**4.2 Melatonin regulates the ripening, aging and preservation of horticultural crops**

Ripening, aging and preservation are contradictory processes in the production of horticultural products. Interestingly, more and more studies have shown that melatonin can be used to flexibly regulate the ripening, aging and storage of horticultural crops under different concentrations and conditions. Sun et al. [36] found that pretreatment of tomato with 50 μmol L<sup>−</sup><sup>1</sup> and 100 μmol L<sup>−</sup><sup>1</sup> melatonin could promote tomato ripening. Its regulatory mechanism can be summarized as follows: melatonin activates the expression of *ACS4* gene and enhances the synthesis of ethylene. Ethylene signal promoted the synthesis of lycopene by expression of *PSY1* gene through the signal transduction pathways such as *NR*, *ETR4*, *EILs* and *ERF2*, which made the tomato turn color, regulated the expression of *TBG4*, *PG2A*, *Exp1*, *XTH5* and *PE1* genes to degrade the cell wall, softened the fruit, regulated the expression of *PIPs* gene to control fruit water loss, and regulated the expression of *ADH2* and *AAT* genes to promote sugar conversion. Thus, phenotypic characteristics promoting tomato ripening were formed. Similar results have been found in grapes [37]. Lei et al. [38] showed that the synthesis of melatonin

in the ripening process of apple fruits was mainly catalyzed by l-tryptophan decarboxylase (TrpDC), tryptophan hydroxylase (T5H), 5-hydroxytryptophan *N*-acetyltransferase (SNAcT) and *N*-acetyl-5-hydroxytryptophan methyltransferase (AcSNMT). Moreover, the research group proposed that during the ripening process of apple fruits, melatonin and malondialdehyde (MDA) concentrations were always negatively correlated. So they speculated that the main role of melatonin in the ripening process of apple fruits was ROS scavenging. When different concentrations of melatonin were used to treat fruits of different species and ripeness, the results were often opposite. Liu et al. [39] found that treatment with melatonin after harvest effectively delayed the senescence of strawberry fruits and reduced the rate of decay, severity of decay and weight loss of strawberry fruits. The main mechanism is to reduce the total soluble solid, H2O2 and MDA concentrations of the fruit, promote the accumulation of total phenolic substances and flavonoids, improve the antioxidant capacity of the fruit, delay its color development, and maintain the hardness and titratable acidity.

Shi et al. [40] showed that the concentrations of endogenous melatonin *Arabidopsis thaliana* increased continuously during the process of seedling, maturation and aging. Exogenous spray of melatonin could significantly inhibit the aging process of *Arabidopsis thaliana*. In view of the close relationship between melatonin and IAA, they found that melatonin reduced the expression of *AXR3* and *IAA17* genes, antagonized by IAA signal, which induced the expression of *SEN4* and *SAG12* genes and led to aging. Thus, high concentration of melatonin inhibits the aging process of plants by reducing the expression of *IAA17* and related genes. Arnao and Hernández-Ruiz [41] found that melatonin treatment (1 mmol L<sup>−</sup><sup>1</sup> ) significantly inhibited the aging and chlorophyll degradation of barley leaves. Through hormone simulation experiment, the research group believed that the main mechanism of melatonin in inhibiting leaf senescence was synergistic kinetin (KT) and antagonistic abscisic acid (ABA). Wang et al. [42] showed that long-term irrigation of melatonin significantly inhibited the aging of apple leaves. Through proteomics analysis, it was found that melatonin inhibited the activities of most hydrolytic enzymes in plasmids of apple leaves, which were involved in the hydrolysis, redox and stress response, transcriptional regulation, photosynthesis and other senescence-related processes of macromolecular proteins.

### **4.3 Effects of melatonin on root development of horticultural crops**

Endogenous melatonin has similar physiological functions with IAA, which promotes root development, elongation and lateral and adventitious root development. Chen et al. [43] found that 0.1 μmol L<sup>−</sup><sup>1</sup> melatonin promoted the elongation of mustard (*Brassica juncea* L.) root, while 100 μmol L<sup>−</sup><sup>1</sup> melatonin could inhibit its elongation, which was consistent with the physiological concentrations of IAA. The results of IAA concentrations' determination showed that melatonin could induce the accumulation of IAA in mustard roots, which indicated that there was an interaction between melatonin and IAA. In *Arabidopsis thaliana*, melatonin increases the appearance of adventurous roots twofold and the appearance of lateral roots by up to threefold, but has no effect on root hair density [44]. In addition, a large number of lateral roots were induced in three *Arabidopsis thaliana* transgenic lines that produced excessive melatonin compared with the WT lines [45]. Zhang et al. [46] showed that exogenous application of 500 μmol L<sup>−</sup><sup>1</sup> melatonin significantly promoted the occurrence of lateral roots of cucumber. Moreover, transcriptome analysis showed that melatonin caused the upregulation of 121 genes and downregulation of 196 genes. Through GO and pathway enrichment analysis, Zhang et al. believed that melatonin could promote lateral root development of cucumber by activating

**115**

to adversity.

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

tissue culture of horticultural crops.

**5. Role of melatonin in stress response of horticultural crops**

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

often changes greatly under the stimulation of stress factors, including light intensity, light quality, temperature, water and oxygen, as well as the stimulation of salinity, ultraviolet (UV-B), paraquat, diseases and insect pests, etc. At the same time, exogenous addition of melatonin or enhancement of plant endogenous melatonin synthesis, through gene editing technology, can improve the plant's resistance

Large numbers of studies have shown that the endogenous melatonin of plants

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

tion of 50 μmol L<sup>−</sup><sup>1</sup>

root-related hormone and transcription factor pathways and reducing oxidative damage caused by respiration during root genesis. Exogenous melatonin could promote the accumulation of nitric oxide (NO) in the periderm of young stem and the tip of new adventitious root, indicating that melatonin-induced NO may be involved in the regulation of adventitious root regeneration. In addition, exogenous applica-

adventitious roots [47]. NO acts as the common downstream signal of melatonin and IAA in the process of melatonin-induced root generation by promoting the synthe-

Although melatonin is closely related to IAA in the process of affecting plant root development, melatonin-induced root morphogenesis is independent of auxin signaling [44]. Melatonin promotes the elongation of the principal root and lateral root in *Arabidopsis thaliana*. However, melatonin and IAA have no signal crosslinking and independent signal transduction pathways during root development via auxin signal response label DR5: *uidA* approach. It can be seen that the interaction between melatonin and IAA, two indolamines with similar chemical structure, is extremely complex. Therefore, the following conclusions were made: melatonin is similar to IAA in the process of affecting plant root construction depending on obvious concentration effect. The melatonin-induced root development is relatively independent of IAA in the signal transduction pathway. However, melatonin can promote root development through IAA synthesis, polar transport and hormone perception. In addition, melatonin plays key role in ROS scavenging for the vigorous development of root system, which ensures a good redox balance and the smooth progress of metabolism in cell. The effects of melatonin on root growth make it a broad application prospect in horticulture industry. For example, cucumber and watermelon root usually develop weakly. Thus, pumpkin or gourd is often used as grafting rootstock in these vegetable productions. Although rootstock application enhances root activity, it tends to decrease the sensory quality of the fruit. If proper amount of melatonin is applied to cucumber and watermelon, it may be a good substitute technology for grafting. The application of melatonin to promote adventitious root production is more extensive, such as the grafting technology of "double broken root" for melon and some other fruit vegetables, the induction of root buds for asexual propagation materials, such as potato (*Solanum tuberosum* L.), the cuttage propagation for fruit trees, vegetables and flowers, and the root induction for

sis, polar transport and hormone signal perception of IAA.

melatonin significantly promotes the regeneration of tomato

### *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*

maintain the hardness and titratable acidity.

senescence-related processes of macromolecular proteins.

of mustard (*Brassica juncea* L.) root, while 100 μmol L<sup>−</sup><sup>1</sup>

showed that exogenous application of 500 μmol L<sup>−</sup><sup>1</sup>

ment. Chen et al. [43] found that 0.1 μmol L<sup>−</sup><sup>1</sup>

**4.3 Effects of melatonin on root development of horticultural crops**

Endogenous melatonin has similar physiological functions with IAA, which promotes root development, elongation and lateral and adventitious root develop-

elongation, which was consistent with the physiological concentrations of IAA. The results of IAA concentrations' determination showed that melatonin could induce the accumulation of IAA in mustard roots, which indicated that there was an interaction between melatonin and IAA. In *Arabidopsis thaliana*, melatonin increases the appearance of adventurous roots twofold and the appearance of lateral roots by up to threefold, but has no effect on root hair density [44]. In addition, a large number of lateral roots were induced in three *Arabidopsis thaliana* transgenic lines that produced excessive melatonin compared with the WT lines [45]. Zhang et al. [46]

moted the occurrence of lateral roots of cucumber. Moreover, transcriptome analysis showed that melatonin caused the upregulation of 121 genes and downregulation of 196 genes. Through GO and pathway enrichment analysis, Zhang et al. believed that melatonin could promote lateral root development of cucumber by activating

melatonin promoted the elongation

melatonin significantly pro-

melatonin could inhibit its

in the ripening process of apple fruits was mainly catalyzed by l-tryptophan decarboxylase (TrpDC), tryptophan hydroxylase (T5H), 5-hydroxytryptophan *N*-acetyltransferase (SNAcT) and *N*-acetyl-5-hydroxytryptophan methyltransferase (AcSNMT). Moreover, the research group proposed that during the ripening process of apple fruits, melatonin and malondialdehyde (MDA) concentrations were always negatively correlated. So they speculated that the main role of melatonin in the ripening process of apple fruits was ROS scavenging. When different concentrations of melatonin were used to treat fruits of different species and ripeness, the results were often opposite. Liu et al. [39] found that treatment with melatonin after harvest effectively delayed the senescence of strawberry fruits and reduced the rate of decay, severity of decay and weight loss of strawberry fruits. The main mechanism is to reduce the total soluble solid, H2O2 and MDA concentrations of the fruit, promote the accumulation of total phenolic substances and flavonoids, improve the antioxidant capacity of the fruit, delay its color development, and

Shi et al. [40] showed that the concentrations of endogenous melatonin *Arabidopsis thaliana* increased continuously during the process of seedling, maturation and aging. Exogenous spray of melatonin could significantly inhibit the aging process of *Arabidopsis thaliana*. In view of the close relationship between melatonin and IAA, they found that melatonin reduced the expression of *AXR3* and *IAA17* genes, antagonized by IAA signal, which induced the expression of *SEN4* and *SAG12* genes and led to aging. Thus, high concentration of melatonin inhibits the aging process of plants by reducing the expression of *IAA17* and related genes. Arnao and Hernández-Ruiz [41] found that melatonin treatment (1 mmol

) significantly inhibited the aging and chlorophyll degradation of barley leaves. Through hormone simulation experiment, the research group believed that the main mechanism of melatonin in inhibiting leaf senescence was synergistic kinetin (KT) and antagonistic abscisic acid (ABA). Wang et al. [42] showed that long-term irrigation of melatonin significantly inhibited the aging of apple leaves. Through proteomics analysis, it was found that melatonin inhibited the activities of most hydrolytic enzymes in plasmids of apple leaves, which were involved in the hydrolysis, redox and stress response, transcriptional regulation, photosynthesis and other

**114**

L<sup>−</sup><sup>1</sup>

root-related hormone and transcription factor pathways and reducing oxidative damage caused by respiration during root genesis. Exogenous melatonin could promote the accumulation of nitric oxide (NO) in the periderm of young stem and the tip of new adventitious root, indicating that melatonin-induced NO may be involved in the regulation of adventitious root regeneration. In addition, exogenous application of 50 μmol L<sup>−</sup><sup>1</sup> melatonin significantly promotes the regeneration of tomato adventitious roots [47]. NO acts as the common downstream signal of melatonin and IAA in the process of melatonin-induced root generation by promoting the synthesis, polar transport and hormone signal perception of IAA.

Although melatonin is closely related to IAA in the process of affecting plant root development, melatonin-induced root morphogenesis is independent of auxin signaling [44]. Melatonin promotes the elongation of the principal root and lateral root in *Arabidopsis thaliana*. However, melatonin and IAA have no signal crosslinking and independent signal transduction pathways during root development via auxin signal response label DR5: *uidA* approach. It can be seen that the interaction between melatonin and IAA, two indolamines with similar chemical structure, is extremely complex. Therefore, the following conclusions were made: melatonin is similar to IAA in the process of affecting plant root construction depending on obvious concentration effect. The melatonin-induced root development is relatively independent of IAA in the signal transduction pathway. However, melatonin can promote root development through IAA synthesis, polar transport and hormone perception. In addition, melatonin plays key role in ROS scavenging for the vigorous development of root system, which ensures a good redox balance and the smooth progress of metabolism in cell. The effects of melatonin on root growth make it a broad application prospect in horticulture industry. For example, cucumber and watermelon root usually develop weakly. Thus, pumpkin or gourd is often used as grafting rootstock in these vegetable productions. Although rootstock application enhances root activity, it tends to decrease the sensory quality of the fruit. If proper amount of melatonin is applied to cucumber and watermelon, it may be a good substitute technology for grafting. The application of melatonin to promote adventitious root production is more extensive, such as the grafting technology of "double broken root" for melon and some other fruit vegetables, the induction of root buds for asexual propagation materials, such as potato (*Solanum tuberosum* L.), the cuttage propagation for fruit trees, vegetables and flowers, and the root induction for tissue culture of horticultural crops.
