Melatonin promotes water-stress tolerance, lateral root formation, and seed germination in cucumber (Cucumis sativus L.)

Research on the Mechanisms of Phytohormone Signaling in Regulating Root Development

Phytohormones are organic compounds produced in trace amounts within plants that regulate their physiological processes. Their physiological effects are highly complex and diverse. They influence processes ranging from cell division, elongation, and differentiation to plant germination and rooting. Therefore, phytohormones play a crucial regulatory role in plant growth and development. Recently, various studies have highlighted the role of PHs, such as auxin, cytokinin (CK), and abscisic acid (ABA), and newer classes of PHs, such as brassinosteroid (BR) and peptide hormone, in the plant responses toward environmental stresses. These hormones not only have distinct roles at different stages of plant growth but also interact to promote or inhibit each other, thus effectively regulating plant development. Roots are the primary organs for water and mineral absorption in plants. During seed germination, the radicle breaks through the seed coat and grows downward to form the primary root. This occurs because the root needs to quickly penetrate the soil to absorb water and nutrients, providing essential support for the plant's subsequent growth. Root development is a highly complex and precisely regulated process influenced by various signals. Changes in root architecture can affect the plant's ability to absorb nutrients and water, which in turn impacts crop yield. Thus, studying the regulation of root development is of great significance. Numerous studies have reported on the role of phytohormones, particularly auxins, in root regulation. This paper reviews recent studies on the regulation of root development by various phytohormones, both individually and in combination, providing a reference for researchers in this field and offering perspectives on future research directions for improving crop yields.

Enhancing drought stress tolerance in horticultural plants through melatonin-mediated phytohormonal crosstalk

KEY MESSAGE

Melatonin and melatonin-mediated phytohormonal crosstalk play a multifaceted role in improving drought stress tolerance via molecular mechanisms and biochemical interactions in horticultural plants. The physical, physiological, biochemical, and molecular characteristics of plants are all affected by drought stress. Crop yield and quality eventually decline precipitously as a result. A phytohormone, melatonin, controls several plant functions during drought stress. However, the interactions between melatonin and other phytohormones, particularly how they control plant responses to drought stress, have not been clearly explored. This review explores the effects of melatonin and particular phytohormones on improving plant tolerance to drought stress. Specifically, the key melatonin roles in improved photosynthetic performance, better antioxidant activities, up-regulated gene expression, increased plant growth, and yield, etc., during drought stress have been elucidated in this review. Furthermore, this review explains how the intricate networks of melatonin-mediated crosstalk phytohormones, such as IAA, BR, ABA, GA, JA, CK, ET, SA, etc., enable horticultural plants to tolerate drought stress. Thus, this research provides a better understanding of the role of phytohormones, mainly melatonin, elucidates phytohormonal cross-talks in drought stress response, and future perspectives of phytohormonal contributions in plant improvements including engineering plants for better drought stress tolerance via targeting melatonin interactions.

Deciphering the Mechanism of Melatonin-Induced Enhancement of Photosystem II Function in Moderate Drought-Stressed Oregano Plants

Melatonin (MT) is considered as an antistress molecule that plays a constructive role in the acclimation of plants to both biotic and abiotic stress conditions. In the present study, we assessed the impact of 10 and 100 μM MT foliar spray, on chlorophyll content, and photosystem II (PSII) function, under moderate drought stress, on oregano (Origanum vulgare L.) plants. Our aim was to elucidate the molecular mechanism of MT action on the photosynthetic electron transport process. Foliar spray with 100 μM MT was more effective in mitigating the negative impact of moderate drought stress on PSII function, compared to 10 μM MT. MT foliar spray significantly improved the reduced efficiency of the oxygen-evolving complex (OEC), and PSII photoinhibition (Fv/Fm), which were caused by drought stress. Under moderate drought stress, foliar spray with 100 μM MT, compared with the water sprayed (WA) leaves, increased the non-photochemical quenching (NPQ) by 31%, at the growth irradiance (GI, 205 μmol photons m-2 s-1), and by 13% at a high irradiance (HI, 1000 μmol photons m-2 s-1). However, the lower NPQ increase at HI was demonstrated to be more effective in decreasing the singlet-excited oxygen (1O2) production at HI (-38%), in drought-stressed oregano plants sprayed with 100 μM MT, than the corresponding decrease in 1O2 production at the GI (-20%), both compared with the respective WA-sprayed leaves under moderate drought. The reduced 1O2 production resulted in a significant increase in the quantum yield of PSII photochemistry (ΦPSII), and the electron transport rate (ETR), in moderate drought-stressed plants sprayed with 100 μM MT, compared with WA-sprayed plants, but only at the HI (+27%). Our results suggest that the enhancement of PSII functionality, with 100 μM MT under moderate drought stress, was initiated by the NPQ mechanism, which decreased the 1O2 production and increased the fraction of open PSII reaction centers (qp), resulting in an increased ETR.

Melatonin mitigates cadmium toxicity by promoting root development, delaying root senescence, and regulating cadmium transport in cotton

Cd ions are absorbed and transported from the soil by crop roots, which are the first organ to be exposed to Cd. This results in an increase in cadmium ions in crops, significantly affecting crop growth and yield. Exogenous melatonin (MT) can help reduce cadmium (Cd) stress in cotton, but the specific contribution of roots to this process remains unclear. In order to address this knowledge gap, an in-situ root phenotyping study was conducted to investigate the the phenotype and lifespan of roots under cadmium stress (Cd) and melatonin treatment (Cd + MT). The results showed that MT alleviated the decreases in plant height, leaf area, SPAD value, stem diameter, stomatal conductance and net photosynthetic rate under Cd stress, which further promoted the biomass accumulation in various cotton organs. What is more, the Cd + MT treatment increased root volume, surface area, and length under Cd stress by 25.63 %, 10.58 %, and 21.89 %, respectively, compared with Cd treatment. Interestingly, compared to Cd treatment, Cd + MT treatment also significantly extended the lifespan of roots and root hairs by 6.68 days and 2.18 days, respectively. In addition, Cd + MT treatment reduced the transport of Cd from roots to shoots, particularly to bolls, and decreased the Cd bioconcentration factor in bolls by 61.17 %, compared to Cd treatment. In conclusion, these findings show that applying MT externally helps reduce Cd stress by delaying root senescence, promoting root development and regulating Cd transport. This method can be an effective approach to managing Cd stress in cotton.

Synergistic effects of melatonin and glycine betaine on seed germination, seedling growth, and biochemical attributes of maize under salinity stress

Salinity stress represents a major threat to crop production by inhibiting seed germination, growth of seedlings, and final yield and, therefore, to the social and economic prosperity of developing countries. Recently, plant growth-promoting substances have been widely used as a chemical strategy for improving plant resilience towards abiotic stresses. This study aimed to determine whether melatonin (MT) and glycine betaine (GB) alone or in combination could alleviate the salinity-induced impacts on seed germination and growth of maize seedlings. Increasing NaCl concentration from 100 to 200 mM declined seed germination rate (4.6-37.7%), germination potential (24.5-46.7%), radical length (7.7-40.0%), plumule length (2.2-35.6%), seedling fresh (1.7-41.3%) and dry weight (23.0-56.1%) compared to control (CN) plants. However, MT and GB treatments lessened the adverse effects of 100 and 150 mM NaCl and enhanced germination comparable to control plants. In addition, results from the pot experiments show that 200 mM NaCl stress disrupted the osmotic balance and persuaded oxidative stress, presented by higher electrolyte leakage, hydrogen peroxide, superoxide radicals, and malondialdehyde compared to control plants. However, compared to the NaCl treatment, NaCl+MT+GB treatment decreased the accumulation of malondialdehyde (24.2-42.1%), hydrogen peroxide (36.2-44.0%), and superoxide radicals (20.1-50.9%) by up-regulating the activity of superoxide dismutase (28.4-51.2%), catalase (82.2-111.5%), ascorbate peroxidase (40.3-59.2%), and peroxidase (62.2-117.9%), and by enhancing osmolytes accumulation, thereby reducing NaCl-induced oxidative damages. Based on these findings, the application of MT+GB is an efficient chemical strategy for improving seed germination and growth of seedlings by improving the physiological and biochemical attributes of maize under 200 mM NaCl stress.

Melatonin Priming Promotes Crop Seed Germination and Seedling Establishment Under Flooding Stress by Mediating ABA, GA, and ROS Cascades

Both seed germination and subsequent seedling establishment are key checkpoints during the life cycle of seed plants, yet flooding stress markedly inhibits both processes, leading to economic losses from agricultural production. Here, we report that melatonin (MT) seed priming treatment enhances the performance of seeds from several crops, including soybean, wheat, maize, and alfalfa, under flooding stress. Transcriptome analysis revealed that MT priming promotes seed germination and seedling establishment associated with changes in abscisic acid (ABA), gibberellin (GA), and reactive oxygen species (ROS) biosynthesis and signaling pathways. Real-time quantitative RT-PCR (qRT-PCR) analysis confirmed that MT priming increases the expression levels of GA biosynthesis genes, ABA catabolism genes, and ROS biosynthesis genes while decreasing the expression of positive ABA regulatory genes. Further, measurements of ABA and GA concentrations are consistent with these trends. Following MT priming, quantification of ROS metabolism-related enzyme activities and the concentrations of H2O2 and superoxide anions (O2 -) after MT priming were consistent with the results of transcriptome analysis and qRT-PCR. Finally, exogenous application of GA, fluridone (an ABA biosynthesis inhibitor), or H2O2 partially rescued the poor germination of non-primed seeds under flooding stress. Collectively, this study uncovers the application and molecular mechanisms underlying MT priming in modulating crop seed vigor under flooding stress.

An ecotype-specific effect of osmopriming and melatonin during salt stress in Arabidopsis thaliana

BACKGROUND

Natural populations of Arabidopsis thaliana exhibit phenotypic variations in specific environments and growth conditions. However, this variation has not been explored after seed osmopriming treatments. The natural variation in biomass production and root system architecture (RSA) was investigated across the Arabidopsis thaliana core collection in response to the pre-sawing seed treatments by osmopriming, with and without melatonin (Mel). The goal was to identify and characterize physiologically contrasting ecotypes.

RESULTS

Variability in RSA parameters in response to PEG-6000 seed osmopriming with and without Mel was observed across Arabidopsis thaliana ecotypes with especially positive impact of Mel addition under both control and 100 mM NaCl stress conditions. Two ecotypes, Can-0 and Kn-0, exhibited contrasted root phenotypes: seed osmopriming with and without Mel reduced the root growth of Can-0 plants while enhancing it in Kn-0 ones under both control and salt stress conditions. To understand the stress responses in these two ecotypes, main stress markers as well as physiological analyses were assessed in shoots and roots. Although the effect of Mel addition was evident in both ecotypes, its protective effect was more pronounced in Kn-0. Antioxidant enzymes were induced by osmopriming with Mel in both ecotypes, but Kn-0 was characterized by a higher responsiveness, especially in the activities of peroxidases in roots. Kn-0 plants experienced lower oxidative stress, and salt-induced ROS accumulation was reduced by osmopriming with Mel. In contrast, Can-0 exhibited lower enzyme activities but the accumulation of proline in its organs was particularly high. In both ecotypes, a greater response of antioxidant enzymes and proline accumulation was observed compared to mechanisms involving the reduction of Na+ content and prevention of K+ efflux.

CONCLUSIONS

In contrast to Can-0, Kn-0 plants grown from seeds osmoprimed with and without Mel displayed a lower root sensitivity to NaCl-induced oxidative stress. The opposite root growth patterns, enhanced by osmopriming treatments might result from different protective mechanisms employed by these two ecotypes which in turn result from adaptive strategies proper to specific habitats from which Can-0 and Kn-0 originate. The isolation of contrasting phenotypes paves the way for the identification of genetic factors affecting osmopriming efficiency.

Evolution and Analysis of Caffeic Acid Transferase (COMT) in Seed Plants

Caffeic acid transferase (COMT) is a key enzyme in the lignin and melatonin synthesis pathways and plays an important role in plant growth and development. All seed plants have two characteristics: they have vascular tissues, phloem, and xylem, and they can produce and reproduce seeds. In order to understand the distribution and evolution of COMTs in seed plants, we performed physicochemical property analysis, subcellular localization, phylogenetic analysis, conserved motif analysis, and protein interaction network analysis of 44 COMT homologs from 26 seed plants through in silico. The results showed that in seed plants, the structure of COMT genes tends to be stable in different plant taxa, while the relationship between the chromosomal positions of different COMT genes in the same plant was more intricate. The conserved distribution of COMT in seed plants reflected its highly specialized function.

Melatonin Interaction with Other Phytohormones in the Regulation of Abiotic Stresses in Horticultural Plants

Horticultural crops play a vital role in global food production, nutrition, and the economy. Horticultural crops are highly vulnerable to abiotic stresses. These abiotic stresses hinder plant growth and development by affecting seed germination, impairing photosynthetic activity, and damaging root development, thus leading to a decrease in fruit yield, quality, and productivity. Scientists have conducted extensive research to investigate the mechanisms of resilience and the ability to cope with environmental stresses. In contrast, the use of phytohormones to alleviate the detrimental impacts of abiotic stresses on horticulture plants has been generally recognized as an effective method. Among phytohormones, melatonin (MT) is a novel plant hormone that regulates various plants' physiological functions such as seedling development, root system architecture, photosynthetic efficiency, balanced redox homeostasis, secondary metabolites production, accumulation of mineral nutrient uptake, and activated antioxidant defense system. Importantly, MT application significantly restricted heavy metals (HMs) uptake and increased mineral nutrient accumulation by modifying the root architecture system. In addition, MT is a naturally occurring, multifunctional, nontoxic biomolecule having antioxidant properties. Furthermore, this review described the hormonal interaction between MT and other signaling molecules in order to enhance abiotic stress tolerance in horticulture crops. This review focuses on current research advancements and prospective approaches for enhancing crop tolerance to abiotic stress.

Seeds Priming with Melatonin Improves Root Hydraulic Conductivity of Wheat Varieties under Drought, Salinity, and Combined Stress

Drought and salinity stress reduce root hydraulic conductivity of plant seedlings, and melatonin application positively mitigates stress-induced damage. However, the underlying effect of melatonin priming on root hydraulic conductivity of seedlings under drought-salinity combined remains greatly unclear. In the current report, we investigated the influence of seeds of three wheat lines' 12 h priming with 100 μM of melatonin on root hydraulic conductivity (Lpr) and relevant physiological indicators of seedlings under PEG, NaCl, and PEG + NaCl combined stress. A previous study found that the combined PEG and NaCl stress remarkably reduced the Lpr of three wheat varieties, and its value could not be detected. Melatonin priming mitigated the adverse effects of combined PEG + NaCl stress on Lpr of H4399, Y1212, and X19 to 0.0071 mL·h-1·MPa-1, 0.2477 mL·h-1·MPa-1, and 0.4444 mL·h-1·MPa-1, respectively, by modulating translation levels of aquaporin genes and contributed root elongation and seedlings growth. The root length of H4399, Y1212, and X19 was increased by 129.07%, 141.64%, and 497.58%, respectively, after seeds pre-treatment with melatonin under PEG + NaCl combined stress. Melatonin -priming appreciably regulated antioxidant enzyme activities, reduced accumulation of osmotic regulators, decreased levels of malondialdehyde (MDA), and increased K+ content in stems and root of H4399, Y1212, and X19 under PEG + NaCl stress. The path investigation displayed that seeds primed with melatonin altered the modification of the path relationship between Lpr and leaf area under stress. The present study suggested that melatonin priming was a strategy as regards the enhancement of root hydraulic conductivity under PEG, NaCl, and PEG + NaCl stress, which efficiently enhanced wheat resistant to drought-salinity stress.

Melatonin-Mediated Molecular Responses in Plants: Enhancing Stress Tolerance and Mitigating Environmental Challenges in Cereal Crop Production

Cereal crops are crucial for global food security; however, they are susceptible to various environmental stresses that significantly hamper their productivity. In response, melatonin has emerged as a promising regulator, offering potential benefits for stress tolerance and crop growth. This review explores the effects of melatonin on maize, sorghum, millet, rice, barley, and wheat, aiming to enhance their resilience to stress. The application of melatonin has shown promising outcomes, improving water use efficiency and reducing transpiration rates in millet under drought stress conditions. Furthermore, it enhances the salinity and heavy metal tolerance of millet by regulating the activity of stress-responsive genes. Similarly, melatonin application in sorghum enhances its resistance to high temperatures, low humidity, and nutrient deficiency, potentially involving the modulation of antioxidant defense and aspects related to photosynthetic genes. Melatonin also exerts protective effects against drought, salinity, heavy metal, extreme temperatures, and waterlogging stresses in maize, wheat, rice, and barley crops by decreasing reactive oxygen species (ROS) production through regulating the antioxidant defense system. The molecular reactions of melatonin upregulated photosynthesis, antioxidant defense mechanisms, the metabolic pathway, and genes and downregulated stress susceptibility genes. In conclusion, melatonin serves as a versatile tool in cereal crops, bolstering stress resistance and promoting sustainable development. Further investigations are warranted to elucidate the underlying molecular mechanisms and refine application techniques to fully harness the potential role of melatonin in cereal crop production systems.

Melatonin - This is important to know

MEL (N-acetyl-5-methoxytryptamine) is a well-known natural compound that controls cellular processes in both plants and animals and is primarily found in plants as a neurohormone. Its roles have been described very broadly, from its antioxidant function related to the photoperiod and determination of seasonal rhythms to its role as a signalling molecule, imitating the action of plant hormones (or even being classified as a prohormone). MEL positively affects the yield and survival of plants by increasing their tolerance to unfavourable biotic and abiotic conditions, which makes MEL widely applicable in ecological farming as a stimulant of growth and development. Thus, it is called a phytobiostimulator. In this review, we discuss the genesis of MEL functions, the presence of MEL at the cellular level and its effects on gene expression and plant development, which can ensure the survival of plants under the conditions they encounter. Moreover, we consider the future application possibilities of MEL in agriculture.

Exogenous application of nano-silicon, potassium sulfate, or proline enhances physiological parameters, antioxidant enzyme activities, and agronomic traits of diverse rice genotypes under water deficit conditions

Water deficit is a critical obstacle that devastatingly impacts rice production, particularly in arid regions under current climatic fluctuations. Accordingly, it is decisive to reinforce the drought tolerance of rice by employing sustainable approaches to enhance global food security. The present study aimed at exploring the effect of exogenous application using different biostimulants on physiological, morphological, and yield attributes of diverse rice genotypes under water deficit and well-watered conditions in 2-year field trial. Three diverse rice genotypes (IRAT-112, Giza-178, and IR-64) were evaluated under well-watered (14400 m3/ha in total for the entire season) and water deficit (9170 m3/ha) conditions and were exogenously sprayed by nano-silicon, potassium sulfate, or proline. The results showed that drought stress substantially decreased all studied photosynthetic pigments, growth traits, and yield attributes compared to well-watered conditions. In contrast, antioxidant enzyme activities and osmoprotectants were considerably increased compared with those under well-watered conditions. However, the foliar application of nano-silicon, potassium sulfate, and proline substantially mitigated the deleterious effects of drought stress and markedly enhanced photosynthetic pigments, antioxidant enzyme activities, growth parameters, and yield contributing traits compared to untreated stressed control. Among the assessed treatments, foliar spray with nano-silicon or proline was more effective in promoting drought tolerance. The exogenous application of proline improved chlorophyll a, chlorophyll b, and carotenoids by 21.4, 19.6 and 21.0% followed by nano-silicon treatment, which enhanced chlorophyll a, chlorophyll b, and carotenoids by 21.1, 17.6 and 9.5% compared to untreated control. Besides, the application of proline demonstrated a superior improvement in the content of proline by 52.5% compared with the untreated control. Moreover, nano-silicon exhibited the maximum enhancement of catalase and peroxidase activity compared to the other treatments. The positive impacts of applied exogenously nano-silicon or proline significantly increased panicle length, number of panicles/plant, number of grains/panicle, fertility percentage, 1000-grain weight, panicle weight, and grain yield, compared to untreated plants under water deficit conditions. In addition, the physiological and agronomic performance of evaluated rice genotypes significantly contrasted under drought conditions. The genotype Giza-178 displayed the best performance under water deficit conditions compared with the other genotypes. Consequently, the integration of applied exogenously nano-silicon or proline with tolerant rice genotype as Giza-178 is an efficient approach to ameliorating drought tolerance and achieving agricultural sustainability under water-scarce conditions in arid environments.

Melatonin as a master regulatory hormone for genetic responses to biotic and abiotic stresses in model plant Arabidopsis thaliana: a comprehensive review

Melatonin is a naturally occurring biologically active amine produced by plants, animals and microbes. This review explores the biosynthesis of melatonin in plants, with a particular focus on its diverse roles in Arabidopsis thaliana , a model species. Melatonin affects abiotic and biotic stress resistance in A. thaliana . Exogenous and endogenous melatonin is addressed in association with various conditions, including cold stress, high light stress, intense heat and infection with Botrytis cinerea or Pseudomonas , as well as in seed germination and lateral root formation. Furthermore, melatonin confers stress resistance in Arabidopsis by initiating the antioxidant system, remedying photosynthesis suppression, regulating transcription factors involved with stress resistance (CBF, DREB, ZAT, CAMTA, WRKY33, MYC2, TGA) and other stress-related hormones (abscisic acid, auxin, ethylene, jasmonic acid and salicylic acid). This article additionally addresses other precursors, metabolic components, expression of genes (COR , CBF , SNAT , ASMT , PIN , PR1 , PDF1.2 and HSFA ) and proteins (JAZ, NPR1) associated with melatonin and reducing both biological and environmental stressors. Furthermore, the future perspective of melatonin rich agri-crops is explored to enhance plant tolerance to abiotic and biotic stresses, maximise crop productivity and enhance nutritional worth, which may help improve food security.

Melatonin Modulates Tomato Root Morphology by Regulating Key Genes and Endogenous Hormones

Melatonin plays a vital role in plant growth and development. In this study, we treated hydroponically grown tomato roots with various concentrations of exogenous melatonin (0, 10, 30, and 50 μmol·L-1). We utilized root scanning and microscopy to examine alterations in root morphology and cell differentiation and elucidated the mechanism by which melatonin regulates these changes through the interplay with endogenous hormones and relevant genes. The results showed that for melatonin at concentrations ranging between 10 and 30 μmol·L-1, the development of lateral roots were significantly stimulated, the root hair growth was enhanced, and biomass accumulation and root activity were increased. Furthermore, we elucidated that melatonin acts as a mediator for the expression of genes, such as SlCDKA1, SlCYCA3;1, SlARF2, SlF3H, and SlKT1, which are involved in the regulation of root morphology changes. Additionally, we observed that melatonin influences the levels of endogenous hormones, including ZT, GA3, IAA, ABA, and BR, which subsequently impact the root morphology development of tomato roots. In summary, this study shows that tomato root morphology can be promoted by the optimal concentration of exogenous melatonin (10-30 μmol·L-1).

Exogenous melatonin induces salt and drought stress tolerance in rice by promoting plant growth and defense system

Due to global climate change, crops are certainly confronted with a lot of abiotic and biotic stress factors during their growth that cause a serious threat to their development and overall productivity. Among different abiotic stresses, salt and drought are considered the most devastating stressors with serious impact on crop's yield stability. Here, the current study aimed to elucidate how melatonin works in regulating plant biomass, oxidative stress, antioxidant defense system, as well as the expression of genes related to salt and drought stress in rice plants. Eight groups of rice plants (3 replicates, 5 plants each) underwent varied treatments: control, melatonin, salt, drought, salt + drought, salt + melatonin, drought + melatonin, and salt + drought + melatonin. Melatonin (100 µM) was alternately applied a week before stress exposure; salt stress received 100 mM NaCl every 3 days for 3 weeks, and drought stress involved 10% PEG. Young leaves were randomly sampled from each group. The results showed that melatonin treatment markedly reduces salt and drought stress damage by promoting root, shoot length, fresh and dry weight, increasing chlorophyll contents, and inhibiting excessive production of oxidative stress markers. Salt and drought stress significantly decreased the water balance, and damaged cell membrane by reducing relative water contents and increasing electrolyte leakage. However, melatonin treated rice plants showed high relative water contents and low electrolyte leakage. Under salt and drought stress conditions, exogenous application of melatonin boosted the expression level of salt and drought stress responsive genes like OsSOS, OsNHX, OsHSF and OsDREB in rice plants. Taken together, our results reveal that melatonin treatment significantly increases salt and drought tolerance of rice plants, by increasing plant biomass, suppressing ROS accumulation, elevating antioxidants defense efficiency, and up-regulating the expression of salt and drought stress responsive genes.

Melatonin as a key regulator in seed germination under abiotic stress

Seed germination (SG) is the first stage in a plant's life and has an immense importance in sustaining crop production. Abiotic stresses reduce SG by increasing the deterioration of seed quality, and reducing germination potential, and seed vigor. Thus, to achieve a sustainable level of crop yield, it is important to improve SG under abiotic stress conditions. Melatonin (MEL) is an important biomolecule that interplays in developmental processes and regulates many adaptive responses in plants, especially under abiotic stresses. Thus, this review specifically summarizes and discusses the mechanistic basis of MEL-mediated SG under abiotic stresses. MEL regulates SG by regulating some stress-specific responses and some common responses. For instance, MEL induced stress specific responses include the regulation of ionic homeostasis, and hydrolysis of storage proteins under salinity stress, regulation of C-repeat binding factors signaling under cold stress, starch metabolism under high temperature and heavy metal stress, and activation of aquaporins and accumulation of osmolytes under drought stress. On other hand, MEL mediated regulation of gibberellins biosynthesis and abscisic acid catabolism, redox homeostasis, and Ca2+ signaling are amongst the common responses. Nonetheless factors such as endogenous MEL contents, plant species, and growth conditions also influence above-mentioned responses. In conclusion, MEL regulates SG under abiotic stress conditions by interacting with different physiological mechanisms.

Synergistic effects of gamma-aminobutyric acid and melatonin on seed germination and cadmium tolerance in tomato

The effects of exogenous γ-aminobutyric acid (GABA) and melatonin (MT) on tomato seed germination and shoot growth exposed to cadmium stress were investigated. On the one hand, treatment with MT (10-200 μM) or GABA (10-200 μM) alone could significantly relieve cadmium stress in tomato seedlings, which is reflected in increasing the germination rate, vigor index, fresh weight, dry weight and radicle lengths of tomato seeds, as well as the soluble content compared to the absence of exogenous treatment, and the alleviating effect reached the peak in the 200 µM GABA or 150 µM MT alone. On the other hand, exogenous MT and GABA showed synergistic effects on the germination of tomato seed under cadmium stress. Moreover, the application of 100 µM GABA combined with 100 µM MT markedly decreased the contents of Cd and MDA by upregulating the activities of antioxidant enzymes, thereby alleviating the toxic effect of cadmium stress on tomato seeds. Collectively, the combinational strategy showed significant positive effects on seed germination and cadmium stress resistance in tomato.

Exogenous Melatonin Promotes Cold Tolerance in Grape Seedlings: Physiological, Transcriptomic, and Functional Evidence

Melatonin (MEL) is an antioxidant molecule that enhances plant tolerance to environmental stress. However, the mechanisms by which MEL regulates cold signaling pathways in grapes under cold stress remain elusive. Here, we investigated the physiological and transcriptomic changes in grape seedlings treated with exogenous MEL to determine their protective role under cold stress. Results showed that 150 μM MEL effectively attenuated cold-induced cell damage by reducing reactive oxygen species (ROS) and preserving the chloroplast structure and function. MEL also inhibited tannin degradation, which contributed to its protective effect. Exogenous MEL promoted the synthesis of endogenous MEL, abscisic acid, auxin, and cytokinin while inhibiting gibberellin. Transcriptomic profiling revealed 776 differentially expressed transcripts in MEL-treated samples compared to controls. Functional analysis of a candidate hub gene, VvHSFA6b, showed that its overexpression in grape calli enhances cold tolerance by activating jasmonic acid synthesis pathway genes, promoting JA accumulation, and inhibiting JAZ-repressed transcription factors.

Current research and future directions of melatonin's role in seed germination

Seed germination is a complex process regulated by internal and external factors. Melatonin (N-acetyl-5-methoxytryptamine) is a ubiquitous signaling molecule, playing an important role in regulating seed germination under normal and stressful conditions. In this review, we aim to provide a comprehensive overview on melatonin's effects on seed germination on the basis of existing literature. Under normal conditions, exogenous high levels of melatonin can suppress or delay seed germination, suggesting that melatonin may play a role in maintaining seed dormancy and preventing premature germination. Conversely, under stressful conditions (e.g., high salinity, drought, and extreme temperatures), melatonin has been found to accelerate seed germination. Melatonin can modulate the expression of genes involved in ABA and GA metabolism, thereby influencing the balance of these hormones and affecting the ABA/GA ratio. Melatonin has been shown to modulate ROS accumulation and nutrient mobilization, which can impact the germination process. In conclusion, melatonin can inhibit germination under normal conditions while promoting germination under stressful conditions via regulating the ABA/GA ratios, ROS levels, and metabolic enzyme activity. Further research in this area will deepen our understanding of melatonin's intricate role in seed germination and may contribute to the development of improved seed treatments and agricultural practices.

Mechanistic Approach on Melatonin-Induced Hormesis of Photosystem II Function in the Medicinal Plant Mentha spicata

Melatonin (MT) is considered a new plant hormone having a universal distribution from prokaryotic bacteria to higher plants. It has been characterized as an antistress molecule playing a positive role in the acclimation of plants to stress conditions, but its impact on plants under non-stressed conditions is not well understood. In the current research, we evaluated the impact of MT application (10 and 100 μM) on photosystem II (PSII) function, reactive oxygen species (ROS) generation, and chlorophyll content on mint (Mentha spicata L.) plants in order to elucidate the molecular mechanism of MT action on the photosynthetic electron transport process that under non-stressed conditions is still unclear. Seventy-two hours after the foliar spray of mint plants with 100 μM MT, the improved chlorophyll content imported a higher amount of light energy capture, which caused a 6% increase in the quantum yield of PSII photochemistry (ΦPSII) and electron transport rate (ETR). Nevertheless, the spray with 100 μM MT reduced the efficiency of the oxygen-evolving complex (OEC), causing donor-side photoinhibition, with a simultaneous slight increase in ROS. Even so, the application of 100 μM MT decreased the excess excitation energy at PSII implying superior PSII efficiency. The decreased excitation pressure at PSII, after 100 μM MT foliar spray, suggests that MT induced stomatal closure through ROS production. The response of ΦPSII to MT spray corresponds to a J-shaped hormetic curve, with ΦPSII enhancement by 100 μM MT. It is suggested that the hormetic stimulation of PSII functionality was triggered by the non-photochemical quenching (NPQ) mechanism that stimulated ROS production, which enhanced the photosynthetic function. It is concluded that MT molecules can be used under both stress and non-stressed conditions as photosynthetic biostimulants for enhancing crop yields.

Melatonin Promotes Iron Reactivation and Reutilization in Peach Plants under Iron Deficiency

The yellowing of leaves due to iron deficiency is a prevalent issue in peach production. Although the capacity of exogenous melatonin (MT) to promote iron uptake in peach plants has been demonstrated, its underlying mechanism remains ambiguous. This investigation was carried out to further study the effects of exogenous MT on the iron absorption and transport mechanisms of peach (Prunus persica) plants under iron-deficient conditions through transcriptome sequencing. Under both iron-deficient and iron-supplied conditions, MT increased the content of photosynthetic pigments in peach leaves and decreased the concentrations of pectin, hemicellulose, cell wall iron, pectin iron, and hemicellulose iron in peach plants to a certain extent. These effects stemmed from the inhibitory effect of MT on the polygalacturonase (PG), cellulase (Cx), phenylalanine ammonia-lyase (PAL), and cinnamoyl-coenzyme A reductase (CCR) activities, as well as the promotional effect of MT on the cinnamic acid-4-hydroxylase (C4H) activity, facilitating the reactivation of cell wall component iron. Additionally, MT increased the ferric-chelate reductase (FCR) activity and the contents of total and active iron in various organs of peach plants under iron-deficient and iron-supplied conditions. Transcriptome analysis revealed that the differentially expressed genes (DEGs) linked to iron metabolism in MT-treated peach plants were primarily enriched in the aminoacyl-tRNA biosynthesis pathway under iron-deficient conditions. Furthermore, MT influenced the expression levels of these DEGs, regulating cell wall metabolism, lignin metabolism, and iron translocation within peach plants. Overall, the application of exogenous MT promotes the reactivation and reutilization of iron in peach plants.

Nitrogen Journey in Plants: From Uptake to Metabolism, Stress Response, and Microbe Interaction

Plants uptake and assimilate nitrogen from the soil in the form of nitrate, ammonium ions, and available amino acids from organic sources. Plant nitrate and ammonium transporters are responsible for nitrate and ammonium translocation from the soil into the roots. The unique structure of these transporters determines the specificity of each transporter, and structural analyses reveal the mechanisms by which these transporters function. Following absorption, the nitrogen metabolism pathway incorporates the nitrogen into organic compounds via glutamine synthetase and glutamate synthase that convert ammonium ions into glutamine and glutamate. Different isoforms of glutamine synthetase and glutamate synthase exist, enabling plants to fine-tune nitrogen metabolism based on environmental cues. Under stressful conditions, nitric oxide has been found to enhance plant survival under drought stress. Furthermore, the interaction between salinity stress and nitrogen availability in plants has been studied, with nitric oxide identified as a potential mediator of responses to salt stress. Conversely, excessive use of nitrate fertilizers can lead to health and environmental issues. Therefore, alternative strategies, such as establishing nitrogen fixation in plants through diazotrophic microbiota, have been explored to reduce reliance on synthetic fertilizers. Ultimately, genomics can identify new genes related to nitrogen fixation, which could be harnessed to improve plant productivity.

Mitigating Cd and bacterial wilt stress in tomato plants through trico-synthesized silicon nanoparticles and Trichoderma metabolites

There has been a growing apprehension in recent years about the harmful effects of environmental pollutants on agricultural output, encompassing both living organisms and non-living factors that cause stress. In this study, the soil application of bulk silicon (Si), silicon nanoparticles (SiNPs) and Trichoderma metabolites (TM) were investigated alone or in combination for the management of an important abiotic stress i.e. Cd toxicity and biotic stress i.e. bacterial wilt (BW) in tomato plants. SiNPs were synthesized by Trichoderma and confirmed through XRD, FTIR, and Ranman spectrum analysis. Results showed that Si, SiNPs and TM were all effective treatments. The combine treatment of SiNPs and TM followed by SiNPs alone were superior over other treatments in mitigating Cd toxicity and reducing BW disease on tomato plants. The soil application of these treatments reduced the Cd toxicity by enhancing Cd-tolerance index, decreasing bioavailability of soil Cd, reducing Cd contents and translocation in plants, improving gaseous exchange, photosynthesis, and increasing the antioxidant enzyme activities and their transcriptions. These treatments significantly suppressed BW pathogen leading to the significant decrease in disease index and severity on plants. In vitro evaluation and scanning electron microscopic (SEM) analysis revealed that SiNPs and TM significantly disrupted the cellular morphology of BW pathogen Ralstonia solanacearum. Findings of this study proposes the possible use of SiNPs and TM in mitigating the Cd and BW stress in tomato plants and possibly in other crops.

Uncovering the synergistic interplay of melatonin and spermidine in the alleviation of nematode stress in Solanum lycopersicum

Worldwide, biotic stress severely degrades agricultural output and increases the risk of starvation. Root-knot nematodes (Meloidogyne incognita) are one of the important endoparasites that adversely affect the growth and development in plants, thus affecting their productivity. Contrarily, humans employ a number of unfriendly techniques, such as chemical applications, to manage biotic stressors. Use of Plant Growth Regulators is an environmentally safe alternative method against chemical pesticides that can be used to defend plants from biotic stressors. Melatonin and polyamines have been broadly found in multiple physiological processes and in diverse biotic and abiotic stresses faced by plants. In the contemporaneous study, we conducted an in vitro experiment which disclosed that pretreated seeds with melatonin and spermidine (a polyamine), decreased root galls in afflicted plants and uplifted the growth of Solanum lycopersicum seedlings. According to our findings, tomato plants' photosynthetic efficiency dropped and reactive oxygen species levels dramatically rose after nematode inoculation. On the other hand, melatonin and spermidine decreased oxidative stress by scavenging hydrogen peroxide and decreased malonaldehyde. The present work investigated improvement in growth characteristics, photosynthetic pigments, antioxidative enzymes and non-antioxidative enzymes in PGR treated tomato seedlings even during the nematode stress. Confocal studies evaluated nuclear damage within root apices and intensity of blue colour was directly proportional to nuclear damage. The findings of the present investigation support the use of plant growth regulators, melatonin and spermidine as seed priming agent to manage nematode stress in plants.

Plant transcriptional memory and associated mechanism of abiotic stress tolerance

Plants face various adverse environmental conditions, particularly with the ongoing changes in global climate, which drastically affect the growth, development and productivity of crops. To cope with these stresses, plants have evolved complex mechanisms, and one of the crucial ways is to develop transcriptional memories from stress exposure. This induced learning enables plants to better and more strongly restart the response and adaptation mechanism to stress when similar or dissimilar stresses reoccur. Understanding the molecular mechanism behind plant transcriptional memory of stress can provide a theoretical basis for breeding stress-tolerant crops with resilience to future climates. Here we review the recent research progress on the transcriptional memory of plants under various stresses and the applications of underlying mechanisms for sustainable agricultural production. We propose that a thorough understanding of plant transcriptional memory is crucial for both agronomic management and resistant breeding, and thus may help to improve agricultural yield and quality under changing climatic conditions.

Melatonin Role in Plant Growth and Physiology under Abiotic Stress

Phyto-melatonin improves crop yield by mitigating the negative effects of abiotic stresses on plant growth. Numerous studies are currently being conducted to investigate the significant performance of melatonin in crops in regulating agricultural growth and productivity. However, a comprehensive review of the pivotal performance of phyto-melatonin in regulating plant morpho-physiological and biochemical activities under abiotic stresses needs to be clarified. This review focused on the research on morpho-physiological activities, plant growth regulation, redox status, and signal transduction in plants under abiotic stresses. Furthermore, it also highlighted the role of phyto-melatonin in plant defense systems and as biostimulants under abiotic stress conditions. The study revealed that phyto-melatonin enhances some leaf senescence proteins, and that protein further interacts with the plant's photosynthesis activity, macromolecules, and changes in redox and response to abiotic stress. Our goal is to thoroughly evaluate phyto-melatonin performance under abiotic stress, which will help us better understand the mechanism by which phyto-melatonin regulates crop growth and yield.

Deciphering the melatonin-mediated response and signalling in the regulation of heavy metal stress in plants

MAIN CONCLUSION

Melatonin has a protective effect against heavy metal stress in plants by immobilizing HM in cell walls and sequestering them in root cell vacuoles, reducing HM's translocation from roots to shoots. It enhances osmolyte production, increases antioxidant enzyme activity, and improves photosynthesis, thereby improving cellular functions. Understanding the melatonin-mediated response and signalling can sustain crop production in heavy metal-stressed soils. Melatonin is a pleiotropic signal molecule that plays a critical role in plant growth and stress tolerance, particularly against heavy metals in soil. Heavy metals (HMs) are ubiquitously found in the soil-water environment and readily taken up by plants, thereby disrupting mineral nutrient homeostasis, osmotic balance, oxidative stress, and altered primary and secondary metabolism. Plants combat HM stress through inbuilt defensive mechanisms, such as metal exclusion, restricted foliar translocation, metal sequestration and compartmentalization, chelation, and scavenging of free radicals by antioxidant enzymes. Melatonin has a protective effect against the damaging effects of HM stress in plants. It achieves this by immobilizing HM in cell walls and sequestering them in root cell vacuoles, reducing HM's translocation from roots to shoots. This mechanism improves the uptake of macronutrients and micronutrients in plants. Additionally, melatonin enhances osmolyte production, improving the plant's water relations, and increasing the activity of antioxidant enzymes to limit lipid peroxidation and reactive oxygen species (ROS) levels. Melatonin also decreases chlorophyll degradation while increasing its synthesis, and enhances RuBisCO activity for better photosynthesis. All these functions contribute to improving the cellular functions of plants exposed to HM stress. This review aims to gain better insight into the melatonin-mediated response and signalling under HM stress in plants, which may be useful in sustaining crop production in heavy metal-stressed soils.

A cysteine-rich secretory protein involves in phytohormone melatonin mediated plant resistance to CGMMV

BACKGROUND

Melatonin is considered to be a polyfunctional master regulator in animals and higher plants. Exogenous melatonin inhibits plant infection by multiple diseases; however, the role of melatonin in Cucumber green mottle mosaic virus (CGMMV) infection remains unknown.

RESULTS

In this study, we demonstrated that exogenous melatonin treatment can effectively control CGMMV infection. The greatest control effect was achieved by 3 days of root irrigation at a melatonin concentration of 50 μM. Exogenous melatonin showed preventive and therapeutic effects against CGMMV infection at early stage in tobacco and cucumber. We utilized RNA sequencing technology to compare the expression profiles of mock-inoculated, CGMMV-infected, and melatonin+CGMMV-infected tobacco leaves. Defense-related gene CRISP1 was specifically upregulated in response to melatonin, but not to salicylic acid (SA). Silencing CRISP1 enhanced the preventive effects of melatonin on CGMMV infection, but had no effect on CGMMV infection. We also found exogenous melatonin has preventive effects against another Tobamovirus, Pepper mild mottle virus (PMMoV) infection.

CONCLUSIONS

Together, these results indicate that exogenous melatonin controls two Tobamovirus infections and inhibition of CRISP1 enhanced melatonin control effects against CGMMV infection, which may lead to the development of a novel melatonin treatment for Tobamovirus control.

Transcriptomic and Physiological Analyses Reveal the Molecular Mechanism through Which Exogenous Melatonin Increases Drought Stress Tolerance in Chrysanthemum

Chrysanthemum (Chrysanthemum morifolium (Ramat.) Hemsl.) is an important species in China's flower industry, and drought stress seriously affects the growth, quality, yield, and geographical distribution of this species. Melatonin (MT) plays a key role in regulating plant abiotic stress responses and stress resistance, but the mechanism through which exogenous MT regulates drought resistance in chrysanthemum remains unclear. This study explored the protective effect of MT on chrysanthemum drought tolerance and its key regulatory pathways. Exogenous MT application increased the photosynthetic capacity (Tr increased by 18.07%; Pn increased by 38.46%; and Gs increased by 26.52%) of chrysanthemum and attenuated decreases in its chlorophyll (19.89%) and relative water contents (26.94%). Moreover, MT increased the levels of osmolarity-related compounds such as soluble sugars (43.60%) and soluble protein (9.86%) under drought stress and increased antioxidant enzyme activity (SOD increased by 20.98%; POD increased by 35.04%; and CAT increased by 26.21%). Additionally, MT increased the endogenous MT (597.96%), growth hormone (45.31% and 92.09%), gibberellic acid (75.92% and 3.79%), salicylic acid (33.02%), and cytokinin contents (1400.00%) under drought stress while decreasing the abscisic acid (50.69% and 56.79%), jasmonate contents (62.57% and 28.31%), and ethylene contents (9.28%). RNA-seq analysis revealed 17,389, 1466, and 9359 differentially expressed genes (DEGs) under three treatments (PEG, MT, and MT _ PEG, respectively) compared with the control. Enrichment analyses of the DEGs identified more than 10 GO terms and 34 KEGG pathways. Nitrogen metabolism, sulfur metabolism, and alanine, aspartate, and glutamate metabolism were significantly increased under all three treatments. The DEGs included many transcription factors, such as MYB, WRKY, and NAC proteins. Our results preliminarily classify candidate genes and metabolic pathways with active roles in the interaction between MT and drought stress and advance the understanding of the molecular mechanism of the response to drought stress under MT conditions, thereby providing a theoretical basis for the breeding of drought-resistant chrysanthemum.

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.

Phytohormones unlocking their potential role in tolerance of vegetable crops under drought and salinity stresses

Globally, abiotic stresses are drastically reducing the productivity of vegetable crops. Among abiotic stresses, drought and salinity are more challenging constraints for the sustainable production of vegetables. A great variety of vegetables are facing dry and hot summer spells, poor water availability, and higher salinity mainly due to irrigation with brackish water. Vegetables are considered higher water-dependent crops, requiring water for proper growth and yield. Drought and salinity impair plant metabolism. The disruption in plant metabolism leads to a reduction in growth, developmental processes, and ultimately crop yield. Appropriate management measures are needed to cope with the adverse effects of drought and salinity. Different agronomic and molecular approaches contributed to improving tolerance. Therefore, the present review significantly explores the impact of phytohormones on vegetable crops under drought and salinity stresses. Phytohormones (salicylic acid, melatonin, jasmonates, Brassinosteroids, ascorbic acid, and numerous others) can be sprayed for improvement of plant growth, yield, and photosynthetic pigments by modulation of physiological and biochemical processes. In this manner, these phytohormones should be explored for sustainable production of vegetable crops growing under abiotic stress conditions.

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

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.

The Content of Phenolic Compounds in Stevia rebaudiana (Bertoni) Plants Derived from Melatonin and NaCl Treated Seeds

Stevia is a plant with many beneficial properties. It contains not only steviol glycosides, which are used as non-caloric natural sweeteners, but also a number of metabolites with antioxidant properties. This study examined the content of both phenolic acids and flavonoids in stevia leaves as an effect of treating seeds with melatonin and conducting germination in NaCl conditions. The results of our research indicated higher amounts of phenolic acids compared to flavonoids in stevia leaves. Among these acids, isochlorogenic, rosmarinic, and chlorogenic acids were accumulated in the largest amounts, regardless of the germination conditions. For 5 and 100 µM of melatonin treatments, the content of both phenolic acids and flavonoids increased. However, in salinity conditions (50 mM NaCl), 500 µM of melatonin had the most favorable effect on the synthesis of phenolic acids. The phenolic acids in that case reached a level three-times higher than that in the samples with the same melatonin concentration but without NaCl. We also found that the content of phenolic compounds varied depending on the age of the leaves. To the best of our knowledge, this is the first study to describe the effect of melatonin and NaCl on the synthesis on phenolic acids and flavonoids in stevia.

Ectopic expression of HIOMT improves tolerance and nitrogen utilization efficiency in transgenic apple under drought stress

Melatonin enhances plant tolerance to various environmental stressors. Although exogenous application of melatonin has been investigated, the role of endogenous melatonin metabolism in the response of apples to drought stress and nutrient utilization remains unclear. Here, we investigated the effects of ectopically expressing the human melatonin synthase gene HIOMT on transgenic apple plants under drought stress conditions. The tolerance of transgenic apple lines that ectopically expressed HIOMT improved significantly under drought conditions. After 10 days of natural drought stress treatment, the transgenic apple plants showed higher relative water content, chlorophyll levels and Fv/Fm, and lower relative electrolyte leakage and hydrogen peroxide accumulation, than wild-type plants. The activities of peroxidase, superoxide dismutase and catalase, as well as genes in the ascorbate-glutathione cycle, increased more in transgenic apple plants than in the wild-type. The ectopic expression of HIOMT also markedly alleviated the inhibitory effects of long-term drought stress on plant growth, photosynthetic rate and chlorophyll concentrations in apple plants. The uptake and utilization of 15N increased markedly in the transgenic lines under long-term moderate drought stress. Drought stress sharply reduced the activity of enzymes involved in nitrogen metabolism, but ectopic expression of HIOMT largely reversed that response. The expression levels of genes of nitrogen metabolism and uptake were more upregulated in transgenic apple plants than the wild-type. Overall, our study demonstrates that ectopic expression of HIOMT enhanced the tolerance of apple plants to drought stress, and transgenic apple plants showed improved growth due to higher nutrient utilization efficiency under drought conditions.

Ectopic expression of MmCYP1A1, a mouse cytochrome P450 gene, positively regulates stress tolerance in apple calli and Arabidopsis

Ectopic expression of MmCYP1A1 gene from Mus musculus in apple calli and Arabidopsis increased the levels of melatonin and 6-hydroxymelatonin, and improved their stress resistance. Melatonin occurs widely in organisms, playing a key regulatory role. CYP1A1 is a cytochrome P450 monooxygenase, involved in the melatonin metabolism, and is responsible for the synthesis of 6-hydroxymelatonin from melatonin. Melatonin and 6-hydroxymelatonin have strong antioxidant activities in animals. Here, we cloned MmCYP1A1 from Mus musculus and found that ectopic expression of MmCYP1A1 improved the levels of melatonin and 6-hydroxymelatonin in transgenic apple calli and Arabidopsis. Subsequently, we observed that MmCYP1A1 increased the tolerance of transgenic apple calli and Arabidopsis to osmotic stress simulated by polyethylene glycol 6000 (PEG 6000), as well as resistance of transgenic Arabidopsis to drought stress. Further, the number of lateral roots of MmCYP1A1 transgenic Arabidopsis were enhanced significantly after PEG 6000 treatment. The expression of MmCYP1A1 remarkably reduced malondialdehyde (MDA) content, electrolyte leakage, accumulation of H₂O₂ and O₂^- during stress treatment. Moreover, MmCYP1A1 enhanced stress tolerance in apple calli and Arabidopsis by increasing the expression levels of resistance genes. MmCYP1A1 also promoted stomatal closure in transgenic Arabidopsis to reduce leaf water loss during drought. Our results indicate that MmCYP1A1 plays a key role in plant stress tolerance, which may provide a reference for future plant stress tolerance studies.

Melatonin Treatment Enhances the Growth and Productivity of Useful Metabolites in the In Vitro Culture of Spirodela polyrhiza

Spirodela polyrhiza (Araceae family) is a duckweed species that serves as a potential resource for feed, food, bioremediation, and pharmaceutical applications. In this study, we assessed the effects of different concentrations of melatonin (0, 0.1, 1, and 10 μM) on the growth of S. polyrhiza during in vitro culture and the metabolic profiles and productivities of useful metabolites using gas chromatography-mass spectrometry coupled with multivariable statistical analysis. We found that exogenous melatonin significantly improved the total dry weight and altered the metabolic profiles of S. polyrhiza cultures. Melatonin significantly enhanced the cellular production of useful metabolites, such as γ-aminobutyric acid, dopamine, threonine, valine, and phytosterols. The volumetric productivities (mg/L) of γ-aminobutyric acid, dopamine, campesterol, β-sitosterol, and stigmasterol were the highest in the presence of 10 μM melatonin on day 12. Moreover, the productivities of ascorbic acid and serotonin were the highest in the presence of 1 μM melatonin on day 12. Therefore, melatonin could be used to enhance the production of biomass and useful metabolites during large-scale S. polyrhiza cultivation in cosmetic, food/feed, and pharmaceutical industries.

Carbon dots promoted soybean photosynthesis and amino acid biosynthesis under drought stress: Reactive oxygen species scavenging and nitrogen metabolism

With global warming and water scarcity, improving the drought tolerance and quality of crops is critical for food security and human health. Here, foliar application of carbon dots (CDs, 5 mg·L^-1) could scavenge reactive oxygen species accumulation in soybean leaves under drought stress, thereby enhancing photosynthesis and carbohydrate transport. Moreover, CDs stimulated root secretion (e.g., amino acids, organic acids, and auxins) and recruited beneficial microorganisms (e.g., Actinobacteria, Ascomycota, Acidobacteria and Glomeromycota), which facilitate nitrogen (N) activation in the soil. Meanwhile, the expression of GmNRT, GmAMT, and GmAQP genes were up-regulated, indicating enhanced N and water uptake. The results demonstrated that CDs could promote nitrogen metabolism and enhance amino acid biosynthesis. Particularly, the N content in soybean shoots and roots increased significantly by 13.2 % and 30.5 %, respectively. The amino acids content in soybean shoots and roots increased by 257.5 % and 57.5 %, respectively. Consequently, soybean yields increased significantly by 21.5 %, and the protein content in soybean kernels improved by 3.7 %. Therefore, foliar application of CDs can support sustainable nano-enabled agriculture to combat climate change.

Melatonin alleviates arsenite toxicity by decreasing the arsenic accumulation in cell protoplasts and increasing the antioxidant capacity in rice

Arsenic (As) is a common environmental pollutant that seriously interferes with the normal growth of organisms. There is an urgent need to take environment-safe and efficient strategies to mitigate As toxicity. Melatonin (MT) is a pleiotropic molecule that regulates plant growth and organ development and alleviates heavy metal stresses. The experiment aims to explore the mechanism of MT in reducing arsenite toxicity by hydroponic rice seedlings. The results showed that MT application reduced the As content in rice roots and shoots by 26.4% and 37.5%, respectively, and mainly decreased As content in the soluble fractions of the rice root cell. MT application also increased the As content of chelated-soluble pectin and alkali-soluble pectin in the cell wall by 14.7% and 74.4%, respectively. It promoted the generation of the functional group of the root cell walls by the FTIR analysis, indicating that MT may promote the fixation of As on the cell wall. Meanwhile, MT contributed to scavenging excess H₂O₂, reducing MDA content, and maintaining normal morphology of root cells by stimulating SOD, POD and CAT activities and increasing the level of GSH. The research deepens our understanding of how MT participates in maintaining redox homeostasis in rice cells, reducing As toxicity, and decreasing As concentration in rice seedlings, thereby providing more possibilities for reducing As accumulation in rice.

Biochemical Mechanism of Fresh-Cut Lotus (Nelumbo nucifera Gaertn.) Root with Exogenous Melatonin Treatment by Multiomics Analysis

Browning limits the commercial value of fresh-cut lotus root slices. Melatonin has been reported to play crucial plant roles in growth and development. However, the mechanisms in repressing the browning of fresh-cut lotuses are still unclear. In this study, fresh-cut lotus root slices were treated with melatonin, the physical signs of browning were tested, and then the selected samples (0 d, 6 d, 12 d) were used in multiomics analysis. Fresh-cut lotus root slices with a thickness of 4 mm were soaked in a 40 mmol/L melatonin solution for 10 min; then, the slices were packed in pallets and packages and stored at 10 ± 1 °C. The results show that the 40 mmol/L melatonin selected for repressing the browning of lotus roots significantly delayed the decrease in water, total soluble solid content, and Vitamin C, decreased the growth of microorganisms, enhanced total phenolic content, improved total antioxidant capacity, and decreased ·OH, H₂O₂, and O₂^-· contents. Moreover, this treatment enhanced phenylalanine ammonialyase, polyphenol oxidase, superoxide dismutase, and catalase activities and reduced peroxidase activities and soluble quinones. NnSOD (104590242), NnCAT (104609297), and some NnPOD genes showed a similar transcript accumulation pattern with enzyme activity. It can be seen from these results that exogenous melatonin accelerated an enhancement in the antioxidant system and AsA-GSH cycle system by regulating ROS-metabolism-related genes, thereby improving the capacity to withstand browning and the quality of lotus root slices. The microbiome also showed that melatonin suppressed the fertility of spoilage organisms, such as Pseudomonas, Tolumonas, Acinetobacter, Stenotrophomonas, and Proteobacteria. Metabonomics data uncovered that the metabolites of flavonoid biosynthesis, phenylpropanoid biosynthesis, tyrosine metabolism, and phenylalanine metabolism were involved in the process.

Melatonin induces drought tolerance by modulating lipoxygenase expression, redox homeostasis and photosynthetic efficiency in Arachis hypogaea L

Melatonin (N-acetyl-5-hydroxy tryptamine), a multipotent biomolecule is well known for its ability to confer tolerance to several abiotic and biotic stresses. The regulation of melatonin-mediated drought tolerance in drought-distinguished varieties can be different due to discriminating redox levels. The present study was focused on assessing the effects of melatonin priming against polyethylene glycol (PEG)-induced stress with respect to the antioxidant system, photosynthetic parameters, lipoxygenase expression, JA and ABA levels in drought-sensitive (Kadiri-7) and drought-tolerant (Kadiri-9) varieties. Exogenous melatonin alleviated the drought stress effects in sensitive variety (Kadiri-7) by increasing the endogenous melatonin content with an improved antioxidant system and photosynthetic attributes. The primed stressed plants of the sensitive variety exhibited reduced expression and activity of the chlorophyll degrading enzymes (Chl-deg PRX, pheophytinase and chlorophyllase) with a concomitant increase in chlorophyll content in comparison to unprimed controls. Interestingly, melatonin priming stimulated higher expression and activity of lipoxygenase (LOX) as well as enhanced the expression of genes involved in the synthesis of jasmonic acid (JA) including its content in drought stressed plants of the sensitive variety. The expression of NCED3 (involved in ABA-biosynthesis) was upregulated while CYP707A2 (ABA-degradation) was downregulated which corresponded with higher ABA levels. Contrastingly, priming caused a decrease in endogenous melatonin content under drought stress in tolerant variety (Kadiri-9) which might be due to feedback inhibition of its synthesis to maintain intracellular redox balance and regulate better plant metabolism. Furthermore, the higher endogenous melatonin content along with improved antioxidant system, photosynthetic efficiency and LOX expression associated with the increased levels of JA and ABA in unprimed stressed plants of the tolerant variety (Kadiri-9) is pointing towards the effectiveness of melatonin in mediating drought stress tolerance. Overall, exogenous melatonin alleviated the adverse effects of drought stress in sensitive variety while having no add-on effect on drought stress responses of tolerant variety which is inherently equipped to withstand the given duration of drought stress treatment.

Screening of cold hardiness-related indexes and establishment of a comprehensive evaluation method for grapevines (V. vinifera)

The goals of this work were to screen physiological and biochemical indexes to assess a set of V. vinifera germplasm resources, to compare evaluation methods for cold hardiness, and to establish a comprehensive method that can be used for more accurate screening for cold hardiness in V. vinifera. Four single methods were used to evaluate the cold hardiness of 20 germplasms resources and 18 physiological and biochemical indexes related to cold hardiness were determined. The LT₅₀ values determined by electrical conductivity (EL), 2,3,5-triphenyltetrazolium chloride staining (TTC), differential thermal analysis (DTA), and recovery growth (RG) methods showed extremely significant positive correlation. Bound water content (BW), proline content (Pro), total soluble sugar content (TSS), malondialdehyde content (MDA), catalase content (CAT), and ascorbic acid content (ASA) exhibited significant correlation with LT₅₀ values measured by different evaluation methods. The comprehensive cold hardiness index calculated by principal component analysis (PCA) combined with subordinate function (SF) was negatively correlated with LT₅₀ values measured by different evaluation methods. Meili and Ecolly exhibited the highest cold hardiness, indicating their potential for use as parents for cold hardiness breeding. EL, DTA, TTC, and RG methods successfully distinguished cold hardiness among different V. vinifera germplasm lines. Measurements of BW, Pro, TSS, MDA, CAT, and ASA in dormant shoots also can be used as main physiological and biochemical indexes related to cold hardiness of V. vinifera. Comprehensive evaluation by PCA combined with SF can accurately screen cold hardiness in V. vinifera. This study provides a reference and accurate identification method for the selection of cold hardiness parents and the evaluation of cold hardiness of germplasm of V. vinifera.

Involvement of ethylene in melatonin-modified photosynthetic-N use efficiency and antioxidant activity to improve photosynthesis of salt grown wheat

The involvement of melatonin in the regulation of salt stress acclimation has been shown in plants in this present work. We found that the GOAL cultivar of wheat (Triticum aestivum L.) was the most salt-tolerant among the investigated cultivars, GOAL, HD-2967, PBW-17, PBW-343, PBW-550, and WH-1105 when screened for tolerance to 100 mM NaCl. The application of 100 μM melatonin maximally reduced oxidative stress and improved photosynthesis in the cv. GOAL. Melatonin supplementation reduced salt stress-induced oxidative stress by upregulating the activity of antioxidant enzymes, such as superoxide dismutase (SOD), ascorbate peroxidase (APX), and glutathione reductase (GR), and reduced the glutathione (GSH) production. This resulted in increased membrane stability, photosynthetic-N use efficiency and photosynthesis in plants. The application of 50 μM of the ethylene biosynthesis inhibitor aminoethoxyvinylglycine (AVG) in the presence of melatonin and salt stress increased H₂ O₂ content but reduced GR activity and GSH, photosynthesis, and plant dry mass. This signifies that melatonin-mediated salt stress tolerance was related to ethylene synthesis as it improved antioxidant activity and photosynthesis of plants under salt stress. Thus, the interaction of melatonin and ethylene bears a prominent role in salt stress tolerance in wheat and can be used to develop salt tolerance in other crops.

Exogenous silicon enhances resistance to 1,2,4-trichlorobenzene in rice

Environmental contamination with 1,2,4-trichlorobenzene (TCB) is a threat to rice growth, and ultimately, to human health. Silicon (Si) plays an important role in plants' stress responses. However, little is known about the effects of Si on the TCB tolerance of rice plants. We investigated the effects of Si on the morphological, physiological, and molecular characteristics of rice plants under TCB stress. First, we compared the TCB tolerance of 13 rice cultivars by measuring seven growth-related and 13 physiological indices across four treatments. Then, six cultivars with contrasting TCB tolerance were selected to study the expression of Si transport and detoxification related genes. Compared with the control, the TCB treatment resulted in decreased growth indices, chlorophyll content, and antioxidant enzyme activities, and increased the superoxide anion content and root electrical conductivity. Application of Si improved rice growth, chlorophyll content and alleviated oxidative damage caused by TCB. The alleviating effect of Si ranged from 4.1 % to 56.72 % among the cultivars, with the strongest alleviating effect on Wuyujing 36. The transcript levels of genes encoding Si transporters and detoxification enzymes were higher in tolerant cultivars than in sensitive cultivars. The TCB treatment induced the expression of GST and Lsi2 in roots and HO-1 in leaves; these genes as well as Lsi1 were differentially expressed in roots and/or leaves in the TCB + Si treatment. Lsi1 played a key role in Si-mediated TCB tolerance in Wuyujing 36. The joint analysis of gene transcript levels in TCB and TCB + Si treatments confirmed that all six genes were associated with TCB tolerance, especially Lsi1 and Lsi2 in roots and GST and CuZn-SOD in leaves. Si can increase rice plants' resistance to TCB stress by improving growth and enhancing superoxide dismutase (SOD) activity and chlorophyll content, and by up-regulating genes involved in Si transport and detoxification.

Towards circular economy: Sustainable soil additives from natural waste fibres to improve water retention and soil fertility

Human activity is accompanied by the introduction of excessive amounts of artificial materials, including geosynthetics, into the environment, causing global environmental pollution. Moreover, climate change continues to negatively affect global water resources. With the intensification of environmental problems, material reusability and water consumption limitations have been proposed. This study replaced synthetic soil additives with biodegradable materials and analysed the potential and sustainable processing of natural fibrous materials, which form problematic waste. Waste fibres are the basis of innovative soil water storage technologies in the form of biodegradable and water-absorbing geocomposites (BioWAG). We analysed the influence of BioWAGs on plant vegetation and the environment through a three-year field experiment. Furthermore, biomass increases, drought effect reductions, and biodegradation mechanisms were analysed. Natural waste fibres had a positive influence, as they released easily accessible nutrients into the soil during biodegradation. BioWAGs had a positive influence on the biometric parameters of grass, increasing biomass growth by 430 %. Our results indicated that this is an effective method of waste fibre management that offers the possibility to manufacture innovative, environmentally friendly materials in compliance with the objectives of circular economy and the expectations of users.

Melatonin Enhances Drought Tolerance in Rice Seedlings by Modulating Antioxidant Systems, Osmoregulation, and Corresponding Gene Expression

Rice is the third largest food crop in the world, especially in Asia. Its production in various regions is affected to different degrees by drought stress. Melatonin (MT), a novel growth regulator, plays an essential role in enhancing stress resistance in crops. Nevertheless, the underlying mechanism by which melatonin helps mitigate drought damage in rice remains unclear. Therefore, in the present study, rice seedlings pretreated with melatonin (200 μM) were stressed with drought (water potential of −0.5 MPa). These rice seedlings were subsequently examined for their phenotypes and physiological and molecular properties, including metabolite contents, enzyme activities, and the corresponding gene expression levels. The findings demonstrated that drought stress induced an increase in malondialdehyde (MDA) levels, lipoxygenase (LOX) activity, and reactive oxygen species (ROS, e.g., O₂⁻ and H₂O₂) in rice seedlings. However, the melatonin application significantly reduced LOX activity and the MDA and ROS contents (O₂⁻ production rate and H₂O₂ content), with a decrease of 29.35%, 47.23%, and (45.54% and 49.33%), respectively. It activated the expression of ALM1, OsPOX1, OsCATC, and OsAPX2, which increased the activity of antioxidant enzymes such as superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX), respectively. Meanwhile, the melatonin pretreatment enhanced the proline, fructose, and sucrose content by inducing OsP5CS, OsSUS7, and OsSPS1 gene expression levels. Moreover, the melatonin pretreatment considerably up-regulated the expression levels of the melatonin synthesis genes TDC2 and ASMT1 under drought stress by 7-fold and 5-fold, approximately. These improvements were reflected by an increase in the relative water content (RWC) and the root-shoot ratio in the drought-stressed rice seedlings that received a melatonin application. Consequently, melatonin considerably reduced the adverse effects of drought stress on rice seedlings and improved rice’s ability to tolerate drought by primarily boosting endogenous antioxidant enzymes and osmoregulation abilities.

Melatonin biosynthesis and signal transduction in plants in response to environmental conditions

Melatonin, the most widely distributed hormone in nature, plays important roles in plants. Many physiological processes in plants are linked to melatonin, including seed germination, anisotropic cell growth, and senescence. Compared with animals, different plants possess diverse melatonin biosynthetic pathways and regulatory networks. Whereas melatonin biosynthesis in animals is known to be regulated by ambient signals, little is known about how melatonin biosynthesis in plants responds to environmental signals. Plants are affected by numerous environmental factors, such as light, temperature, moisture, carbon dioxide, soil conditions, and nutrient availability at all stages of development and in different tissues. Melatonin content exhibits dynamic changes that affect plant growth and development. Melatonin plays various species-specific roles in plant responses to different environmental conditions. However, much remains to be learned, as not all environmental factors have been studied, and little is known about the mechanisms by which these factors influence melatonin biosynthesis. In this review, we provide a detailed, systematic description of melatonin biosynthesis and signaling and of the roles of melatonin in plant responses to different environmental factors, providing a reference for in-depth research on this important issue.

Anabolism and signaling pathways of phytomelatonin

Phytomelatonin is a small multifunctional molecule found ubiquitously in plants, which plays an important role in plant growth, development, and biotic and abiotic stress responses. The classical biosynthetic and metabolic pathways of phytomelatonin have been elucidated, and uncovering alternative pathways has deepened our understanding of phytomelatonin synthesis. Phytomelatonin functions mainly via two pathways. In the direct pathway, phytomelatonin mediates the stress-induced reactive oxygen species burst through its strong antioxidant capacity. In the indirect pathway, phytomelatonin acts as a signal to activate signaling cascades and crosstalk with other plant hormones. The phytomelatonin receptor PMTR1/CAND2 was discovered in 2018, which enhanced our understanding of phytomelatonin function. This review summarizes the classical and potential pathways involved in phytomelatonin synthesis and metabolism. To elucidate the functions of phytomelatonin, we focus on the crosstalk between phytomelatonin and other phytohormones. We propose two models to explain how PMTR1 transmits the phytomelatonin signal through the G protein and MAPK cascade. This review will facilitate the identification of additional signaling molecules that function downstream of the phytomelatonin signaling pathway, thus improving our understanding of phytomelatonin signal transmission.

Phytomelatonin as a central molecule in plant disease resistance

Melatonin is an essential phytohormone in the regulation of many plant processes, including during plant development and in response to stress. Pathogen infections cause serious damage to plants and reduce agricultural production. Recent studies indicate that melatonin plays important roles in alleviating bacterial, fungal, and viral diseases in plants and post-harvest fruits. Herein, we summarize information related to the effects of melatonin on plant disease resistance. Melatonin, reactive oxygen species, and reactive nitrogen species form a complex loop in plant-pathogen interaction to regulate plant disease resistance. Moreover, crosstalk of melatonin with other phytohormones including salicylic acid, jasmonic acid, auxin, and abscisic acid further activates plant defense genes. Melatonin plays an important role not only in plant immunity but also in alleviating pathogenicity. We also summarize the known processes by which melatonin mediates pathogenicity via negatively regulating the expression levels of genes related to cell viability as well as virulence-related genes. The multiple mechanisms underlying melatonin influences on both plant immunity and pathogenicity support the recognition of the essential nature of melatonin in plant-pathogen interactions, highlighting phytomelatonin as a critical molecule in plant immune responses.