**6. Melatonin regulates the signal transduction network of plant growth and stress resistance**

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

transduction network regulated by melatonin (**Figure 2**). The main function of melatonin is to promote the biosynthesis of IAA and cooperate with IAA to promote the elongation and expansion of cells, which is manifested in the induction of root growth, lateral root occurrence, adventitious root occurrence and fruit expansion. Although both of them are indoleamine compounds, melatonin and IAA do not share a set of signal transduction networks [44]. Interestingly, melatonin inhibits the expression of auxin antagonistic transcription factor *IAA17*, which blocks auxin signaling [40]. This is equivalent to melatonin indirectly amplifying the IAA signal. In this regard, melatonin and IAA are not absolutely isolated, but have a signal dialogue. In addition, our study also showed that melatonin can promote the polar transport and perception of IAA in tomato root development [47]. In addition, melatonin can induce GA and inhibit ABA synthesis during seed germination [70]. The physiological effects of GA are mainly growth stimulation and aging delay, while ABA is an aging induction hormone. Unfortunately, the relationship of melatonin-GA-ABA is currently limited to germination experiments, and no more conclusions have been drawn to support its regulatory mechanism. Researchers have recently revealed the antioxidant function of melatonin and the inhibitory function of *IAA17* in plant anti-aging process [40, 41]. We believe that melatonin can delay the degradation of chlorophyll, maintain a good redox balance in leaf tissue, and synergistically promote the functions of aging antagonistic hormones, which are the main regulatory mechanisms to inhibit the aging of plant leaves. Healthy leaves can carry out photosynthesis better, which explains that melatonin promotes plant growth from the perspective of carbon nutrition and improves the physiological mechanism of yield. Another interesting issue is that melatonin can induce the release of Eth during the ripening process of tomato fruits, which promotes the ripening of fruits and improves the quality of commodities [36]. However, Eth is also a plant senescence-inducing hormone, which seems to contradict the abovementioned idea. We speculate that melatonin may have spatio-temporal specificity of developmental period and tissues due to its multipathway properties in synthesis, which needs systematic analysis and experimental verification from the composition of promoter elements. Unfortunately, this area of research is still blank.

**Figure 2.**

*The signal transduction network of melatonin in plant growth, development and stress tolerance.*

**121**

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

(*CAMTA1*) [60], plasma membrane Na+

new horticultural germ plasm rich in melatonin.

/H+

**7. Conclusion and prospect**

membrane Na+

Environmental stress and hormones can induce plant cells to produce polyamine

/H+

reverse transporters (*NHX*) and also Na+

rate and growth potential of plants under adverse conditions.

etc. [59]. In conclusion, these signal transduction network pathways of melatonin systematically enhance the plant's resistance to adversity and improve the survival

Melatonin is widely found in plant tissues, but its concentration in plants is still very low and has obvious tissue specificity. The pan-frying, deep-frying, stir-frying, steaming and stewing techniques commonly used in Chinese food culture are not conducive to the preservation of melatonin in food. Therefore, the food sources mainly focusing on the acquisition of melatonin nutrition are mainly focused on gardening crops like fruits and vegetables suitable for fresh eating. However, melatonin is a substance with similar hormone activity in plants. The use of gene editing technology to comprehensively increase the melatonin concentrations in plants may destroy the balance of melatonin metabolism in plants and bring some adverse effects on the growth and development of plants. Based on this, we propose two suggestions for improving the concentrations of melatonin in horticultural crops: (1) inducing the expression of edible organs or specific developmental periods in horticultural crops by using tissue-specific or inducible promoters combined with melatonin synthesis of key genes. For example, tomato E8 promoter could be used to specifically express the key gene of melatonin synthesis in the fruit, and the effect of excessive melatonin accumulation on the plant was reduced on the basis of increasing the concentrations of melatonin in tomato fruit. Another example is that the chemical-induced expression system TetR combined with melatonin synthesis of key genes can be used to induce the expression of horticultural products in a time period prior to harvesting, which can improve the melatonin concentrations of the harvested products. (2) The *Arabidopsis thaliana* mutant library was used to dig out the mutant with excessive melatonin accumulation, locate the key gene and use CRISPR-Cas9 to conduct targeted gene editing on horticultural crops to create a

The study of melatonin can promote the improvement of global ecological environment. In the horticultural production system under great pressure in the natural environment and frequent outbreak of biological stress, the study on melatonin

reverse transporters (*SOS*), vacuole

transporters (*HKT*),

(PAs) and NO. However, both PAs and NO are free radicals with strong reactivity. As two active small molecule signaling substances, they are easy to gain and lose electrons. Lei et al. [55] found that melatonin can induce the synthesis of polyamine, under low-temperature stress, and enhance cold resistance in carrot. However, NO is produced by melatonin during the induction of disease resistance [78], alkali resistance [71] and rooting [47], and NO is needed as the downstream signal. In recent years, we have found that NO is the downstream signal of PAs in tomato stress response, and it can activate several plant stress tolerance signaling pathways including antioxidant system [81]. Of course, the mechanical strengthening of melatonin on cell wall tissues [79] and the contact reaction between melatonin and ROS [82] also contributed to the acquisition of plant resilience traits. In addition, melatonin can directly regulate the expression of stress-related functional genes through transcription factor activation, such as senescence-associated genes (SAGs) [41], C-repeat binding factor (*CBFs*) and low-temperature response gene (*COR15a*) [60], heat shock factor (*HSFAs*) and heat shock proteins (*HSPs*) [53], drought response binding factor (*DREBs*) and drought stress resistance genes

#### *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*

*The signal transduction network of melatonin in plant growth, development and stress tolerance.*

transduction network regulated by melatonin (**Figure 2**). The main function of melatonin is to promote the biosynthesis of IAA and cooperate with IAA to promote the elongation and expansion of cells, which is manifested in the induction of root growth, lateral root occurrence, adventitious root occurrence and fruit expansion. Although both of them are indoleamine compounds, melatonin and IAA do not share a set of signal transduction networks [44]. Interestingly, melatonin inhibits the expression of auxin antagonistic transcription factor *IAA17*, which blocks auxin signaling [40]. This is equivalent to melatonin indirectly amplifying the IAA signal. In this regard, melatonin and IAA are not absolutely isolated, but have a signal dialogue. In addition, our study also showed that melatonin can promote the polar transport and perception of IAA in tomato root development [47]. In addition, melatonin can induce GA and inhibit ABA synthesis during seed germination [70]. The physiological effects of GA are mainly growth stimulation and aging delay, while ABA is an aging induction hormone. Unfortunately, the relationship of melatonin-GA-ABA is currently limited to germination experiments, and no more conclusions have been drawn to support its regulatory mechanism. Researchers have recently revealed the antioxidant function of melatonin and the inhibitory function of *IAA17* in plant anti-aging process [40, 41]. We believe that melatonin can delay the degradation of chlorophyll, maintain a good redox balance in leaf tissue, and synergistically promote the functions of aging antagonistic hormones, which are the main regulatory mechanisms to inhibit the aging of plant leaves. Healthy leaves can carry out photosynthesis better, which explains that melatonin promotes plant growth from the perspective of carbon nutrition and improves the physiological mechanism of yield. Another interesting issue is that melatonin can induce the release of Eth during the ripening process of tomato fruits, which promotes the ripening of fruits and improves the quality of commodities [36]. However, Eth is also a plant senescence-inducing hormone, which seems to contradict the abovementioned idea. We speculate that melatonin may have spatio-temporal specificity of developmental period and tissues due to its multipathway properties in synthesis, which needs systematic analysis and experimental verification from the composition of promoter elements. Unfortunately, this area of research is still blank.

**120**

**Figure 2.**

Environmental stress and hormones can induce plant cells to produce polyamine (PAs) and NO. However, both PAs and NO are free radicals with strong reactivity. As two active small molecule signaling substances, they are easy to gain and lose electrons. Lei et al. [55] found that melatonin can induce the synthesis of polyamine, under low-temperature stress, and enhance cold resistance in carrot. However, NO is produced by melatonin during the induction of disease resistance [78], alkali resistance [71] and rooting [47], and NO is needed as the downstream signal. In recent years, we have found that NO is the downstream signal of PAs in tomato stress response, and it can activate several plant stress tolerance signaling pathways including antioxidant system [81]. Of course, the mechanical strengthening of melatonin on cell wall tissues [79] and the contact reaction between melatonin and ROS [82] also contributed to the acquisition of plant resilience traits. In addition, melatonin can directly regulate the expression of stress-related functional genes through transcription factor activation, such as senescence-associated genes (SAGs) [41], C-repeat binding factor (*CBFs*) and low-temperature response gene (*COR15a*) [60], heat shock factor (*HSFAs*) and heat shock proteins (*HSPs*) [53], drought response binding factor (*DREBs*) and drought stress resistance genes (*CAMTA1*) [60], plasma membrane Na+ /H+ reverse transporters (*SOS*), vacuole membrane Na+ /H+ reverse transporters (*NHX*) and also Na+ transporters (*HKT*), etc. [59]. In conclusion, these signal transduction network pathways of melatonin systematically enhance the plant's resistance to adversity and improve the survival rate and growth potential of plants under adverse conditions.
