The role of phytomelatonin receptor 1-mediated signaling in plant growth and stress response

Integrated analysis of physiological and metabolic data uncovers essential dynamic mechanisms involved in the maturation of cigar tobacco leaves

The quality of cigar tobacco leaves is profoundly affected by the timing of their harvest, with both early and late collections resulting in inferior characteristics. While the relationship between maturity and physiological metabolic processes is acknowledged, a comprehensive understanding of the physiological behavior of cigar leaves harvested at different stages remains elusive. This research investigated the physiological and metabolomic profiles of the cigar tobacco variety CX-014, grown in Danjiangkou City, Hubei Province, with leaves sampled at 35 (T1), 42 (T2), 49 (T3), and 56 (T4) days post-inflorescence removal. Leaf color transitioned from green to yellow, accompanied by the appearance of white mature spots. Notable increases in photosynthetic pigments and gas exchange parameters occurred between T1 and T2, followed by decline at T3 and T4. The optimal sugar-to-nicotine and potassium-to-chlorine ratios, critical determinants of smoking quality and tobacco combustibility, were observed at T3, indicating a superior chemical balance in leaves harvested at this stage. Metabolomic analysis revealed 2153 distinct metabolites, with the most significant changes occurring between T2 and T3, highlighting critical physiological transformations during this interval. Pathway enrichment analysis via KEGG pinpointed notable shifts in amino acid synthesis pathways, particularly those involving tryptophan, alanine, and aspartate. Tryptophan metabolism and zeatin biosynthesis were substantially altered, with compounds like indolepyruvic acid, N-formylpurine nucleotide, isopentenyladenine nucleotide, and dihydrozeatin showing marked reductions at T3. This study also explored how the timing of lower leaf harvest influences the physiological processes of middle leaves, finding that a plethora of metabolites associated with the breakdown of arachidonic acid-a primitive metazoan signaler implicated in plant stress and defense networks-were abundant in T3 leaves when lower leaves were harvested 43-38 days prior. Overall, these findings elucidates the complex physiological dynamics of cigar leaves during maturation, highlighting critical metabolites involved in essential metabolic pathways.

Melatonin extends shelf life in postharvest okra via delaying fruit softening and reducing weight loss

BACKGROUND

Melatonin, a hormone present in animals and some plants, has garnered attention for its potential in preserving harvested produce. Softening due to changes in cell wall composition and wilting caused by weight loss are the major reasons for the loss of commercial value in postharvest okra. This study aimed to evaluate the impact of melatonin on the softening and weight loss of postharvest okra.

RESULTS

The results revealed that the application of melatonin had a significant influence on the maintenance of fruit firmness by inhibiting the breakdown and dissolution of cell wall polysaccharides by suppressing the expression of specific genes responsible for cell wall degradation in okra. Conversely, melatonin treatment positively influenced the expression of genes involved in the synthesis of cell wall components. Furthermore, the treatment exhibited notable benefits in reducing weight loss in okra, which was accomplished by promoting the closure of stomata - the tiny pores on the surface of the fruit.

CONCLUSION

Melatonin could serve as a novel approach to reduce water loss, delay fruit softening and extend the shelf life of okra. © 2024 Society of Chemical Industry.

Presence of melatonin in foods of daily consumption: The benefit of this hormone for health

Melatonin (MLT) is a hormone that exists in all living organisms, including bacteria, yeast, fungi, animals, and plants, many of which are ingested daily in the diet. However, the exact concentrations of melatonin in each of the foods and the effect on health of the intake of foods rich in MLT are not known. Therefore, the aim of this review was to gather the available information on the melatonin content of different foods and to evaluate the effect that this hormone has on different pathologies. The amount of MLT may vary depending on the variety, origin, heat treatment, processing, and analysis technique, among other factors. Dietary interventions with foods rich in MLT report health benefits, but there is no evidence that hormone is partially responsible for the clinical improvement. Therefore, it is necessary to evaluate the MLT content in more foods, as well as the effect that cooking/processing has on the amount of MLT, to estimate its total intake in a typical diet and better explore its potential impact on the health.

Melatonin Affects Peucedanum praeruptorum Vegetative Growth and Coumarin Synthesis by Modulating the Antioxidant System, Photosynthesis, and Endogenous Hormones

The dried root of Peucedanum praeruptorum is often used medicinally and has high pyran- and furanocoumarin content. Although exogenous melatonin (MT) impacts the regulation of plant growth, stress responses, secondary metabolism, etc., it remains unclear whether MT regulates the vegetative growth and development of P. praeruptorum. Thus, the aim of the current study is to characterize the effects of different exogenous MT concentrations on the physiological functions, photosynthesis, antioxidant systems, hormone induction, and coumarin synthesis of P. praeruptorum. Different MT concentrations exert distinct regulatory effects on P. praeruptorum growth and the expression of genes related to coumarin synthesis. Treatment of P. praeruptorum with low concentrations of MT increases photosynthesis and leaf growth compared to the control, while high concentrations reduce root vitality and elongation and decrease the expression of photosynthetic system genes. Low concentrations of MT also significantly increase antioxidant enzyme activity and photosynthetic pigment content and modulate the levels of IAA, gibberellic acid, salicylic acid, jasmonic acid, abscisic acid, and endogenous MT. Moreover, MT increases the activity of the MT synthesis enzymes tryptophan decarboxylase, tryptophan hydroxylase, tryptamine-5-hydroxylase, serotonin N-acetyltransferase, acetylserotonin O-methyltransferase, and caffeic acid O-methyltransferase, and promotes the accumulation of isoscopoletin, scopoletin, peucedanocoumarin II, praeruptorin A, praeruptorin B, and praeruptorin E. MT also upregulates most genes associated with coumarin synthesis, including PAL1, C4H, 4CL-3, C3H-1, F6H-1, CCoAMT, OMT-1, CYP71AJ1, CYP84A1-1, S8H-1, PT-1, and COSY-1. These findings demonstrate that MT may improve P. praeruptorum growth and development while promoting the synthesis of coumarin components.

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

KEY MESSAGE

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

Identification and functional analysis of two serotonin N-acetyltransferase genes in maize and their transcriptional response to abiotic stresses

Introduction

Melatonin, a tryptophan-derived indoleamine metabolite with important roles in plant growth and defense, has recently been regarded as a new plant hormone. Maize is one of the most important cereal crops in the world. Although the melatonin receptor gene, ZmPMTR1, has already been identified, the genetic basis of melatonin biosynthesis in maize has still not been elucidated. Serotonin N-acetyltransferase (SNAT) is the enzyme that converts serotonin to N-acetylserotonin (NAS) or 5-methoxytryptamine (5MT) to melatonin in Arabidopsis and rice, but no SNAT encoding gene has been identified yet in maize.

Methods

The bioinformatics analysis was used to identify maize SNAT genes and the enzyme activity of the recombinant proteins was determined through in vitro assay. The expression levels of ZmSNAT1 and ZmSNAT3 under drought and heat stresses were revealed by public RNA-seq datasets and qRT-PCR analysis.

Results

We first identified three maize SNAT genes, ZmSNAT1, ZmSNAT2, and ZmSNAT3, through bioinformatics analysis, and demonstrated that ZmSNAT2 was present in only eight of the 26 cultivars analyzed. We then determined the enzyme activity of ZmSNAT1 and ZmSNAT3 using their recombinant proteins through in vitro assay. The results showed that both ZmSNAT1 and ZmSNAT3 could convert serotonin to NAS and 5-MT to melatonin. Recombinant ZmSNAT1 catalyzed serotonin into NAS with a higher catalytic activity (K m, 8.6 mM; V max, 4050 pmol/min/mg protein) than ZmSNAT3 (K m, 11.51 mM; V max, 142 pmol/min/mg protein). We further demonstrated that the 228th amino acid Tyr (Y228) was essential for the enzymatic activity of ZmSNAT1. Finally, we revealed that the expression of ZmSNAT1 and ZmSNAT3 varied among different maize cultivars and different tissues of a plant, and was responsive to drought and heat stresses.

Discussion

In summary, the present study identified and characterized the first two functional SNAT genes in maize, laying the foundation for further research on melatonin biosynthesis and its regulatory role in plant growth and response to abiotic stresses.

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

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

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

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

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

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