Introduction

Melatonin is an amine hormone that is produced by the pineal gland in mammals and humans. The chemical name is N-acetyl-5-methoxytryptamine, which was first discovered in the pineal gland of cattle in 1917 and extracted from it by Lerner in 1959 (Lerner et al. 1959, 1958; McCord and Allen 1917). The content of melatonin in the body is in the range of pg (1 × 10–12 g). In recent years, melatonin has been studied widely as a dietary supplement. The main physiological functions of melatonin include anti-aging, anti-tumor, regulating immunity, promoting sleep, regulating jet lag, and so on (Hardeland et al. 2012; Lin et al. 2020; Moroni et al. 2021). In 1995, the existence of melatonin in higher plants was discovered. Since then, the regulatory effects of melatonin in plants have been researched. Plants synthesize melatonin mainly through the following four steps: ① Tryptophan is converted to tryptamine by TDC or to 5-hydroxytryptophan by TPH; ② tryptamine forms N-acetyltryptamine under the action of SNAT or 5-hydroxytryptamine under the action of T5H. This step can also involve 5-hydroxytryptophan conversion to 5-hydroxytryptamine under the action of TDC; ③ N-acetyltryptamine is catalyzed by T5H or 5-hydroxytryptamine by SNAT to form N-acetyl-5-hydroxytryptamine. On the other hand, 5-hydroxytryptamine can be changed to 5-methoxytryptamine under the catalysis of ASMT or COMT; ④ N-acetyl-5-hydroxytryptamine is converted to melatonin by ASMT or COMT, whereas melatonin formation from 5-methoxytryptamine is catalyzed by SNAT (Nawaz et al. 2016). In plants, the research on the function and the regulatory mechanisms of melatonin have become a hot topic in recent years.

Tomato is a crop with high economic value, which is native to South America but is grown throughout the world. In China, it is planted widely in the north and the south. Under the appropriate conditions, the tomato has a high yield, produces delicious fruits rich in vitamins and sugars, and has high nutritional value. It is not only a delicious vegetable but also an after-dinner fruit, which is deeply loved by consumers. However, the resistance of tomatoes to stressful environments such as low temperature, drought and high salinity is weak, so they cannot grow in many places. As a new plant-active substance, melatonin plays an important role in the growth and development of tomatoes and the resistance to abiotic and biotic stresses (Arnao and Hernandez-Ruiz 2021b; Rehaman et al. 2021). However, the research on, and application of, melatonin in tomato production is still in an early stage. Therefore, this paper summarizes the recent research results on melatonin in tomatoes, providing a theoretical basis for future research.

Detection and anabolism of melatonin in tomato

Detection of melatonin in tomato

Melatonin is the main hormone produced by the pineal gland in vertebrate animals. Among its functions, melatonin synchronizes the circadian rhythm, initiates immune function, prolongs lifespan, and prevents tumor progression and cancer cell growth; it is a strong antioxidant and hydroxyl radical scavenger (Cipolla-Neto et al. 2014; Manchester et al. 2015). However, the research progress on melatonin in plants, especially in fruits and vegetables, has been slow. Initially, melatonin was detected in nine fruits and vegetables by radioimmunoassay and gas chromatography/mass spectrometry, ranging between 0 and 862 pg/mg protein (Dubbels et al. 1995). Eating plant materials with high melatonin content can change the level of melatonin in the blood, which is of great significance in preventing oxidative damage (Dubbels et al. 1995). The two main functions of melatonin in animals are antioxidant protection and dark signal (Van Tassel et al. 2001). Due to the changes in the oxidation state during fruit ripening, the measurement of melatonin levels in tomato fruits at different growth stages found that the fruit melatonin content usually increased from ripening to full ripening. However, due to the limitations of experimental methods, there is no conclusive evidence that melatonin in these plant organs increases significantly at night compared to the daylight, as it does in many animals (Van Tassel et al. 2001).

Initially, the determination of melatonin was done by the enzyme-linked immunosorbent assay following extraction by the acetone-methanol method. In tomato roots, stems, flowers and other tissues, the melatonin content ranges from 1.5 to 66.6 ng/g (FW), and the seeds had the highest content. The concentration in leaves and fruits varied with developmental stages, which strongly suggested that melatonin plays a role in these developmental processes (Okazaki and Ezura 2009). With the recent development of analytical methodology, LC–MS technology is used to clearly identify melatonin in a variety of tomatoes and strawberries. The melatonin concentration of the tomato samples ranged from 4.11 to 114.52 ng/g (FW), and the melatonin concentration of the strawberry samples ranged from 1.38 to 11.26 ng/g (FW). It was suggested that melatonin might be a new biologically active compound in tomatoes and strawberries (Sturtz et al. 2011).

The melatonin levels in tomato roots, stems, and leaves fluctuated with the light–dark cycle (Arnao and Hernandez-Ruiz 2013). When investigating the effects of solar radiation and cultivars on tomato and pepper melatonin content, it was found that the content of melatonin in red pepper fruits was 31.0 to 93.4 ng/g (DW), and the content in tomato fruits was 7.47 to 249.98 ng/g (DW). The melatonin content in pepper fruits was influenced more by cultivar than by the ripening stage. In the shading treatment, the melatonin content showed an upward trend in tomato (up to 135%) and a downward trend in pepper genotypes (64%). Moreover, the content of melatonin in the fruit was not related to the carbon flux in the leaves (Riga et al. 2014). In summary, the literatures contain a substantial amount of information on the melatonin content and detection methods in tomato and related fruits. However, whether the melatonin in fruits only originates from leaves or is biosynthesized in fruits and which factors affect melatonin transport in fruits remains to be studied further.

Synthesis and metabolism of melatonin in tomato

The scientists also explored the melatonin synthesis in tomatoes. In Micro-Tom, constitutively expressing the Chlamydomonas reinhardtii N-acetyltransferase (the possible rate-limiting enzyme of melatonin biosynthesis) gene CrAANAT resulted in a higher melatonin content in transgenic plants, which indicated that melatonin was synthesized through N-acetyl 5-hydroxytryptamine, but no significant phenotypic changes were observed (Okazaki et al. 2009). To clarify the role and metabolism of melatonin in plants, a more in-depth analysis of the development of fruits, leaves and seedlings of the transgenic Micro-Tom rich in melatonin is needed. In addition, the overexpression of indoleamine 2,3-dioxygenase (IDO), the enzyme metabolizing melatonin in rice, reduced the level of melatonin in tomato plants. It showed that the heterologous expression of the gene encoding melatonin-metabolizing enzyme can be used to regulate the content of endogenous melatonin, and the transgenic lines can be used in further studies (Okazaki et al. 2010). Subsequently, studies have found that the melatonin content in Micro-Tom plants was increased by heterologously expressing the sheep melatonin synthesis genes oAANAT and oHIOMT. Because tryptophan is the precursor of both melatonin and indole acetic acid, the transgenic plants showed reduced IAA content and increased branching. The oAANAT overexpression lines lost their apical dominance, indicating that melatonin may have auxin activity. The melatonin content was higher in the oHIOMT than in the oAANAT overexpression lines, indicating that N-acetylserotonin methyltransferase (ASMT) plays an important role in melatonin biosynthesis. Furthermore, the drought resistance of the oHIOMT overexpression lines was enhanced, suggesting that the increased melatonin content of transgenic plants was associated with enhanced drought tolerance. It shows that melatonin can be used as a potential auxin or auxin regulator, contributing to the improved drought tolerance of plants and increased branching (Wang et al. 2014).

Through the integration of structural characteristics and expression profiles during growth and development as well as biotic stress, the analysis of the acetylserotonin methyltransferase gene family revealed that at least 14 members of the tomato genome encode acetylserotonin methyltransferase (including three possible pseudogenes), and the family may be expanded by a tandem repeat. Gene expression analysis based on RNA-seq and qRT-PCR found that the tandem-repeated SlASMT genes showed different expressions, indicating that potential functional differences in these genes may have developed in the evolutionary process. Moreover, some SlASMT genes are induced by multiple pathogens and may be related to the tomato’s response to biotic stress (Liu et al. 2017). Similarly, a systematic analysis of tryptophan decarboxylase has shown that the SlTrpDC gene family in tomatoes contained five members, of which SlTrpDC1 and SlTrpDC2 had tissue expression specificity, which laid the foundation for the interpretation of the functions of SlASMT and TrpDCs family members in melatonin synthesis in tomato plants (Pang et al. 2018) (Fig. 1). Chloroplasts and mitochondria are the main organelles of melatonin biosynthesis in plants. Many isomers of melatonin biosynthetic enzymes exist in different plant species. The melatonin synthesis gene ASMT evolved into COMT. The COMT obtained a new function of catalyzing lignin biosynthesis. It participated in vascular bundle formation and organ (root) diversification, and further promoted plant colonization of land (Mannino et al. 2021).

Fig. 1
figure 1

The role of melatonin in tomato stress response, growth and development

Full size image

Although the metabolism of melatonin in animals has been widely studied (Tan et al. 2007b), it is a new field being explored in plants. The main obstacles to this research are the difficulty of extracting specific melatonin metabolites from plant tissues because these metabolites are unstable. The first melatonin metabolite found in plants is AFMK (in aquatic plant water hyacinth) (Tan et al. 2007a). In this species, AFMK shows a circadian rhythm similar to that of melatonin, apart from a short delay in the melatonin peak. This indicates that a portion of melatonin is converted to AFMK. So far, no homologue of animal IDO has been found in plants. It is speculated that AFMK is the product of the interaction between melatonin and O2 ·- and singlet oxygen because both are produced in large quantities in the process of photosynthesis.

The 2-, 4- and 6-hydroxymelatonin have been identified in plants. However, the contents of 4- and 6-hydroxymelatonin in plants are very low. Surprisingly, the average ratio of 2-hydroxymelatonin to melatonin in rice is about 368:1 (Byeon et al. 2015). In plants, 2-hydroxymelatonin has much stronger protective effects against abiotic stress than melatonin and can protect plants from combined or multiple abiotic stresses, such as cold and drought (Back and Lee 2019; Lee and Back 2016). It is formed by the enzyme M2H, which has not been found in animals. Due to the lack of this enzyme in aquatic plants, the ability to synthesize this enzyme may have evolved during the transition from aquatic plants to terrestrial plants (Back and Lee 2019). M2H has not been reported in animals. In contrast, 6-hydroxymelatonin is the main metabolite of melatonin in animals (Skene et al. 2001). Cyclic 3-hydroxymelatonin (c3OHM) has recently been found in plants where it is produced by melatonin 3-hydroxylase (M3H), which has been cloned from plants (Lee et al. 2016). Interestingly, if the expression of M2H gene is inhibited, the production of c3OHM increases significantly. This suggests a potential association between c3OHM and 2-hydroxymelatonin in melatonin metabolism. The metabolic pathway of melatonin in plants is shown in Fig. 2.

Fig. 2
figure 2

Synthesis and metabolism patterns of melatonin in plants

Full size image

The above studies showed that heterologous or homologous expression of genes encoding the melatonin synthesis enzymes can change the endogenous melatonin content of transgenic tomatoes. However, the research on related transgenic lines has been reported only at the phenotype and physiological and biochemical levels. In addition, the molecular mechanism of genes encoding enzymes in the melatonin anabolic pathway in tomato growth and development also needs to be further in-depth analyzed. No report on melatonin metabolism in tomatoes is found in the literature.

Melatonin can improve crop resistance to abiotic stress

Drought

Drought is severe and often fatal stress to plants. There are many published reports on the positive effects of melatonin in tomato drought resistance. For example, pre-treating tomato seeds and seedlings of "Jinpeng No. 1" with 0.1 mM melatonin resulted in increased stomatal conductance, photochemical efficiency, and seedling health index, thus significantly reducing the impact of drought stress, which provides a theoretical basis for the production of more drought-tolerant tomato (Liu et al. 2015a) (Table 1). However, we need more research to determine the mechanism of melatonin promoting drought tolerance.

Table 1 Studys on the role of melatonin in tomato resistance to different stress
Full size table

The cuticle on the plant leaves is waxy, limiting water loss. Under drought conditions, exogenous melatonin increased the expression of plant wax synthesis genes KCS1, LTP1 and CER3, thus significantly increasing the content of leaf epidermal wax. The increase in the cuticle led to a decrease in water permeability, thereby inhibiting non-stomatal water loss and delaying leaf senescence during the drought period (Ding et al. 2018) (Table 1). However, a mechanism underlying the regulatory role of melatonin in cuticle formation is still unclear. In addition, studies have found that exogenous melatonin can relieve the impact of drought stress on tomato plants by reducing reactive oxygen species. The melatonin concentration of 20 mg/kg was more useful than 40 mg/kg. At the same time, exogenous melatonin could improve the yield and quality of tomato fruit (Ibrahim et al. 2020). In summary, melatonin plays an important role in improving crop drought resistance as well as crop quantity and quality. However, most of these studies only focused on the short-term effects on seeds, seedlings and fruits, with the underlying mechanism of the melatonin regulatory role in cuticle formation remaining unclear.

When exogenous melatonin was sprayed on the leaves of tomato seedlings treated with polyethylene glycol (PEG) to simulate drought stress, the contents of total chlorophyll and coumaric acid increased together with the activities of antioxidative enzymes superoxide dismutase, glutathione reductase and ascorbic acid peroxidase, and consequently the content of MDA was diminished. It shows that exogenous melatonin can reduce the harmful effect of PEG by enhancing antioxidant-mediated defense (Karaca and Cekic 2019; Tiwari et al. 2021).

Heavy metal

On a global scale, farmland polluted by heavy metals threatens food safety. Nickel stress causes severe plant growth retardation, impairs photosynthesis and the root system, and results in an imbalance of mineral dynamics and excessive reactive oxygen species (ROS) production, which reduces the tolerance to nickel toxicity in plants (Jahan et al. 2020). Moreover, melatonin-treated seedlings increased the production of secondary metabolites that were mainly involved in heavy metal chelation and inhibited the formation of ROS, thereby alleviating the growth inhibition induced by nickel (Altaf et al. 2021a). The aforementioned studies suggest that melatonin enhances tolerance to nickel in plants by increasing photosynthesis efficiency (up-regulating chlorophyll biosynthesis genes), enhancing mineral homeostasis (reducing nickel uptake by plants), and reducing oxidative damage (inhibiting ROS and improving antioxidant defense systems).

However, the mechanism of melatonin-mediated enhancement of plant tolerance to nickel needs further research, and the alleviating effects of melatonin on other environmental pollutants such as zinc, lead and chromium in different plant species also need to be characterized further.

Exogenous melatonin and cadmium stress induced the accumulation of endogenous melatonin that had positive effects on plant growth, active oxygen scavenging, antioxidant capacity, sulfhydryl compound biosynthesis and the sequestration of cadmium in vacuoles (Hasan et al. 2015). Although exogenous melatonin had no obvious effect on the cadmium content in roots, it significantly inhibited the upward transport of cadmium from roots and reduced the cadmium content in leaves, indicating that exogenous melatonin could regulate cadmium transport (Hasan et al. 2015). A comprehensive analysis of heat shock factor A1a (HsfA1a) using genetic and molecular methods found that cadmium stress induced the expression of HsfA1a (Table. 2). HsfA1a binds directly to the caffeic acid O-methyltransferase 1 (COMT1) gene promoter to promote its expression and increase the biosynthesis of melatonin. Hence, HsfA1a is a positive regulator of COMT1 transcription and melatonin accumulation. Melatonin alleviated cadmium stress by promoting the biosynthesis of glutathione and phytochelatin and thus sequestration of cadmium in vacuoles, thereby enhancing cadmium tolerance to tomato (Cai et al. 2017).

Table 2 Melatonin-related genes studied in tomato
Full size table

Sulfur metabolism is also crucial in cadmium detoxification. The use of melatonin and sulfur combined strengthened the tomato seedlings antioxidant and photosynthetic capacity. Melatonin also promotes the biosynthesis of downstream sulfur metabolites cysteine and glutathione, and ultimately improves plant cadmium tolerance by regulating chelation capacity and redox homeostasis (Hasan et al. 2019). Studies have also found that selenium promotes the synthesis of melatonin and sulfide under cadmium stress, thereby improving the cadmium tolerance of plants and alleviating growth suppression and reducing electrolyte leakage (Li et al. 2016).

Lanthanum toxicity can induce ROS production in tomato seedlings, aggravating lipid peroxidation and leading to growth inhibition and lower photosynthetic parameters. Alleviation of lanthanum toxicity was achieved more effectively with exogenous application of melatonin and sulfur combined than melatonin or sulfur alone. Melatonin and sulfur can increase the tolerance of plants to lanthanum by up-regulating the activity of precursor enzymes (δ-ALAD), inhibiting chlorophyll degradation, increasing the activity of photosynthesis-related enzymes to improve photosynthetic efficiency, and promoting the synthesis of antioxidant enzymes (APX, SOD, MDHAR, DHAR, and GR) (Siddiqui et al. 2019b).

Vanadium inhibits plant growth and biomass, which is mainly due to a damage to the photosynthetic system, root properties and nutrient balance. The application of melatonin can improve the tolerance to vanadium stress by promoting photosynthesis, biomass production, redox balance, nutrient absorption, and root traits and by reducing the absorption of vanadium by tomato seedlings (Altaf et al. 2021c; Hoque et al. 2021).

Melatonin is an important regulator of sulfur metabolism, which can not only improve the cadmium tolerance of tomato but also regulate the transfer of cadmium to the shoots. Selenium can promote the synthesis of melatonin and sulfide under cadmium stress, thereby improving the cadmium tolerance of tomato. It shows that both selenium and melatonin can reduce the absorption of cadmium by plants, and melatonin can also regulate the lanthanum tolerance of tomato. Therefore, controlling the level of melatonin or selenium to regulate the absorption and assimilation of sulfur is potentially an efficient strategy to improve the safety and eco-friendliness of food produced in heavy metal-contaminated soil (Li and Yan 2021). However, further molecular and genetic evidence is still needed to support the mechanism regulating a role of melatonin in sulfur metabolism and lanthanum tolerance, and the mechanism by which selenium induces melatonin biosynthesis and improves cadmium tolerance.

pH

Acid rain is a widespread environmental problem that seriously affects the normal growth of crops. Through conjoint analysis of transcriptome data and secondary metabolites of tomato plants treated with simulated acid rain and melatonin, it was found that ERF, WRKY, MYB, and bZIP-related transcription factors are involved in antioxidant activation, secondary metabolite regulation and gene expression regulation under acid rain stress (Debnath et al. 2020). In tomatoes, melatonin significantly increased the photosynthesis and the antioxidant capacity to scavenge ROS, thereby improving the tolerance to acid rain stress (Debnath et al. 2018a). In addition, melatonin regulates the quality and yield of tomato fruits under acid rain stress (Debnath et al. 2018b). Therefore, exogenous melatonin can be developed as a beneficial resource to enhance resistance to acid rain stress, and it can improve tomato fruit quality and yield in regions where this abiotic stress is limiting crop production. However, the molecular mechanism needs to be studied further.

Salinization is increasingly influential in crop growth and productivity in arid areas. In tomatoes, salt-alkaline stress can significantly reduce Fv/Fm, chlorophyll content, antioxidative enzyme activity, and biomass accumulation, accompanied by exacerbated lipid peroxidation, membrane damage and ROS. It was found that 0.5 μM exogenous melatonin could protect chlorophyll from degradation and maintain high Fv/Fm by inducing activity of antioxidantive enzymes and accumulation of ascorbic acid (AsA) and glutathione (GSH). Under saline-alkali stress, 0.5 μM exogenous melatonin had the most significant effect on promoting tomato plant growth and reducing oxidative stress, indicating a concentration-dependent effect of melatonin on improving the resistance of tomato to salt-alkali stress. In addition, under salt-alkali stress, in tomato leaves melatonin decreased Na+ and increased K+ concentrations, maintained ion homeostasis, thus significantly enhancing the tolerance of tomatoes to salt-alkali stress (Liu et al. 2015c).

The concentration of NO in tomato roots under salt-alkali stress can be induced by exogenous melatonin, increasing the salt-alkali tolerance in tomatoes. Chemical removal of NO can reduce the salt-alkali stress tolerance induced by melatonin, indicating that NO (as a downstream signal) is involved in the salt-alkali tolerance of tomato induced by melatonin (Liu et al. 2015b). The above-mentioned tolerance is based on reducing the accumulation of Na+, activating the expression of defense response genes, increasing the absorption of K+, and reducing the negative effects of saline-alkali stress on growth and photosynthesis. The crosstalk between melatonin and NO creates a new signal pathway that improves plant salt-alkali tolerance. However, detailed mechanisms and interactions need to be characterized further using advanced molecular techniques and mutation analysis. By establishing the transcription network of melatonin-induced hormones, transcription factors and functional genes, as well as using VIGS methods, it has been proven that IAA3 and DREB1 are the master downstream transcription factors for melatonin-mediated salt-alkali resistance. The transcription of IAA3 and DREB1 can be induced by saline-alkali or melatonin; DREB1 can directly bind to the IAA3 promoter to up-regulate IAA3 expression, revealing the direct connection between melatonin, DREB1 and IAA3 under salt-alkali stress. However, the relationship between the melatonin-DREB1-IAA3 signaling pathway and other unknown pathways regulating growth and resistance, as well as the energy distribution and mechanism still need to be studied further (Yan et al. 2019a).

Temperature

Rising global temperatures pose a serious threat to food security, making it necessary to further explore the important biomolecules that confer heat resistance to plant species. When plants are subjected to heat stress, a series of metabolic changes occur, including photoinhibition, excessive ROS generation, destruction of the structure and function of biofilms, denaturation of some proteins (such as chloroplast proteins), inhibition of protein biosynthesis, and protein oxidation and misfolding. Therefore, enhancing the heat tolerance of plants should rely on protecting proteins from oxidation and misfolding. In tomato plants, the endogenous melatonin concentration gradually increases with an increase in temperature and reaches its peak at 40 °C. Proteomics analysis found that under heat stress, wild-type plants accumulated the aggregated proteins, whereas exogenous melatonin or the endogenous N-acetyl serotonin methyltransferase gene ASMT overexpression reduced ubiquitination and accumulation of aggregated proteins induced by heat, indicating that melatonin can protect cells by inducing heat shock proteins and autophagy, and refolding or degrading denatured proteins, thereby enhancing the heat resistance of plants (Xu et al. 2016). In addition, heat stress can produce photoinhibition.

The melatonin content in tomato plants engineered to have melatonin biosynthesis gene COMT1 silenced was decreased together with photosynthetic pigment content and CO2 assimilation. By contrast, exogenous melatonin can compensate for the decreased photosynthetic capacity caused by COMT1 silencing and increase the PSII activity of tomato plants. This genetic evidence indicates that under heat stress, melatonin addition can maintain the photosynthesis of tomato plants, and is therefore expected to have a beneficial effect on the management of horticultural crops under a warming climate (Ahammed et al. 2018).

Melatonin irrigation can reduce the accumulation of ROS in anthers, enhance the expression of heat shock proteins and induce antioxidant enzymes to protect organelles, which reduces high temperature-induced pollen inactivation and pollen germination inhibition, indicating that melatonin has a potential regulatory role in plant reproduction (Qi et al. 2018). These findings provide new insights into the mechanism by which melatonin regulates plant reproduction and stress tolerance, but the molecular mechanisms must be studied further.

The polyamines (PAs) can eliminate the impact of environmental stress on plants. However, how polyamines and melatonin interact under heat stress remains unclear. Pretreatment (7 days) of tomato seedlings with 100 μM melatonin induced the ascorbate–glutathione cycle, reprogramming the polyamine metabolism and NO biosynthesis. This helps remove excess ROS and increase the stability of cell membranes, ultimately reducing heat-induced oxidative stress and improving the tolerance of tomato plants (Jahan et al. 2019). These findings also revealed the existence of crosstalk between melatonin, polyamines and NO, providing new insights into tomato heat tolerance. Further research is needed to determine how these three compounds work together to reduce the damage caused by heat stress (Jahan et al. 2019).

The SNAT enzyme is involved in the biosynthesis of melatonin; increases or decreases in the endogenous melatonin content of tomato occur by overexpressing or silencing, respectively, the tomato SlSNAT gene. Overexpression of SlSNAT in tomatoes significantly increased the heat tolerance, improved the photosystem II (PSII) maximum photochemical quantum yield (Fv/Fm) and decreased the heat damage compared with wild-type. HSP40 is related to the SNAT enzyme. HSP40 interacts with SlSNAT and participates in the melatonin-related heat resistance regulation in tomatoes (Wang et al. 2020b).

Abscisic acid (ABA) and heat stress can induce leaf senescence, whereas melatonin and gibberellin (GA) can inhibit leaf senescence. Plant tolerance to different stresses is related to leaf longevity. However, the molecular mechanism underpinning the signal interaction among ABA, melatonin and GA regarding heat-induced leaf senescence has not yet been determined. The inhibition of leaf senescence mediated by melatonin is related to the inhibition of ABA biosynthesis as well as activation of melatonin and GA biosynthesis in tomatoes (Jahan et al. 2021b). The positive contribution of melatonin and GA to enhancing heat tolerance can be demonstrated by improved physiological properties and the inhibition of ROS overproduction. Further research through genetic modification or VIGS method is needed to gain an insight into the interactions and mechanisms between melatonin and GA (Jahan et al. 2021b).

Low temperature can damage tomato plants and cause huge yield losses. Seeking an effective method to alleviate chilling injury is very important for tomato production. Under cold stress, compared with untreated plants, melatonin can improve cold tolerance in tomatoes mainly through: (1) Decrease oxidative stress and increase SBPase activity to enhance photosynthetic carbon fixation, (2) Improve antioxidant capacity, (3) Induce the expression of cold-responsive genes, and (4) Increase accumulation of sucrose, proline and polyamines (Ding et al. 2017a). Non-photochemical quenching (NPQ) is a key process for heat dissipation of excess light energy, and plants use it as a protective mechanism against the impairment of Photosystem II. Exogenous melatonin can reduce the photoinhibition of tomato seedlings under moderate light during cold storage. Melatonin-mediated increase in the violaxanthin deep oxidase (VDE) transcription and the concentration of ascorbic acid contribute to higher VDE activity in tomato seedlings during cold storage, which increases the de-epoxidation state of the lutein cycle and induces NPQ. The enhancement of NPQ alleviated the photoinhibition of melatonin-pretreated tomato seedlings under cold stress (Ding et al. 2017b). Furthermore, melatonin treatment of roots or leaves can not only decrease the damage of cold stress to tomato seedlings, but also markedly enhance the photosynthetic performance of leaves, and promote the recovery of photosynthesis after cold stress, with the effect of spraying melatonin on leaves being better than applying it to roots. In summary, melatonin is essential for antioxidant capacity and redox balance, and improves tomato photosynthesis under cold stress. Melatonin is expected to be an effective measure to increase plant resistance to cold and reduce photoinhibition. However, the mechanism by which melatonin regulates and initiates the photosynthetic electron transport chain requires extensive research (Wang et al. 2020a; Yang et al. 2018).

Tomato seedlings were treated with melatonin under high-temperature stress (42 °C for 24 h). It was found that melatonin could enhance glucose metabolism, improve photosynthetic efficiency, up-regulate melatonin biosynthesis, and weaken the photoinhibition of tomatoes induced by heat stress (Jahan et al. 2021a).

Salinity

The soils affected by salinity can cause significant yield reductions, causing considerable economic losses. Salt stress can result in a decrease in photosynthetic function, plant chlorophyll content and PSII activity, and an increase in ROS accumulation and photoinhibition. The reasons for these changes are the hindered electron transfer and decreased activity of the oxygen evolution complex (OEC) in the PSII (Yin et al. 2019a). Melatonin has beneficial effects on root length, FW and DW, branch length, FW and DW, leaf area, and physiological and biochemical parameters [Proline (Pro) and total soluble carbohydrate (TSC) content, chlorophyll a and b, and increase in the activity of carbonic anhydrase (CA) and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco)] (Siddiqui et al. 2019a). Melatonin can promote PSII repair and electron transfer, and balance photosynthetic electron distribution to reduce ROS generation, induce the expression of the thiol compound TRXf, regulate the abundance of TRXf and TRXm gene products, and promote photosynthetic repair. It can also diminish ROS by inducing the activity of enzymes in the AsA-GSH cycle, thereby increasing plant salinity tolerance (Yin et al. 2019b; Zhou et al. 2016).

The melatonin biosynthetic enzyme caffeic acid O-methyltransferase 1 (SlCOMT1) is located in the cytoplasm and is highly expressed in fruits. The transcription of SlCOMT1 was positively correlated with the melatonin content, and overexpression of SlCOMT1 improved the production of melatonin in tomato plants. The transgenic lines showed higher tolerance to salt stress. Under salt treatment, overexpression of SlCOMT1 can activate the SOS pathway to maintain K+/Na+ balance and reduce ion damage, thereby protecting the photosynthetic system and enhancing antioxidant capacity. It shows that SlCOMT1 and endogenous melatonin can improve the tolerance of plants to salinity stress, which provides clues for coping with the increasingly serious salinization problem in global agricultural production (Liu et al. 2019c; Sun et al. 2020b). In addition, the application of exogenous melatonin and GA3 alone or in combination can increase the tolerance of plants to salinity, and the combined application is more effective than each application alone. It shows that melatonin and GA3 are important regulators activating antioxidant systems, methylglyoxal (MG) detoxification and the production of GSH and ASC, thereby reducing salt-induced oxidative stress by maintaining redox homeostasis and promoting osmotic regulation (Siddiqui et al. 2020). The application of 100 μM melatonin can compensate for the growth inhibition of tomato seedlings caused by salt stress and promote root growth, thus opening up new opportunities for its application in boosting crop salt tolerance (Altaf et al. 2021b, 2020). Under hydroponic experimental conditions, the application of exogenous melatonin to the seedlings of two tomato varieties (Rome and FM 9) relieved short-term salinity stress by improving antioxidant capacity and ion homeostasis, and the Rome variety was more salt-tolerant than FM9 (Ali et al. 2020). In addition, ABA, melatonin and ABA + melatonin can regulate osmotic pressure, promote photosynthesis and increase antioxidant capacity to enhance tomato salt tolerance. Notably, melatonin can improve the stomatal conductance by enhancing the AsA-GSH cycle. The ABA treatment can close stomata and increase proline to minimize plant water loss. The melatonin + ABA treatment alleviates salinity stress by regulating hormone metabolism and the expression of the salinity tolerance gene (Hu et al. 2021).

Hydrogen sulfide (H2S) is a potential regulator of plant metabolism under abiotic stress. During salt stress, the melatonin signal in the roots of tomato seedlings works through an endogenous H2S-dependent pathway, in which the secondary active transport stimulated by H+-ATPase regulates K+–Na+ homeostasis. The increase in L-DES activity and the accumulation of endogenous H2S during the melatonin treatment can induce enzymatic antioxidant defense, reducing ROS, improving RWC and increasing K+, indicating that melatonin and H2S can synergistically regulate salt stress tolerance (Siddiqui et al. 2021). In summary, melatonin has a significant role in nutrient absorption and transport, protection of photosynthesis and antioxidative processes, and can interact with H2S to promote plant salt tolerance. It provides a basis for future research on melatonin and H2S, which are two important agronomic regulators and trigger molecules that coordinately regulate abiotic stress. However, further studies of the molecular mechanisms of melatonin action are still needed.

Calcium nitrate stress can lead to significant changes in the light energy distribution of tomato seedlings, resulting in photoinhibition and slow growth. Exogenous melatonin can reduce the inhibition of the photosynthetic electron transport chain, protect photosystem II from light damage caused by excessive light energy, and effectively alleviate the growth of tomato seedlings under calcium nitrate stress (Xie et al. 2020).

NaCl stress (120 mm) inhibited the elongation of tomato hypocotyl, promoted the growth of primary roots and increased the electrolyte leakage from tomato seedlings. The H2S donor (100 μM; NaHS) treatment can reverse these reactions, and melatonin treatment can make the reversal effect more significant. Melatonin treatment reduced the accumulation of H2S in cotyledons of young seedlings under NaCl stress, accompanied by an increase in L-des activity. Therefore, melatonin-mediated regulation of H2S homeostasis in cotyledons of young tomato seedlings under NaCl stress is also carried out through a pathway independent of L-des activity. In addition, melatonin regulates the environmental concentration of H2S in seedling cotyledons (treated with NaCl), indicating the importance of H2S catabolic pathway and providing evidence for the role of melatonin in regulating H2S homogenization. Deciphering the molecular mechanism of melatonin and H2S crosstalk in plants may further strengthen the importance of these two biomolecules in agriculture (Mukherjee and Bhatla 2021). Exogenous melatonin regulates endogenous H2S homeostasis and L-cysteine desulfurase activity in cotyledons of tomato seedlings under salt stress.

By measuring plant growth, H2O2 content, electrolyte leakage, antioxidative system, gas exchange, pigment content, and chloroplast ultrastructure of salt-sensitive genotype and salt-resistant genotype under CK (control), spraying melatonin, salt and spraying melatonin under salt stress, it was found that spraying melatonin reduced the salt tolerance gap between SG and RG through photosynthesis regulation rather than the antioxidative system. This indicates that the positive effect of melatonin on tomato plants under salt stress depends on genotype sensitivity (Zhou et al. 2022).

Combined stresses

Salt and heat, cold and drought

The response of plants to combined stress is unique and cannot be inferred from the response to each individual stress. A decrease in ROS accumulation can effectively enhance plant tolerance to abiotic stress. Under the combined stresses of salt and heat, melatonin helps prevent damage to proteins and cell membranes, mainly by avoiding oxidation and regulating the expression of antioxidative enzymes, especially GR, GPX, Ph-GPX, and APX. The photosynthesis parameters and the performance of the photosystem of the plants treated with exogenous melatonin are also improved (Martinez et al. 2018). Tomato plants were grown under the atmospheric concentration of CO2 and high concentration of CO2, and half of the plants were pretreated with melatonin, followed by a combination of drought stress and cold stress. The results indicated that melatonin increased the net photosynthetic rate of plants and enhanced tomato biomass accumulation at high CO2 concentrations during drought and cold stress recovery. Under cold stress and high concentration of CO2, melatonin treatment increased chlorophyll content in plants (Zhou et al. 2020). These findings provide new insights for improving plant resistance in future climate warming and carbon dioxide-rich environments. However, the biochemical mechanism of melatonin under abiotic stress combinations, as well as the molecular pathways that respond to abiotic stresses in the reproductive stage, need to be studied further to clarify the underlying mechanisms. The application of melatonin is expected to increase plant yield and productivity in the field environment.

Sulfur assimilation and H 2 O 2 signal

Low sulfur can lead to a decrease in biomass, photosynthesis and chlorophyll content, severely inhibiting growth. Supplementing melatonin to plants lacking sulfur can reduce the changes in cell ultrastructure and DNA damage caused by oxidative stress. Melatonin can regulate the expression of the sulfur transport and metabolism-related genes to promote sulfur absorption, indicating that melatonin may be an upstream molecule in the sulfur metabolism pathway, thereby improving the sulfur utilization efficiency of plants (Hasan et al. 2018). In addition, when studying the relationship of melatonin in the signal transduction pathway in plant stress response, a new mechanism was discovered. Melatonin can reduce the levels of NO and SNOs, and then activate NADPH oxidase (RBOH). The activity of RBOH is first activated by denitrosylation, and higher RBOH activity leads to the generation of H2O2, which initiates the protein kinase cascade and then activates some transcription factors and gene expression related to stress tolerance. Melatonin increases abiotic stress tolerance by regulating H2O2 signaling (Gong et al. 2017). It provides new insights into the potential mechanism of melatonin regulating sulfur homeostasis under sulfur-deficient conditions and lays a foundation for improving crop quality and yield in sulfur-depleted soils.

Overall, little is known about the interaction between genes and the environment. The QTLs identified in one background do not perform well in different other backgrounds. Similarly, transgenic plants developed to resist high temperature and drought stress still have uncertainties because they were not tested under field conditions. Using conditional promoters to drive gene expression in specific developmental stages, specific tissues / organs and / or in response to specific environmental cues can enable specific transgenic crops to grow under specific abiotic stresses with minimal yield loss (Fahad et al. 2017).

Carbendazim

The function of melatonin in pesticide metabolism is still unknown. The widely used fungicide carbendazim (MBC) was selected as a model. After the application of exogenous melatonin or overexpression of SlCOMT1, the increased melatonin level significantly enhanced the activity of antioxidative enzymes and activated the ascorbic acid-glutathione cycle that is involved in the detoxification of pesticides mediated by glutathione S-transferase. Grafting experiments showed that the rootstocks of transgenic plants increased the accumulation of melatonin in wild-type scions, suggesting that melatonin can be transported long distances in plants, stimulating the metabolism of the fungicide carbendazim (MBC) in the scions. Using rootstocks of the COMT1 overexpression lines not only provided melatonin-rich tomato fruits but also reduced pesticide residues. It provided the first evidence that melatonin induced pesticide metabolism, and a new method to reduce pesticide residues in crops using a plant self-detoxification mechanism. Of course, whether the increase in melatonin content of WT scion was induced by the transgenic rootstock or was transferred from the transgenic rootstock needs further study. We should pay more attention to pesticide metabolism in plants (Yan et al. 2019b).

Melatonin can improve crop resistance to biotic stress

Biotic stresses have caused tremendous damage to agricultural products around the world and have increased the risk of hunger in many countries. Biotic stresses include viruses, bacteria, parasitic nematodes, fungi, weeds, insects, and other native or cultivated plants. Many factors affect the severity of these damages, such as environmental changes. Plants themselves tolerate biotic stresses through a variety of pathways and plant resistance proteins. In addition, humans use non-eco-friendly chemical pesticides to minimize or eliminate biotic stresses. In plant-fungus interaction model, melatonin can help tomatoes resist Botrytis cinerea, which can increase Phytophthora infestans tolerance (Liu et al. 2019a). Furthermore, melatonin can eliminate the apple stem groove virus from apple buds and reduce the concentration of tobacco mosaic virus in tomato seedlings. Melatonin has an antibacterial effect (Moustafa-Farag et al. 2019). Compared to pesticides, melatonin is an environmentally friendly alternative strategy to protect plants from pathogens. There is insufficient evidence on the effects of melatonin on nematodes, viruses and insects that damage plants, so further research in this area is needed.

Melatonin was found to inhibit the growth of food-borne Bacillus species on a cherry tomato, including Bacillus cereus, Bacillus licheniformis and Bacillus subtilis. Interestingly, this result was attributed to two complementary effects. On the one hand, the high concentration of melatonin (≥ 1000 μM) had strong antibacterial activity against Bacillus subtilis by inhibiting cell division and oxidative phosphorylation. In addition, melatonin reduced the swimming capacity and biofilm formation of Bacillus subtilis. On the other hand, low concentrations (50 and 100 μM) of melatonin enhanced the antioxidant capacity of cherry tomato and induced the defense response based on ethylene. The effect of melatonin on oxidative stress resistance was observed in the absence and presence of Bacillus subtilis, but the defense response was detected only in the presence of Bacillus subtilis. The application of melatonin was found to delay the senescence of cherry tomatoes. There is a potential for applying melatonin in the control of food-borne pathogens in fruits after harvest (Zhu et al. 2021).

The role of melatonin in tomato growth

Fruit development and ripening

The process of fruit ripening is very complicated, and it is characterized by huge changes in the flavor, color, aroma, and texture of the fruit. When studying the effect of long-term application of melatonin on the fruit quality and yield of 'Zhefen 701' tomato, it was found that the exogenous melatonin irrigation increased tomato yield by 4%, and the fruit quality also improved, including the increase in SSC, organic acids, ascorbic acid, and lycopene. In addition, soaking tomato seeds in melatonin had little effect on fruit quality, mainly affecting the content of AsA and lycopene. However, the fruit yield of plants grown from seeds soaked in melatonin increased by 13%. Melatonin also played a pivotal role in regulating the mineral composition of plants (Liu et al. 2016). Therefore, apart from being an antioxidant hormone, melatonin also greatly improved the quality and yield of tomato fruits. Proteomic analysis of tomato fruits treated with 50 µM melatonin (M50) and control (CK) revealed that fatty acid metabolism, carbohydrate metabolism and amino acid and flavonoid biosynthesis and phosphorylation were regulated by melatonin. Melatonin can enhance the stress resistance and fruit quality of tomatoes by producing more anthocyanins. In addition, the reason why melatonin improves fruit ripeness and quality is an increase in the expression of ripening-related proteins. In-depth understanding of melatonin-promoted fruit ripening at the protein level requires further research (Sun et al. 2016).

In tomatoes, red color (carotenoids and lycopene) is a pivotal commercial trait. It was found that melatonin can promote lycopene accumulation and ethylene production. By studying the carotenoid change profile in control (CK) and melatonin-treated fruits (M50), it was found that the levels of α- and β-carotene and lycopene in M50 were significantly increased, and the transcription levels of SlRIN, SlCNR, and SlNOR were up-regulated. Exogenous melatonin cannot restore the inhibited accumulation of lycopene in Nr (ethylene-insensitive mutant, Never ripe) fruits, indicating melatonin-induced lycopene in tomatoes may rely on NR gene. The above results indicate that there is a crosstalk between ethylene and melatonin in the synthesis of tomato fruit lycopene, and applying melatonin is a potential strategy that can improve the commercial value of tomatoes (Sun et al. 2020a). The most important light is blue (B) and red (R) light for plant growth and development. As the fruit ripens, melatonin decreases, accompanied by an increase in ethylene biosynthesis, fruit softening, carbohydrate conversion, as well as respiration rate and lycopene accumulation. Compared with monochromatic blue light and red light, blue and red lights mixed significantly increase the accumulation of melatonin in tomato fruits and regulate the synthesis of ethylene. Melatonin mainly improves respiration rate and fruit softening through phytochrome signals, and promotes fruit ripening, increasing the accumulation of carbohydrates and lycopene, but delaying fruit senescence. Thus, RB-regulated endogenous melatonin and melatonin are involved in tomato fruit ripening under RB light. The above results broadened our understanding of the role of melatonin in tomato fruit ripening under different light qualities. In future work, melatonin combined with light qualities can be a useful strategy to promote fruit ripeness and lengthen shelf life (Li et al. 2021) (Fig. 3).

Fig. 3
figure 3

The role of melatonin in tomato fruit ripening and storage. On the one hand, the spraying of exogenous melatonin can transmit signals into cells through the receptor PMTR1, pass calcium ions to NADPH in the cells, inhibit the accumulation of reactive oxygen species and H2O2, reduce the oxidative damage of tomatoes, and improve stress resistance. On the other hand, exogenous melatonin can increase the expression of the melatonin synthase gene in tomatoes, increase the content of endogenous melatonin, promote ethylene synthesis and signal transduction process, then promote ripening; When resisting adverse factors, melatonin can crosstalk with JA and MeJA to resist biotic and abiotic stresses in the environment

Full size image

Leaf senescence

The last stage of leaf development is senescence, which can limit crop growth and yield, resulting in agricultural losses. Therefore, it is very important to find strategies to postpone senescence. Melatonin can reduce chlorophyll degradation by down-regulating the chlorophyll degradation gene, and improve antioxidant activity in leaf senescence induced by Methyl Jasmonate. It can control the level of ROS and reduce membrane damage, and finally reduce the tomato leaf senescence induced by Methyl Jasmonate. Therefore, melatonin is expected to be a biologically active factor that delays crop senescence in agricultural practice (Wang et al. 2019).

Root development

Melatonin can trigger the accumulation of NO through down-regulating S-nitrosoglutathione reductase GSNOR expression and up-regulating nitrate reductase expression. NO induces the accumulation of IAA, which in turn affects the formation of adventitious roots in tomato seedlings. The transport and distribution of auxin regulated by melatonin in turn regulate the formation of adventitious roots. Exogenous melatonin has a positive or negative effect on the formation of adventitious roots, depending on the applied melatonin concentration, in particular, 50 µM melatonin has the best positive effect because melatonin has the function of inducing IAA accumulation and regulating auxin transport (Wen et al. 2016).

More work is needed to more accurately understand the melatonin signaling pathway in the regulation of adventitious root formation. In tomato roots, melatonin can stimulate SlRboh3/4 and SlPAO1 expression to increase the production of apoplast O2‾ and H2O2, respectively. In turn, it regulates the expression of cell cycle-related genes and initiates lateral root primordia development and lateral roots formation. In addition, H2O2 can regulate cell cycle gene expression and stimulate lateral root development. However, the role of specific Ca2+ channels that may be involved in this signal still needs to be elucidated (Chen et al. 2019).

The role of melatonin in fruit storage

Compared with the control, the 50 µM melatonin treatment of mature green fruits increased lycopene by 5.8 times. The expression of the major genes regulating fruit color development, such as carotenoid isomerase (CRTISO) and phytoene synthase 1 (PSY1), increased twofold. The water loss rate of tomato fruits increased by 8.3%, and the expression of SlPIP21Q, SlPIP12Q, SlPIP22, and SlPIPQ was increased by 2 to 3 times. Furthermore, 50 μM melatonin reduced pro-pectin by 19.5%, increased water-soluble pectin by 22.5%, enhanced fruit softening, and increased the expression of β-galactosidase (TBG4), polygalacturonase (PG), Pectin Esterase 1 (PE1), and Expansion Protein 1 (Exp1). Melatonin can induce ethylene synthesis (increased 27.1%) and affects the ethylene signaling pathway. The expression of ACS4 affects ethylene synthesis. Melatonin (50 µM) can upregulate the expression of SlERF2, SlEIL3, SlEIL1, NR, and SlETR4. Melatonin can regulate ethylene and influence tomato fruit ripening and quality. Melatonin application has broad prospects in fruit ripening and post-harvest fruit management (Sun et al. 2015).

Treatment with 100 μM melatonin promoted the defense response of tomato fruits to cold stress. The expression of T5H, TDC, ASMT, and SNAT in tomato fruits was increased by exogenous melatonin, promoting the accumulation of endogenous melatonin and eliminating ROS during cold storage. The PMTR1 receptor on the plasma membrane senses melatonin signaling, causing the increase in cytoplasmic Ca2+ and signaling the accumulation of H2O2 through NADPH oxidase activity (Sharafi et al. 2019). In addition, in tomatoes, the high enzymatic activities of SDH, Ca-ATPase, CCO, and H-ATPase provide enough intracellular ATP. The higher level of linoleic acid in tomatoes is consistent with the lower accumulation of oleic acid, stearic acid, and palmitic acid. Higher FAD7 and FAD3 gene expression and lower LOX and PLD enzyme activities maintain membrane fluidity. These changes may be the reason why the application of melatonin reduces the cold damage of tomato fruits (Jannatizadeh et al. 2019). By studying the effect of applying 100 µM exogenous melatonin on tomato fruit cold tolerance during storage of tomato fruits at 4 °C for 28 days, it was found that the fruits of the treatment group maintained good membrane integrity, with low electrolyte leakage and MDA accumulation, and the expression of SlZAT12/2/6 induces the expression of CBF1. Exogenous melatonin induces the expression of ornithine aminotransferase, ornithine decarboxylase, and arginine decarboxylase, promotes endogenous polyamines and proline, and causes a higher accumulation of NO, thereby maintaining membrane stability. Therefore, melatonin activates the CBF1 signaling pathway to trigger the activity of the arginine pathway, likely due to up-regulation of SlZAT12/2/6 (Aghdam et al. 2019). Consequently, melatonin can be used as an important substance to alleviate the cold stress of tomato fruits in cold storage.

The tomato MG fruit was immersed in 0.1 mM melatonin for one hour and then stored at 22 ± 1 °C, which improved the disease resistance of cherry tomato fruits to Botrytis caused by Botrytis cinerea during storage. Increased endogenous melatonin content activates the SA signaling pathway and the phenylpropane pathway, thereby increasing the content of lignin, phenolic compounds and flavonoids in cherry tomato fruits, which enhanced tomato disease resistance (Li et al. 2019). In addition, treatment of tomato fruits with 50 μM melatonin significantly increased the activity of defense-related enzymes PAL, GLU, PPO, and CHI, and reduced the H2O2 content, which can induce the activity of antioxidative enzymes. Melatonin also increases the JA defense signal and MeJA content, a key pathway of pathogen resistance (Liu et al. 2019b). The above results indicate that melatonin plays an active role in the resistance of tomato fruits to gray mold by regulating ROS and H2O2 levels, and the JA and SA signaling pathways. Therefore, melatonin can be used as a natural and economical inducer to control fruit rot after harvest.

Summary and prospect

In recent years, melatonin has received extensive attention due to its multiple functions in plants. The review of studies on melatonin in tomatoes found that melatonin can improve stress resistance in plants and fruits, for example, by restoring tomato photosynthesis and enhancing antioxidant capacity against abiotic stresses such as drought, pH, high temperature, cold, and salt. In addition, melatonin can enhance the chelation of heavy metals by inducing the synthesis of metallothionein and chelating proteins in plants, thereby reducing the concentration of heavy metals in plant roots, maintaining the balance of osmotic regulation and mineral nutrition, protecting the integrity of cell membrane structure and chloroplast structure, enhancing antioxidant defense system and improving photosynthesis, and finally promote plant growth. Besides, by affecting the synthesis, transport and metabolism of hormones and signaling molecules in plants, a complex regulatory network is formed to improve the resistance of plants to heavy metal stress. Melatonin can also regulate fruit ripening by regulating the ethylene signaling pathway and the expression of genes related to fruit cell wall softening, and increase the content of lignin, phenolic compounds and flavonoids in tomato fruit by activating SA, JA, MeJA and phenylpropane pathways, thereby enhancing the resistance of tomato to Botrytis cinerea, tobacco mosaic virus and foodborne Bacillus and prolonging the shelf life of tomato fruit after harvest. Additionally, melatonin can regulate root development by regulating auxin concentration, slow leaf senescence by reducing the expression of chlorophyll degradation genes, promote fruit ripening by inducing the expression of fruit ripening-related proteins, and improve tomato quality by regulating the synthesis of organic acids, ascorbic acid and lycopene.

However, from reviewing previous studies, we do not know much about the molecular mechanism of melatonin acting in tomatoes. For example, the response mechanism of melatonin to different stresses, the regulation of melatonin metabolism, the interaction of melatonin with other hormones in plants under stress and other external environments, and the signaling network of melatonin regulating the absorption, transport and storage of heavy metals still need to be further studied.

For actual production, the application of melatonin is mainly through exogenous spraying during crop growth, improving crop stress resistance and related hormone interactions to increase yield, improve fruit quality and storage tolerance. Effective doses, optimal formulations and timing of melatonin used as a foliar fertilizer for individual plant species and cultivation conditions, the half-life of melatonin in postharvest fruit, the metabolism of plant melatonin and its interaction with phytonutrients (carotenoids, chlorophyll, flavonoids, fibers, etc.) need to be further investigated.

In addition, for the environment, the possible interactions between melatonin and the microbiota and invertebrates are also significant. Although melatonin is classified as harmless, specific studies on melatonin persistence in plant tissues, plant by-products, soil and fresh water are recommended for obtaining key information.

Detailed studies are needed to determine the highest safe levels of plant and animal food for human use before the relevant authorities approve melatonin for use in improving agricultural production and post-harvest preservation. Also, the role of melatonin in reducing damage from climate change to ensure the future use of food crops still needs to be elucidated (Arnao and Hernandez-Ruiz 2021a). In Arabidopsis thaliana, the specific receptor of melatonin (AtPMTR1) was found, and the role of melatonin in regulating plant circadian rhythm was characterized (Li et al. 2020; Wei et al. 2018). The corresponding melatonin receptor ZmPMTR1 was found in maize (Wang et al. 2021). However, for tomatoes and the like plants, their melatonin-specific receptors have not been found yet. Further work on the signal mechanism of melatonin in tomatoes is very necessary for understanding how melatonin causes its effects.

In conclusion, after conducting rigorous safety evaluation studies, melatonin is expected to become a natural, effective and economical crop growth regulator for modern agriculture.