Bacterial and Fungal Communities Are Differentially Modified by Melatonin in Agricultural Soils Under Abiotic Stress

Views and perspectives on the indoleamines serotonin and melatonin in plants: past, present and future

In the decades since their discovery in plants in the mid-to-late 1900s, melatonin (N-acetyl-5-methoxytryptamine) and serotonin (5-methoxytryptamine) have been established as their own class of phytohormone and have become popular targets for examination and study as stress ameliorating compounds. The indoleamines play roles across the plant life cycle from reproduction to morphogenesis and plant environmental perception. There is growing interest in harnessing the power of these plant neurotransmitters in applied and agricultural settings, particularly as we face increasingly volatile climates for food production; however, there is still a lot to learn about the mechanisms of indoleamine action in plants. A recent explosion of interest in these compounds has led to exponential growth in the field of melatonin research in particular. This concept paper aims to summarize the current status of indoleamine research and highlight some emerging trends.

Integrative transcriptome and metabolome analysis reveals the mechanism of exogenous melatonin alleviating drought stress in maize roots

Melatonin (MT) is essential for plant development and drought adaptation. However, the molecular and metabolic mechanisms underlying MT-induced drought tolerance in maize roots remain largely unclear. Herein, we investigated the effects of MT on drought tolerance in maize roots using integrated transcriptomic and metabolomic analyses, and identified MT-induced genes and metabolites associated with drought resistance. Compared with the untreated control plants, MT application alleviated the deleterious effects of drought on roots, by decreasing the malondialdehyde level and increasing the solute potential, eventually promoting root growth. Transcriptome and metabolome analysis demonstrated that MT significantly upregulates the expression of genes related to flavonoid biosynthesis (PAL, C4H, 4CL, HCT, CHS, CHI, F3'5'H, and DFR), activates drought-responsive transcription factors (ERFs, NACs, MYBs, and bHLHs), and regulates hormone signaling-related genes, especially ethylene response factors (ERF4, ERF81, and ERF110). Moreover, MT increased the accumulation of flavonoid metabolites, particularly apigenin, luteolin, and quercetin, under drought-stress conditions. These findings were further supported by quantitative real-time polymerase chain reaction analysis and total flavonoid measurements. Altogether, our findings suggest that MT promotes maize root growth during drought by regulating flavonoid synthesis pathways, transcription factors, and plant hormone signals. This study provides new insights into the complex mechanisms by which MT enhances crop resistance to drought damage.

Melatonin production by rhizobacteria native strains: Towards sustainable plant growth promotion strategies

In the current context of climate change and water deficit, the selection of native beneficial microorganisms, such as plant growth-promoting rhizobacteria (PGPR), has become a trend for sustainable agriculture due to their ability to improve plant-bacteria interaction with a minimal adverse effect on the soil microbiota compared to commercial PGPR. Until now, the production of phytohormones like melatonin (MT) by native PGPR and their effect on endogenous MT levels in plants have been poorly studied. MT is a ubiquitous phytohormone that protects plants against biotic and abiotic stress by improving the tolerance of stressed plants. In this work, the production of MT by two native PGPR, Enterobacter 64S1 and Pseudomonas 42P4, was evaluated and both PGPR were applied in Arabidopsis thaliana plants grown under drought conditions to assess the inoculation effects. Parameters such as plant growth, leaf cellular membrane damage, leaf protective compounds, and endogenous MT levels under drought and irrigation conditions were evaluated. The results demonstrated that the native strains Pseudomonas 42P4 and Enterobacter 64S1 produce MT and increase the content of endogenous MT in A. thaliana plants under drought. These native strains improved the tolerance of arabidopsis plants to drought by preventing oxidative and membrane damages and improving plant growth. To the best of our knowledge, this is the first report on MT production by native PGPR and their effects on endogenous MT levels in arabidopsis plants, setting the bases to elucidate the role of native PGPR on water deficit conditions.

Role of Melatonin and Nitrogen Metabolism in Plants: Implications under Nitrogen-Excess or Nitrogen-Low

Melatonin is a new plant hormone involved in multiple physiological functions in plants such as germination, photosynthesis, plant growth, flowering, fruiting, and senescence, among others. Its protective role in different stress situations, both biotic and abiotic, has been widely demonstrated. Melatonin regulates several routes in primary and secondary plant metabolism through the up/down-regulation of many enzyme/factor genes. Many of the steps of nitrogen metabolism in plants are also regulated by melatonin and are presented in this review. In addition, the ability of melatonin to enhance nitrogen uptake under nitrogen-excess or nitrogen-low conditions is analyzed. A model that summarizes the distribution of nitrogen compounds, and the osmoregulation and redox network responses mediated by melatonin, are presented. The possibilities of using melatonin in crops for more efficient uptake, the assimilation and metabolization of nitrogen from soil, and the implications for Nitrogen Use Efficiency strategies to improve crop yield are also discussed.

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.

Exogenous Melatonin Reprograms the Rhizosphere Microbial Community to Modulate the Responses of Barley to Drought Stress

The rhizospheric melatonin application-induced drought tolerance has been illuminated in various plant species, while the roles of the rhizosphere microbial community in this process are still unclear. Here, the diversity and functions of the rhizosphere microbial community and related physiological parameters were tested in barley under the rhizospheric melatonin application and drought. Exogenous melatonin improved plant performance under drought via increasing the activities of non-structural carbohydrate metabolism enzymes and activating the antioxidant enzyme systems in barley roots under drought. The 16S/ITS rRNA gene sequencing revealed that drought and melatonin altered the compositions of the microbiome. Exogenous melatonin increased the relative abundance of the bacterial community in carbohydrate and carboxylate degradation, while decreasing the relative abundance in the pathways of fatty acid and lipid degradation and inorganic nutrient metabolism under drought. These results suggest that the effects of melatonin on rhizosphere microbes and nutrient condition need to be considered in its application for crop drought-resistant cultivation.

Basic Cognition of Melatonin Regulation of Plant Growth under Salt Stress: A Meta-Analysis

Salt stress severely restricts the growth of plants and threatens the development of agriculture throughout the world. Worldwide studies have shown that exogenous melatonin (MT) can effectively improve the growth of plants under salt stress. Through a meta-analysis of 549 observations, this study first explored the effects of salt stress characteristics and MT application characteristics on MT regulated plant growth under salt stress. The results show that MT has a wide range of regulatory effects on plant growth indicators under salt stress, of which the regulatory effect on root indexes is the strongest, and this regulatory effect is not species-specific. The intensity of salt stress did not affect the positive effect of MT on plant growth, but the application effect of MT in soil was stronger than that in rooting medium. This meta-analysis also revealed that the foliar application of a concentration between 100–200 μM is the best condition for MT to enhance plant growth under salt stress. The results can inspire scientific research and practical production, while seeking the maximum improvement in plant salt tolerance under salt stress.

Rhizosphere melatonin application reprograms nitrogen-cycling related microorganisms to modulate low temperature response in barley

Rhizospheric melatonin application has a positive effect on the tolerance of plants to low temperature; however, it remains unknown whether the rhizosphere microorganisms are involved in this process. The aim of this study was to investigate the effect of exogenous melatonin on the diversity and functioning of fungi and bacteria in rhizosphere of barley under low temperature. The results showed that rhizospheric melatonin application positively regulated the photosynthetic carbon assimilation and redox homeostasis in barley in response to low temperature. These effects might be associated with an altered diversity of microbial community in rhizosphere, especially the species and relative abundance of nitrogen cycling related microorganisms, as exemplified by the changes in rhizosphere metabolites in the pathways of amino acid synthesis and metabolism. Collectively, it was suggested that the altered rhizospheric microbial status upon melatonin application was associated with the response of barley to low temperature. This suggested that the melatonin induced microbial changes should be considered for its application in the crop cold-resistant cultivation.

Melatonin as a plant biostimulant in crops and during post-harvest: a new approach is needed

A great amount of data covering a wide variety of plant species and experimental conditions has demonstrated the beneficial actions that melatonin exerts on many aspects of plant development, including germination, photosynthesis and water economy. Melatonin behaves especially well as a plant biostimulator against biotic and abiotic stressors, increasing stress tolerance. The present contribution sets out possible future multidisciplinary studies, in which the impact of using melatonin with respect to agriculture, food technology, human nutrition and the environment needs to be clearly established. In crops, the effective dose and best formulations for individual plant species and cultivation conditions should be studied. As regards post-harvest, the focus should be on the half-life time of melatonin in fruits and water-residue treatments. Detailed studies are lacking on the human intake of phytomelatonin in different diets. Studies on the metabolization of phytomelatonin and the combined effect with other phytonutrients such as carotenoids, chlorophylls, flavonoids, fibers, etc., would also be of interest. In soils, the possible interaction between melatonin and microbiome and non-vertebrate animals is of primordial interest. In terms of the environment, although melatonin is classified as a non-hazardous agent, its limitations as a possible animal hormone disruptor have been suggested. Specific studies on the permanence of melatonin in plant tissues, plant by-products, soil, freshwater and honeybees, amongst others, are proposed to obtain crucial information. © 2021 Society of Chemical Industry.

Phytomelatonin in stress management in agriculture

Melatonin was discovered as a pineal gland hormone in animals and is now more significantly known as a signaling molecule in plants' biotic and abiotic stressors. Melatonin has been traced back to prokaryotic organisms during evolution and its primary function of antioxidant scavenging free radicals in photosynthetic prokaryotic bacteria is a lesser explored and exciting area for further research globally. The authors at IIT Delhi are trying to establish its potential role in stress management in agriculture. The present manuscript addresses the biosynthetic pathways hitherto suggested by scientists. In this manuscript, the potential scope of melatonin in agriculture as a growth promoter, post-harvest loss inhibitor, and signaling and quality improvement molecule is envisaged.

A Systematic Review of Melatonin in Plants: An Example of Evolution of Literature

Melatonin (N-acetyl-5-methoxy-tryptamine) is a mammalian neurohormone, antioxidant and signaling molecule that was first discovered in plants in 1995. The first studies investigated plant melatonin from a human perspective quantifying melatonin in foods and medicinal plants and questioning whether its presence could explain the activity of some plants as medicines. Starting with these first handful of studies in the late 1990s, plant melatonin research has blossomed into a vibrant and active area of investigation and melatonin has been found to play critical roles in mediating plant responses and development at every stage of the plant life cycle from pollen and embryo development through seed germination, vegetative growth and stress response. Here we have utilized a systematic approach in accordance with the preferred reporting items for systematic reviews and meta-analyses (PRISMA) protocols to reduce bias in our assessment of the literature and provide an overview of the current state of melatonin research in plants, covering 1995-2021. This review provides an overview of the biosynthesis and metabolism of melatonin as well as identifying key themes including: abiotic stress responses, root development, light responses, interkingdom communication, phytohormone and plant signaling. Additionally, potential biases in the literature are investigated and a birefringence in the literature between researchers from plant and medical based which has helped to shape the current state of melatonin research. Several exciting new opportunities for future areas of melatonin research are also identified including investigation of non-crop and non-medicinal species as well as characterization of melatonin signaling networks in plants.

Melatonin Inhibits the Progression of Oral Squamous Cell Carcinoma via Inducing miR-25-5p Expression by Directly Targeting NEDD9

Melatonin exerts anti-cancer roles in various types of cancers. However, to the best of our knowledge, its role in oral squamous cell carcinoma (OSCC) is unknown. The present study aimed to investigate the role of melatonin and its underlying mechanism in OSCC. MTT, colony formation, wound healing, and transwell invasion assays proved that melatonin played anti-tumor effects in OSCC cells by inhibiting cell viability, proliferation, migration, and invasion in a concentration-dependent manner. The RT-qPCR analysis showed that miR-25-5p was significantly upregulated after melatonin treatment. Further, miR-25-5p might be involved in melatonin-induced inhibitory effects on the biological behavior of OSCC. The expression of miR-25-5p was decreased in tumor tissues and OSCC cells detected by RT-qPCR. MTT assay, colony formation assay, and TUNEL staining indicated miR-25-5p overexpression inhibited OSCC cell viability, proliferation, and induced OSCC cell apoptosis. Furthermore, wound healing, transwell invasion assay, and animal experiments suggested that miR-25-5p might exert suppressive effects on the migration, invasion, and tumor formation of OSCC cells, while miR-25-5p knockdown exhibited the opposite effects in OSCC cells. Bioinformatics analysis, western blot analysis, and luciferase reporter assay suggested that neural precursor cell expressed developmentally downregulated protein 9 (NEDD9) was proved to be a putative target for miR-25-5p. The role of NEDD9 in inhibiting OSCC cell proliferation, invasion, and migration was verified with NEDD9 siRNA transfection. Thus, melatonin exerted anti-proliferative, anti-invasive, and anti-migrative effects on OSCC via miR-25-5p/NEDD9 pathway. Melatonin could be applied as a potential novel drug on treating OSCC.