**7. Conclusion and prospect**

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 new horticultural germ plasm rich in melatonin.

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

promoting growth and anti-stress regulation mechanism and the successful cases of exogenous melatonin promoting crop growth and enhancing resistance are expected to make great contributions to the high yield and high-quality production of horticultural crops. Based on the current research status, we make a prediction on the application of melatonin in horticulture production, hoping to attract more and more attention. (1) For cucumbers and others prone to premature aging, horticultural crops can be applied by exogenous melatonin to alleviate the aging process. (2) Leek, asparagus and most fruit trees show aging trend with the extension of cultivation year. The application of exogenous melatonin also plays a role in the rejuvenation of this kind of aging horticultural crops. (3) Taking advantage of the anti-aging and drought resistance properties of melatonin, it can be used for fresh cut flowers, fruits and leafy vegetables. (4) Melatonin can promote root growth and withstand low temperature in plants, alleviating the reduction in root growth and absorption of fertilizer and water that are caused by the excessive low ground temperature, thereby reducing heating costs, improving the utilization rate of CO2, and promoting the efficacy of fertilizer. (5) At present, shade means are often used in the over-summer cultivation of ginger and other light-tolerant and shade-tolerant horticultural crops to reduce leaf senescence and root loss caused by the damage of strong light to leaf photosynthetic organs. Melatonin is introduced into the cultivation technology of this type of crops by utilizing the mechanism of accumulation of melatonin in chloroplast and protection of photosynthetic organs and inhibition of leaf senescence. (6) In cucumber and pumpkin grafting, the technology of double broken root grafting is often adopted. Melatonin can promote the occurrence of adventitious root and prevent the infection of wound. At the same time, melatonin also has the function of promoting graft wound healing, which has great application potential in some fruit trees and flowers with low graft survival rate. (7) Taking advantage of the characteristics of melatonin in delaying senescence, MT is used as a flower-/fruit-preserving agent for vegetable crops in winter and summer to improve pollen vitality, increase fruit setting rate and reduce malformation fruit rate. (8) As a plant growth regulator, melatonin can be used in plant growth promotion and disease prevention to reduce the use of pesticides. Moreover, as a new type of growth regulation substance, it has the characteristics of high efficiency and environmental protection. Its development and utilization can greatly promote the healthy growth of horticultural crops, improve the fertilizer utilization ratio and reduce the occurrence of plant diseases and insect pests and pesticide dosage. It can play an important role in the process of using fewer chemical fertilizers and pesticides in China, and has a great development potential in promoting the healthy development of horticulture industry.
