Initially Study On The Effect Of Melatonin On Fruit Post-harvest In Tomato(Solarium Lycopersicum L.)

Melatonin, or N-acetyl-5-methoxytryptamine, is a hormone found in animals, plants, and microbes. Melatonin concentration varies among plant organs depending on the given physiological and environmental conditions. Besides its function as synchronizer of the biological clock, melatonin is also a powerful free-radical scavenger and wide-spectrum antioxidant. Melatonin plays regulatory roles in plant metabolism, acts as a growth-regulatory signal similar to auxin, delays flower induction, slows root formation, and promotes adventitious and lateral root regeneration. However, the role of melatonin in tomato fruit ripening is not well understood.In this study, the effect of melatonin on the postharvest ripening and quality improvement of tomato fruit was carried out. The tomatoes were immersed in exogenous melatonin for 2 h, and then the related physiological indicators and the expression of genes during post-harvest life were evaluated. Compared with control check (CK), the 50 μM melatonin treatment significantly increased lycopene levels by 5.8-fold. Meanwhile, the key genes involved in fruit colour development, including phytoene synthasel (PSY1) and carotenoid isomerase (CRTISO), showed a 2-fold increase in expression levels. The rate of water loss from tomato fruit also increased 8.3%, and the expression of aquaporin genes, such as S1PIP12Q, S1PIPQ, S1PIP21Q, and S1PIP22, was up-regulated 2- to 3-fold under 50 μM melatonin treatment. In addition,50 μM melatonin treatment enhanced fruit softening, increased water-soluble pectin by 22.5%, and decreased protopectin by 19.5%. The expression of the cell wall modifying proteins polygalacturonase (PG), pectin esterasel (PE1), β-galactosidase (TBG4), and expansinl (Expl) was up-regulated under 50 μM melatonin treatment. Melatonin increased ethylene production by 27.1%, accelerated the climacteric phase, and influenced the ethylene signalling pathway. Alteration of ethylene production correlated with altered 1-aminocyclopropane-l-carboxylic acid (ACC) synthase (ACS4) expression. The expression of ethylene signal transduction-related genes such as NR, S1ETR4, S1EIL1, S1EIL3, and S1ERF2, was enhanced by 50μM melatonin. The effect of melatonin on ethylene biosynthesis, ethylene perception, and ethylene signalling may contribute to fruit ripening and quality improvement in tomato. This research may promote the application of melatonin on postharvest ripening and quality improvement of tomato fruit as well as other horticultural productions in the future.To better understand the function of melatonin in tomato fruit ripening and quality improvement, a label-free quantitation method was used to investigate the proteins that differ between the control (CK) and 50μM melatonin treatment (M50) fruits. Proteomics data identified 241 proteins that were significantly influenced by melatonin. These proteins were involved in several ripening-related pathways, including cell wall metabolism, oxidative phosphorylation, carbohydrate, and fatty acid metabolism. Moreover, the application of exogenous melatonin increased eight proteins that are related to anthocyanin accumulation during fruit ripening. Additionally, the affected protein levels correlated with the corresponding gene transcript levels. Further, the total anthocyanin content from M50 increased by 52%,48%, and 50% at 5,8, and 13 DAT (day after melatonin treatment), respectively. The melatonin-mediated promotion of fruit ripening and quality might be due to the altered proteins involved in processes associated with ripening. In this work, we indicated that a senescence-related protein was downregulated in the M50 fruit, while a cell apoptosis inhibitor (API5) protein was upregulated. In addition, peroxidases (POD9, POD12, peroxidase p7-like) and catalase (CAT3) significantly increased in the M50 fruits. Based on the previous studies and our data, we inferred that melatonin might be positively related to fruit ripening but negatively related to fruit senescence. This research provides insights into the physiological and molecular mechanisms underlying melatonin-mediated fruit ripening as well as the anthocyanin formation process in tomato fruit at the protein concentration level, and we reveal possible candidates for regulation of anthocyanin formation during fruit ripening.In order to further understand if the accelerated red color development in melatonin-treated tomato fruits is only dependent on ET signaling or not, we explore the role of ethylene on melatonin-induced lycopene accumulation by treatment of Nr mutant fruits with exogenous melatonin. Although melatonin-treated Nr released markedly more ethylene, it did not accumulate many more pigments. The expression of SIDXS, SIPSY1, SIPDS and SIZDS was only significantly higher in the M50-Nr than the CK2 at 4 DAT. The transcript accumulation of SIGGPS and SICRTISO has no significant change in M50-Nr and CK2 at all mature stages. Meanwhile, the transcript accumulation of all three lycopene β-cyclases (β-LCY1, β-LCY2, CYCβ) was almost no difference in the M50-Nr and the CK2. The results demonstrated that the perception of ET is required in the induction of lycopene biosynthesis by melatonin.This research may promote the application of melatonin on postharvest ripening and quality improvement of tomato fruit as well as other horticultural productions in the future. Moreover, it provides insights into the physiological and molecular mechanisms underlying melatonin-mediated fruit ripening as well as the anthocyanin formation process in tomato fruit at the protein concentration level, and we reveal possible candidates for regulation of anthocyanin formation during fruit ripening.