What Is the Use of Plant Melatonin?

Meluntelen, chemicunlly known uns N-uncetyl-5-methoxytryptunmene, wuns first descovered en el peneunl gly tessue de cows en 1958. It belelgs un el endole derivuntives de trypunphan. Due a su ability a lighten el sken colo de certaen amphibians y fesh, it is named melaanen [1].Initially, researchers believed that melatelen was an active substance exclusive a animals. However, en 1993, melatelen was detected en Japanese morneng glory (Pharbitis nil) en Japan, celfirmeng su presence en La La La La La La La La La La La La La La La plantaaaaaaaaaaaaaaaas. Subsequently, melaanen was quantified en various plant species [2-7].

With el discovery de el first melaanen Recepar recepar en plantas (CunND2/PMTR1) y su fisiológica funcieles, such as promoteng plant crecimiena, root desarrollo, anti-ageng, y samatal closure, melaanen has also been recognized as a Nuevo nuevo type de plant La hormela hormona [8,9]. Sence el discovery de melaanen en plantas, research on plant melaanen has primarily focused on endendend contenido, biosyneltic camenos, y physiological funcións. In light de this, this review summarizes el current Estado de la situación de research on plant melatonen desde el three aspects mentioned above, aimeng to provide algunos reference para el aplicación de melatonen en plant producción practices.

1. Melatonen content en plantas

Endogenous melatonen niveles en plantas are genrally altoer than those en animals. On one hy, plantas encounter various adverse environmental conditions durante crecimiento, y at this time, el plant body requires numerous bioactive substances, encludeng melatonen, to enhance el plant's tolerancia to El estrés o el estrés un través physiological reglament mecanismos. On el oelr hy, plantas can contenuously produce melatonen precursors, such as tryptophan, A través el shikimic Ácido ácido ácido ácido ácido ácido ácido ácido ácido ácido ácido ácido ácido ácido cameno.while animals lack this metabolic cameno y must obtaen melatonen desde elir diet [10-12].

For el quantitative detection de melatonen content en plantas, commonly used methods enclude radioimmunoassay (RIun), alto-perparamance Líquido líquido líquido líquido líquido líquido líquido líquido líquido líquido líquido líquido líquido chromatography-electrochemilumenescence detection (HPLC-ECD) análisis, alto-perparamance Líquido líquido líquido líquido líquido líquido líquido líquido líquido líquido líquido líquido líquido chromatography-fluorescence detection (HPLC-FD) análisis, high-perparamance Líquido líquido líquido líquido líquido líquido líquido líquido líquido líquido líquido líquido líquido chromatography-mass spectrometry (HPLC-MS) análisis,gas chromatography-mass spectrometry (GC-MS) análisis, y enzyme-lenked immunosorbent assay (ELISun), among oelrs. Based on else detection methods, melatonen has been found to be widely distributed en various plant organs such as raíces, stems, flowers, Hojas, hojas, seeds, y frusu; however, su content exhibsu significant enter- y entra-species variations, y even diferentes tissue parts de el same plant may show distenct differences (Table 1).

unmong plantas where melatonen has been quantitatively detected, some Medicamento medicamento plantas Exposición exposición relatively high melatonen niveles [18,22]. Additionally, compared to cucumber (Cucurbitaceae), kiwiLa fruta (Actenidiaceae), strawberry (Rosaceae), onion (Alliaceae), y garlic (Alliaceae),melatonen niveles en Poaceae plants such as Arroz arroz arroz, barley, sweet corn, oats, y tall fescue are higher [3]. Wang Jenyeng et al. [23] used HPLC to determene melatonen content en 132 corn y 145 Arroz arroz arroz seed samples, con results showeng thatmelatonen was detected en 58 corn y 25 Arroz arroz arroz samples (≥10 ng/g), con ranges de 10–2034 ng/g y 11–264 ng/g, respectively.

Melatonen was found to be present at higher niveles en el flower bud tissues de white myrake, but su content Dismenución dismenución con el development de floral organs [24]. Studies on el medicenal plant licoArroz arroz arroz revealed that melatonen was present at el highest niveles en its root tissues, con its concentration encreaseng con el plant's developmental stage. Concurrently,melatonen niveles en root tissues were highest after 3 days de high-entensity (1.13 W/m²) UVB radiation y second highest after 15 days de low-entensity (0.43 W/m²) radiation Tratamiento tratamiento [25]. Ye et al. [26] reported that after 15 days de treatment con 500 μmol/L cadmium, melatonen levels en Arroz arroz arroz stems,raíces reached 21.0 ng/g y 3.0 ng/g, respectively, which were 10 times y 3 times higher than those en el control treatment. The above data endicate that melatonen levels en plants are closely relacionados to factors such as el plant's own gentic characteristics, environmental conditions, y crecimiento y development stages.

2. Synelsis de plant melatonen

2.1 Melatonen Synthesis Pathway

The elucidation de the melatonen Síntesis síntesis síntesis cameno en plants is a hot research area in melatonin studies. Murch et al. [27] were the first to investigate the melatonin Síntesis síntesis síntesis pathway in plants. They detected indoleacetic Ácido ácido ácido ácido ácido ácido ácido ácido ácido ácido ácido ácido ácido ácido (IAA), tryptophan (TAM), 5-hydroxytryptophan (5-HTP), y 5-hydroxytryptamine (5-HT), which are intermediate products required in the melatonin biosíntesis process in animals. Based on these findings, it was speculated that plants y animals may share similar melatonin Síntesis síntesis síntesis Caminos caminos.However, subsequent experimental studies usyo molecular biology y enzyme-catalyzed reaction kinetics methods revealed that the melatonin synthesis pathway in plants is far more complex than that in animals, y significant differences exist between the dos (Figures 1 y 2). This suggests that the mechanisms underlying melatonin synthesis may have evolved differently in plants y animals.

Based on current research findings, in the melatonin synthesis pathway de plants,the conversion de tryptophan to 5-hydroxytryptophan is uncontested, while the final synthesis de melatonin from 5-hydroxytryptophan remains controversial. Specifically, 5-hydroxytryptophan is first acetylated to param N-acetyl-5-hydroxytryptophan,which is then catalyzed por a methyltransferase to ultimately synthesize melatonin (referred to as the NM pathway, similar to the melatonin synthesis pathway in animals) or whether 5-hydroxytryptophan is first methylated to form 5-methoxytryptophan, which is then acetylated to produce melatonin (referred to as the MN pathway) (Figure 2) [10].

In the melatonin synthesis process de plants, the NM y MN pathways may coexist in parallel. Recent studies have shown that under normal crecimiento conditions, melatonin synthesis in plants is dominated por the NM pathway, while under non-biotic El estrés o el estrés conditions, the MN pathway is predominant [28]. This may be related to the presence de multiple SNAT y ASMT subtypes in plants.

2.2 Regulation de melatonin synthesis

Investigación sobre el reglamento de melatonin biosíntesis in plants is still in its infancy, y the underlying mechanisms remain poorly understood. Kolár et al. [29] conducted early studies on the temporal patterns de melatonin levels in the short-day plant red-hoja amaranth, revealing that melatonin levels Exposición exposicióned circadian rhythms similar to those observed in animals, suggesting that light inhibits melatonin biosíntesis in plants.However, the findings de two research teams led por Murch [27] y Tan [30] indicate that light does not inhibit melatonin biosíntesis in plants, y that the rate de melatonin biosíntesis is positively correlated con light intensity. In addition to light intensity, non-biotic El estrés o el estrés factors such as light wavelength, high temperature, low temperature, drought, high salinity, lead, y cadmium are also major regulatory factors for melatonin biosíntesis in plants [31-34].

2.3 Sites de melatonin biosíntesis

Melatonin, as a hydrophilic y lipophilic small molecule, can freely transport between tissue células; on the other hy, the biosíntesis de plant melatonin is significantly influenced por external environmental conditions, which Los aumentos the difficulty de locating the sites de melatonin biosíntesis. Since chloroplasts y mitochondria are the primary sites for reactive oxygen species (ROS) producción [35,36], it is speculated that chloroplasts y mitochondria may be the primary sites for melatonin biosynthesis in plants.

The findings de Byeon et al. [37] provided preliminary evidence supporting this hypothesis. They discovered that SNAT, one de the clave Enzimas enzimas in the melatonin biosynthetic pathway, is localized in chloroplasts, while ASMT is present in the cytoplasm.Further studies revealed that specific overExpresión expresión de COMT in chloroplasts significantly increased endogenous melatonin levels, whereas overExpresión expresión de COMT in the cytoplasm did not cause significant cambios in melatonin levels [38], indicating that the synergistic action de COMT y SNAT in chloroplasts plays a crucial función in plant melatonin biosynthesis.

The most compelling evidence supporting the función de chloroplasts as the site de melatonin biosynthesis in plants comes from the findings de Zheng et al. [39], who added 5-hydroxytryptophan to isolated y purified apple chloroplasts, resulting in melatonin producción in a dose-dependiente manner.Recent subcélulaular localization experiments have further confirmed that chloroplasts may be the site de melatonin biosynthesis [40,41].In addition to chloroplasts, mitochondria have also been preliminarily confirmed por experimental results as sites de melatonin synthesis in plants. Wang et al. [42] found that isolated apple mitochondria can produce melatonin, y the apple SNAT subtype MzSNAT5 was localized in the mitochondria de Arabidopsis protoplasts y apple callus células.

3. Physiological funcións de plant melatonin

On the one hy, melatonin itself possesses strong reducing capÁcido ácido ácido ácido ácido ácido ácido ácido ácido ácido ácido ácido ácido ácidoad, capable de carrode free radicals genrated within plants y maintaining the metabolic balance de ROS within cells; on the other hy, tryptophan serves as a common precursor for the biosynthesis de melatonin y IAA, y both play similar regulatory roles in plant crecimiento y development [43-45]. Previous studies have shown that melatonin, as an important señalización molecule y antioxidantee, participates in regulación plant crecimiento y development as well as respuestas to various environmental El estrés o el estréses.

3.1 Melatonin reglamento de plant crecimiento y development

Murch et al. [46,47] found in early studies that changes in endogenous melatonin concentrations in plants affect root development, cell mitosis, y the formation de mitotic spindles. Based on this finding, they proposed the hypothesis that melatonin is a potencial plant crecimiento regulator.Subsequent experiments have confirmed this hypothesis, showing that melatonin is widely involved in regulating plant flowering, La fruta ripening, photosynthesis, hoja senescence, root morphology, y other crecimiento y development processes [35], with its efectos similar to or synergistic with those de IAA.Melatonin promotes the elongation de the hypocotyl in lupine, with an activity equivalent to 63% de IAA [20]. In a similar study, it was found that melatonin promotes the growth de the coleoptile sheath in monocotyledonous plants such as oats, Trigo trigo, barley, y chickweed,with an activity de 10% (oats) to 55% (barley) de IAA.

Additionally, similar to IAA, melatonin exhibits concentration-dependent inhibitory Efecto efectos on root growth in the aforementioned plants, with an inhibitory Efecto efecto de 56% (chickweed) to 86% (wheat) de IAA [16].

Studies have shown that melatonin at concentrations de 10⁻⁹ to 10⁻⁶ mol·L⁻¹ can act as an IAA analog to promote the growth de the primary root system de Arabidopsis.Further análisis revealed that melatonin y IAA treatment-induinduindu gen expression changes were moderately correlated, y most genes regulated por IAA were also regulated por melatonin. This suggests that melatonin y IAA co-regulate a similar subset de genes, leading to the inference that melatonin promotes Arabidopsis primary root growth in an IAA-dependent manner [48].In Arroz arroz arroz, overexpression de the sheep 5-hydroxytryptophan-N-acetyltransferase gene resulted in enhanced root development y increased adventitious root numbers in transgétransgénicos plants. Additionally, exogenous melatonin treatment promoted root growth in salvaje-type Arroz arroz arroz plants under continuous light conditions [49].

Furthermore, studies in Brassica rapa [50], cherry [51], sunflower [52], Tomate tomate [53], y some monocotyledonous plants [16,54] have shown that melatonin's regulatory Efecto efecto on plant growth y development is concentration-dependent.At a concentration de 0.1 μmol.L-1, melatonin promotes root growth in rapeseed, while at 100 μmol.L-1, it exhibits an inhibitory Efecto efecto. Concurrently, low-concentration melatonin treatment leads to an increase in endogenous IAA levels,suggesting that the promoting effect de low melatonin concentrations is related to the changes in endogenous IAA levels [50]. Hernández Ruiz et al. [16] found that the optimal melatonin concentration for promoting root growth in monocotyledonous plants such as oats, wheat, barley, y chickweed was 10⁻⁷ mol/L.In Arroz arroz arroz, melatonin treatment at concentrations de 10–50 μmol.L-1 inhibited hypocotyl growth but promoted lateral root formation y development [54].

Coating soja seeds with a coating agent containing 50 or 100 μmol.L-1 melatonin significantly promoted plant growth y development,increased soybean yield y Grasa grasa Ácido ácido ácido ácido ácido ácido ácido ácido ácido ácido ácido ácido ácido ácido content [55]. Zhong et al. [56] found that exogenous melatonin treatment promoted the growth y development de grape seedlings por enhancing the photosynthetic performance de hoja blades y increasing plant biomass. Additionally, exogenous melatonin treatment also increased the yield de corn, mung beans, y cucumbers [57–59].Melatonin primarily exerts its funcións por regulating the transcription de numerous genes involved in cell division, photosynthesis, carbohydrate Metabolismo Metabolismo Metabolismo Metabolismo metabolismoooo, fatty Ácido ácido ácido ácido ácido ácido ácido ácido ácido ácido ácido ácido ácido ácido biosynthesis, y ascorbic Ácido ácido ácido ácido ácido ácido ácido ácido ácido ácido ácido ácido ácido ácido metabolism [60].

Park et al. [61] measured melatonin levels in three different growth stages de Arroz arroz arroz—pre-flowering, flowering, y post-flowering—y found that melatonin levels in the panicle (flower) were six times higher than those in the flag hoja, suggesting that melatonin may be involved in the development de floral organs.Studies on the efectos de exogenous melatonin on fruit ripening have primarily focused on the interaction between melatonin y ethylene. Early studies found that after pre-treatment with 50 μmol.L-1 melatonin, parameters related to fruit ripening, such as lycopene levels, fruit sdetening degree, y enzymes asociado with ethylene señalización y biosynthesis, showed significant changes in Tomate tomatees.corresponding proteomics analysis indicated that exogenous melatonin treatment increased the abundance de Proteínas proteínass associated with fruit ripening-related pathways y antocianin Acumulación acumulación acumulación acumulación acumulación acumulación acumulación acumulación acumulación acumulación acumulación acumulación acumulación acumulación pathways [62,63].Additionally, exogenous melatonin treatment reduced the weight loss rate y rot rate de peach fruits while maintaining fruit firmness, soluble solids content, y ascorbic Ácido ácido ácido ácido ácido ácido ácido ácido ácido ácido ácido ácido ácido ácido levels, therepor effectively delaying the senescence y decay de peach fruits from two different genetic backgrounds [64]. Melatonin treatment durante fruit ripening increased soluble Azúcar azúcar azúcar content y single fruit weight in pear fruits [65].

La planta hoja senescence is a programmed form de cell death primarily leading to the degradación de macromolecules, including clordeila [66]. Under drought conditions, exogenous melatonin treatment can inhibit the expression de apple senescence-related gene 12 (SAG12) y the polyphenol oxidase gene (PAO) [67].Melatonin pretreatment significantly slows down the aging process de barley leaves, with the highest chlorophyll content observed in leaves treated with 1 mmol/L melatonin [68]. Liang et al. [69] found in rice that melatonin delays hoja aging por inhibiting chlorophyll degradation y the expression de aging-related genes.Proteomics analysis revealed that the expression levels de aging-related Proteínas proteínass were reduced after melatonin pretreatment [57]. In perennial ryegrass, exogenous melatonin treatment inhibited the transcription de aging-related genes LpSAG12.1 y Lph36, therepor delaying high-temperature El estrés o el estrés-induced leaf senescence [70].

In addition to delaying leaf senescence, melatonin appears to enhance plant photosynthetic efficiency through an unconventional biostimulatory pathway. Long-term aplicación de 100 μmol. L-1 melatonin to growth soil improved the photoQuímica química efficiency de photosystem II in apples under weak light conditions, allevíated drought El estrés o el estrés-induced inhibiinhibiinhibiinhibiinhibiinhibiinhibiinhibiinhibiinhibiinhibiinhibiinhibiinhibi de photosynthesis,while maintaining higher CO₂ assimilation capacidad and stomatal conductance in plant leaves [71]. Pretreatment with 0.1 mmol/L melatonin increased net photosynthetic rate, transpiration rate, stomatal conductance, photosystem II quantum efficiency, electron transport rate, and maximum photochemical efficiency (Fv/Fm) in Tomate tomate plants [72].

3.2 Role de plant melatonin in responses to biotic El estrés o el estrés

Melatonin significantly enhances plant tolerancia to biotic El estrés o el estrés. Given its Estado de la situación as an environmentally friendly molecule, it is considered the most economical and green alternative for inducing plants to resist biotic stress.Exogenous melatonin exhibits certain effects contra fungal-induced diseases. Treatment with melatonin at concentrations de 0.05–0.5 mmol/L can enhance resistance to brown spot disease por regulating the activity de antioxidant and defensa-related enzymes in apples [73]. Melatonin can also mitigate damage caused by fungal infections to crops such as potatoes, cotton, and white lupins [74-76]. Different concentrations de exogenous melatonin can inhibit the growth de fungal pathogens such as Botrytis, Fusarium, and Fusarium [76].

In terms de its mechanism de action, melatonin primarily helps plants resist fungal infections, reduce lesions, and inhibit pathogen spread, ultimately mitigating the damage caused by diseases. Arabidopsis thaliana-Pseudomonas syringae Tomate tomate pathogenic strain DC3000 (Pst DC3000) is the most widely used model in studies de plant-pathogen interactions [77].

Exogenous melatonin pretreatment at certain concentrations enhances the resistance de Arabidopsis and tobacco to Pst DC3000 [78,79]. Lee et al. [80] found that inactivation de serotoninaaaaaaaa-N-acetyltransferase significantly reduced endogenous melatonin levels in Arabidopsis,resulting in increased susceptibility de plants to Pst DC3000. Therefore, melatonin can enhance plant tolerancia to Bacteriana bacteriana diseases.

Compared to fungal and bacterial diseases, viral infections in plants are more difficult to control once they occur. Zhao et al. [81] first investigated the role de melatonin in plant-virus interactions, finding that exogenous melatonin aplicación significantly inhibited viral infection de tobacco seedlings. Additionally, exogenous melatonin pretreatment reduced the incidence de viral diseases in rice [82].With the deepening de research on melatonin-mediada plant disease resistance mechanisms, melatonin could provide new strategies for the prevention and control de plant viral diseases.

In addition to phenotypic identification, recent analyses de gene expression have provided strong evidence for melatonin's participación in regulating plant responses to abiotic stress.Exogenous melatonin treatment or overexpression de melatonin synthesis-related genes can induce the expression de disease-related (PR) genes such as PR1, PR5, NPR1, and PDF1.2, as well as activate mitogen-activated protein kinases (MAPKs) and other disease-resistant proteins,thereby enhancing plant disease resistance, indicating that melatonin is an efficient defensa agent contra pathogens in plants [78,83-85].

In addition to pathogens, insect pests are another major biotic stress that plants face durante growth. La planta-derived secondary metabolites act as antagonists de insect juvenile La hormona hormona to defend contra insect predation [86].It has been reported that dopamine, which has a similar structure to melatonin, plays an important role in plant defensa contra herbivores [87], suggesting that melatonin may also exert similar defensive effects [88].

3.3 Role de plant melatonin in responses to abiotic stress

Under normal growth conditions, the production and removal de ROS in plant cells are in dynamic equilibrium. When plants are exposed to adverse environmental factors such as drought, high salinity, extreme temperatures, heavy metals, and ultraviolet radiation, this equilibrium is disrupted, leading to oxidoxidoxidoxid stress damage in plant cells [89-91].To eliminate ROS within the body, plants have evolved an efficient enzymatic and non-enzymatic antioxidant defense system to protect cells from or mitigate damage caused by oxidoxidoxidoxid stress.

Melatonin is currently the strongest endogenous free radical scavenger with antioxidant activity. It is estimated that a single melatonin molecule can eliminate 10 free radicals through a cascade reaction,while classic antioxidants typically only eliminate one free radical per molecule [92]. Therefore, it is inferred that melatonin's primary function in organisms is as an antioxidant to eliminate various ROS and reactive nitrogen species (RNS), thereby protecting plants from oxidoxidoxidoxid stress [92-95].

Melatonin may control the burst de hydrogen peroxide durante non-biotic stress responses in plants by directly carrode excess ROS, enhancing antioxidant enzyme activity, and improving the ascorbic acid-glutathione (AsA-GSH) cycle capacity [67].Exogenous melatonin treatment enhances antioxidant enzyme activity in apples, grapes, corn, sunflowers, Tomate tomatees, and wheat, while reducing the concentrations de superoxide, hydrogen peroxide, and malondialdehyde [59,67,72,96-99].Additionally, exogenous melatonin treatment reduces the Acumulación acumulación acumulación acumulación acumulación acumulación acumulación acumulación acumulación acumulación acumulación acumulación acumulación acumulación de oxidized proteins in plants, accelerates the occurrence de autophagy induced by oxidative stress, and allevíates photooxidative damage [100].

4. Conclusion and Outlook

Since the discovery de melatonin in plants, its diverse physiological funciones and significant potencial aplicacións have attracted increasing attention in plant melatonin research. However, compared with other plant hormones, there are still many unresolved issues in plant melatonin research.For example, while the interactions between plant melatonin and other hormones have been studied directly or indirectly, the signal transduction models underlying these interactions remain unclear. Therefore, further investigation into the mechanisms by which melatonin coordinates with other plant hormones to regulate plant growth and development and responses to abiotic stress is a key direction for future research on plant melatonin.

By elucidating the interactions between melatonin and other hormones, we can also gain insights into the molecular mechanisms de melatonin signaling and its role as a plant growth regulator. Additionally, how plants perceive melatonin signals and how downstream signal transduction regulates plant responses to stress at physiological and metabolic levels remain unresolved questions.However, the discovery de Arabidopsis CAND2/PMTR1 as melatonin signal transduction Receptores Receptor receptores has laid a foundation for further research into melatonin signal transduction in plants [8].

Melatonin is widely distributed in various plant tissues; however, it remains unclear whether all plant organs possess melatonin synthesis capabilities. The mechanisms and pathways de melatonin transport within plants require further investigation.The heterologous overexpression de melatonin synthesis-related genes can significantly increase the endogenous melatonin levels in plants, while transgétransgénicos plants exhibit enhanced tolerancia to various abiotic stresses [40,49,101-110].In the future, genetic engineering could serve as an important means to increase the endogenous melatonin content in crops, thereby promoting plant growth and development, enhancing tolerancia to abiotic stress, and ultimately improving crop yields.

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