Unlocking the sustainable role of melatonin in fruit production and stress tolerance: a review

Abstract

While melatonin, a vital player in plant physiology, initially attracted recognition due to its involvement in animal circadian rhythms, the molecule appears to be a multifunctional molecule requiring substantial attention for prospective applications in sustainable horticulture. It has been identified and recorded in numerous fruit crops, and its significance in physiological functions is critical for crop productivity. It is critical in safeguarding plants in response to reactive oxygen species in oxidative stress, one of the most damaging stressors to plant life in adverse conditions. Melatonin also cooperates with plants in boosting stress resistance, which concerns abiotic stress factors, e.g. low and high temperature, drought stress, toxicity of heavy metals, and biotic stress factors, including pests and pathogens. The anti-senescence properties of melatonin in aging leaves may be explained by its widespread antioxidant activity and its function in maintaining chlorophyll. The function of melatonin in controlling the production of genes linked to ethylene to modify postharvest fruit ripening has been the subject of an astounding amount of research. Additionally, recent research has shown that melatonin works with other phytohormones and well-known chemicals like nitric oxide and reactive oxygen species to assist plants in responding to biotic stress.The present review emphasizes a perspective that examining the role of melatonin in fruit crop physiology and stress responses may be a promising research direction in prospective fruit crop yield. In particular, this perspective is well supported by the following: melatonin is involved in the antioxidant response of fruit crops and can thus be used to mitigate the stressful impact of various environmental conditions; melatonin influences the development of plants and, consequently, affects fruit yield and quality; and applying melatonin is feasible for mitigating the impact of abiotic factors, such as cold, drought, heavy metals, and biotic factors, pests, and pathogens.

Introduction

According to Food and Agriculture Organization, to meet estimated food demands, there might be a one-third rise in the global population between now and 2050, so agricultural productivity will need to rise by 60% (Tyczewska et al. 2018). Production of fruit crops is crucial for several reasons, including cultural aspects, economic growth, environmental sustainability, and human nutrition. Fruits are an excellent means of fiber, antioxidants, vitamins, and minerals vital to human health (Omotayo and Aremu 2020). A variety of fruit intake can help people away from diseases and enhance general health. The production of fruit crops adds to dietary diversity by providing a diverse array of tastes, textures, and nutritional profiles. This diversity lowers the possibility of nutrient deficiencies by ensuring a diet that is both balanced and rich in nutrients. A substantial part of the agricultural sector, fruit production boosts both the local and national economies. Fruit sales and exports boost rural livelihoods, provide revenue for farmers, and increase the Gross domestic product (GDP) of the entire nation (Golovin et al. 2020). Particularly in rural areas, the production, harvesting, processing, and marketing of fruits creates job opportunities for a diverse workforce. Numerous nations participate in the international fruit trade. Fruit imports and exports support global trade by giving localities access to a wide range of fruits and enabling them to take part in the world economy (Cramer and Chisoro-Dube 2021). In many societies, fruits are symbolic and cultural objects. Some fruit trees also help to improve soil fertility and reduce soil erosion. Fruit orchards create tourists and support rural tourism, particularly those with beautiful surroundings. Certain fruit crops are resistant to environmental changes because they are suited to a particular climate (Gyawali et al. 2022. Research and innovation in fields like breeding, disease and pest management, and cultivation techniques are driven by the production of fruit crops. Technological developments support higher yields, better fruit quality, and sustainability. Sustainable methods of growing fruit, like agroforestry and organic farming, protect the environment, use fewer toxic chemicals, and improve soil health. Fruits support food security because they are a vital component of a balanced diet (Gamage et al. 2023).

Melatonin has a significantly smaller history in plants than it does in animals. Nevertheless, melatonin study in plants has advanced significantly in recent years. Melatonin can reduce oxidative stress and is specifically involved in many physiological processes in plants throughout the vegetative and reproductive phases. Melatonin’s role in biological cycles and photoperiodic traits is also supported by evidence. Melatonin has the potential to regulate gene expression, growth, and development, protect plants from external biological stress, and facilitate interactions between plants and biotic factors (Agathokleous et al. 2019). It has been observed that melatonin, a hormone best known for controlling animal sleep–wake cycles, also has captivating functions in plants. In plants, melatonin functions as an efficient antioxidant that aids in the removal of reactive oxygen species (ROS). This is essential for shielding plant cells from environmental stressors like pollution, Ultravoilet (UV) rays, and other environmental factors that cause oxidative stress. Melatonin is a hormone that plants release in reaction to a variety of stresses, such as infections, severe weather, and drought (Ramasamy et al. 2023). Stressed plants have been found to have higher melatonin levels, which may play a part in their ability to adapt to harsh conditions. Plant defense mechanisms seem to involve melatonin. It can improve the expression of genes linked to pathogen defense, strengthening the plant’s resistance to infection. Melatonin is believed to regulate plant growth and development and could influence seed germination, root growth, and flower development. Studies have shown that exogenous melatonin treatment affects key developmental processes (Ivanov et al. 2021). Melatonin may have an impact on how plants interact with other living things. It may be involved in the interactions between pollinators and plants as well as the attracting the beneficial insects. Furthermore, melatonin may impact both mutualistic and pathogenic relationships in plant–microbe interactions. It has been suggested that melatonin increases the vigor of seedlings (Cao et al. 2024).

Melatonin has a well-established role in plants as a plant growth regulator (PGR), impacting processes such as fruit set, seed germination, root proliferation, flowering, and ripening of fruits as shown in Table 1. Melatonin can also keep postharvest fruit fresher longer and increase its shelf life (Dai et al. 2020). Apart from its impact on these routine physiological functions, research has demonstrated that melatonin possesses antioxidant characteristics that are essential for eliminating reactive oxygen species (ROS) and reactive nitrogen species (RNS). It can offer resilience against many biotic and abiotic stressors, such as microbial infections (bacteria, viruses, and fungi), cold temperatures, salt, hot temperatures, and drought. Melatonin has been used to lessen the buildup of toxic metals and traces of pesticides in food (Tijero et al. 2019). Melatonin regulates growth and development and affects physiological processes essential for fruit output and quality. Studies imply its role in seed germination, flowering, fruit ripening, and post-harvest preservation. Melatonin may boost crop productivity and handle environmental changes by altering these mechanisms.

Table 1 Effect of melatonin on different fruit crops

Plant resistance to abiotic and biological stresses has been enhanced by melatonin. Plants always produce melatonin in proportion to the degree of stress. In the same way, plants exhibit noticeably higher melatonin levels in response to a variety of stressors, such as salinity, drought, excessive watering, cold, high temperature, Ultraviolet radiation, and pollution (Zhang et al. 2022a, b). The transcription factors encoded by C-repeat binding factors (CBFs) and dehydration-responsive element-binding proteins (DREB) are crucial for many environmental stimuli, such as salt, drought, fungal infection, freezing, and cold stress. The expression of certain cold-responsive genes can be regulated by binding these CBF proteins to their promoter regions. In a similar vein, overexpressing AtCBF3, AtCBF2, and AtCBF1 increased resistance to freezing, salt, and drought by triggering several downstream genes, including those that are dehydration-responsive and cold-inducible. The defensive functions of melatonin and ABA in reaction to a variety of environmental stressors in different crops have been revealed with notable advancements in the last several years (Tiwari et al. 2021a, b).

The specific impact of melatonin on fruit production varies based on the species of the plant, the environment, and the methods of culture. Melatonin performs many roles in plant physiology and against various stresses (Ayyaz et al. 2022). Melatonin's role in seedling growth and germination has been explored and found that early development and ideal germination are two important elements that can influence total fruit production (Bulgari et al. 2019). For fruit set and pollination to be successful, flowering must occur at the appropriate time. Melatonin may affect the expression of genes involved in the formation of flowers. A possible role of melatonin in the ripening stages of fruit has been investigated in certain studies. The ripening genes’ expression may be modulated by melatonin, which could impact the time and quality of fruit maturation (Arnao and Hernandez-Ruiz 2020). Antioxidant-rich melatonin has been shown to aid plants withstand environmental challenges. Melatonin may indirectly improve overall plant health and fruit output by lowering oxidative stress. Fruits’ post-harvest qualities may be preserved by the antioxidant and anti-aging effects of melatonin (Khan et al. 2020). Despite these insights, there remains much to unravel regarding the intricate mechanisms underlying melatonin’s actions in plants. Clarifying its signaling pathways, interactions with other phytohormones, and mode of action at the molecular level is critical for harnessing its full potential sustainably. In this review, we aim to consolidate current knowledge on melatonin's roles in fruit production and stress tolerance, highlighting its implications for sustainable agriculture. By elucidating its mechanisms and exploring practical applications, we can pave the way for innovative strategies to enhance crop resilience and yield in the face of escalating environmental challenges.

It has a significant effect on agriculture worldwide. It makes crops more resilient to environmental stressors such salinity, drought, and harsh temperatures, which can lead to much higher fruit yields and better quality fruits even in challenging circumstances. As a result, food supplies become more steady, promoting food security in the face of climate change. Additionally, melatonin usage promotes more environmentally friendly and sustainable farming methods by lowering the demand for chemical inputs like pesticides and fertilizers. These advantages help achieve the Sustainable Development Goals (SDGs) on a bigger scale, especially the ones that deal with ending hunger, responsible production and consumption, and climate action. In the end, melatonin integration into farming methods can improve food system sustainability, lessen environmental effects, and sustain farmers' livelihoods throughout the world.

Role sustainable development goal (SDGs)

The Sustainable Development Goals (SDGs) play a key role to advance science and its use in agriculture especially especially when it comes to strengthening the long-term benefits of melatonin for stress tolerance and fruit production. Plants naturally produce melatonin, which is well-known for its antioxidant qualities and capacity to improve crop stress tolerance Melatonin incorporation into agricultural operations is consistent with multiple SDGs, including Goals 2 (Zero Hunger), 12 (Responsible Consumption and Production), and 13 (Climate Action). By encouraging the incorporation of melatonin in fruit production, these SDGs promote sustainable agricultural techniques that boost crop productivity and resistance to environmental stressors such as drought and salinity. This strategy not only helps to ensure food security, but it also minimizes the need for artificial fertilizers and pesticides, promoting more sustainable consumption and production practices. Furthermore, improved crop resilience to climate-induced stresses is consistent with climate action programs, which help alleviate the effects of climate change on agriculture. Overall, the SDGs provide a framework for melatonin study and application in agriculture, promoting environmentally friendly techniques and global food systems.

Melatonin: an overview

Melatonin, also known as N-acetyl-5-methoxytryptamine, is a modified indoleamine that is generated from tryptophan and is extensively present in living, distantly related creatures (Juhnevica‐Radenkova et al. 2020). In 1958, the isolation of this substance from the pineal gland of a cow. Melatonin has been considered to be an animal neurohormone for many years and its effects on seasonal reproductive cycles, mammalian immune system modulation, and circadian rhythm regulation are now old (Shao et al. 2024). Melatonin was also found in a variety of non-vertebrates in the 1980s. In 1989, it was discovered in a phototrophic unicell called Gonyaulax polyedra (now known as Lingulodinium polyedrum), and it was then thoroughly examined. Later, in 1995, it was discovered that certain plant species contained melatonin (Salehi et al. 2019).

Action mechanism of melatonin

The adaptable molecule melatonin, which is well known for its functions in animals, is equally essential to plants, especially when it comes to growth control and stress response. Melatonin’s multipathway action mechanism in plants enables it to carry out a variety of tasks that promote plant development and resilience.

• Antioxidant activity: Melatonin functions as a strong antioxidant by scavenging reactive oxygen species (ROS) such hydrogen peroxide, hydroxyl radicals, and superoxide anions directly. This lessens oxidative stress, which is frequently brought on by a variety of abiotic stressors such as severe heat, salinity, and dehydration. Superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) are examples of antioxidant enzymes whose activity is increased by melatonin. By preserving cellular redox equilibrium, these enzymes further detoxify ROS (Tiwari et al. 2024).

• Gene expression regulation: Genes linked to stress tolerance are influenced by melatonin in their expression. As an example, it increases the expression of genes related to the manufacture of osmoprotectants, which aids in preserving osmotic equilibrium in stressful situations. Genes associated in plant growth, such as those governing root development, seed germination, and blooming, are expressed in a way that is regulated by melatonin. This modulation guarantees that even in less-than-ideal conditions, plants can maximize their growth (Gao et al. 2023). RNA-Seq was used to assess the genome-wide gene expression patterns of kiwifruit seedlings that had been primed with or without melatonin at 45 °C. Furthermore, qRT-PCR demonstrated that MT dramatically increased the expression of the carotenoid biosynthesis gene, which corresponded to an increase in carotenoid content. Furthermore, MT significantly increased the expression of 10 heat shock proteins (HSPs) (Xia et al. 2021).

• Signal transduction pathways: Melatonin is a signaling substance that initiates many pathways related to the stress response. It triggers signaling cascades of Mitogen-Activated Protein Kinase (MAPK), which are essential for delivering stress messages and starting the right kinds of cellular reactions. It regulates growth and stress responses by interacting with gibberellins, auxins, and abscisic acid, among other plant hormones. Crosstalk adjusts how plants react to external stimuli (Khalil et al. 2023).

• Photosynthesis and chloroplast function: Under stressful circumstances, melatonin prevents chlorophyll from degrading, preserving photosynthetic activity. For plants to produce energy and be healthy overall, this is essential. By increasing light absorption and energy transfer inside the chloroplasts, melatonin raises the efficiency of photosystems I and II. Better growth and productivity result from this, even in challenging circumstances (Hussain et al. 2024).

• Root system architecture: Melatonin increases the elongation and development of lateral roots, which enhances a plant's capacity to absorb nutrients and water, particularly during droughts. It has an impact on the root microbiome as well, encouraging the development of advantageous microbes that improve stress tolerance and nutrient absorption (Wang et al. 2022a, b).

• Molecular and epigenic modifications: Melatonin has the ability to cause epigenetic modifications, such as histone modification and DNA methylation, which affect the patterns of gene expression and provide long-term stress tolerance. In addition, melatonin affects how proteins are modified post-translationally, such as via phosphorylation, which modifies the function of important enzymes and proteins involved in stress reactions (Kaya and Ugurlar 2023).

The ways that various organisms synthesize melatonin have varied over billions of years of evolution. Plants can create more melatonin than animals, and in general, their synthesis of the hormone is more sophisticated. The study employed isotope tracer technology to provide exogenous C14-labelled tryptophan. Ultimately, radioactive tryptamine, 5-hydroxytryptophan, indoleacetic acid, and melatonin were found in Hypericum monogynum (Wu et al. 2021a, b). The production of phytomelatonin involves four sequential enzymatic processes, the majority of which are catalyzed by five different enzymes (Mannino et al. 2021). The most crucial stage in the synthesis of melatonin is the alteration of tryptamine to serotonin, which is catalyzed by tryptophan decarboxylase (TDC) in the first two steps (Liu et al. 2022) as shown in Figs. 1 and 2. Afterward, tryptamine is converted to tryptamine by tryptamine-5-hydroxylase (T5H). Plants have two separate routes that lead from serotonin to melatonin in the following two phases (Erland et al. 2017). One pathway uses serotonin-N-acetyltransferase (SNAT) to catalyze the conversion of serotonin to N-acetylserotonin. Then, N-acetylserotonin methyltransferase (ASMT; formerly known as hydroxyindole-O-methyltransferase, HIOMT) or caffeic acid O-methyltransferase (COMT) catalyzes the conversion of N-acetylserotonin to melatonin (Sharma et al. 2023). The second route, which is catalyzed by ASMT/COMT and SNAT, goes from serotonin to 5-methoxytryptamine and subsequently to melatonin. To further investigate the melatonin biosynthesis route in plants, a range of transgenic plants for melatonin synthesis have been created, and all the essential synthesis enzyme-coding genes have been cloned in plants thus far (Mir et al. 2020).

Fig. 1

Role of melatonin application in fruit crops (Pan et al. 2023)

Fig. 2

Biosynthesis pathways of melatonin

Different subcellular sites, including the cytoplasm, endoplasmic reticulum, and chloroplasts, generate melatonin intermediates. The cytoplasm may be the ultimate subcellular site of melatonin synthesis, depending on the pathway (Tan and Reiter 2020). This could have a different impact on the way that melatonin acts in plants. Both mitochondria and chloroplasts contain SNAT. These melatonin synthesis enzyme subcellular locations imply that melatonin production sites expanded during evolution to include the cytoplasm and endoplasmic reticulum, becoming more diversified (Suresh et al. 2023).

Melatonin production in plants

Melatonin is a universal amphiphilic antioxidant molecule that has strong solubility in both water and lipids and a modest size that allows it to enter all cell compartments. ROS and RNS species can be directly scavenged by melatonin, which also accelerates the function of antioxidant enzymes, protects against oxidative damage, antioxidants to produce beneficial effects, and improves electron transport capability in the mitochondrial respiratory chain to reduce electron loss and restrict the synthesis of free radicals (Roychoudhury 2023). Moreover, melatonin and its byproducts serve as naturally occurring electron donors that are highly efficient in averting oxidative stress. It can produce several radical scavengers that lead to the oxidation of molecules like β-hydroxy melatonin, cyclic β-hydroxy melatonin, and N1-acetyl-N2-formyl-5-methoxykynuramine (AFMK), as well as help to eliminate ROS (Checker et al. 2021). This property sets melatonin apart from other antioxidants and allows it to effectively shield organisms from oxidative stress, even at low concentrations. Aerobic cells’ ability to efficiently perform their metabolism depends on their mitochondria, which are responsible for maintaining a proper redox status (Sachdev et al. 2021).

Melatonin has been detected in apples, barley, beans, cucumbers, grapes, lupine, maize, rice, and tomato (Salehi et al. 2019). Numerous studies conducted on 108 kinds of herbs that are commonly used in Chinese medicine have shown evidence of melatonin at concentrations ranging from thousands to a few nanograms per gram of tissue. This suggests that these herbs are great natural supplies of this important chemical for physiology (Wijesinghe et al. 2023). It has also been verified by examining melatonin’s anticipated protective effect on plant photosynthesis. In general, melatonin influences the efficiency of photosynthetic processes by doing the following: slowing down the breakdown of chlorophyll, boosting the uptake of CO2, and quickening the electron transport. Beyond its ability to prevent oxidative damage, which results from both abiotic and biotic stresses, melatonin also activates several distinct plant defense mechanisms at the proteomic and genetic levels. These mechanisms protect plants from various biotic and abiotic stressors containing heavy metals, pathogens, cold, drought, heat, and light (Nawaz et al. 2023). Melatonin is a usual auxin generated from tryptophan structurally similar to indole-3-acetic acid (IAA). It has a role in the development and regulation of plant growth. Melatonin may also be involved in photoperiodic responses and the control of the circadian rhythm in plants, according to earlier studies; however, further investigation is required to completely comprehend these phenomena (Murch and Erland 2021). Studies on the chemical melatonin in plants show that it is crucial to their survival. Therefore, it is not surprising that melatonin can be found in food products and beverages made from plants, as it has been found in a large variety of edible plants and herbs (Hossain et al. 2022).

Similarities and differences between animal and plant melatonin

While the structure and functions of the chemical melatonin are similar in both plants and animals, there are distinctions in its production, regulation, and activities within each kingdom.

Similarities concerning chemical structure, biosynthetic Pathway, functions as a signaling molecule, antioxidant Properties, and regulation by Light and Circadian Rhythms. The molecular structure is conserved in both plants and animals. It is a derivative of tryptophan, an amino acid, and all living things have it in their basic structural makeup. In general, both plants and mammals share a similar melatonin biosynthesis route. Through a sequence of enzyme processes, tryptophan is converted to serotonin and subsequently to melatonin (Zhao et al. 2019). Melatonin functions as a signaling molecule in both plants and animals, helping to regulate several physiological activities. It is involved in coordinating sleep–wake cycles, circadian rhythms, and reactions to stresses in the environment. In both plants and animals, melatonin demonstrates antioxidant qualities. It contributes to cellular protection by scavenging reactive oxygen species (ROS) and reducing oxidative stress. Circadian rhythms and light are two examples of external factors that affect the generation of melatonin in plants and animals. Plants have a variety of organs that can generate melatonin in reaction to light and other environmental stimuli, but animals typically produce it in the pineal gland in response to the light–dark cycle (Tan and Reiter 2020).

Differences concerning tissues of Production, regulatory Points, physiological Roles, and localization within Cells. Animals produce melatonin in several tissues, but the pineal gland is the primary site of synthesis. Melatonin is not only produced in one organ of plants but also in a variety of tissues throughout the plant. Plants and mammals can differ in the regulatory points and enzymes involved in melatonin synthesis, even though the overall biosynthetic pathway is conserved. For instance, compared to animals, plants may have various forms and sites for N-acetylserotonin O-methyltransferase (ASMT), the enzyme in charge of the last stage of melatonin synthesis. Animals and plants can have different physiological functions for melatonin (Saha et al. 2019).

Distribution in fruit crops

Melatonin presence in various fruit crops

To meet our daily needs for the major vitamins and minerals, we frequently eat fruits and vegetables. Furthermore, fruits include several phytochemicals that control several physiological and biochemical processes, protecting against a range of chronic illnesses. After melatonin was discovered in the kingdom of plants, many researchers started looking into the amounts and purposes of this hormone in various plants. As a result, a variety of plant parts have been assessed, including seeds, roots, bulbs, and fruits. Fruits are essential foods that have also had their melatonin concentrations assessed. First noted that melatonin was present in dicotyledonous plants and described its function as a strong radical scavenger (Debnath et al. 2019). Melatonin has been found in a variety of consumable fruits. Fruits vary significantly in terms of melatonin content, even within types of the same fruit and during different phases of development as discussed in Table 2. Many techniques have been used to detect melatonin in plants, including high-performance liquid chromatography (HPLC) with electrochemical detection (HPLC-ECD), radioimmunoassay (RIA), gas chromatography-mass spectrometry (GC–MS), enzyme-linked immunosorbent assay (ELISA), and fluorescence detection (HPLC-FD). The sensitivity and specificity of each of these techniques vary (Galijašević et al. 2023).

Table 2 Melatonin concentration present in fruits and species

Physiological functions of melatonin in plants

Indoleamines such as tryptophan are the precursor molecules of melatonin, and they should play a role in regulating development and growth. Because of its many physiological roles, melatonin—which is best known to assist animals in regulating their sleep and wake cycles—has become an intriguing molecule in the field of plant biology. It is produced in a variety of plant organs, such as the roots, shoots, leaves, and seeds. It works through a combination of receptor-dependent and receptor-independent pathways. Melatonin's activity as an antioxidant is one of its primary physiological roles in plants. Environmental factors like drought, salinity, high temperatures, and pollution are continual threats to plants, and they can cause oxidative stress and reactive oxygen species (ROS). Strong ROS scavenger melatonin neutralizes ROS and shields plants from oxidative damage. Furthermore, melatonin strengthens the antioxidant defense system of plants by promoting the activity of antioxidant enzymes like catalase, peroxidases, and superoxide dismutase. Apart from its antioxidant characteristics, it controls other facets of plant growth and development. It controls flowering time, strengthens root development, and encourages seed germination and seedling growth. Additionally, it regulates stomatal mobility, which impacts plant water loss and gas exchange. It also affects how plants react to biotic stressors, such as disease invasion and herbivore assault. It has a wide range of physiological roles in plants, including growth regulation, stress responses, and antioxidant defense (Muhammad et al. 2021).

One of the most well-known properties of melatonin is its ability to scavenge free radicals triggered by oxidative stress. Furthermore, it not only controls blooming, fruit ripening, photosynthesis, senescence, apoptosis, growth cycles, root morphogenesis, seed germination, and other growth and development processes, but it also governs biotic and abiotic stress responses. As an important growth regulator for life, directly associated to plant growth and development, as well as stress responses. In plants, it can control development by influencing metabolism. It promotes kiwifruit leaf senescence by increasing the concentration of flavonoids (Zhang et al. 2021).

Antioxidant properties

The climate shift throughout the crop’s growth phases makes the crops more susceptible to various abiotic stresses. Plants that are under stress produce 1–2% of the oxygen they consume into reactive oxygen species, which includes singlet oxygen (1O2), hydrogen peroxide (H₂O₂), superoxide radical (O2⋅ −), and hydroxyl radical (⋅OH). Melatonin can also boost the function of antioxidant enzymes in plants. These enzymes, which include superoxide dismutase, catalase, and peroxidases, are crucial in ROS detoxification. Melatonin can promote the production and activity of these enzymes, improving the plant's antioxidant capacity. lants are continually exposed to a variety of environmental challenges, including drought, salinity, severe temperatures, and pollution, all of which can cause ROS and oxidative stress. It helps plants to deal with these challenges by scavenging ROS and maintaining cellular homeostasis. Its antioxidant function helps the plant adapt and withstand unfavorable environmental circumstances. It has been found to enhance photosynthetic efficiency in plants under stress situations (Sachdev et al. 2021). MT increases the body's natural antioxidants, like glutathione and ascorbic acid, to scavenge free radicals created under stressful conditions as discussed in Table 3. Melatonin aids in maintaining the ideal levels of photosynthetic activity, which is necessary for plant growth and productivity, by lowering oxidative damage to chloroplasts and other photosynthetic machinery. Many plant species have been shown to benefit from melatonin in terms of seed germination and seedling growth. Because of its antioxidant qualities, it may help shield seeds and young seedlings from oxidative damage during the germination and early growth stages, promoting the plant’s successful establishment and growth (Hasanuzzaman et al. 2019). Melatonin administration enhanced antioxidant activity and promoted GABA production in yellow-flesh peach fruit. It drastically lowered H2O2 and MDA levels while enhancing the non-enzymatic antioxidant system (Wu et al. 2023). The application of Melatonin (100 µmol L⁻¹) applied two weeks before harvest minimized red drupelet reversion in blackberry fruit, enhancing antioxidant capacity and sustaining postharvest quality for 12 days of cold storage (Shah et al. 2023). The effect was seen with melatonin (1 µM) application with 100 mM NaCl on growth, antioxidant enzyme activity and ion hemeostasis in maize plant. The findings indicated that improvements in photosynthetic ability, antioxidant capacity, and ion homeostasis in leaves due to melatonin which enhanced salt tolerance in maize (Jiang et al. 2016).

Table 3 Effect of different concentrations of melatonin on antioxidant and overall quality in fruits

Regulation of growth and development

Plant growth regulators (PGRs) are vital to plant development, growth, and adaptation to environmental stimuli. There are multiple widely recognized types of PGRs in plants. Salicylic and abscisic acid, ethylene, jasmonic acid, brassinosteroids, auxins, cytokinins, gibberellins, and ethylene are some of these PGRs. Auxin gradients are thought to be the most influential on deciding the consequence of morphological patterns, as they support apical dominance, cell elongation, and rooting, among other functions. On the other hand, cytokinins act as major individuals in organ development, germination, death, and the keeping up of circadian rhythms (Erland et al. 2015).

Effect of melatonin on fruit crops

Complex signaling networks control the growth and development of higher plants. Auxin, cytokinin, ethylene, and other plant hormones, together with signal molecules like calcium ions, hydrogen sulfide, and nitric oxide, work together to control essential biological processes in plants, including photosynthesis, growth, development, responses to their environment, and even senescence. At now, there is a consensus based on many lines of evidence derived from both basic and applied research that melatonin, also known as N-acetyl-5-methoxytryptamine, is an endogenous chemical that is highly conserved and plays a crucial role in various biological processes in plants (Qiao et al. 2019). The investigation of melatonin in plants began with the discovery of phytomelatonin, which is present in various plants and plant-derived products. Researchers have identified the essential elements of phytomelatonin production piece by piece in recent years. Compared to animal synthesis, phytomelatonin synthesis in plants is far more versatile. It has been reported that melatonin enhanced the growth of wheat seedlings in both N-sufficient and -deficient environments.

Melatonin treatment enhanced the growth parameters such as root elongation, plant height, and leaf surface area of maize seedlings (Erdal 2019).

Different developmental processes affected by melatonin

Phytomelatonin acts as a plant growth stimulator, stress protector, and flowering and fruit ripening regulator by regulating various elements related to the redox network or interfering with other phytohormones. The primary function of melatonin in plants is to serve as the first line of defense against oxidative stress, which occurs under almost all adverse environmental conditions. In cells, ROS, NO, and phytomelatonin form an interesting triad that regulates redox homeostasis, and melatonin might be the key component in the center of the redox network (Arnao and Hernández-Ruiz 2019a, 2019b). Thus, the effect of melatonin on the redox network could be used as a stimulator to develop fortified plants with, for example, anti-senescence genes and improved photosynthesis.

Abiotic stress responses

Melatonin’s role in coping with environmental stressors

In terms of nutrition for humans, fruit crops are the main source of carbohydrates, minerals, micronutrients, proteins, lipids, fiber, organic acids, pigments, and antioxidants. In addition to being an essential part of a healthy diet, fruit crops help farmers in underdeveloped countries generate revenue and reduce poverty. These crops have the potential to support developing countries’ economies and diversify agriculture. It is anticipated that the demand for fruits will rise significantly in the future due to a steady rise in living standards and diet. The production and yield of these crops must be increased immediately, but it is also crucial to reduce losses from abiotic stressors and pathogen infection. Drought, salinity, heat, and chilling are the main abiotic factors that can cause losses in horticultural crops of up to 50–70% as shown in Fig. 3. Typically, plants have an internal defense mechanism to lessen these pressures, but it only works up to a certain point. The natural defense mechanism is compromised by extreme stress, which results in damage and aberrant physiological conditions inside the plant cell, both of which eventually lower yields (Nowroz et al. 2024). It is anticipated that under stress conditions, melatonin biosynthesis will be upregulated, demonstrating melatonin’s antistress qualities. Additionally, melatonin has a variety of physiological regulatory functions that influence plants’ ability to withstand abiotic stress. Because of its detoxifying qualities, exogenously administered melatonin has a major impact on plants' capacity to withstand stress (Kaur et al. 2015).

Fig. 3

Comparison between stress and Melatonin application under stressful conditions

Effect of melatonin treatment on heat stress management

Heat stress is now one of the main factors restricting fruit crop output as a result of climate change. Heat stress causes morphological, biochemical, and physiological alterations in plants that impede their ability to grow. Major enzymes are rendered inactive by high temperatures, which also restricts protein synthesis, causes photoinhibition, and accelerates the degradation of chloroplast membranes and proteins. The main negative impacts of heat stress are oxidative damage to cell proteins, lipids, and DNA, just like drought stress. Plants commonly use heat stress-induced protein in the form of self-defense (dos Santos et al. 2022).

Effect of melatonin on mitigation of salt stress

One of the most severe abiotic stresses to horticulture crops is salt stress. Salt stress has already harmed 45 million hectares (19.5% of all irrigated land) globally (Bharti and Ray 2023). Ion toxicity, osmotic stress, reactive oxygen species-mediated damage, and nutritional imbalance are among the negative impacts of salinity stress on plants. ROS overproduction is linked to membrane damage, and in salinity conditions, Na+ accumulation causes ion toxicity. In recent times, numerous studies have revealed the vital and indispensable function of melatonin in fruit crops' mitigation of salt stress, as it has been clear that the hormone is a widely distributed growth regulator that is essential for numerous abiotic stress resistance in plants. In salt-stressed plants, exogenous melatonin treatment may modulate both enzymatic and nonenzymatic antioxidant systems to reduce ROS-mediated damage (Gu et al. 2022). Melatonin reduced salt damage in alfalfa plants under salt stress. It significantly reduced electrolyte leakage, malondialdehyde (MDA) content, and H2O2 content, while increasing the activities of catalase (CAT), peroxidase (POD), and Cu/Zn superoxide dismutase (Cu/Zn-SOD), as well as upregulating genes related to melatonin and antioxidant enzymes biosynthesis. It functions primarily as an antioxidant, scavenging H₂O₂ and increasing the activity of antioxidant enzymes to improve alfalfa plant salt tolerance (Cen et al. 2020). Three types of grapevines were used to isolate and identify root-dwelling bacteria. It was discovered that certain bacterial strains released tryptophan-ethyl ester and melatonin when they were cultivated in a lab setting. We discovered that, in grapevine plantlets subjected to stress from salt or drought, B. amyloliquefaciens SB-9 colonization enhanced the upregulation of melatonin synthesis and its intermediates, while decreasing the upregulation of grapevine tryptophan decarboxylase genes (VvTDCs) and a serotonin N-acetyltransferase gene (VvSNAT) transcription (Jiao et al. 2016). Plants were able to sustain a higher photosynthetic capacity after being pretreated with 0.1 μm melatonin, which considerably reduced the growth inhibition in Malus hupehensis Rehd. under high-salinity conditions. Significant up-regulation of dNHX1 and MdAKT1 was seen in the leaves, which may have aided in maintaining ion homeostasis and enhancing salinity tolerance in plants exposed to exogenous melatonin (Li et al. 2012).

Some fruit crops have been shown to exhibit improved photosynthesis and other associated mechanisms when exposed to salt stress discussed in (Table 4).

Table 4 Case studies or examples showcasing stress tolerance enhancement in fruits

Effect of melatonin on mitigation of cold stress

Low temperature stress can warm-climate plants and cause chilling injury which is a key abiotic problem. Both the physiological and metabolic dynamics of the cell are impacted by cold stress. The majority of the detrimental effects are similar to signs of drought stress, including damage to cell membranes, disruptions in the electron transport chain, excessive production of reactive oxygen species, unbalanced ion homeostasis, etc. Carbon fixation is hampered by cold-induced ROS buildup because it inactivates enzymes that control the Calvin cycle (Tiwari et al. 2020). Horticultural crops may exhibit morphological reactions such as uneven seed germination, reduced seedling vigor, and delayed ontogenic development, which can lead to a decrease in yield. Like other abiotic stressors, cold stress is also naturally defended against plants through enzymatic and non-enzymatic antioxidant systems, as well as increased glucose and amino acid storage to prevent osmotic damage (Zahra et al. 2021). By increasing antioxidant defense capability and modifying the DREB/CBF—COR pathway, the application of melatonin (100 μM) protects the strawberry plants against the harm caused by cold stress (Hayat et al. 2022). Melatonin treatment after harvesting kiwiberries might increase both the exterior and interior quality of the fruit and prevent senescence via altering sugar metabolism under cold storage (Zhang et al. 2023). The study examined the impact of melatonin (10 and 30 μm) on the model plant Arabidopsis thaliana, which was subjected to a cold stress at 4 °C for 72 and 120 h. The findings indicated that melatonin up-regulated genes associated with cold signaling, which could potentially promote the production of compounds that shield plants from cold stress and lead to the higher growth of plants treated with exogenous melatonin (Bajwa et al. 2014). In this work, priming-induced alterations in the maize embryo proteome were analyzed, and proteins related with priming and melatonin were identified in chilling-treated maize seeds (Kołodziejczyk et al. 2016).

Melatonin under drought stress

Plants undergo several physiological, biochemical, and molecular processes that are altered primarily by drought. Drought conditions will be harsher and last longer for plants cultivated in arid and semi-arid areas. Promoting adaptive agricultural techniques is critically needed in rapidly changing environmental conditions since various organs exhibit varying degrees of vulnerability to water stress. Enzymes involved in photosynthetic processes, including fructose-1,6-bisphosphatase (FBP) and ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco), are much less active during drought stress (Dalal 2021). These enzymes are also crucial for the Benson–Calvin cycle. Notably, excessive harmful reactive oxygen species (ROS) accumulation is the consequence of drought-induced over-reduction of the electron transport chain (ETC). Specifically, excessive accumulation of hazardous reactive oxygen species (ROS) is caused by drought-induced over-reduction of the electron transport chain (ETC). ROS may harm plants by oxidizing membrane lipids, photosynthetic pigments, proteins, and nucleic acids. Plants respond to drought stress by raising the amount of immobile nutrients through their root systems. To help with maintaining water and adapting to dry conditions, they can also change their metabolism and collect advantageous metabolites, such as proline and soluble sugar. Recent climate models predict that drought will increase in frequency throughout a large portion of the planet. Exogenous administration of plant growth regulators is one method to increase plants’ resistance to drought (Farooqi et al. 2020). Pretreatment with 100 µM melatonin reduced antioxidant enzymes (POD, SOD, etc.) and osmoregulation (soluble sugars and proline) in tomato seedlings by regulating gene expression. It also improved seedling growth, root characteristics, and leaf photosynthesis under drought stress (Altaf et al. 2022). The sweet cheery trees were sprayed with melatonin at 100, 200, and 300 µM on their leaves, and their soil field capacity (FC) was measured at 75, 50, and 25%.The highest levels of AsA and phenolic contents were produced by trees exposed to drought at (50–5% FC) and treated with 200 µM of melatonin topically. Oleic acid (55.18%–68.05%) was the predominant fatty acid component, and melatonin elevated its levels while the drought lowered them (Hojjati et al. 2024). Both drought-tolerant Malus prunifolia and drought-sensitive M. hupehensis plants exhibit dramatically increased tolerance when pre-treated with melatonin. By up-regulating the expression of the melatonin synthesis genes MdTDC1, MdAANAT2, MdT5H4, and MdASMT1, plants can efficiently manage their water balance during drought circumstances.Drought-stressed plants have lower ABA concentrations because melatonin preferentially down-regulates the gene that makes ABA, MdNCED3, and up-regulates the genes that catabolize ABA, MdCYP707A1 and MdCYP707A2 (Li et al. 2015).

Melatonin under heavy metal stress

Heavy element pollutants in soil agriculture have increased as a result of the construction of factories and industries, which lowers agricultural production and economics. Heavy metal addition from diverse sources changes growth, yield, nitrogen metabolism, and photosynthesis inhibition. Melatonin helps plants grow by boosting their defense mechanisms against oxidative stress and lowering the buildup of heavy metals at the rhizospheric surface through the use of its chelating activity (Iqbal et al. 2023). Heavy metals (HMs), such as iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), molybdenum (Mo), nickel (Ni), and cobalt (Co), are necessary for plants in particular amounts, but they are harmful when are present in excess amount (Varma and Jangra 2021). Cadmium (Cd) is one of the most hazardous heavy metals for plants. Melatonin increases growth, photosynthesis, redox homeostasis, and secondary metabolites in strawberry seedlings, which may assist to mitigate phytotoxicity caused by Cadium (Saqib et al. 2023).

Biotic stress responses

Melatonin’s interactions with pests and pathogens

In the horticultural system, biotic stress is commonly defined as fruit and vegetable loss resulting from infectious diseases produced by bacteria, viruses, nematodes, and insects. These biotic stressors cause plants to die and decompose in addition to directly obtaining nutrients from the plants. Pathogenic stress is a significant factor in pre- and postharvest loss of fruits. Researchers are still working to establish effective control for several viral infections that affect horticultural crops. Researchers are still trying to find efficient ways to manage various viral infections that impact horticulture crops and the surrounding environment (Umamaheswari et al. 2020). Chemical control is the most popular and successful approach because it provides quick protection and offers a wide range of management choices for its use and application. Nonetheless, there is a great need for comparatively safer compounds to manage diseases due to consumer concerns about having a food crop free of pesticides and environmental sustainability. Melatonin can serve as a biocide against certain fungi and bacteria, although there is no solid evidence on its mode of action. Melatonin has antimicrobial effect in vitro against antibiotic-resistant pathogen strains 103, whereas considerably higher concentrations have antibacterial activity against ten different human pathogenic bacteria (Arnao Hernandez-Ruiz 2015). Melatonin is an eco-friendly substance that might be used as a different tactic to help plants defend themselves against biotic stressors as shown in Fig. 4. Melatonin’s preventive effect against bacterial and viral illnesses has previously been documented in animal science (Sun et al. 2021a, b). It is highly effective in both directly inhibiting the pathogen and triggering the host plant's defense response. The primary mechanisms that enable biotic stress generated by melatonin in plants are defense gene activation, cell wall thickening, and ROS scavenging. A severe threat to the worldwide horticultural production system is insect infestations. They inflict indirect costs by spreading dangerous bacterial and viral infections, as well as direct losses by eating the plants. These herbivorous insects cause the loss of one-fifth of the world's crop production annually. Melatonin is a good tool to fight insect pests, as evidenced by the growing concern about using fewer pesticides and instead relying on natural and sustainable management solutions (Giraldo Acosta et al. 2022). It is previously known that endogenous melatonin exists in the nematoda and insecta which primarily impacts circadian oscillations, photoperiodism, moulting, and seasonal rhythms (Tiwari et al. 2021a, b).

Fig. 4

Melatonin application and their entry into cellular membranes and immediately eliminates reactive oxygen species (ros) and ameliorate the osmolytes, safeguards the photosynthetic apparatus, preserves the redox balance, and influences the signalling transduction and gene expression associated with various stresses to promote stress tolerance. Adopted from (Hassan et al. 2022) under the terms of the Creative Commons Attribution License (CC BY)

An Investigation was done to inducing resistance mechanism of melatonin treatment in a table grape. The grapes were submerged in 0.02, 0.2, and 2 mmol L⁻¹ melatonin, and then 48 h later, Botrytis cinerea suspension injections were given. The findings indicated that exogenous melatonin treatment could stimulate defense mechanisms to fight off B. cinerea infection in post-harvest grapes (Li et al. 2022).

Melatonin in fruit ripening and quality

Melatonin’s impact on fruit maturation

Fruit ripening is a multifaceted process controlled by hormones and genes that involves several metabolic, physiological, and structural alterations (Table 5). When it comes to hormones, ethylene is crucial in regulating a variety of the mechanisms involved in fruit ripening. In climacteric fruits, the release of ethylene and respiration rate rise during fruit ripening (Li et al. 2021). A typical climacteric fruit takes around four weeks to ripen, and ethylene is a key regulator of the flavor, aroma, color, and softening changes that occur during this time and ultimately determine the fruit’s final quality. Other chemicals that have regulatory roles in fruit ripening are also integrated, such as auxin, salicylic acid, jasmonic acid, and abscisic acid. Physiological nitro-oxidative stress associated with fruit ripening has been discovered recently. This stress affects numerous subcellular processes, causing some metabolic pathways to be up and down-regulated. These effects include delaying fruit senescence, reducing chilling injury, reducing fungal decay, and enhancing nutritional quality (Fenn and Giovannoni 2021). The study examined the growth and ripening as well as the impact of melatonin (500 µM) on the apple tree. Melatonin treatment dramatically enhanced ethylene production while also increasing fruit size, weight, sugar content, and firmness. Melatonin appears to enhance fruit ripening by inducing ethylene production, whereas melatonin treatments prior to ripening increase ultimate fruit quality (Verde et al. 2020a, b).

Table 5 Potential applications of melatonin to overcome pest and disease management in fruit crops

Implications for fruit quality and post-harvest characteristics

Fruit's delicious flavor and abundance of nutrients (such as vitamins, organic acids, and polyphenolic compounds) have made it a popular meal choice for people all over the world. Fruits are perishable and need quick postharvest storage, which diminishes their taste and nutritional value and negatively impacts consumer acceptance as well as the financial value of fresh produce (Palumbo et al. 2022). Thus, scientists have always been concerned with how to preserve the quality of postharvest fruit throughout storage. In contrast to fruit ripening, senescence is characterized by a progressive decline in fruit quality. Fruit senescence is a phase in which the tissue structure and nutritional components gradually deteriorate as the ability to maintain the internal environment and adapt to the storage environment gradually declines. The senescence of postharvest during storage has been proven to be considerably delayed by exogenous melatonin therapy, extending the shelf life (Zhang et al. 2019a, b). Fruit firmness was preserved by melatonin treatment, which also decreased weight loss, respiration rate, and weight loss while maintaining levels of TSS and soluble sugar and preventing browning of the peel and flesh (Chen et al. 2022). The study examined melatonin affected the postharvest quality of nectarine fruit (cv. Fantasia). Fruits were stored for 40 days at 0–1 °C and 85–90% relative humidity after being immersed in melatonin concentrations (control, 250, 500, and 1000 µmol L⁻¹). It prolonged the postharvest life of nectarine fruits, keeping their appearance and nutritional content, and decreasing the loss of health-promoting compounds (Bal 2021). An investigation was done on exogenous melatonin treatment which greatly inhibited postharvest physiological deterioration (PPD) of cassava tuberous roots by lowering H2O2 levels and enhancing catalase and peroxidase activity. Taken together, this study gives new insights into melatonin's influence and underlying process on PPD delay, as well as a viable technique for prolonging shelf life and improving cassava tuberous roots (Hu et al. 2016).

Research gaps, challenges future prospects, and implications

A naturally occurring substance in food is melatonin. Research has indicated that melatonin is available in various fruit types and that the precise amount is determined by several variables, such as the type of fruit and its level of ripeness, genetic characteristics, growing circumstances, environmental stressors, and laboratory technique. Though the controlled processes of melatonin based on fruit ripening remain incompletely understood, recent studies have focused more on studying the links between melatonin content and the fruit ripening stage. Surprisingly, there hasn’t been any research done on how postharvest modifications that extend fruit shelf life affect melatonin levels. The comprehension of melatonin's physiological functions during ripening could be enhanced by this information. The most difficult aspect of the research on melatonin in fruits is still obtaining established analytical methodologies with sufficient sample preparation to reliably estimate the amount of melatonin in fruits. More precise analytical techniques are required to get consistent results in fruits so that data on the effects of dietary melatonin may be gathered. Even though a recent study confirmed that SNAT is present in plants for the first time and that ASMT is the enzyme that limits the pace at which melatonin is biosynthesized, more research needs to be done on the melatonin synthesized pathway in fruits. It’s uncertain if the proposed plant biosynthetic pathway is widespread or not. According to studies, melatonin in fruits primarily acts as the primary protection against oxidative stressors, which are put on by toxins from internal and from the environment. It is still unclear whether melatonin’s potential to act as an antioxidant in fruits depends just on its direct interaction with ROS or if melatonin receptors also play a role. There is no evidence of melatonin receptors in fruits. In addition to its role as an antioxidant, melatonin has the potential to enhance plants' ability to remove various contaminants, including heavy metals, through phytoremediation. Finally, because fruit has biological functions, it may affect the endogenous melatonin level and thereby improve human health.

Melatonin has been shown in recent research to have a wide range of potential applications in horticultural fields, such as food safety, virus-free plant development, vegetative reproduction, molecular breeding, and crop processing. Important plant functions like parthenocarpy, autophagy, and arbuscular mycorrhizal symbiosis can all be impacted by melatonin. Fruit growth and defense have been linked to endogenous melatonin, whose concentrations change as the fruit ripens, peaking before veraison and falling over the rest of the fruit's development. Research has shown that applying this treatment to plants increases fruit weight and quantity, which in turn increases crop yield. Direct application reduced oxidative stress in fruit and encouraged the suppression of fruit senescence. The fruit that has been treated has a higher ratio of unsaturated to saturated fatty acids, which has been shown to enhance its resistance to chilling stress. In-vitro fruit pathogen-fighting properties of melatonin have been demonstrated against oomycete and fungal infections (Wang et al. 2020a).

Conclusion and way forward

The evolution of new varieties and the application of melatonin as an innovative, environmentally friendly agrochemical have an enormous ability to address agricultural difficulties by controlling plant growth, prolonging the conservation of postharvest crops, and boosting resilience to biotic and abiotic stresses.. Some climacteric fruits, like mangos, bananas, kiwifruits, avocados, and bananas, must be harvested before the climacteric phase to prolong their shelf life. These fruits can also be artificially ripened with ethephon. Melatonin is anticipated to be utilized, similar to other PGRs, to enhance the agronomic qualities of horticulture plants. In addition to its health benefits, melatonin can enhance the body’s natural production of other advantageous compounds including phenols and anthocyanins. Additionally possessing significant antioxidant qualities, phenolic substances may offer protection against neurological and cardiovascular illnesses. Finally, several horticultural crop examples have indicated the creation of novel crop cultivars, and the application of melatonin as a PGR, antibacterial, and preservative has a significant ability to tackle major issues faced by the horticulture sector. A naturally occurring substance called melatonin helps plants withstand environmental stresses including salinity, drought, and high temperatures, which improves fruit quality and yield. Because of its antioxidant qualities, plants may respond to stress better and adjust to shifting climates more successfully. Furthermore, supplementing with melatonin can improve growth conditions, lessen the need for artificial agrochemicals, and encourage environmentally friendly farming methods. In light of these issues, melatonin integration into agricultural practices provides a sustainable alternative to increase fruit yield, guaranteeing environmental preservation and food security. Therefore, field experiments should be used to evaluate any possible uses for melatonin. It is also necessary to obtain melatonin from less costly sources.