Melatonin (N-acetyl-5-methoxytryptamine) is an indole molecule widely found in animals and plants. Melatonin is well-known to improve plant resistance to various biotic and abiotic stresses due to its potent free radical scavenging ability while being able to modulate plant signaling and response pathways through mostly unknown mechanisms. In recent years, more and more studies have shown that melatonin plays a crucial role in improving crop quality and yield by participating in the regulation of various aspects of plant growth and development. This article mainly reviews the effects of melatonin on plant vegetative growth and reproductive development, and systematically summarizes the molecular regulatory network of melatonin. At the same time, the effective concentrations of exogenously applied melatonin in different crops or different growth stages of the same crop are analyzed. In addition, this article compares endogenous phytomelatonin concentrations in various crops and different organs, and analyzes the potential function of phytomelatonin in plant circadian rhythms. The prospects of different approaches in regulating crop yield and quality through exogenous application of appropriate concentrations of melatonin, endogenous modification of phytomelatonin metabolism-related genes, and the use of nanomaterials and other technologies to improve melatonin utilization efficiency are also discussed.

Melatonin functions in multiple aspects of plant growth, development, and stress response. Nonetheless, the mechanism of melatonin in plant carbon metabolism remains largely unknown. In this study, we investigated the influence of melatonin on the degradation of starch in tomato leaves. Results showed that exogenous melatonin attenuated carbon starvation‐induced chlorophyll degradation and leaf senescence. In addition, melatonin delayed leaf starch degradation and inhibited the transcription of starch‐degrading enzymes after sunset. Interestingly, melatonin‐alleviated symptoms of leaf senescence and starch degradation were compromised when the first key gene for starch degradation, α‐glucan water dikinase (GWD), was overexpressed. Furthermore, exogenous melatonin significantly upregulated the transcript levels of several microRNAs, including miR171b. Crucially, the GWD gene was identified as a target of miR171b, and the overexpression of miR171b ameliorated the carbon starvation‐induced degradation of chlorophyll and starch, and inhibited the expression of the GWD gene. Taken together, these results demonstrate that melatonin promotes plant tolerance against carbon starvation by upregulating the expression of miR171b, which can directly inhibit GWD expression in tomato leaves.

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Increasing carbon dioxide (CO2) promotes photosynthesis and mitigates heat stress‐induced deleterious effects on plants, but the regulatory mechanisms remain largely unknown. Here, we found that tomato (Solanum lycopersicum L.) plants treated with high atmospheric CO2 concentrations (600, 800, and 1000 µmol mol−1) accumulated increased levels of melatonin (N‐acetyl‐5‐methoxy tryptamine) in their leaves and this response is conserved across many plant species, including Arabidopsis, rice, wheat, mustard, cucumber, watermelon, melon, and hot pepper. Elevated CO2 (eCO2; 800 µmol mol−1) caused a 6.8‐fold increase in leaf melatonin content, and eCO2‐induced melatonin biosynthesis preferentially occurred through chloroplast biosynthetic pathways in tomato plants. Crucially, manipulation of endogenous melatonin levels by genetic means affected the eCO2‐induced accumulation of sugar and starch in tomato leaves. Furthermore, net photosynthetic rate, maximum photochemical efficiency of photosystem II, and transcript levels of chloroplast‐ and nuclear‐encoded photosynthetic genes, such as rbcL, rbcS, rbcA, psaD, petB, and atpA, significantly increased in COMT1 overexpressing (COMT1‐OE) tomato plants, but not in melatonin‐deficient comt1 mutants at eCO2 conditions. While eCO2 enhanced plant tolerance to heat stress (42°C) in wild‐type and COMT1‐OE, melatonin deficiency compromised eCO2‐induced thermotolerance in comt1 plants. The expression of heat shock proteins genes increased in COMT1‐OE but not in comt1 plants in response to eCO2 under heat stress. Further analysis revealed that eCO2‐induced thermotolerance was closely linked to the melatonin‐dependent regulation of reactive oxygen species, redox homeostasis, cellular protein protection, and phytohormone metabolism. This study unveiled a crucial mechanism of elevated CO2‐induced thermotolerance in which melatonin acts as an essential endogenous signaling molecule in tomato plants.