enhanced superoxide radical and hydroxyl radical levels. SA improved the efficiency of enzymatic antioxidants (ascorbate and guaiacol peroxidases [APX, GPX], superoxide dismutases [SOD], and catalases [CAT]) at 50-μM Pb²⁺ concentration. However, SA pre-treatment could not improve the efficiency of CAT and APX at 500 μM of Pb²⁺ treatment. Elevated levels of ascorbate and glutathione were observed in A. cepa roots pre-treated with SA and exposed to 50 μM Pb²⁺ treatment, except for oxidized glutathione. Nuclear membrane integrity test demonstrated the ameliorating effect of SA by reducing the number of dark blue-stained nuclei as compared to Pb²⁺ alone treatments. SA was successful in reducing DNA damage in cell exposed to higher concentration of Pb²⁺ (500 μM) as observed through comet assay. The study concludes that SA played a major role in enhancing defense mechanism and protecting against DNA damage by acclimatizing the plant to Pb²⁺-induced toxicity.

Melatonin improves heat tolerance in Actinidia deliciosa via carotenoid biosynthesis and heat shock proteins expression

The whole-genome molecular mechanisms of melatonin (MT)-mediated enhancement of thermotolerance in plants has rarely been studied. In this study, the genome-wide gene expression profiles of kiwifruit seedlings primed with MT and non-MT at 45°C were analyzed by RNA-Seq. A total of 3299 differentially expressed genes (DEGs) were screened between MT and non-MT treatment, in which carotenoid biosynthesis was one of the high-enrichment pathways revealed by Kyoto Encyclopedia of Genes and Genomes analysis. Further, qRT-PCR verified that MT significantly induced the upregulated expression of the carotenoid biosynthesis gene, which was consistent with the increase of carotenoid content. In addition, 10 heat shock proteins (HSPs) were identified to have a highly upregulated expression by MT. These findings provide a set of informative and fundamental data on the role of MT in heat resistance.

Comparison of fruit morphology and nutrition metabolism in different cultivars of kiwifruit across developmental stages

Kiwifruit (Actinidia) is becoming increasingly popular worldwide due to its favorable flavour and high vitamin C content. However, quality parameters vary among cultivars. To determine the differences in quality and metabolic parameters of kiwifruit, we monitored the growth processes of 'Kuilv' (Actinidia arguta), 'Hongyang' (Actinidia chinensis) and 'Hayward' (Actinidia deliciosa). We found that 'Kuilv' required the shortest time for fruit development, while 'Hayward' needed the longest time to mature. The fruit size of 'Hayward' was the largest and that of 'Kuilv' was the smallest. Furthermore, 'Hongyang' showed a double-S shape of dry matter accumulation, whereas 'Kuilv' and 'Hayward' showed a linear or single-S shape pattern of dry matter accumulation during development. The three cultivars demonstrated the same trend for total soluble solids accumulation, which did not rise rapidly until 90-120 days after anthesis. However, the accumulation of organic acids and soluble sugars varied among the cultivars. During later fruit development, the content of glucose, fructose and quinic acid in 'Kuilv' fruit was far lower than that in 'Hongyang' and 'Hayward'. On the contrary, 'Kuilv' had the highest sucrose content among the three cultivars. At maturity, the antioxidative enzymatic systems were significantly different among the three kiwifruit cultivars. 'Hongyang' showed higher activities of superoxide dismutase than the other cultivars, while the catalase content of 'Hayward' was significantly higher than that of 'Hongyang' and 'Kuilv'. These results provided knowledge that could be implemented for the marketing, handling and post-harvest technologies of the different kiwifruit cultivars.

Phytomelatonin: An overview of the importance and mediating functions of melatonin against environmental stresses

Recently, melatonin has gained significant importance in plant research. The presence of melatonin in the plant kingdom has been known since 1995. It is a molecule that is conserved in a wide array of evolutionary distant organisms. Its functions and characteristics have been found to be similar in both plants and animals. The review focuses on the role of melatonin pertaining to physiological functions in higher plants. Melatonin regulates physiological functions regarding auxin activity, root, shoot, and explant growth, activates germination of seeds, promotes rhizogenesis (growth of adventitious and lateral roots), and holds up impelled leaf senescence. Melatonin is a natural bio-stimulant that creates resistance in field crops against various abiotic stress, including heat, chemical pollutants, cold, drought, salinity, and harmful ultra-violet radiation. The full potential of melatonin in regulating physiological functions in higher plants still needs to be explored by further research.

Mechanistic insights on melatonin-mediated drought stress mitigation in plants

Drought stress imposes a serious threat to crop productivity and nutritional security. Drought adaptation mechanisms involve complex regulatory network comprising of various sensory and signaling molecules. In this context, melatonin has emerged as a potential signaling molecule playing a crucial role in imparting stress tolerance in plants. Melatonin pretreatment regulates various plant physiological processes such as osmoregulation, germination, photosynthesis, senescence, primary/secondary metabolism, and hormonal cross-talk under water deficit conditions. Melatonin-mediated regulation of ascorbate-glutathione (AsA-GSH) cycle plays a crucial role to scavenge reactive oxygen species generated in the cells during drought. Here, in this review, the current knowledge on the role of melatonin to ameliorate adverse effects of drought by modulating morphological, physiological, and redox regulatory processes is discussed. The role of melatonin to improve water absorption capacity of roots by regulating aquaporin channels and hormonal cross-talk involved in drought stress mitigation are also discussed. Overall, melatonin is a versatile bio-molecule involved in growth promotion and yield enhancement under drought stress that makes it a suitable candidate for eco-friendly crop production to ensure food security.

Pre treatment with Bacillus subtilis mitigates drought induced photo-oxidative damages in okra by modulating antioxidant system and photochemical activity

Growth promoting potential of Bacillus subtilis (BS) in drought stressed Abelmoschus esculentus (L.) Moench (okra) was assessed by measuring the chlorophyll stability index (CSI), chlorophyll a (Chl-a) fluorescence, leaf osmotic potential and lipid peroxidation by malondialdehyde content, emission of reactive oxygen species (ROS), osmolyte content and the activity of non-enzyme and enzyme antioxidants. BS treatment significantly increased the leaf osmotic potential, osmolyte production and the activity of non-enzyme and enzyme antioxidants under drought stress. BS treatment mitigated the drought-induced reduction in Chl a fluorescence and CSI. Concomitant increase in total sugar, proline, non-enzyme antioxidants [glutathione and ascorbate] and enzyme antioxidants like superoxide dismutase, catalase, ascorbate peroxidase, glutathione reductase, monodehydroascorbate reductase and dehydroascorbate reductase modulate the intracellular ROS concentration in okra to resist the stress induced oxidative damage in BS treated plants led to fast recovery and less photodamage.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-021-00982-8.

A Beginner's Guide to Osmoprotection by Biostimulants

Water is indispensable for the life of any organism on Earth. Consequently, osmotic stress due to salinity and drought is the greatest threat to crop productivity. Ongoing climate change includes rising temperatures and less precipitation over large areas of the planet. This is leading to increased vulnerability to the drought conditions that habitually threaten food security in many countries. Such a scenario poses a daunting challenge for scientists: the search for innovative solutions to save water and cultivate under water deficit. A search for formulations including biostimulants capable of improving tolerance to this stress is a promising specific approach. This review updates the most recent state of the art in the field.

Pure Organic Active Compounds Against Abiotic Stress: A Biostimulant Overview

Biostimulants (BSs) are probably one of the most promising alternatives nowadays to cope with yield losses caused by plant stress, which are intensified by climate change. Biostimulants comprise many different compounds with positive effects on plants, excluding pesticides and chemical fertilisers. Usually mixtures such as lixiviates from proteins or algal extracts have been used, but currently companies are interested in more specific compounds that are capable of increasing tolerance against abiotic stress. Individual application of a pure active compound offers researchers the opportunity to better standarise formulations, learn more about the plant defence process itself and assist the agrochemical industry in the development of new products. This review attempts to summarise the state of the art regarding various families of organic compounds and their mode/mechanism of action as BSs, and how they can help maximise agricultural yields under stress conditions aggravated by climate change.

The application of artificial neural networks in modeling and predicting the effects of melatonin on morphological responses of citrus to drought stress

Drought stress as one of the most devastating abiotic stresses affects agricultural and horticultural productivity in many parts of the world. The application of melatonin can be considered as a promising approach for alleviating the negative impact of drought stress. Modeling of morphological responses to drought stress can be helpful to predict the optimal condition for improving plant productivity. The objective of the current study is modeling and predicting morphological responses (leaf length, number of leaves/plants, crown diameter, plant height, and internode length) of citrus to drought stress, based on four input variables including melatonin concentrations, days after applying treatments, citrus species, and level of drought stress, using different Artificial Neural Networks (ANNs) including Generalized Regression Neural Network (GRNN), Radial basis function (RBF), and Multilayer Perceptron (MLP). The results indicated a higher accuracy of GRNN as compared to RBF and MLP. The great accordance between the experimental and predicted data of morphological responses for both training and testing processes support the excellent efficiency of developed GRNN models. Also, GRNN was connected to Non-dominated Sorting Genetic Algorithm-II (NSGA-II) to optimize input variables for obtaining the best morphological responses. Generally, the validation experiment showed that ANN-NSGA-II can be considered as a promising and reliable computational tool for studying and predicting plant morphological and physiological responses to drought stress.

Melatonin-Induced Water Stress Tolerance in Plants: Recent Advances

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.

Melatonin-Induced Water Stress Tolerance in Plants: Recent Advances

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.