Exogenous Melatonin Application Delays Senescence of Kiwifruit Leaves by Regulating the Antioxidant Capacity and Biosynthesis of Flavonoids

Melatonin in crop plants: from biosynthesis through pleiotropic effects to enhanced stress resilience

Melatonin plays a crucial role in enhancing plant resilience to environmental stresses by regulating physiological and biochemical responses. This review provides an overview of melatonin biosynthesis, signaling pathways, and its interactions with phytohormones, highlighting its multifunctional roles across various crop species. We summarize recent discoveries regarding the biosynthetic pathways of melatonin and its crucial metabolites, emphasizing the importance of tryptophan and serotonin in this process. Furthermore, we discuss the intricate crosstalk between melatonin and phytohormones, particularly auxins, cytokinins, and brassinosteroids, which collectively influence root development, growth, and stress tolerance, among other traits. The antioxidant activity of melatonin and its derivatives, along with their impact on photosynthesis, has also been thoroughly discussed. Notably, melatonin's regulatory actions promote root growth, thereby improving water and nutrient absorption under stress conditions. The identification of candidate genes and a putative receptor provides a foundation for future studies aimed at elucidating the molecular mechanisms underlying melatonin signaling in crop species. Ultimately, this review underscores the potential of harnessing melatonin in crop improvement strategies to enhance resilience to abiotic stresses while promoting sustainable agricultural practices.

Two AcMYB22 Alleles Differently Regulate Flavonoid Biosynthesis Resulting in Varied Flesh Color in Kiwifruit

Flavonoids are essential nutrient compounds in kiwifruit, yet the specific regulatory mechanism governing their biosynthesis remains poorly understood. In this study, we identified an R2R3-MYB transcription factor (TF), AcMYB22, associated with flavonoid biosynthesis in kiwifruit. Two alleles of AcMYB22 were isolated: AcMYB22-1 is exclusively present in the cultivar "Hongyang", while both AcMYB22-1 and AcMYB22-2 were identified in its mutant "H-16", with nine single nucleotide polymorphisms. Overexpression of AcMYB22 in kiwifruit resulted in enhanced yellow flesh coloration with a b* hue value and a significant increase in flavonoid content. Moreover, transgenic kiwifruit plants of overexpressing AcMYB22 exhibited more pronounced yellow leaves with red margins accompanied by significant increases in total flavonoid and anthocyanin levels. The expression levels of flavonoid biosynthesis genes were significantly upregulated in transgenic plants with notably higher increases in AcMYB22-2 overexpressing plants compared to those overexpressing AcMYB22-1. Furthermore, yeast one-hybrid assays, electrophoretic mobility shift assays (EMSAs), and GUS activity assays confirmed that both AcMYB22-1 and AcMYB22-2 can physically bind to the promoters of AcF3H and AcUFGT, positively activating their transcription, with AcMYB22-2 exhibiting stronger activation activity than AcMYB22-1. These findings provide new insights into the regulatory mechanism of flavonoid biosynthesis in kiwifruit.

Exogenous 24-epibrassinolide mitigates damage in grape seedlings under low-temperature stress

Grapes are cultivated worldwide and have a high economic value as fruit trees. However, winter frost damage and spring cold damage have limited the sustainability of the table grape industry. A novel plant growth regulator, 24-epibrassinolide (EBR), exhibits an essential regulatory function in plant life cycles, especially in its unique mechanism against various environmental stresses. We treated 'Lihongbao' grapes with exogenous EBR (0.2 μM), brassinazole (BRZ, 10 μM), EBR + BRZ (0.2 μM +10 μM), and deionized water (CK). We investigated the effect of exogenous EBR on 'Lihongbao' grape seedlings under low-temperature stress (4°C) at different periods (0 h, 12 h, 24 h, 48 h, and 96 h). We explored physiological mitigation mechanisms of exogenous EBR in grape seedlings with low-temperature injury by observing the impacts of EBR treatment on the physical and biochemical indices such as phenotypes and anatomical structures, photosynthetic characteristics, chlorophyll fluorescence parameters, antioxidant systems, and osmoregulatory substances. Exogenous EBR had an inhibitory effect on cold stress in grape seedlings at different treatment periods compared with the CK group. Based on plant phenotype and anatomical structure, the leaves of the grape seedlings treated with exogenous EBR showed no signs of water loss or wilting. At 96 h under low-temperature stress, the lower epidermal thickness (LET), palisade tissue thickness (PT), palisade-to-sea ratio (P/S), and blade structural compactness (CTR) of the exogenous EBR-treated grape leaves were significantly reduced by 6.71%, 19.59%, 14.52%, and 11.65% compared with the CK group, respectively. Chlorophyll a (Chl a), chlorophyll b (Chl b), total chlorophyll (Chl total), carotenoids (carotenoid), transpiration rate (Tr), and stomatal conductance (Gs) contents of exogenous EBR-treated grape leaves were significantly upregulated by 30.24%, 48.52%, 39.75%, 34.67%, 704.66%, and 277.27%, respectively. The intercellular CO2 concentration (Ci) and non-photosynthetic burst coefficient (NPQ) of exogenous EBR-treated grape leaves were significantly downregulated by 16.29% and 25.83%, respectively. Glutathione (GSH) contents of exogenous EBR-treated grape leaves were significantly upregulated by 33.63%, superoxide dismutase (SOD), peroxidase (POD), and ascorbate peroxidase (APX) activities of exogenous EBR-treated grape leaves were significantly increased by 42.70%, 27.60%, and 28.64%, respectively. However, hydrogen peroxide (H2O2), superoxide anion (O2·-), and malondialdehyde (MDA) contents of exogenous EBR-treated grape leaves were reduced by 29.88%, 23.66%, and 47.96%, respectively, and significantly. Catalase (CAT) activity of exogenous EBR-treated grape leaves significantly increased by 15.03%. Soluble sugar and free proline contents increased by 5.29% and 19.44%, respectively, and significantly. Exogenous EBR could effectively alleviate growth inhibition caused by regulating the antioxidant system indices in grape seedlings under cold temperature. The results offer a theoretical basis for enhancing grape cold tolerance.

Melatonin maintains postharvest quality by modulating the metabolisms of cell wall and sugar in flowering Chinese cabbage

Melatonin is known to delay postharvest nutrient loss. However, its specific roles in regulating cell wall and sugar metabolism to maintain postharvest quality in flowering Chinese cabbage remain unclear. In this study, postharvest flowering Chinese cabbage was treated with melatonin (100 µmol L- 1) to investigate the cell wall and sugar metabolisms. Our research found that melatonin supplementation increased levels of important nutrients like ascorbic acid, sugars, soluble protein, carotenoid, and glucosinolates. It also enhanced various cell wall components (cellulose, hemicellulose, lignin, and protopectin) while reducing water-soluble pectin accumulation. Melatonin helped stabilize cell wall by reducing the expression of genes related to cell wall degradation (BrBGAL4, BrPG, BrCEL3, and BrPME3), inhibiting activities of cell wall-degrading enzymes like pectin methylesterase and cellulase, and improving expressions of cellulose biosynthesis-related genes (BrCesA1, BrCesA3.1, and BrCesA3.2). Additionally, it boosted sugar accumulation by increasing activities of sucrose synthetase and sucrose phosphate synthetase, upregulating expressions of BrSUS1, BrSUS5, BrSPS1F, BrSPS3F, BrSPS4F, and suppressing activities of neutral invertase and acidic invertase and expressions of genes involved in sugar metabolism (BrSUS3, BrINV, BrSWEET2, BrSWEET4, and BrSWEET17). This study highlights melatonin's vital roles in maintaining postharvest quality and offers practical insights for improving market performance of flowering Chinese cabbage.

The impact of genetic similarity and environment on the flavonoids variation pattern of Cyclocarya paliurus

The leaves of Cyclocarya paliurus (Batalin) Iljinskaja, an endemic tree with a scattered distribution in subtropical China, are rich in flavonoids with beneficial, health-promoting properties. To understand the impact of environment and genetic similarity on the variation pattern of flavonoids in this species, we analyzed C. paliurus germplasm resources from 26 different populations previously sampled from the main distribution area. Environmental, genetic and biochemical data was associated by genetic structure analysis, non-parametric tests, correlation analysis and principal component analysis. We found that populations with higher flavonoid contents were distributed at higher elevations and latitudes and fell into two groups with similar genetic diversities. Significant accumulations of isoquercitrin and kaempferol 3-O-glucoside were detected in the higher flavonoid-content resources. In addition, the genetic clusters with higher flavonoid contents exhibited broader environmental-adaptive capacities. Even in the presence of environmental factors promoting C. paliurus flavonoid accumulation, only those populations having a specific level of genetic similarity were able to exploit such environments.

Interactions between Brassinosteroids and Strigolactones in Alleviating Salt Stress in Maize

Exogenous brassinolide (BR) and strigolactones (SLs) play an important role in alleviating salt stress in maize. We studied the morphological and physiological responses of the salt-sensitive genotype PH4CV and salt-tolerant genotype Zheng58 to BR (1.65 nM), SL (1 µM), and BS (1.65 nM BR + 1 µM SL) under salt stress. Phenotypic analysis showed that salt stress significantly inhibited the growth of maize seedlings and significantly increased the content of Na+ in the roots. Exogenous hormones increased oxidase activity and decreased Na+ content in the roots and mitigated salt stress. Transcriptome analysis showed that the interaction of BR and SL is involved in photosynthesis-antenna proteins, the TCA cycle, and plant hormone signal transduction pathways. This interaction influences the expression of chlorophyll a/b-binding protein and glucose-6-phosphate isomerase 1 chloroplastic, and aconitase genes are affected. Furthermore, the application of exogenous hormones regulates the expression of genes associated with the signaling pathways of cytokinin (CK), gibberellins (GA), auxin (IAA), brassinosteroid (BR), abscisic acid (ABA), and jasmonic acid (JA). Additionally, exogenous hormones inhibit the expression of the AKT2/3 genes, which are responsible for regulating ion transduction and potassium ion influx. Four candidate genes that may regulate the seedling length of maize were screened out through WGCNA. Respective KOG notes concerned inorganic ion transport and metabolism, signal transduction mechanisms, energy production and conversion, and amino acid transport and metabolism. The findings of this study provide a foundation for the proposition that BR and SL can be employed to regulate salt stress alleviation in maize.

Integrated Metabolomic-Transcriptomic Analyses of Flavonoid Accumulation in Citrus Fruit under Exogenous Melatonin Treatment

The flavonoids in citrus fruits are crucial physiological regulators and natural bioactive products of high pharmaceutical value. Melatonin is a pleiotropic hormone that can regulate plant morphogenesis and stress resistance and alter the accumulation of flavonoids in these processes. However, the direct effect of melatonin on citrus flavonoids remains unclear. In this study, nontargeted metabolomics and transcriptomics were utilized to reveal how exogenous melatonin affects flavonoid biosynthesis in "Bingtangcheng" citrus fruits. The melatonin treatment at 0.1 mmol L-1 significantly increased the contents of seven polymethoxylated flavones (PMFs) and up-regulated a series of flavonoid pathway genes, including 4CL (4-coumaroyl CoA ligase), FNS (flavone synthase), and FHs (flavonoid hydroxylases). Meanwhile, CHS (chalcone synthase) was down-regulated, causing a decrease in the content of most flavonoid glycosides. Pearson correlation analysis obtained 21 transcription factors co-expressed with differentially accumulated flavonoids, among which the AP2/EREBP members were the most numerous. Additionally, circadian rhythm and photosynthesis pathways were enriched in the DEG (differentially expressed gene) analysis, suggesting that melatonin might also mediate changes in the flavonoid biosynthesis pathway by affecting the fruit's circadian rhythm. These results provide valuable information for further exploration of the molecular mechanisms through which melatonin regulates citrus fruit metabolism.

The role of phytomelatonin in plant homeostasis, signaling, and crosstalk in abiotic stress mitigation

In recent years, there has been an increase in the study of phytomelatonin. Having numerous functions in animals, melatonin produced by plants (phytomelatonin) is also a multi-regulatory molecule with great potential in plant physiology and in mitigating abiotic stresses, such as drought, salinity, chilling, heat, chemical contamination, and UV-radiation stress. This review highlights the primary functions of phytomelatonin as an anti-stress molecule against abiotic stress. We discuss the role of phytomelatonin as a master regulator, oxidative stress manager, reactive oxygen species and reactive nitrogen species regulator, and defense compounds inducer. Although there exist a handful of reviews on the crosstalk of phytomelatonin with other signaling molecules like auxin, cytokinin, gibberellin, abscisic acid, ethylene, nitric oxide, jasmonic acid, and salicylic acid, this review looks at studies that have reported a few aspects of phytomelatonin with newly discovered signaling molecules along with classical signaling molecules with relation to abiotic stress tolerance. The research and applications of phytomelatonin with hydrogen sulfide, strigolactones, brassinosteroids, and polyamines are still in their nascent stage but hold a promising scope for the future. Additionally, this review states the recent developments in the signaling of phytomelatonin with nitrogen metabolism and nitrosative stress in plants.

Single-cell and spatial RNA sequencing reveal the spatiotemporal trajectories of fruit senescence

The senescence of fruit is a complex physiological process, with various cell types within the pericarp, making it highly challenging to elucidate their individual roles in fruit senescence. In this study, a single-cell expression atlas of the pericarp of pitaya (Hylocereus undatus) is constructed, revealing exocarp and mesocarp cells undergoing the most significant changes during the fruit senescence process. Pseudotime analysis establishes cellular differentiation and gene expression trajectories during senescence. Early-stage oxidative stress imbalance is followed by the activation of resistance in exocarp cells, subsequently senescence-associated proteins accumulate in the mesocarp cells at late-stage senescence. The central role of the early response factor HuCMB1 is unveiled in the senescence regulatory network. This study provides a spatiotemporal perspective for a deeper understanding of the dynamic senescence process in plants.

Flavonol profiles of mature leaves allow discriminating Toona sinensis Roem from different north-south geographical origins across China with varied antioxidant activities

Toona sinensis (A. Juss.) Roem, a multipurpose economic tree, is widely cultivated across Asia, but its high-yielding mature leaves are largely overlooked. This study systematically analysed the flavonols in the mature leaves of T. sinensis from 44 different geographic locations across China, using HPLC-DAD and HPLC-ESI-MS2 techniques. In total, 18 flavonols were detected, among which 6 (f1, f3, f7, f14, f15, and f17) were firstly identified in this plant. Significant variations in quality among different T. sinensis varieties were observed (p < 0.01). Through OPLS-DA analysis, all samples could be clearly categorised into two distinct geographical groups. The northern varieties (N1-N20) exhibited concise flavonol fingerprints with higher total flavonol content (TFC) (727.55 ± 22.79 mg/100 g fresh weight, FW), predominantly non-acylated flavonols (705.95 ± 21.65 mg/100 g FW), particularly quercetin glycosides (614.60 ± 22.76 mg/100 g FW). In contrast, the southern varieties (S1-S24) presented more intricate flavonol profiles with lower TFC (622.81 ± 21.82 mg/100 g FW) and balanced amounts of quercetin (344.75 ± 16.41 mg/100 g FW) and kaempferol glycosides (278.06 ± 12.29 mg/100 g FW). Notably, the southern samples possessed higher content of acylated flavonols (184.50 ± 12.87 mg/100 g FW), especially galloylated ones, which contributed to their heightened antioxidant activities. Quercetin 3-O-rhamnoside (f11') and kaempferol 3-O-galloyglucoside (f11) were determined to be the crucial biomarkers for quality discrimination. Considering quality control of mature T. sinensis leaves as potential resources for natural flavonol extraction, this study suggested that their northern/southern geographic origins should be distinguished first. Additionally, the flavonol profiles allow for discriminating the origin and assessing the quality of T. sinensis.

Melatonin and 14-hydroxyed brassinosteroid combined promote kiwifruit seedling growth by improving soil microbial distribution, enzyme activity and nutrients uptake

Kiwifruit, a nutrient-dense fruit, has become increasingly popular with consumers in recent decades. However, kiwifruit trees are prone to stunted growth after a few years of planting, called early tree decline. In this study, melatonin (MT), pollen polysaccharide (SF), 14-hydroxyed brassinosteroid (14-HBR) were applied alone or in combination to investigate their influence on plant growth, nutrition absorption and rhizosphere bacterial abundance in kiwifruit seedlings. The results revealed that MT, SF and 14-HBR alone treatments significantly increased leaf chlorophyll content, photosynthetic capacity and activities of dismutase and catalase compared with the control. Among them, MT treatment significantly increased the dry root biomass by 35.7%, while MT+14-HBR treatment significant enhanced the dry shoot biomass by 36.9%. Furthermore, both MT and MT+14-HBR treatments markedly improved the activities of invertase, urease, protease and phosphatase in soil, as well as the abundance of Proteobacteria and Acidobacteria in rhizosphere microorganisms based on 16S rDNA sequencing. In addition, MT treatment improved the content of available K and organic matter in soil, and increased the uptake of P, K and Fe by seedlings. In summary, 14-HBR and MT combined had the best effect on promoting rhizosphere bacterial distribution, nutrient absorption and plant growth. These findings may provide valuable guidance for solving growth weakness problem in kiwifruit cultivation.

Effects of light spectra on morphological characteristics, primary and specialized metabolites of Thymus vulgaris L

Light is a crucial environmental factor that profoundly influences the growth and development of plants. However, the precise mechanisms by which light affects biochemical processes and growth and development factors in Thymus vulgaris remain unknown, necessitating further investigation. Hence, this study aimed to investigate the impact of different light spectra, including red, blue, red-blue, and white lights, on the morphological characteristics, primary, and specialized metabolites of T. vulgaris. Compared to white light, red light significantly increased leaf area (by 64 %), the number of branches (by 132 %), and dry weight (by 6.2 %), although a 40 % reduction in fresh weight was observed under red light conditions. Red-blue light notably enhanced canopy width, fresh weight, and dry weight. Gas chromatography/mass spectrometry (GC/MS) analysis of the plant's essential oil (EO) revealed that p-Cymene and γ-Terpinene were present at the highest levels. Notably, p-Cymene exhibited the highest concentrations under white light and blue light treatments, reaching 60.92 % and 59.53 %, respectively. Moreover, under the same light conditions, phenol and antioxidant levels were significantly elevated. Overall, these findings indicate that red and red-blue light spectra are the most favorable for thyme production.

The Roles of Mepiquate Chloride and Melatonin in the Morpho-Physiological Activity of Cotton under Abiotic Stress

Cotton growth and yield are severely affected by abiotic stress worldwide. Mepiquate chloride (MC) and melatonin (MT) enhance crop growth and yield by reducing the negative effects of abiotic stress on various crops. Numerous studies have shown the pivotal role of MC and MT in regulating agricultural growth and yield. Nevertheless, an in-depth review of the prominent performance of these two hormones in controlling plant morpho-physiological activity and yield in cotton under abiotic stress still needs to be documented. This review highlights the effects of MC and MT on cotton morpho-physiological and biochemical activities; their biosynthetic, signaling, and transduction pathways; and yield under abiotic stress. Furthermore, we also describe some genes whose expressions are affected by these hormones when cotton plants are exposed to abiotic stress. The present review demonstrates that MC and MT alleviate the negative effects of abiotic stress in cotton and increase yield by improving its morpho-physiological and biochemical activities, such as cell enlargement; net photosynthesis activity; cytokinin contents; and the expression of antioxidant enzymes such as catalase, peroxidase, and superoxide dismutase. MT delays the expression of NCED1 and NCED2 genes involved in leaf senescence by decreasing the expression of ABA-biosynthesis genes and increasing the expression of the GhYUC5, GhGA3ox2, and GhIPT2 genes involved in indole-3-acetic acid, gibberellin, and cytokinin biosynthesis. Likewise, MC promotes lateral root formation by activating GA20x genes involved in gibberellin catabolism. Overall, MC and MT improve cotton's physiological activity and antioxidant capacity and, as a result, improve the ability of the plant to resist abiotic stress. The main purpose of this review is to present an in-depth analysis of the performance of MC and MT under abiotic stress, which might help to better understand how these two hormones regulate cotton growth and productivity.

Synergistic action of Pseudomonas fluorescens with melatonin attenuates salt toxicity in mustard by regulating antioxidant system and flavonoid profile

Salt stress is an alarming abiotic stress that reduces mustard growth and yield. To attenuate salt toxicity effects, plant growth-promoting rhizobacteria (PGPR) offers a sustainable approach. Among the various PGPR, Pseudomonas fluorescens (P. fluorescens NAIMCC-B-00340) was chosen for its salt tolerance (at 100 mM NaCl) and for exhibiting various growth-promoting activities. Notably, P. fluorescens can produce auxin, which plays a role in melatonin (MT) synthesis. Melatonin is a pleiotropic molecule that acts as an antioxidant to scavenge reactive oxygen species (ROS), resulting in stress reduction. Owing to the individual role of PGPR and MT in salt tolerance, and their casual nexus, their domino effect was investigated in Indian mustard under salt stress. The synergistic action of P. fluorescens and MT under salt stress conditions was found to enhance the activity of antioxidative enzymes and proline content as well as  promote the production of secondary metabolites. This led to reduced oxidative stress following effective ROS scavenging, maintained photosynthesis, and improved growth. In mustard plants treated with MT and P. fluorescens under salt stress, eight flavonoids showed significant increase. Kaempferol and cyanidin showed the highest concentrations and are reported to act as antioxidants with protective functions under stress. Thus, we can anticipate that strategies involved in their enhancement could provide a better adaptive solution to salt toxicity in mustard plants. In conclusion, the combination of P. fluorescens and MT affected antioxidant metabolism and flavonoid profile that could be used to mitigate salt-induced stress and bolster plant resilience.

Melatonin as a therapeutic agent for alleviating endothelial dysfunction in cardiovascular diseases: Emphasis on oxidative stress

The vascular endothelium is vital in maintaining cardiovascular health by regulating vascular permeability and tone, preventing thrombosis, and controlling vascular inflammation. However, when oxidative stress triggers endothelial dysfunction, it can lead to chronic cardiovascular diseases (CVDs). This happens due to oxidative stress-induced mitochondrial dysfunction, inflammatory responses, and reduced levels of nitric oxide. These factors cause damage to endothelial cells, leading to the acceleration of CVD progression. Melatonin, a natural antioxidant, has been shown to inhibit oxidative stress and stabilize endothelial function, providing cardiovascular protection. The clinical application of melatonin in the prevention and treatment of CVDs has received widespread attention. In this review, based on bibliometric studies, we first discussed the relationship between oxidative stress-induced endothelial dysfunction and CVDs, then summarized the role of melatonin in the treatment of atherosclerosis, hypertension, myocardial ischemia-reperfusion injury, and other CVDs. Finally, the potential clinical use of melatonin in the treatment of these diseases is discussed.

Role of melatonin in enhancing arbuscular mycorrhizal symbiosis and mitigating cold stress in perennial ryegrass (Lolium perenne L.)

Melatonin is a biomolecule that affects plant development and is involved in protecting plants from environmental stress. However, the mechanisms of melatonin's impact on arbuscular mycorrhizal (AM) symbiosis and cold tolerance in plants are still unclear. In this research, AM fungi inoculation and exogenous melatonin (MT) were applied to perennial ryegrass (Lolium perenne L.) seedlings alone or in combination to investigate their effect on cold tolerance. The study was conducted in two parts. The initial trial examined two variables, AM inoculation, and cold stress, to investigate the involvement of the AM fungus Rhizophagus irregularis in endogenous melatonin accumulation and the transcriptional levels of its synthesis genes in the root system of perennial ryegrass under cold stress. The subsequent trial was designed as a three-factor analysis, encompassing AM inoculation, cold stress, and melatonin application, to explore the effects of exogenous melatonin application on plant growth, AM symbiosis, antioxidant activity, and protective molecules in perennial ryegrass subjected to cold stress. The results of the study showed that compared to non-mycorrhizal (NM) plants, cold stress promoted an increase in the accumulation of melatonin in the AM-colonized counterparts. Acetylserotonin methyltransferase (ASMT) catalyzed the final enzymatic reaction in melatonin production. Melatonin accumulation was associated with the level of expression of the genes, LpASMT1 and LpASMT3. Treatment with melatonin can improve the colonization of AM fungi in plants. Simultaneous utilization of AM inoculation and melatonin treatment enhanced the growth, antioxidant activity, and phenylalanine ammonia-lyase (PAL) activity, while simultaneously reducing polyphenol oxidase (PPO) activity and altering osmotic regulation in the roots. These effects are expected to aid in the mitigation of cold stress in Lolium perenne. Overall, melatonin treatment would help Lolium perenne to improve growth by promoting AM symbiosis, improving the accumulation of protective molecules, and triggering in antioxidant activity under cold stress.

Phytomelatonin interferes with flavonols biosynthesis to regulate ROS production and stomatal closure in tobacco

Flavonols are well-known antioxidants that prevent stomatal closure via interfering with ROS signaling. Phytomelatonin regulates stomatal closure, but the signaling pathways are still largely unknown. Here, we investigated the role of flavonols in phytomelatonin-mediated stomatal closure in tobacco plants. The application of melatonin induced stomatal closure through NADPH oxidase-mediated ROS production. Transgenic tobacco plants overexpressing soybean GmSNAT1 (coding for serotonin N-acetyltransferase that catalyzes the penultimate step in phytomelatonin biosynthesis) had higher phytomelatonin concentration, accumulated more ROS in guard cells and were more sensitive to melatonin-induced stomatal closure than the wild-type plants, which was associated with the higher expression of PMTR1-homologous genes. Exogenous melatonin decreased flavonol concentrations in guard cells and the expression of flavonoid-related genes in wild-type and transgenic tobacco plants, and these inhibitory effects were more obvious in GmSNAT1-overexpressing plants than the wild type. However, the melatonin-mediated stomatal closure and ROS production were diminished by the application of kaempferol (a type of flavonol). Additionally, transgenic tobacco plants with increased expression of NtFLS (encoding flavonol synthase) were less sensitive to melatonin-induced stomatal closure. In conclusion, phytomelatonin hampers the biosynthesis of flavonols in guard cells, which results in high concentration of ROS and induces stomatal closure in tobacco plants.

Soil Amendments and Foliar Melatonin Reduced Pb Uptake, and Oxidative Stress, and Improved Spinach Quality in Pb-Contaminated Soil

Amending Pb-affected soil with biochar (BH) and magnesium potassium phosphate cement (MKC) reduces Pb uptake in plants. Moreover, foliar applications of melatonin and proline are also known to reduce plant oxidative stress and Pb uptake. However, little is known about combining both techniques, i.e., adding a combo immobilizing dose (CIA = mixture of BH and MKC at 50:50 ratio) in Pb-polluted soil and foliar application of proline and melatonin for reducing Pb uptake and oxidative stress in spinach. Control, proline, melatonin, CIA, CIA+proline, and CIA+melatonin were the treatments utilized in this pot study to see their effects on reducing plant oxidative stress, Pb uptake, and improving spinach quality in Pb-polluted soil. Moreover, Pb bioavailability, enzymatic activities, and numbers of bacteria, fungi, and actinomycetes in the soil were also evaluated. The effect of CIA on reducing Pb in the soil-plant system and improving soil enzymes and microbial numbers was more pronounced than melatonin alone. The most effective treatment was CIA+melatonin reducing Pb availability in soil (77%), shoots (95%), and roots (84%), alleviating oxidative stress, and improving plant biomass (98%) and nutrients. Soil enzymatic activities and the number of microorganisms in the rhizosphere were also highest with CIA+melatonin. Results highlight the significance of CIA+melatonin, as an inexpensive approach, in remediating Pb-polluted soil and improving spinach quality. However, further research is needed to understand the significance of CIA+melatonin on different crops and various soil Pb concentrations before employing this technique commercially in agriculture and environment sectors.

Melatonin enhanced the heavy metal-stress tolerance of pepper by mitigating the oxidative damage and reducing the heavy metal accumulation

Heavy metals (HMs), like vanadium (V), chromium (Cr), cadmium (Cd), and nickel (Ni) toxicity due to anthropogenic, impair plant growth and yield, which is a challenging issue for agricultural production. Melatonin (ME) is a stress mitigating molecule, which alleviates HM-induced phytotoxicity, but the possible underlying mechanism of ME functions under HMs' phytotoxicity is still unclear. Current study uncovered key mechanisms for ME-mediated HMs-stress tolerance in pepper. HMs toxicity greatly reduced growth by impeding leaf photosynthesis, root architecture system, and nutrient uptake. Conversely, ME supplementation markedly enhanced growth attributes, mineral nutrient uptake, photosynthetic efficiency, as measured by chlorophyll content, gas exchange elements, chlorophyll photosynthesis genes' upregulation, and reduced HMs accumulation. ME treatment showed a significant decline in the leaf/root V, Cr, Ni, and Cd concentration which was about 38.1/33.2%, 38.5/25.9%, 34.8/24.9%, and 26.6/25.1%, respectively, when compared with respective HM treatment. Furthermore, ME remarkably reduced the ROS (reactive oxygen species) accumulation, and reinstated the integrity of cellular membrane via activating antioxidant enzymes (SOD, superoxide dismutase; CAT, catalase; APX, ascorbate peroxidase; GR, glutathione reductase; POD, peroxidase; GST, glutathione S-transferase; DHAR, dehydroascorbate reductase; MDHAR, monodehydroascorbate reductase) and as well as regulating ascorbate-glutathione (AsA-GSH) cycle. Importantly, oxidative damage showed efficient alleviations through upregulating the genes related to key defense such as SOD, CAT, POD, GR, GST, APX, GPX, DHAR, and MDHAR; along with the genes related to ME biosynthesis. ME supplementation also enhanced the level of proline and secondary metabolites, and their encoding genes expression, which may control excessive H₂O₂ (hydrogen peroxide) production. Finally, ME supplementation enhanced the HM stress tolerance of pepper seedlings.

Reduction in the cadmium (Cd) accumulation and toxicity in pearl millet (Pennisetum glaucum L.) by regulating physio-biochemical and antioxidant defense system via soil and foliar application of melatonin

Cadmium (Cd) is among the toxic pollutants that harms the both animals and plants. The natural antioxidant, melatonin can improve Cd-stress tolerance but its potential role in reducing Cd stress and resilience mechanisms in pearl millet (Pennisetum glaucum L.) is remain unclear. The present study suggests that Cd causes severe oxidative damage by decreasing photosynthesis, and increasing reactive oxygen species (ROS), malondialdehyde content (MDA), and Cd content in different parts of pearl millet. However, exogenous melatonin (soil application and foliar treatment) mitigated the Cd toxicity and enhanced the growth, antioxidant defense system, and differentially regulated the expression of antioxidant-responsive genes i. e superoxide dismutase SOD-[Fe] 2, Fe-superoxide dismutase, Peroxiredoxin 2C, and L-ascorbate peroxidase-6. The results showed that foliar melatonin at F-200/50 significantly increased the plant height, chlorophyll a, b, a+b and carotenoids by 128%, 121%, 150%, 122%, and 69% over the Cd treatment, respectively. The soil and foliar melatonin at S-100/50 and F-100/50 reduced the ROS by 36%, and 44%, and MDA by 42% and 51% over the Cd treatment, respectively. Moreover, F200/50 significantly boosted the activities of antioxidant enzymes i. e SOD by 141%, CAT 298%, POD 117%, and APX 155% over the Cd treatment. Similarly, a significant reduction in Cd content in root, stem, and leaf was found on exposure to higher concentrations of exogenous melatonin. These findings suggest that exogenous melatonin may significantly and differentially improve the tolerance to Cd stress in crop plants. However, field applications, type of plant species, concentration of dose, and type of stress may vary with the degree of tolerance in crop plants.

Potential Role of Biochar and Silicon in Improving Physio-Biochemical and Yield Characteristics of Borage Plants under Different Irrigation Regimes

Silicon (Si) and biochar (Bc) are key signaling conditioners that improve plant metabolic processes and promote drought tolerance. However, the specific role of their integrative application under water restrictions on economical plants is not yet well understood. Two field experiments throughout 2018/2019 and 2019/2020 were conducted to examine the physio-biochemical modifications and yield attributes of borage plants mediated by Bc (9.52 tons ha^-1) and/or Si (300 mg L^-1) under different irrigation regimes (100, 75, and 50% of crop evapotranspiration). Catalase (CAT) and peroxidase (POD) activity; relative water content, water, and osmotic potential; leaf area per plant and yield attributes; and chlorophyll (Chl) content, Chl_a/chlorophyllide_a (Chlid_a), and Chl_b/Chlid_b were considerably reduced within the drought condition. On the other hand, oxidative biomarkers, as well as organic and antioxidant solutes, were increased under drought, associated with membrane dysfunction, superoxide dismutase (SOD) activation, and osmotic adjustment (OA) capacity as well as a hyperaccumulation of porphyrin intermediates. Supplementation of Bc and Si lessens the detrimental impacts of drought on several plant metabolic processes associated with increasing leaf area and yield attributes. Their application under normal or drought conditions significantly elicited the accumulation of organic and antioxidant solutes as well as the activation of antioxidant enzymes, followed by lessening the formation of free radical oxygen and mitigating oxidative injuries. Moreover, their application maintained water status and OA capacity. Si and/or Bc treatment reduced protoporphyrin, magnesium-protoporphyrin, and protochlorophyllide while increasing Chl_a and Chl_b assimilation and boosting the ratio of Chl_a/Chlid_a and Chl_b/Chlid_b, resulting in a rise in leaf area per plant and yield components following these modifications. These findings highlight the significance of Si and/or Bc as (a) stress-signaling molecule(s) in regulating defensive systems in drought-affected borage plants by boosting antioxidant aptitude, regulating water status, and accelerating chlorophyll assimilation, thus leading to increasing leaf area and productivity.

Melatonin-priming enhances maize seedling drought tolerance by regulating the antioxidant defense system

Drought stress (DS) challenges sustainable agriculture production by limiting crop growth and development. The objective of the study was to evaluate the effect of melatonin-priming on enzymatic and non-enzymatic antioxidant defense mechanisms and its relation with leaf ultrastructure and stomatal traits in maize (Zea mays L) seedlings under DS (PEG-6000). DS drastically decreased seed germination, plant growth, and leaf chlorophyll content due to excessive reactive oxygen species (ROS) production. Melatonin-priming significantly (P < 0.05) increased seed germination, root length, shoot length, fresh seedling weight, proline content, total soluble protein content, sugar content, chlorophyll content, and stomatal aperture size by 101%, 30%, 133%, 51%, 22%, 59%, 54%, 20%, and 424%, compared to no priming (NP) under DS, respectively. Similarly, priming improved leaf ultrastructure and reduced the amount of chlorophyll loss and oxidative damage in maize seedlings. Melatonin seed priming with 500 µM melatonin (M2) greatly increased superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), glutathione, and ascorbate (AsA) activity, by 65%, 63%, 94%, 41%, and 55% compared to NP under DS and by 0.26%, 8%, 33%, 42%, and 15% under no-stress (NS), respectively. Melatonin-priming also reduced malondialdehyde content, electrolyte leakage, hydrogen peroxide (H2O2) content, and superoxide anion (O2-) content by 26%, 31%, 31%, and 33% compared to NP under DS and by 8%, 18%, 10%, and 11% under NS, respectively. In response to DS, melatonin-priming also stabilized the chloroplast structure, sustained cell expansion, protected cell walls, and greatly improved stomatal traits, including stomatal number, length, and width. Our results suggest that melatonin-priming improves drought tolerance in maize seedlings by alleviating the negative effect of ROS.

Exogenous Melatonin Enhances the Yield and Secondary Metabolite Contents of Prunella vulgaris by Modulating Antioxidant System, Root Architecture and Photosynthetic Capacity

Melatonin (MT) plays a number of key roles in regulating plant growth and secondary metabolite accumulation. Prunella vulgaris is an important traditional Chinese herbal medicinal plant which is used for the treatment of lymph, goiter, and mastitis. However, the effect of MT on the yield and medicinal component content of P. vulgaris remains still unclear. In this research, we have examined the influence of different concentrations of MT (0, 50, 100, 200, 400 μM) on the physiological characteristics, secondary metabolite contents, and yield of P. vulgaris biomass. The results showed that 50-200 μM MT treatment had a positive effect on P. vulgaris. MT treatment at 100 μM greatly increased the activities of superoxide dismutase and peroxidase, the contents of soluble sugar and proline, and obviously decreased the relative electrical conductivity, the contents of malondialdehyde and hydrogen peroxide of leaves. Furthermore, it markedly promoted the growth and development of the root system, increased the content of photosynthetic pigments, improved the performance of photosystems I and II and the coordination of both photosystems, and enhanced the photosynthetic capacity of P. vulgaris. In addition, it significantly increased the dry mass of whole plant and spica and promoted the accumulation of total flavonoids, total phenolics, caffeic acid, ferulic acid, rosmarinic acid, and hyperoside in the spica of P. vulgaris. These findings demonstrated that the application of MT could effectively activate the antioxidant defense system of P. vulgaris, protect the photosynthetic apparatus from photooxidation damage, and improve the photosynthetic capacity and the root absorption capacity, thereby promoting the yield and accumulation of secondary metabolites in P. vulgaris.

Melatonin Delays Postharvest Senescence through Suppressing the Inhibition of BrERF2/BrERF109 on Flavonoid Biosynthesis in Flowering Chinese Cabbage

Flowering Chinese cabbage is prone to withering, yellowing and deterioration after harvest. Melatonin plays a remarkable role in delaying leaf senescence and increasing flavonoid biosynthesis. However, the underlying molecular mechanisms of melatonin procrastinating postharvest senescence by regulating flavonoid biosynthesis remain largely unknown. In this study, melatonin could promote flavonoid accumulation and delay the postharvest senescence of flowering Chinese cabbage. Surprisingly, we observed that BrFLS1 and BrFLS3.2 were core contributors in flavonoid biosynthesis, and BrERF2 and BrERF109 were crucial ethylene response factors (ERFs) through the virus-induced gene silencing (VIGS) technique, which is involved in regulating the postharvest senescence under melatonin treatment. Furthermore, yeast one-hybrid (Y1H), dual luciferase (LUC), and β-glucuronidase (GUS) tissue staining experiments demonstrated that BrERF2/BrERF109 negatively regulated the transcripts of BrFLS1 and BrFLS3.2 by directly binding to their promoters, respectively. Silencing BrERF2/BrERF109 significantly upregulated the transcripts of BrFLS1 and BrFLS3.2, promoting flavonoid accumulation, and postponing the leaf senescence. Our results provided a new insight into the molecular regulatory network of melatonin delaying leaf senescence and initially ascertained that melatonin promoted flavonoid accumulation by suppressing the inhibition of BrERF2/BrERF109 on the transcripts of BrFLS1 and BrFLS3.2, which led to delaying the leaf senescence of postharvest flowering Chinese cabbage.

Polyamines effectively mitigate senescence in persistent leaves of Berginia ciliata - a novel model system

Plant leaves provide a unique insight into the changes that occur in organs, tissues and cells as they approach senescence. As part of the parental outlay, plants instigate leaf senescence to reallocate resources from older tissues to new organs towards the termination of the growing season. The aim of crop breeding initiatives is to optimize senescence for specific species. Considering hormonal regulation and their crosstalk during leaf senescence through integration of developmental signals, this work examines the efficacy of polyamines (PAs) in modulating several biochemical and physiological aspects with an ultimate aim to delay leaf senescence in leaf discs of Berginia ciliata (Haw.) sternb. Leaf discs were treated with putrescine (Put), spermidine (Spd) and spermine (Spm) at 20μM, 20μM and 15μM concentration, respectively. A set of leaf discs kept in distilled water served as the control. Leaf discs treated with PAs were green and fresh by about 4 days compared to the control, thus exhibited delayed senescence. This delayed leaf senescence corroborated with the maintenance of high activity of reactive oxygen species (ROS) scavenging antioxidant enzymes viz , superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and higher content of chlorophylls. A marked increase was also observed in membrane stability and soluble proteins in leaf discs treated with PAs. Exogenous PAs reduced oxidative stress in the leaf discs, as revealed by lower malondialdehyde (MDA) level, which is manifested as reduced lipid peroxidation (LPO). Improved membrane stability was proportional to lower LPO, as measured by the membrane stability index (MSI).

Melatonin-mediated development and abiotic stress tolerance in plants

Melatonin is a multifunctional molecule that has been widely discovered in most plants. An increasing number of studies have shown that melatonin plays essential roles in plant growth and stress tolerance. It has been extensively applied to alleviate the harmful effects of abiotic stresses. In view of its role in regulating aspects of plant growth and development, we ponder and summarize the scientific discoveries about seed germination, root development, flowering, fruit maturation, and senescence. Under abiotic and biotic stresses, melatonin brings together many pathways to increase access to treatments for the symptoms of plants and to counteract the negative effects. It has the capacity to tackle regulation of the redox, plant hormone networks, and endogenous melatonin. Furthermore, the expression levels of several genes and the contents of diverse secondary metabolites, such as polyphenols, terpenoids, and alkaloids, were significantly altered. In this review, we intend to examine the actions of melatonin in plants from a broader perspective, explore the range of its physiological functions, and analyze the relationship between melatonin and other metabolites and metabolic pathways.

Effect of exogenous melatonin on improvement of chlorophyll content and photochemical efficiency of PSII in mallow plants (Malva parviflora L.) treated with cadmium

Melatonin is a growth regulator that improves the growth and chlorophyll (chl) content in plants. This study aims to investigate the effect of melatonin pretreatment on chl synthesis and fluorescence parameters in Malva parviflora exposed to cadmium (Cd). The 42-day-old plants were transferred to nutrient solutions containing 50 μM melatonin. After two days, some plants were exposed to 50 μM Cd. Eight days after Cd treatment, some indicators related to chl fluorescence and some biochemical parameters were measured. In this study, melatonin increased chl content and chl a/pheophytin a (pheo a) ratio, chlorophyllide a (chlide a), porphyrin compounds, and 5-aminolevulinic acid (5-ALA) in the presence of Cd. However, it decreased chl a/chlide a ratio under these conditions. Whereas Cd treatment resulted in significant reductions in photochemical activity and electron transfer rate in PSII, melatonin improved photochemical efficiency of PSII by reducing the toxic effect of Cd on the activity of the oxygen evolving complex (OEC) on the electron donor site and reducing non-photochemical quenching (NPQ). Based on the results, it appears that melatonin can maintain the chl content of plants exposed to Cd by increasing the precursors of the chl biosynthesis pathway and reducing its degradation rate. These results may, at least in our experimental conditions, partly explain the reason for the improved yield and growth of Cd-exposed plants when pretreated with melatonin.

Flavonoid Metabolism in Tetrastigma hemsleyanum Diels et Gilg Based on Metabolome Analysis and Transcriptome Sequencing

Tetrastigma hemsleyanum Diels et Gilg, known as a "plant antibiotic", possesses several attractive properties including anti-inflammatory, anti-tumor, and antioxidant effects, with its efficacy being attributed to flavonoids. However, the flavonoid biosynthesis of T. hemsleyanum has rarely been studied. In this study, we investigated the flavonoid metabolism of T. hemsleyanum through metabolome analysis and transcriptome sequencing. The metabolomic results showed differences in the flavonoids of the leaves and root tubers of T. hemsleyanum. A total of 22 flavonoids was detected, and the concentrations of most flavonoids in the leaves were higher than those in the root tubers. Transcriptome analysis revealed that differentially expressed genes (DEGs) in the leaves and root tubers were enriched in photosynthesis-antenna proteins. Pearson correlation analysis indicated that the expression levels of chalcone isomerase (CHI) and UDP-glycose flavonoid glycosyltransferase (UFGT) were highly correlated with the concentrations of most flavonoids. Further, this study found that the photosynthesis-antenna proteins essentially contributed to the difference in the flavonoids in T. hemsleyanum. The gene expressions and concentrations of the total flavonoids of leaves and root tubers in Hangzhou, Jinhua, Lishui, and Taizhou in Zhejiang Province, China, showed that CHI (CL6715.Contig1_All, Unigene19431_All, CL921.Contig4_All) and UFGT (CL11556.Contig3_All, CL11775.Contig1_All) were the potential key genes of accumulation of most flavonoids in T. hemsleyanum.

Application of Exogenous Melatonin Improves Tomato Fruit Quality by Promoting the Accumulation of Primary and Secondary Metabolites

Melatonin plays key roles in improving fruit quality and yield by regulating various aspects of plant growth. However, the effects of how melatonin regulates primary and secondary metabolites during fruit growth and development are poorly understood. In this study, the surfaces of tomato fruit were sprayed with different concentrations of melatonin (0, 50, and 100 µmol·L^-1) on the 20th day after anthesis; we used high-performance liquid chromatography (HPLC) and liquid chromatography/mass spectrometry (LC/MS) to determine the changes in primary and secondary metabolite contents during fruit development and measured the activity of sucrose metabolizing enzymes during fruit development. Our results showed that 100 µmol·L^-1 melatonin significantly promoted the accumulation of soluble sugar in tomato fruit by increasing the activities of sucrose synthase (SS), sucrose phosphate synthase (SPS), and acid convertase (AI). The application of 100 µmol·L^-1 melatonin also increased the contents of ten amino acids in tomato fruit as well as decreased the contents of organic acids. In addition, 100 µmol·L^-1 melatonin application also increased the accumulation of some secondary metabolites, such as six phenolic acids, three flavonoids, and volatile substances (including alcohols, aldehydes, and ketones). In conclusion, melatonin application improves the internal nutritional and flavor quality of tomato fruit by regulating the accumulation of primary and secondary metabolites during tomato fruit ripening. In the future, we need to further understand the molecular mechanism of melatonin in tomato fruit to lay a solid foundation for quality improvement breeding.

Transcriptome and Metabolome Analysis of a Late-Senescent Vegetable Soybean during Seed Development Provides New Insights into Degradation of Chlorophyll

(1) Background: Senescence represents the final stage of plant growth and development, which transfers nutrients to growing seeds and directly affects the yield and quality of crops. However, little is known about chlorophyll degradation in developing and maturing seeds, in contrast to leaf senescence; (2) Methods: RNA-Seq was used to analyze the differentially expressed genes of different late-senescent germplasms. A widely untargeted metabolic analysis was used to analyze differential metabolites. In addition, qRT-PCR was conducted to detect gene expression levels; (3) Results: Transcriptome analysis revealed that ZX12 seeds have a higher expression level of the chlorophyll synthesis genes in the early stage of maturity, compared with ZX4, and have a lower expression level of chlorophyll degradation genes in the late stage of maturity. Flavonoids were the primary differential metabolites, and ZX12 contains the unique and highest expression of three types of metabolites, including farrerol-7-O-glucoside, cyanidin-3-o-(6'-o-feruloyl) glucoside, and kaempferide-3-o-(6'-malonyl) glucoside. Among them, farrerol-7-O-glucoside and cyanidin-3-o-(6'-o-feruloyl) glucoside are flavonoid derivatives containing mono and dihydroxy-B-ring chemical structures, respectively; and (4) Conclusions: It is speculated that the two metabolites can slow down the degradation process of chlorophyll by scavenging oxygen-free radicals in the chloroplast.

Exogenous melatonin treatment affects ascorbic acid metabolism in postharvest 'Jinyan' kiwifruit

Ascorbic acid (AsA) is an important nutritious substance in fruits, and it also can maintain the biological activity of fruits during storage. This research investigated the effect of exogenous melatonin (MT) on AsA metabolism in postharvest kiwifruit. Our results indicated that exogenous MT delayed the decrease of fruit firmness and titratable acid (TA), inhibited the increase of soluble solids content (SSC), reduced the respiration rate and ethylene production, and maintained a higher AsA content in kiwifruit during storage. The high expression of L-galactose pathway key genes in the early storage and regeneration genes in the later storage maintained the AsA content in postharvest kiwifruit. MT treatment enhanced the expression levels of AsA biosynthesis (AcGME2, AcGalDH, and AcGalLDH) and regeneration (AcGR, AcDHAR, and AcMDHAR1) genes. Meanwhile, the expression of the degradation gene AcAO was inhibited in MT-treated kiwifruits.

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.

Recent advances and mechanistic insights on Melatonin-mediated salt stress signaling in plants

Salinity stress is one of the major abiotic constraints that limit plant growth and yield, which thereby is a serious concern to world food security. It adversely affects crop production by inducing hyperosmotic stress and ionic toxicity as well as secondary stresses such as oxidative stress, all of which disturb optimum physiology and metabolism. Nonetheless, various strategies have been employed to improve salt tolerance in crop plants, among which the application of Melatonin (Mel) could also be used as it has demonstrated promising results. The ongoing experimental evidence revealed that Mel is a pleiotropic signaling molecule, which besides being involved in various growth and developmental processes also mediates environmental stress responses. The current review systematically discusses and summarizes how Mel mediates the response of plants under salt stress and could optimize the balance between plant growth performances and stress responses. Specifically, it covers the latest advances of Mel in fine-tuning the signaling in plants. Furthermore, it highlights plant-built tolerance of salt stress by manifesting the biosynthesis of Mel, its cross talks with nitric oxide (NO), and Mel as a multifaceted antioxidant molecule.

Dark secrets of phytomelatonin

Phytomelatonin is a newly identified plant hormone, and its primary functions in plant growth and development remain relatively poorly appraised. Phytomelatonin is a master regulator of reactive oxygen species (ROS) signaling and acts as a darkness signal in circadian stomatal closure. Plants exhibit at least three interrelated patterns of interaction between phytomelatonin and ROS production. Exogenous melatonin can induce flavonoid biosynthesis, which might be required for maintenance of antioxidant capacity under stress, after harvest, and in leaf senescence conditions. However, several genetic studies have provided direct evidence that phytomelatonin plays a negative role in the biosynthesis of flavonoids under non-stress conditions. Phytomelatonin delays flowering time in both dicot and monocot plants, probably via its receptor PMTR1 and interactions with the gibberellin, strigolactone, and ROS signaling pathways. Furthermore, phytomelatonin signaling also functions in hypocotyl and shoot growth in skotomorphogenesis and ultraviolet B (UV-B) exposure; the G protein α-subunit (Arabidopsis GPA1 and rice RGA1) and constitutive photomorphogenic1 (COP1) are important signal components during this process. Taken together, these findings indicate that phytomelatonin acts as a darkness signal with important regulatory roles in circadian stomatal closure, flavonoid biosynthesis, flowering, and hypocotyl and shoot growth.

Phytomelatonin: an unexpected molecule with amazing performances in plants

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

Melatonin enhanced oilseed rape growth and mitigated Cd stress risk: A novel trial for reducing Cd accumulation by bioenergy crops

Melatonin (M) is a pleiotropic molecule that improves plant growth and increases heavy metal tolerance. The role of M for improving plant growth and tolerance under cadmium (Cd) stress, and mitigation of Cd-induced toxicity has not yet been sufficiently examined. Therefore, here we conducted a glasshouse experiment to explore the influence of various M dosages on Cd detoxification and stress-tolerance responses of Brassica napus under high Cd content (30 mg kg^-1). The effects of M on the modulation of Cd tolerance in B. napus plants have been investigated using various growth attributes, Cd accumulation and tolerance indices, and secondary metabolic parameters. We found that Cd stress inhibited root growth (by 11.9%) as well as triggered reactive oxygen species accumulation (by 31.2%) and MDA levels (by 18.7%); however, exogenous M substantially alleviated the adverse effect of oxidative stress by decreasing levels of H₂O₂ (by 38.7%), MDA (by 13.8%) and EL (by 1.8%) in the Cd-stressed plants, as compared to the M-untreated plants (control). Interestingly, exogenous M reduced Cd accumulation in roots (∼48.2-58.3-fold), stem (∼2.9-5.0-fold) and leaves (∼4.7-6.6-fold) compared to control plants, which might be due to an M-induced defense and/or detoxification response involving a battery of antioxidants. Overall, addition of the exogenous M to the Cd-stressed plants profoundly enhanced Cd tolerance in B. napus relative to control plants. These results suggested the biostimulatory role (at the physiological and molecular level) of M in improving growth, Cd tolerance, and Cd detoxification in B. napus, which indicate the potentiality of M for green remediation of Cd contaminated soils. This green trial would provide a reference for producing renewable bioenergy crops under Cd stress in contaminated soils. However, these recommendations should be verified under field conditions and the potential mechanisms for the interaction between Cd and M should be explicitly explored.

Exogenous melatonin increases salt tolerance in bitter melon by regulating ionic balance, antioxidant system and secondary metabolism-related genes

BACKGROUND

Melatonin is a multi-functional molecule widely employed in order to mitigate abiotic stress factors, in general and salt stress in particular. Even though previous reports revealed that melatonin could exhibit roles in promoting seed germination and protecting plants during various developmental stages of several plant species under salt stress, no reports are available with respect to the regulatory acts of melatonin on the physiological and biochemical status as well as the expression levels of defense- and secondary metabolism-related related transcripts in bitter melon subjected to the salt stress.

RESULTS

Herewith the present study, we performed a comprehensive analysis of the physiological and ion balance, antioxidant system, as well as transcript analysis of defense-related genes (WRKY1, SOS1, PM H⁺-ATPase, SKOR, Mc5PTase7, and SOAR1) and secondary metabolism-related gene expression (MAP30, α-MMC, polypeptide-P, and PAL) in salt-stressed bitter melon (Momordica charantia L.) plants in response to melatonin treatment. In this regard, different levels of melatonin (0, 75 and 150 µM) were applied to mitigate salinity stress (0, 50 and 100 mM NaCl) in bitter melon. Accordingly, present findings revealed that 100 mM salinity stress decreased growth and photosynthesis parameters (SPAD, ^Fv/_Fo, Y(II)), RWC, and some nutrient elements (K⁺, Ca²⁺, and P), while it increased Y(NO), Y(NPQ), proline, Na⁺, Cl^-, H₂O₂, MDA, antioxidant enzyme activity, and lead to the induction of the examined genes. However, prsiming with 150 µM melatonin increased SPAD, ^Fv/_Fo, Y(II)), RWC, and K⁺, Ca²⁺, and P concentration while decreased Y(NO), Y(NPQ), Na⁺, Cl^-, H₂O₂, and MDA under salt stress. In addition, the antioxidant system and gene expression levels were increased by melatonin (150 µM).

CONCLUSIONS

Overall, it can be postulated that the application of melatonin (150 µM) has effective roles in alleviating the adverse impacts of salinity through critical modifications in plant metabolism.

Exogenous Melatonin Improves Seed Germination of Wheat (Triticum aestivum L.) under Salt Stress

Melatonin (MT) can effectively reduce oxidative damage induced by abiotic stresses such as salt in plants. However, the effects of MT on physiological responses and molecular regulation during wheat germination remains largely elusive. In this study, the response of wheat seeds to MT under salt stress during germination was investigated at physiological and transcriptome levels. Our results revealed that application of MT significantly reduced the negative influence of salt stress on wheat seed germination. The oxidative load was reduced by inducing high activities of antioxidant enzymes. In parallel, the content of gibberellin A3 (GA3) and jasmonic acid (JA) increased in MT-treated seedling. RNA-seq analysis demonstrated that MT alters oxidoreductase activity and phytohormone-dependent signal transduction pathways under salt stress. Weighted correlation network analysis (WGCNA) revealed that MT participates in enhanced energy metabolism and protected seeds via maintained cell morphology under salt stress during wheat seed germination. Our findings provide a conceptual basis of the MT-mediated regulatory mechanism in plant adaptation to salt stress, and identify the potential candidate genes for salt-tolerant wheat molecular breeding.

LsMYB15 Regulates Bolting in Leaf Lettuce (Lactuca sativa L.) Under High-Temperature Stress

High temperature is one of the primary environmental stress factors affecting the bolting of leaf lettuce. To determine the potential role of melatonin in regulating high-temperature induced bolting in leaf lettuce (Lactuca sativa L.), we conducted melatonin treatment of the bolting-sensitive cultivar "S39." The results showed that 100 μmol L^-1 melatonin treatment significantly promoted growth, and melatonin treatment delayed high-temperature-induced bolting in lettuce. RNA-seq analysis revealed that the differentially expressed genes (DEGs) involved in "plant hormone signal transduction" and "phenylpropanoid biosynthesis" were significantly enriched during high-temperature and melatonin treatment. Quantitative reverse transcription polymerase chain reaction (qRT-PCR) analysis suggested that the expression patterns of abscisic acid (ABA)-related genes positively correlated with stem length during leaf lettuce development. Furthermore, weighted gene co-expression network analysis (WGCNA) demonstrated that MYB15 may play an important role in melatonin-induced resistance to high temperatures. Silencing the LsMYB15 gene in leaf lettuce resulted in early bolting, and exogenous melatonin delayed early bolting in leaf lettuce at high temperatures. Our study provides valuable data for future studies of leaf lettuce quality.

Application of exogenous melatonin in vitro and in planta: a review of its effects and mechanisms of action

Melatonin is a natural indolamine that regulates many physiological functions in plants. The most prominent role of melatonin in plants has been its ability to work as an anti-stressor agent. Exogenous melatonin can prevent cell death and promote cell proliferation through its antioxidant properties, enhancement of polyamine biosynthesis, and the ability to shift cell metabolism in case of stressors like sugar starvation. Melatonin scavenges reactive oxygen species and thus preventing damage to cell membranes and other organelles. Its application in different plant culture systems reveals its important physiological and biochemical roles during the growth and development of these cultures. It has been observed that the exogenous melatonin protects callus culture, reduces cold-induced apoptosis in cell suspension, and stimulates adventitious and lateral roots formation. This review presents the physiological and biochemical effects of exogenous melatonin on in vitro culture systems, including its impact on biomass accumulation, growth, and development of plants.

Exogenous Melatonin Improves Pear Resistance to Botryosphaeria dothidea by Increasing Autophagic Activity and Sugar/Organic Acid Levels

Pear is an important fruit tree worldwide, but it is often infected by the pathogen Botryosphaeria dothidea, which causes pear ring rot disease. To explore the effect of exogenous melatonin on the disease resistance of pear, we treated inoculated pear fruits with different concentrations of melatonin. The results showed that 100 μΜ of melatonin had the most significant effect with resistance to B. dothidea. In addition, melatonin treatment significantly reduced the diameter of disease lesions and enhanced the endogenous melatonin content in pears inoculated with B. dothidea. Compared with the control treatment, melatonin treatment suppressed increases in reactive oxygen species (ROS) and activated ROS-scavenging enzymes. Treatment with exogenous melatonin maintained ascorbic acid-glutathione at more reductive status. The expression levels of core autophagic genes and autophagosome formation were elevated by melatonin treatment in pear fruits. Silencing of PbrATG5 in Pyrus pyrifolia conferred sensitivity to inoculation that was only slightly attenuated by melatonin treatment. After inoculation with B. dothidea, exogenous melatonin treatment led to higher levels of soluble sugars and organic acids in pear fruits than H₂O treatment. Overall, our results demonstrate that melatonin enhances resistance to B. dothidea by increasing autophagic activity and soluble sugar/organic acid accumulation.

The phytomelatonin receptor PMTR1 regulates seed development and germination by modulating abscisic acid homeostasis in Arabidopsis thaliana

Melatonin is known to involve multiple physiological actions in plants. Herein, we found that exogenous melatonin inhibited the Arabidopsis seedling growth through the elevated abscisic acid (ABA) levels, and the elevated ABA was ascribed to the upregulation of 9-cis-epoxycarotenoid dioxygenase genes (NCEDs) in the ABA biosynthesis pathway. We also found that the overexpression lines of the melatonin receptor gene PMTR1 (also known as Cand2) yielded smaller seeds and germinated slower than the wild type, whereas PMTR1-knockout mutants produced larger seeds and germinated faster than the wild type. During the seed development, the accumulation peak of ABA was higher in the PMTR1-knockout mutant, while it was lower in the PMTR1-overexpression line than that in the wild type. In the dry seeds and imbibed seeds, the PMTR1-overexpression line accumulated higher ABA levels, while the PMTR1-knockout contained less ABA than the wild type. In summary, our findings suggest that PMTR1 is involved in ABA-mediated seed development and germination in Arabidopsis.

Comparative analysis of flavonoids in white and red table grape cultivars during ripening by widely targeted metabolome and transcript levels

The flavonoid metabolites were compared between red 'Summer Black' (SB) and white 'Shine Muscat' (SM) table grapes during fruit development based on widely targeted metabolome. A total of 134 flavonoids were identified in two cultivars, including 37 flavones, 33 flavonols, and 11 anthocyanidins, and so on. From young to veraison, the composition and the content of most flavonoids were decreasing in both cultivars but increased at maturation in SB. In general, SB has higher flavonoid compositions and content than SM during the whole fruit development, especially the content of anthocyanin after veraison. While the SM had higher content of flavonols such as quercetin, kaempferol and their derivatives. The expression of anthocyanin-related genes such as UFGT, OMT, GST, MATE, MYBA1, and MYBA2 was remarkably higher in SB than those in SM, which may attribute to higher anthocyanin content, while the higher expression of F3H and FLS resulted higher level of flavonols in SM. These results improve our understanding of flavonoid profiles and molecular mechanism in table grape cultivars.

Dynamics of Etiolation Monitored by Seedling Morphology, Carotenoid Composition, Antioxidant Level, and Photoactivity of Protochlorophyllide in Arabidopsis thaliana

Although etiolated Arabidopsis thaliana seedlings are widely used as a model to study the de-etiolation process, the etiolation itself at the molecular level still needs elucidation. Here, we monitored the etiolation dynamics for wild type A. thaliana seedlings and lutein-deficient (lut2) mutant between 2 and 12 days of their growth in the absence of light. We analyzed the shape of the apex, the growth rate, the carotenoids and protochlorophyllide (Pchlide) accumulation, and the light-dependent protochlorophyllide oxidoreductase (LPOR) transcripts. Differences concerning the apical hook curvature and cotyledon opening among seedlings of the same age were observed, mostly after day 6 of the culture. We categorized the observed apex shapes and presented quantitatively how distribution among the categories changed during 12 days of seedling growth. The Pchlide₆₅₄/Pchlide₆₃₃ ratio, corresponding to the amount of the photoactive Pchlide, was the highest in the youngest seedlings, and decreased with their age. LPORA, LPORB, and LPORC transcripts were detected in etiolated seedlings, and their content decreased during seedling growth. Expression of SAG12 or SAG13 senescence markers, depletion in antioxidants, and excess ion leakage were not observed during the etiolation. Lack of lutein in the lut2 mutant resulted in slow Pchlide accumulation and affected other xanthophyll composition.

Identification of Suitable Reference Genes for qRT-PCR Normalization in Kiwifruit

Reference genes are used for the correction of qRT-PCR data, and it is necessary to investigate the optimum reference gene under certain conditions. The expression levels of seven traditional reference genes ACT1, ACT2, GAPDH, 18S rRNA, UBQ, TUB and CYP were analyzed using qRT-PCR in different varieties, tissues, developmental stages and hormone (or pollen polysaccharide) treatments in kiwifruit. Gene expression stability was assessed with the help of three common software (geNorm, NormFinder, BestKeeper), and the minimum number of reference genes necessary for normalization was also determined. GAPDH, ACT1 and ACT2 were selected as reference genes for different genotypes of kiwifruit. GAPDH and UBQ were the best combinations of reference genes for root, stem, leaf, flower and fruit. GAPDH and ACT1 could be the preferred reference genes for normalization of qRT-PCR data during fruit development. The pairing of ACT1 and UBQ constituted the optimal combination of reference genes in kiwifruit treated with different hormones (or pollen polysaccharide). This study provides a new and reliable option for the use of reference genes in the analysis of gene expression patterns of interest in kiwifruit.

Extraction and characterization of chitin from Oratosquilla oratoria shell waste and its application in Brassica campestris L.ssp

Mantis shrimp waste (Oratosquilla oratoria) is a good source of chitin. The applicability of microwave-assisted organic acids and proteases for extracting chitin from mantis shrimp shell waste was evaluated, and the extracted-chitin was characterized by scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and thermogravimetric analysis (TGA). Moreover, the effects of nanochitin on the growth of pak choi (Brassica campestris L.ssp.) were also investigated. The results indicated that alkaline protease (4000 U/g, microwave heating at 60 °C, 10 min) and malic acid (5%, 320 W, 5 min) exhibited excellent potential for deproteinizing and demineralizing shells. The deproteinization and demineralization yields were 92.78% and 94.11%, respectively, and the chitin yield was 15.6%. The extracted-chitin had a highly porous structure and exhibited excellent crystallinity and thermostability compared with chitin prepared by traditional chemical methods. Furthermore, 0.003% nanochitin significantly enhanced photosynthesis, which improved the pak choi fresh weight by 22.94%, and improved the accumulation of isothiocyanates in its leaves. This study provides an alternative approach for the high-value utilization of mantis shrimp waste, and reveals the potential of chitin for application in agricultural production.

Transcriptome analysis of melatonin regulating the transformation of glucoraphanin to sulforaphane in broccoli hairy roots

Sulforaphane (SF) is one of the most effective natural products in preventing and fighting cancer, found in cruciferous plants. In this study, broccoli hairy roots grown for 20 d were used as the experimental material, and it was treated with 500 μmol/L melatonin (MT) for 0, 12 and 32 h to explore the effect of MT on the conversion of glucoraphanin (GRA) to SF. Results showed that the yields of GRA and SF were the largest under MT treatment for 12 h, which were 1.53 and 1.93-fold, respectively, compared to 0 h. However, Myrosinases activity was the highest under MT treatment for 32 h, which was 1.42-fold compared to that of the 0 h. The differential expression of key genes involved in GRA conversion to SF in broccoli hairy roots was identified transcriptome sequencing, and the path of the transformation from GRA to SF was simulated, which provided a theoretical basis for establishing an efficient transformation system from GRA to SF.

Exogenous Application of Melatonin and Methyl Jasmonate as a Pre-Harvest Treatment Enhances Growth of Barhi Date Palm Trees, Prolongs Storability, and Maintains Quality of Their Fruits under Storage Conditions

Fresh date palm fruits (cv. Barhi) have received much attention due to their sweet taste and popularity in marketing. There is a critical need to prolong their storability, as well as maintain their quality during the postharvest and marketing periods. In this study, the effects of spraying date palm trees with melatonin (Mt) and/or methyl jasmonate (Mj) at 10, 20, and 50 ppm, on their growth and yield were investigated. In addition, impacts on quality and storability of the fruits were also studied. In general, application of Mt was mostly more effective than that of Mj, even at 50 ppm, with regard to all evaluated parameters. However, the dual treatment at 50 ppm recorded the highest relative chlorophyll and nutrient content in date palm leaves, as well as the yield and its components. Regarding the date palm fruits stored at 4 °C for 28 days, this dual treatment recorded the lowest weight loss and fruit decay values (0.14 and 2%, respectively), the highest firmness (6 g·cm⁻²), total soluble solids content (36 °Brix), total sugar content (32.5 g/100 g fresh weight), and the lowest total acidity (0.16 g citric acid/100 mL juice). Moreover, the highest total phenolic content and activity of peroxidase and polyphenol oxidase enzymes in the stored fruits were also recorded for the dual treatment. In contrast to the untreated fruits, scanning electron microscopy observations showed that the sprayed fruits had a very good microstructure, showing intact and thick exocarp tissue with a dense layer of epicuticular wax. The mesocarp tissue showed a normal and clear cellular framework with well organized and arranged cells, after 28 days storage at 4 °C. Based on these results, we can conclude that application of the dual treatment (Mt + Mj) at 50 ppm is a promising way to prolong the storability of date palm fruits and maintain their quality during storage periods.