High Concentration of Melatonin Regulates Leaf Development by Suppressing Cell Proliferation and Endoreduplication in Arabidopsis

Melatonin - This is important to know

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

Shoot differentiation from Dendrocalamus brandisii callus and the related physiological roles of sugar and hormones during shoot differentiation

Only a few calli regeneration systems of bamboos were successfully established, which limited the research on physiological mechanism of callus differentiation. In this study, we successfully established the callus differentiation systems of Dendrocalamus brandisii via seeds. The results showed that the best medium for callus induction of D. brandisii seeds was basal MS media amended with 5.0 mg L-1 2,4-D and 0.5 mg L-1 KT, and the optimal medium for shoot differentiation was the basal MS media supplemented with 4.0 mg L-1 BA and 0.5 mg L-1 NAA. Callus tissues had apparent polarity in cell arrangement, and developed their own meristematic cell layers. α-amylase, STP and SUSY played a dominant role in carbohydrates degradation in callus during shoot differentiation. PPP and TCA pathways up-regulated in the shoot-differentiated calli. The dynamics of BA and KT contents in calli was consistent with their concentrations applied in medium. IAA synthesis and the related signal transduction were down-regulated, while the endogenous CTKs contents were up-regulated by the exogenous CTKs application in shoot-differentiated calli, and their related synthesis, transport and signal transduction pathways were also up-regulated. The downregulated signal transduction pathways of IAA and ABA revealed that they did not play the key role in shoot differentiation of bamboos. GAs also played a role in shoot differentiation based on the down-regulation of DELLA and the up-regulation of PIF4 genes. The overexpression of DbSNRK2 and DbFIF4 genes further confirmed the negative role of ABA and the positive role of GAs in shoot differentiation.

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.

Melatonin improves the photosynthesis in Dracocephalum kotschyi under salinity stress in a Ca2+/CaM-dependent manner

This study investigated: (1) the effects of various concentrations of melatonin (MT) and Ca2+; and (2) the impact of crosstalk between these signal molecules on photosynthesis and salt tolerance of Dracocephalum kotschyi Boiss. Results indicated that 5mM CaCl2, as well as 100μM MT were the best concentrations for increasing shoot dry weight, leaf area, SPAD index, maximum quantum efficiency of photosystem II (Fv/Fm), and decreasing malondialdehyde content under salinity stress. The impact of MT on growth and photosynthesis was closely linked to its effect on enhancing antioxidant enzyme activities in leaves. Application of p-chlorophenylalanine, as an inhibitor of MT biosynthesis, negated the impacts of MT on the aforementioned attributes. Salinity and MT boosted cytosolic Ca2+ concentration. Exogenous MT, as well as Ca2+, enhanced tolerance index, membrane stability, leaf area, the content of chlorophyll (Chl) a, Chl b, and carotenoids (Car), Fv/Fm, and stomatal conductance under salinity stress. These impacts of MT were eliminated by applying a calmodulin antagonist, a Ca2+ chelator and a Ca2+ channel blocker. These novel findings indicate that the MT-induced effects on photosynthetic parameters and salt-evoked oxidative stress were mediated through calcium/calmodulin (Ca2+/CaM) signalling.

Melatonin Application Modifies Antioxidant Defense and Induces Endoreplication in Maize Seeds Exposed to Chilling Stress

The aim of the study was to demonstrate the biostimulating effect of exogenous melatonin (MEL) applied to seeds via hydroconditioning. It was indicated that only well-chosen application technique and MEL dose guarantees success concerning seed germination and young seedlings growth under stress conditions. For maize seed, 50 μM of MEL appeared to be the optimal dose. It improved seed germination and embryonic axes growth especially during chilling stress (5 °C/14 days) and during regeneration after its subsided. Unfortunately, MEL overdosing lowered IAA level in dry seeds and could disrupt the ROS-dependent signal transduction pathways. Very effective antioxidant MEL action was confirmed by low level of protein oxidative damage and smaller quantity of lipid oxidation products in embryonic axes isolated from seeds pre-treated with MEL and then exposed to cold. The stimulatory effects of MEL on antioxidant enzymes: SOD, APX and GSH-PX and on GST-a detoxifying enzyme, was also demonstrated. It was indicated for the first time, that MEL induced defence strategies against stress at the cytological level, as appearing endoreplication in embryonic axes cells even in the seeds germinating under optimal conditions (preventive action), but very intensively in those germinating under chilling stress conditions (intervention action), and after stress removal, to improve regeneration.

Structural and Molecular Dynamics Analysis of Plant Serotonin N-Acetyltransferase Reveal an Acid/Base-Assisted Catalysis in Melatonin Biosynthesis

Serotonin N-acetyltransferase (SNAT) is the key rate-limiting enzyme in melatonin biosynthesis. It mediates melatonin biosynthesis in plants by using serotonin and 5-methoxytryptamine (5-MT), but little is known of its underlying mechanisms. Herein, we present a detailed reaction mechanism of a SNAT from Oryza sativa through combined structural and molecular dynamics (MD) analysis. We report the crystal structures of plant SNAT in the apo and binary/ternary complex forms with acetyl-CoA (AcCoA), serotonin, and 5-MT. OsSNAT exhibits a unique enzymatically active dimeric fold not found in the known structures of arylalkylamine N-acetyltransferase (AANAT) family. The key residues W188, D189, D226, N220, and Y233 located around the active pocket are important in catalysis, confirmed by site-directed mutagenesis. Combined with MD simulations, we hypothesize a novel plausible catalytic mechanism in which D226 and Y233 function as catalytic base and acid during the acetyl-transfer reaction.

Analysis of melatonin regulation of germination and antioxidant metabolism in different wheat cultivars under polyethylene glycol stress

Melatonin is effective in enhancing various abiotic stress resistances of plants. However, its underlying mechanisms in drought-resistance in winter wheat (Triticum aestivum L.) is not clear. The goal of this work was to investigate the effect of melatonin on seed germination and to evaluate leaf antioxidant physiology for two wheat varieties. Experiments included 20% PEG, melatonin plus 20% PEG and a control using two contrasting wheat varieties (JM22– drought sensitive and HG35– drought resistant). Melatonin levels were 0, 1, 10, 100 and 300 μmol L^-1. Results revealed that 300 μmol L^-1 of melatonin alleviated the negative effect of water stress on germination and increased radicle length, radicle number, and plumule length of the germinated seeds. Principal component analysis showed a significant change in amino acid content during germination and this change was dependent on melatonin concentration and the variety. Lysine (Lys) content in wheat seeds under the PEG plus 300 μmol L^-1 melatonin treatment increased compared with that of the seeds under PEG alone. There was a significant and positive correlation between Lys content and morphological index of germination. During seedling growth, soluble protein was involved in osmotic adjustment and superoxide dismutase (SOD) activity was increased to mitigate the damage in the cytomembrane of JM 22 leaf under 300 μmol L^-1 melatonin plus PEG treatment. The effect of melatonin was dependent on SOD activity increasing significantly for HG35—a drought resistant variety. The results of this work lays a foundation for further studies to determine if melatonin can be economically used to mitigate the impact of dry planting conditions on wheat productivity in North China Plain.

Foliar application of melatonin delay leaf senescence in maize by improving the antioxidant defense system and enhancing photosynthetic capacity under semi-arid regions

Melatonin is an important plant growth regulator which plays a key role in plant growth and development. The objective of the current research was to evaluate the effect of foliar application of melatonin (MF) on photosynthetic efficiency, antioxidant defense mechanism, and its relation with leaf senescence in maize crop grown in a semi-arid region. A field experiment was conducted during 2017 and 2018 growth season, where melatonin was applied to the foliage at concentrations of 0 (MF₀), 25 (MF₁), 50 (MF₂), and 75 (MF₃) μM at the ninth leaf stage. Foliar application of melatonin significantly improved chlorophyll content, net photosynthetic rate, soluble sugar content, and soluble protein content during the process of leaf senescence. The application of melatonin also enhanced antioxidant enzyme activities including superoxide dismutase, catalase, and peroxidase, while reduced malondialdehyde and reactive oxygen species accumulation. Melatonin foliar application also increased total leaf area per plant, grains per ear, thousand grain weight and grain yield of maize crop in a semi-arid region. The application of melatonin significantly improved photosynthetic activity, antioxidant defense mechanism, and yield of maize crop in a semi-arid region, where the most effective treatment was MF2.

Knockout of Arabidopsis Serotonin N-Acetyltransferase-2 Reduces Melatonin Levels and Delays Flowering

Melatonin plays roles in both plant growth and defense. Serotonin N-acetyltransferase (SNAT) catalyzes formation of N-acetylserotonin (NAS) from serotonin. Plants contain two SNAT isogenes, which exhibit low-level amino acid homology. We studied the ArabidopsisthalianaSNAT2 (AtSNAT2) gene; we prepared recombinant SNAT2 protein and characterized a snat2 knockout mutant. The SNAT2 protein exhibited 27% amino acid homology with SNAT1; the Km was 232 μM and the Vmax was 2160 pmol/min/mg protein. Melatonin inhibited SNAT enzyme activity in vitro. SNAT2 mRNA was abundantly expressed in flowers; the melatonin content of flowers of the snat2 mutant was significantly less than that of wild-type flowers. The mutant exhibited delayed flowering and reductions in leaf area and biomass compared to the wild type. Delayed flowering was attributable to reductions in the expression levels of the gibberellin biosynthetic genes ent-kaurene synthase (KS) and FLOWERING LOCUS T (FT).

Melatonin enhances thermotolerance of maize seedlings (Zea mays L.) by modulating antioxidant defense, methylglyoxal detoxification, and osmoregulation systems

Melatonin (MT), derived from tryptophan, is an amazing signaling molecule with multiple functions in plants. Heat stress (HS) induced by high temperature is a major stress factor that limits metabolism, growth, development, and productivity of plants. However, whether MT could enhance the thermotolerance of maize seedlings and the underlying mechanisms is not completely known. In this study, treatment of maize seedlings with MT enhanced the survival percentage of maize seedlings under HS conditions, mitigated an increase in malondialdehyde (MDA, product of membrane lipid peroxidation) and electrolyte leakage, and improved tissue vitality compared with the control without MT treatment, indicating that MT treatment could enhance the theromotolerance of maize seedlings. To understand the mechanisms underlying MT-enhanced thermotolerance of maize seedlings, the antioxidant defense (guaiacol peroxidease: GPX; glutathione reductase: GR; catalase: CAT; ascorbic acid: AsA; and glutathione: GSH), methylglyoxal (MG) detoxification (glyoxalase I: Gly I; and glyoxalase II: Gly II), and osmoregulation (proline: Pro; trehalose: Tre; and total soluble sugar: TSS) systems were assayed. The results showed that MT treatment stimulated the activities of antioxidant enzymes (GPX, GR, and CAT) and MG detoxification enzymes (Gly I and Gly II), increased the contents of nonenzyme antioxidants (AsA and GSH) and osmolytes (Pro, Tre, and TSS) in maize seedlings under normal culture conditions, and maintained a higher abovementioned enzyme activity and antioxidant and osmolyte contents under HS conditions compared with the control. This work reported that MT could enhance the thermotolerance of maize seedlings by modulating the antioxidant defense, MG detoxification, and osmoregulation systems.

Exogenous melatonin improves lead tolerance of bermudagrass through modulation of the antioxidant defense system

Lead (Pb) is a major anthropogenic contaminant that can be devastating on both animals and plants. It is essential to develop methods to decrease the Pb contaminant in soil by phytoremediation using plants that are tolerance to Pb. In this study, we investigated the tolerance of bermudagrass (Cynodon dactylon (L.) Pers.) and the role of exogenous application of melatonin for improving its tolerance to Pb. Bermudagrass growing in soil treated with Pb at 1,000 or 2,000 mg kg^-1 were assessed with or without melatonin pretreatment at various concentrations. Under Pb stresses, bermudagrass plants showed stunted growth and increased cellular oxidative stress. Pre-treating bermudagrass plants with melatonin at 20 or 100 μM significantly increased the activities of antioxidant enzymes (superoxide dismutase, catalase, peroxidase, ascorbate peroxidase, and glutathione reductase) and the contents of non-enzymatic antioxidants (ascorbic acid and glutathione), and decreased reactive oxygen species (hydrogen peroxide, superoxide), and ultimately reduced membrane lipid peroxidation and permeability. These changes contributed to improvements in the water status, photosynthetic pigment synthesis, and biomass production of bermudagrass under Pb stresses. Our study provides the first evidence that melatonin may be a promising tool for enhancing Pb tolerance and phytoremediation potential of bermudagrass.

Hypocotyl Elongation Inhibition of Melatonin Is Involved in Repressing Brassinosteroid Biosynthesis in Arabidopsis

Melatonin functions as a plant hormone/regulator in the regulation of growth and development. However, the underlying mechanisms are still unclear. In this study, we found that a high dose of melatonin inhibited hypocotyl elongation in a dose-dependent manner in Arabidopsis. An expression profile analysis showed that hypocotyl growth inhibition by melatonin was involved in reprograming the expression of cell elongation genes and brassinosteroid (BRs) biosynthetic genes. Furthermore, similar to BR biosynthetic inhibitor brassinazole (BRZ), a high concentration of melatonin upregulated BR-biosynthetic genes and downregulated BR-induced genes involved in cell elongation, while melatonin was inefficient in brassinazole-resistant mutants like the bzr1-1D and bes1-D in hypocotyl inhibition. The comparative expression profile analysis showed an opposite expression mode in the co-regulated genes between melatonin and BZR1 or melatonin and brassinolide (BL). Additionally, exogenous BL rescued the repressive phenotype of BR biosynthesis-deficient mutant like det2-1 even in the presence of high-dose melatonin, but not BR receptor mutant bri1-5 or signal transduction mutant bin2-1. A biochemical analysis further confirmed that melatonin reduced endogenous BR levels in a dose-dependent manner in Arabidopsis. Taken together, these results indicate that melatonin inhibits BR biosynthesis but does not block BR signaling in the inhibition of hypocotyl elongation and extends insights on the role of melatonin in cross-talking with plant hormone signaling.

Hydrogen peroxide acts downstream of melatonin to induce lateral root formation

BACKGROUND AND AIMS

Although several studies have confirmed the beneficial roles of exogenous melatonin in lateral root (LR) formation, the molecular mechanism is still elusive. Here, the role of hydrogen peroxide (H2O2) in the induction of LR formation triggered by melatonin was investigated.

METHODS

Alfalfa (Medicago sativa 'Biaogan') and transgenic Arabidopsis seedlings were treated with or without melatonin, diphenyleneiodonium (DPI, NADPH oxidase inhibitor), N,N'-dimethylthiourea (DMTU, H2O2 scavenger), alone or combined. Then, H2O2 content was determined with 2',7'-dichlorofluorescein diacetate (H2DCFDA)-dependent fluorescence and spectrophotography. Transcript levels of cell cycle regulatory genes were analysed by real-time reverse transcription-PCR.

KEY RESULTS

Application of exogenous melatonin not only increased endogenous H2O2 content but also induced LR formation in alfalfa seedlings. Consistently, melatonin-induced LR primordia exhibited an accelerated response. These inducible responses were significantly blocked when DPI or DMTU was applied. Compared with the wild-type, transgenic Arabidopsis plants overexpressing alfalfa MsSNAT (a melatonin synthesis gene) increased H2O2 accumulation and thereafter LR formation, both of which were blocked by DPI or DMTU. Similarly, melatonin-modulated expression of marker genes responsible for LR formation, including MsCDKB1;1, MsCDKB2;1, AtCDKB1;1 and AtCDKB2;1, was obviously impaired by the removal of H2O2 in both alfalfa and transgenic Arabidopsis plants.

CONCLUSIONS

Pharmacological and genetic evidence revealed that endogenous melatonin-triggered LR formation was H2O2-dependent.

Melatonin and serotonin: Mediators in the symphony of plant morphogenesis

Melatonin and serotonin are important signaling and stress mitigating molecules that play important roles across growth and development in plants. Despite many well-documented responses, a systematic investigation of the entire metabolic pathway (tryptophan, tryptamine, and N-acetylserotonin) does not exist, leaving many open questions. The objective of this study was to determine the responses of Hypericum perforatum (L.) to melatonin, serotonin, and their metabolic precursors. Two well-characterized germplasm lines (#4 and 112) created by mutation and a haploid breeding program were compared to wild type to identify specific responses. Germplasm line 4 has lower regenerative and photosynthetic capacity than either wild type or line 112, and there are documented significant differences in the chemistry and physiology of lines 4 and 112. Supplementation of the culture media with tryptophan, tryptamine, N-acetylserotonin, serotonin, or melatonin partially reversed the regenerative recalcitrance and growth impairment of the germplasm lines. Quantification of phytohormones revealed crosstalk between the indoleamines and related phytohormones including cytokinin, salicylic acid, and abscisic acid. We hypothesize that melatonin and serotonin function in coordination with their metabolites in a cascade of phytochemical responses including multiple pathways and phytohormone networks to direct morphogenesis and protect photosynthesis in H. perforatum.