**5.1 Regulation of melatonin resistance to abiotic stress**

The efficient utilization of light energy for plant growth, development, yield and quality by light has always been the core of scientific research in horticulture production. Melatonin, on the other hand, has been shown to have a circadian rhythm in mammals, so it is inferred that there should also be a myriad of links

between endogenous melatonin of plant and light environmental factors. Melatonin improved the growth performance of yeast under UV radiation and reduced the mortality [48]. The mechanism proposed suggests that melatonin increased the expression of antioxidant genes and DNA-repairing genes.

Kolar et al. [49] used 100 mmol L<sup>−</sup><sup>1</sup> and 500 mmol L<sup>−</sup><sup>1</sup> of melatonin solutions to treat the cotyledon and germ of the short-day plant *Chenopodium rubrum* (*Chenopodium rubrum* L.), which could significantly inhibit the flowering rate under the short-term conditions, and only applied melatonin before the end of the light treatment or the first half of the dark treatment was effective. This suggests that melatonin levels are influenced by circadian rhythms and regulate the photoperiod of plants, which in turn affects some early steps in flowering. In addition, the application of shading technology in the cultivation of capsicum can significantly reduce the melatonin level of capsicum fruits, which indicates that solar radiation can cause the increase of melatonin concentrations [50]. Studies in wheat also show that the concentrations of melatonin in leaves under light condition are significantly higher than they are under dark condition. At the same time, the light treatment significantly increases the concentrations of melatonin synthesis precursors (tryptamine, 5-hydroxytryptamine and *N*-acetyl-5-hydroxytryptamine) in leaves [36]. This suggests that light promotes plants to convert l-tryptophan into melatonin. Light quality also affects the endogenous melatonin concentrations of licorice (*Glycyrrhiza uralensis* L.), following the rule that red light > blue light > white light [51]. The induced effect of light on melatonin is consistent with the hypothesis that chloroplast is the key organelle for melatonin synthesis [52], which is based on the following: pepper peel and wheat leaves are rich in chloroplasts, while light will inevitably produce excess light energy while promoting plant photosynthesis, resulting in the combination of free electrons and O2 to form ROS. In addition, single light masses, such as red light and blue light, may specifically increase the conversion efficiency of light energy to electric energy at photosystem I (PSI) or photosystem II (PSII), thus increasing the imbalance of electron transfer between PSII and PSI, creating conditions for free electrons to combine with O2 to generate ROS. Since melatonin is an important antioxidant in organisms, it is of certain biological significance to induce chloroplasts to produce a large amount of melatonin to remove excess ROS so as to reduce the oxidative pressure faced by cells. Unfortunately, at present, there is no direct evidence for the systematic studies about the light quality and light intensity that affect melatonin concentrations for alleviating the strong light damage in plants. Therefore, the mechanism of melatonin metabolism on light in plants and melatonin response to light needs further exploration and verification.

Temperature is a key environmental factor that affects horticultural crops, especially vegetable crops that are planted off season. Inappropriate temperature leads to substantial loss of yield and poor quality of horticultural crops. For this reason, horticultural researchers have been working on temperature adaptation mechanisms and efficient and safe plant growth regulators to cope with sudden temperature changes. Shi et al. [53] confirmed that melatonin was induced by high temperature in *Arabidopsis*, and the application of 20 μmol L<sup>−</sup><sup>1</sup> melatonin significantly improved the expression of heat shock factor (*HSFA1s* and *HSFA2s*) and heat shock protein (*HSP90* and *HSP101*), which led to increased survival rate of *Arabidopsis* under high temperature stress. Jia et al. found that application of 29.0 mg L<sup>−</sup><sup>1</sup> melatonin can enhance the high temperature stress tolerance of cherry radish (*Raphanus sativus* L. var. *radculus pers*), which increased the biomass to 12.9% and the soluble protein to 18.7%. At the same time, the activity of antioxidant enzyme, especially for POD, was enhanced and the lipid peroxidation was reduced under adverse conditions [54]. For melatonin-regulated plant cold tolerance, Lei et al. [55] found that very low concentration of exogenous melatonin (21.5 nmol L<sup>−</sup><sup>1</sup> ) could significantly

**117**

reduce water loss.

*Review of Melatonin in Horticultural Crops DOI: http://dx.doi.org/10.5772/intechopen.90935*

ment of Kiwifruit seedlings with 100 μmol L<sup>−</sup><sup>1</sup>

activation of related metabolic processes.

improve the cell viability of carrots, under low-temperature stress, enhance the stability of cell membrane structure, and inhibit orderly degradation of DNA caused by programmed cell death. When tomato was treated with low temperature, the melatonin concentrations and the expression of synthetic control genes were significantly increased [56]. With the extension of stress time, the melatonin concentrations showed an increasing trend, indicating that melatonin played an important role in plant resistance to low-temperature stress. Appropriate osmotic stress can improve the germination rate of cucumber seeds under low-temperature stress [57]. At the same time, osmotic stress intensity was positively correlated with endogenous melatonin concentrations in cucumber seed germination. Further studies showed that the endogenous melatonin, induced by osmotic stress, was beneficial to remove peroxidation damage and stabilize membrane structure under low-temperature stress. However, the excessive endogenous melatonin induced by hyperosmotic stress destroyed the oxidation equilibrium state of protein, but reduced the resistance of cucumber shoots to low temperature. In tomato, the maximum quantum yield (Fv/Fm) of PSII was significantly reduced during chilling, which was effectively alleviated by exogenous melatonin. This is because melatonin induces the expression of violaxanthin de-epoxidase gene, enhances the enzyme activity of violaxanthin de-epoxidase and the enhancement of de-epoxidation state of xanthophyll pigments, promotes the non-photochemical quenching and alleviates the photoinhibition during chilling [58]. In addition, studies on the improvement of plant cold tolerance by exogenous melatonin have also been reported in Kiwifruit (*Actinidia Chinensis*) and *Arabidopsis thaliana*. Wang et al. [59] found that the treat-

the growth inhibition and chlorophyll degradation, improve antioxidant enzyme activity and eliminate ROS under low-temperature stress. Exogenous melatonin can induce the expression of a series of low-temperature responsive transcription factors (*CBFs*, *DREBs*, *COR15a*, *CAMTA1* and *ZATs*) [60], which indicates that melatonin has a physiological function in responding to low temperature and transcriptional

Horticultural crops require lots of water in their cultivation. Water stress or physiological drought will affect the growth and development of crops and make a significant impact on the yield. Exogenous MT promoted the accumulation of soluble sugar and protein under stress, thereby alleviating the damage of rapeseed seedlings under drought stress [61]. Many orchards in arid/semi-arid areas (especially in mountainous areas) are in a state of long-term water shortage. Although fruit trees can grow, their productions are affected. Melatonin treatment significantly improved the drought resistance of wheat seedlings, including reduced membrane damage, more complete chloroplast grana lamella, higher photosynthetic rate, maximum efficiency of photosystem II, and higher cellular turgor and water-holding capacity [62]. Zuo et al. [45] cloned *AcSNMT*, a key gene for drought-induced melatonin synthesis, from drought-tolerant apple rootstock (*Malus zumi* Mats) and heterologous expressed it in *Arabidopsis thaliana*. The subcellular localization analysis showed that *AcSNMT* gene was mainly located in the nucleus and cell membrane, and the synthesized melatonin could effectively remove drought-induced ROS and improve the growth potential and survival rate of transgenic plants under drought stress. Gong et al. [63] also verified the regulation of melatonin on ROS metabolism under drought stress and the drought-resistant mechanism in tomato. In addition, Meng et al. [64] also found that melatonin could protect the chloroplast membrane structure and grana lamella structure of grape under drought stress, increase the thickness of leaves and the tightness of palisade tissue, and regulate stomatal closure to

melatonin could significantly relieve

### *Review of Melatonin in Horticultural Crops DOI: http://dx.doi.org/10.5772/intechopen.90935*

*Melatonin - The Hormone of Darkness and Its Therapeutic Potential and Perspectives*

to treat the cotyledon and germ of the short-day plant *Chenopodium rubrum* (*Chenopodium rubrum* L.), which could significantly inhibit the flowering rate under the short-term conditions, and only applied melatonin before the end of the light treatment or the first half of the dark treatment was effective. This suggests that melatonin levels are influenced by circadian rhythms and regulate the photoperiod of plants, which in turn affects some early steps in flowering. In addition, the application of shading technology in the cultivation of capsicum can significantly reduce the melatonin level of capsicum fruits, which indicates that solar radiation can cause the increase of melatonin concentrations [50]. Studies in wheat also show that the concentrations of melatonin in leaves under light condition are significantly higher than they are under dark condition. At the same time, the light treatment significantly increases the concentrations of melatonin synthesis precursors (tryptamine, 5-hydroxytryptamine and *N*-acetyl-5-hydroxytryptamine) in leaves [36]. This suggests that light promotes plants to convert l-tryptophan into melatonin. Light quality also affects the endogenous melatonin concentrations of licorice (*Glycyrrhiza uralensis* L.), following the rule that red light > blue light > white light [51]. The induced effect of light on melatonin is consistent with the hypothesis that chloroplast is the key organelle for melatonin synthesis [52], which is based on the following: pepper peel and wheat leaves are rich in chloroplasts, while light will inevitably produce excess light energy while promoting plant photosynthesis, resulting in the combination of free electrons and O2 to form ROS. In addition, single light masses, such as red light and blue light, may specifically increase the conversion efficiency of light energy to electric energy at photosystem I (PSI) or photosystem II (PSII), thus increasing the imbalance of electron transfer between PSII and PSI, creating conditions for free electrons to combine with O2 to generate ROS. Since melatonin is an important antioxidant in organisms, it is of certain biological significance to induce chloroplasts to produce a large amount of melatonin to remove excess ROS so as to reduce the oxidative pressure faced by cells. Unfortunately, at present, there is no direct evidence for the systematic studies about the light quality and light intensity that affect melatonin concentrations for alleviating the strong light damage in plants. Therefore, the mechanism of melatonin metabolism on light in plants and melatonin response to light needs further exploration and verification. Temperature is a key environmental factor that affects horticultural crops, especially vegetable crops that are planted off season. Inappropriate temperature leads to substantial loss of yield and poor quality of horticultural crops. For this reason, horticultural researchers have been working on temperature adaptation mechanisms and efficient and safe plant growth regulators to cope with sudden temperature changes. Shi et al. [53] confirmed that melatonin was induced by high temperature

expression of antioxidant genes and DNA-repairing genes.

Kolar et al. [49] used 100 mmol L<sup>−</sup><sup>1</sup>

in *Arabidopsis*, and the application of 20 μmol L<sup>−</sup><sup>1</sup>

the expression of heat shock factor (*HSFA1s* and *HSFA2s*) and heat shock protein (*HSP90* and *HSP101*), which led to increased survival rate of *Arabidopsis* under high

enhance the high temperature stress tolerance of cherry radish (*Raphanus sativus* L. var. *radculus pers*), which increased the biomass to 12.9% and the soluble protein to 18.7%. At the same time, the activity of antioxidant enzyme, especially for POD, was enhanced and the lipid peroxidation was reduced under adverse conditions [54]. For melatonin-regulated plant cold tolerance, Lei et al. [55] found that very

temperature stress. Jia et al. found that application of 29.0 mg L<sup>−</sup><sup>1</sup>

low concentration of exogenous melatonin (21.5 nmol L<sup>−</sup><sup>1</sup>

between endogenous melatonin of plant and light environmental factors. Melatonin improved the growth performance of yeast under UV radiation and reduced the mortality [48]. The mechanism proposed suggests that melatonin increased the

and 500 mmol L<sup>−</sup><sup>1</sup>

of melatonin solutions

melatonin significantly improved

) could significantly

melatonin can

**116**

improve the cell viability of carrots, under low-temperature stress, enhance the stability of cell membrane structure, and inhibit orderly degradation of DNA caused by programmed cell death. When tomato was treated with low temperature, the melatonin concentrations and the expression of synthetic control genes were significantly increased [56]. With the extension of stress time, the melatonin concentrations showed an increasing trend, indicating that melatonin played an important role in plant resistance to low-temperature stress. Appropriate osmotic stress can improve the germination rate of cucumber seeds under low-temperature stress [57]. At the same time, osmotic stress intensity was positively correlated with endogenous melatonin concentrations in cucumber seed germination. Further studies showed that the endogenous melatonin, induced by osmotic stress, was beneficial to remove peroxidation damage and stabilize membrane structure under low-temperature stress. However, the excessive endogenous melatonin induced by hyperosmotic stress destroyed the oxidation equilibrium state of protein, but reduced the resistance of cucumber shoots to low temperature. In tomato, the maximum quantum yield (Fv/Fm) of PSII was significantly reduced during chilling, which was effectively alleviated by exogenous melatonin. This is because melatonin induces the expression of violaxanthin de-epoxidase gene, enhances the enzyme activity of violaxanthin de-epoxidase and the enhancement of de-epoxidation state of xanthophyll pigments, promotes the non-photochemical quenching and alleviates the photoinhibition during chilling [58]. In addition, studies on the improvement of plant cold tolerance by exogenous melatonin have also been reported in Kiwifruit (*Actinidia Chinensis*) and *Arabidopsis thaliana*. Wang et al. [59] found that the treatment of Kiwifruit seedlings with 100 μmol L<sup>−</sup><sup>1</sup> melatonin could significantly relieve the growth inhibition and chlorophyll degradation, improve antioxidant enzyme activity and eliminate ROS under low-temperature stress. Exogenous melatonin can induce the expression of a series of low-temperature responsive transcription factors (*CBFs*, *DREBs*, *COR15a*, *CAMTA1* and *ZATs*) [60], which indicates that melatonin has a physiological function in responding to low temperature and transcriptional activation of related metabolic processes.

Horticultural crops require lots of water in their cultivation. Water stress or physiological drought will affect the growth and development of crops and make a significant impact on the yield. Exogenous MT promoted the accumulation of soluble sugar and protein under stress, thereby alleviating the damage of rapeseed seedlings under drought stress [61]. Many orchards in arid/semi-arid areas (especially in mountainous areas) are in a state of long-term water shortage. Although fruit trees can grow, their productions are affected. Melatonin treatment significantly improved the drought resistance of wheat seedlings, including reduced membrane damage, more complete chloroplast grana lamella, higher photosynthetic rate, maximum efficiency of photosystem II, and higher cellular turgor and water-holding capacity [62]. Zuo et al. [45] cloned *AcSNMT*, a key gene for drought-induced melatonin synthesis, from drought-tolerant apple rootstock (*Malus zumi* Mats) and heterologous expressed it in *Arabidopsis thaliana*. The subcellular localization analysis showed that *AcSNMT* gene was mainly located in the nucleus and cell membrane, and the synthesized melatonin could effectively remove drought-induced ROS and improve the growth potential and survival rate of transgenic plants under drought stress. Gong et al. [63] also verified the regulation of melatonin on ROS metabolism under drought stress and the drought-resistant mechanism in tomato. In addition, Meng et al. [64] also found that melatonin could protect the chloroplast membrane structure and grana lamella structure of grape under drought stress, increase the thickness of leaves and the tightness of palisade tissue, and regulate stomatal closure to reduce water loss.

There are about 831 million hectares of saline-alkali land in the world, including 397 million hectares of neutral saline soil and 434 million hectares of alkaline saline soil, accounting for 10% of the world's arable land [65]. Salinity stress can lead to the reduction of water availability and nutrient imbalance, seriously restricting agricultural production. In addition, facility horticulture is also faced with the problem of soil secondary salinization due to the closed environment, lack of rain water leaching in the soil, excessive fertilization and other factors. Therefore, how to improve the salinity tolerance of horticultural crops becomes a key link in the development of characteristic horticulture industry in saline and alkaline areas. Ke et al. [66] demonstrated that melatonin pretreatment regulated polyamine metabolism in wheat and reduced the damage of salt stress. They also believed that melatonin induces enzyme activity that stimulates ROS to clear antioxidant defenses in response to salinity. In addition, exogenous melatonin can also prevent the accumulation of triacylglycerol and promote fatty acid β-oxidation and energy conversion under salt stress conditions. So it is helpful for improving PM H<sup>+</sup> -ATPase activity, activating gene expression of Na<sup>+</sup> -K<sup>+</sup> reverse transporter, and maintaining K<sup>+</sup> /Na<sup>+</sup> homeostasis of sweet potato (*Solanum tuberosum* L.) [67]. Application of 0.5 μmol L<sup>−</sup><sup>1</sup> exogenous melatonin can alleviate symptoms of green leaf loss, reduce the accumulation of Na<sup>+</sup> , improve phenols, ascorbic acid and glutathione etc., and promote the related activity of antioxidant enzymes, to alleviate the oxidative damage in salt-stressed tomato [68]. Similarly, root treatment with melatonin alleviated the damage of photosynthetic capacity and oxidative stress and improved antioxidant enzyme activity in salt-stressed watermelon [69]. Zhang et al. [70] also demonstrated in cucumber that exogenous melatonin can upregulate the gibberellin (GA) signaling pathway through the upregulated GA biosynthesis genes (*GA20ox* and *GA3ox*) and inhibit abscisic acid (ABA) signaling pathway through the upregulation of ABA catabolism genes (*CYP707A1* and *CYP707A2*) and downregulation of an ABA biosynthesis gene (*NECD2*), thus promoting the germination rate of cucumber seeds under salt stress. The accumulation of endogenous melatonin in sunflower can be induced by salt stress for 48 h, and the distribution of melatonin accumulation induced by salt stress in the root vascular bundles and cortex is also regionalized. For example, the concentrations of melatonin in sunflower cotyledons and oil-rich tissues are significantly higher than those in other tissues [71]. Moreover, exogenous melatonin can promote the elongation of hypocotyl and root growth of sunflower seedlings under salt stress, and alleviate the inhibition of salt stress on root development of sunflower seedlings to some extent. In addition, our study showed that exogenous addition of 0.5 μmol L<sup>−</sup><sup>1</sup> melatonin could significantly improve the biomass of tomato seedlings, protect photosynthetic organs, promote the activity of antioxidant system, and balance the Na<sup>+</sup> -K<sup>+</sup> metabolism of tomato plants under the stress of alkaline salt (NaHCO3) [68]. It has been confirmed that melatonin regulates the physiological processes, such as Na<sup>+</sup> detoxification, dehydration resistance, high pH buffering and ROS scavenging through DREB1 a and IAA3 pathways [72]. These basic studies on exogenous melatonin improving the saline-alkali tolerance of horticultural crops provide theoretical support for the innovation of horticultural crop cultivation technology in saline-alkali areas.

#### **5.2 The regulation of melatonin resistance to biological stress**

Plants are often attacked by fungi, bacteria, viruses and pests during their growth and development. Under biological stress, plants produce endogenous hormone-regulated responses, such as salicylic acid (SA), jasmonic acid (JA), ethylene (Eth) and abscisic acid (ABA). Studies in recent years have shown that

**119**

defense enzymes.

**growth and stress resistance**

*Review of Melatonin in Horticultural Crops DOI: http://dx.doi.org/10.5772/intechopen.90935*

response chain in plants.

et al. [77] found that spraying of 50 mol L<sup>−</sup><sup>1</sup>

melatonin can interact with the signaling pathways of biological stress regulated by

The bacterial disease model strains *Pseudomonas syringe pv.* Tomato DC3000 (DC3000) can induce the accumulation of endogenous melatonin, which may play an important role in plant disease resistance response [73, 74]. When 10 μmol L<sup>−</sup><sup>1</sup> solution of melatonin was sprayed on *Arabidopsis* or tobacco leaves, gene expression of disease-course related proteins and defense-related genes activated by SA and Eth signals were induced to reduce the incidence of DC3000. However, exogenous melatonin has no disease-resistant induction effect on *Arabidopsis thaliana* SA and Eth signal mutants of *npr1*, *ein2* and *mpk6*, which indicates that melatonin has close communication with SA and Eth signals. In addition, exogenous melatonin can promote the translocation of the inhibitor *NPR1* of pathophysiology-related protein (*PR1*) from the cytoplasm to the nucleus [75]. All these evidences indicate that melatonin acts as the upstream signal of SA to activate the disease-resistant

Application of exogenous melatonin to cotton could induce the expression of phenylpropanoid mevalonate (MVA), gossypol and other pathway-related genes, thus leading to increased lignin and gossypol concentrations in metabolites of this pathway and thus enhancing the resistance of cotton to Verticillium wilt [76]. Liu

can induce and enhance the activity of disease-resistant proteases CHI, GLU, PAL and PPO, and significantly enhance disease resistance to *Botrytis cinerea* by regulating ROS accumulation and JA defense signaling pathways. Shi et al. [78] showed that DC3000 could promote the accumulation of melatonin and NO in *Arabidopsis thaliana*, while exogenous melatonin could induce the production of NO. Moreover, both exogenous melatonin and NO treatment of *Arabidopsis thaliana* can enhance its disease resistance and activate the expression of defense genes related to SA signal. This indicates that NO, as the downstream signal of melatonin, acts as the second messenger, communicates the signal network between melatonin and SA,

In addition to the disease resistance mechanism mediated by SA signal, there is mechanical disease resistance that consists of epidermal tissue, cell wall, phenylalanine pathway-mediated disease resistance mechanism, etc. in plants. Zhao et al. [79] indicated that exogenous melatonin could downregulate the expression of invertase inhibitors of *Arabidopsis thaliana*, activate cell wall invertase activity, promote sucrose metabolism, and accumulate hexose. However, melatonin-induced cell wall invertase activity can increase the strength of cell wall through synthesis of cellulose, xylose and galactose and promote the deposition of callose in cell wall. Melatonin can improve the disease resistance of plants by improving cell wall composition and structure. In the same year, the group of Yin et al. [80] found that exogenous melatonin can reduce marssonina apple blotch and its main regulation mechanism for melatonin can maintain intracellular redox state after infection, improving the phenylalanine ammonia enzyme, polyphenol oxidase, chitinase and glucanase course related to the activity of

and activates the disease-resistant regulatory network of plants.

**6. Melatonin regulates the signal transduction network of plant** 

As mentioned above, melatonin is widely involved in the regulation of plant growth, development and resistance. Based on this, we sorted out the signal pathways involved in melatonin and summarized the schematic diagram of the signal

of melatonin solution on tomato fruits

SA and JA, and negatively regulate plant resistance to biological stress.

#### *Review of Melatonin in Horticultural Crops DOI: http://dx.doi.org/10.5772/intechopen.90935*

*Melatonin - The Hormone of Darkness and Its Therapeutic Potential and Perspectives*


thione etc., and promote the related activity of antioxidant enzymes, to alleviate the oxidative damage in salt-stressed tomato [68]. Similarly, root treatment with melatonin alleviated the damage of photosynthetic capacity and oxidative stress and improved antioxidant enzyme activity in salt-stressed watermelon [69]. Zhang et al. [70] also demonstrated in cucumber that exogenous melatonin can upregulate the gibberellin (GA) signaling pathway through the upregulated GA biosynthesis genes (*GA20ox* and *GA3ox*) and inhibit abscisic acid (ABA) signaling pathway through the upregulation of ABA catabolism genes (*CYP707A1* and *CYP707A2*) and downregulation of an ABA biosynthesis gene (*NECD2*), thus promoting the germination rate of cucumber seeds under salt stress. The accumulation of endogenous melatonin in sunflower can be induced by salt stress for 48 h, and the distribution of melatonin accumulation induced by salt stress in the root vascular bundles and cortex is also regionalized. For example, the concentrations of melatonin in sunflower cotyledons and oil-rich tissues are significantly higher than those in other tissues [71]. Moreover, exogenous melatonin can promote the elongation of hypocotyl and root growth of sunflower seedlings under salt stress, and alleviate the inhibition of salt stress on root development of sunflower seedlings to some extent. In addition, our study showed that exogenous addition of 0.5 μmol L<sup>−</sup><sup>1</sup> melatonin could significantly improve the biomass of tomato seedlings, protect photosynthetic organs, promote the activity of antioxidant system, and balance

metabolism of tomato plants under the stress of alkaline salt (NaHCO3)

detoxification, dehydration resistance, high pH buffering and ROS

[68]. It has been confirmed that melatonin regulates the physiological processes,

Plants are often attacked by fungi, bacteria, viruses and pests during their growth and development. Under biological stress, plants produce endogenous hormone-regulated responses, such as salicylic acid (SA), jasmonic acid (JA), ethylene (Eth) and abscisic acid (ABA). Studies in recent years have shown that

scavenging through DREB1 a and IAA3 pathways [72]. These basic studies on exogenous melatonin improving the saline-alkali tolerance of horticultural crops provide theoretical support for the innovation of horticultural crop cultivation

**5.2 The regulation of melatonin resistance to biological stress**

/Na<sup>+</sup>

leaf loss, reduce the accumulation of Na<sup>+</sup>

There are about 831 million hectares of saline-alkali land in the world, including 397 million hectares of neutral saline soil and 434 million hectares of alkaline saline soil, accounting for 10% of the world's arable land [65]. Salinity stress can lead to the reduction of water availability and nutrient imbalance, seriously restricting agricultural production. In addition, facility horticulture is also faced with the problem of soil secondary salinization due to the closed environment, lack of rain water leaching in the soil, excessive fertilization and other factors. Therefore, how to improve the salinity tolerance of horticultural crops becomes a key link in the development of characteristic horticulture industry in saline and alkaline areas. Ke et al. [66] demonstrated that melatonin pretreatment regulated polyamine metabolism in wheat and reduced the damage of salt stress. They also believed that melatonin induces enzyme activity that stimulates ROS to clear antioxidant defenses in response to salinity. In addition, exogenous melatonin can also prevent the accumulation of triacylglycerol and promote fatty acid β-oxidation and energy conversion under salt stress conditions. So it is helpful for improving


, improve phenols, ascorbic acid and gluta-

homeostasis of sweet potato (*Solanum tuberosum* L.) [67].

exogenous melatonin can alleviate symptoms of green

reverse transporter,

**118**

the Na<sup>+</sup>

such as Na<sup>+</sup>


technology in saline-alkali areas.

PM H<sup>+</sup>

and maintaining K<sup>+</sup>

Application of 0.5 μmol L<sup>−</sup><sup>1</sup>

melatonin can interact with the signaling pathways of biological stress regulated by SA and JA, and negatively regulate plant resistance to biological stress.

The bacterial disease model strains *Pseudomonas syringe pv.* Tomato DC3000 (DC3000) can induce the accumulation of endogenous melatonin, which may play an important role in plant disease resistance response [73, 74]. When 10 μmol L<sup>−</sup><sup>1</sup> solution of melatonin was sprayed on *Arabidopsis* or tobacco leaves, gene expression of disease-course related proteins and defense-related genes activated by SA and Eth signals were induced to reduce the incidence of DC3000. However, exogenous melatonin has no disease-resistant induction effect on *Arabidopsis thaliana* SA and Eth signal mutants of *npr1*, *ein2* and *mpk6*, which indicates that melatonin has close communication with SA and Eth signals. In addition, exogenous melatonin can promote the translocation of the inhibitor *NPR1* of pathophysiology-related protein (*PR1*) from the cytoplasm to the nucleus [75]. All these evidences indicate that melatonin acts as the upstream signal of SA to activate the disease-resistant response chain in plants.

Application of exogenous melatonin to cotton could induce the expression of phenylpropanoid mevalonate (MVA), gossypol and other pathway-related genes, thus leading to increased lignin and gossypol concentrations in metabolites of this pathway and thus enhancing the resistance of cotton to Verticillium wilt [76]. Liu et al. [77] found that spraying of 50 mol L<sup>−</sup><sup>1</sup> of melatonin solution on tomato fruits can induce and enhance the activity of disease-resistant proteases CHI, GLU, PAL and PPO, and significantly enhance disease resistance to *Botrytis cinerea* by regulating ROS accumulation and JA defense signaling pathways. Shi et al. [78] showed that DC3000 could promote the accumulation of melatonin and NO in *Arabidopsis thaliana*, while exogenous melatonin could induce the production of NO. Moreover, both exogenous melatonin and NO treatment of *Arabidopsis thaliana* can enhance its disease resistance and activate the expression of defense genes related to SA signal. This indicates that NO, as the downstream signal of melatonin, acts as the second messenger, communicates the signal network between melatonin and SA, and activates the disease-resistant regulatory network of plants.

In addition to the disease resistance mechanism mediated by SA signal, there is mechanical disease resistance that consists of epidermal tissue, cell wall, phenylalanine pathway-mediated disease resistance mechanism, etc. in plants. Zhao et al. [79] indicated that exogenous melatonin could downregulate the expression of invertase inhibitors of *Arabidopsis thaliana*, activate cell wall invertase activity, promote sucrose metabolism, and accumulate hexose. However, melatonin-induced cell wall invertase activity can increase the strength of cell wall through synthesis of cellulose, xylose and galactose and promote the deposition of callose in cell wall. Melatonin can improve the disease resistance of plants by improving cell wall composition and structure. In the same year, the group of Yin et al. [80] found that exogenous melatonin can reduce marssonina apple blotch and its main regulation mechanism for melatonin can maintain intracellular redox state after infection, improving the phenylalanine ammonia enzyme, polyphenol oxidase, chitinase and glucanase course related to the activity of defense enzymes.
