Melatonin promotes ripening and improves quality of tomato fruit during postharvest life

Abstract

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 2h, 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 synthase1 (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 SlPIP12Q, SlPIPQ, SlPIP21Q, and SlPIP22, 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 esterase1 (PE1), β-galactosidase (TBG4), and expansin1 (Exp1) 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-1-carboxylic acid (ACC) synthase (ACS4) expression. The expression of ethylene signal transduction-related genes such as NR, SlETR4, SlEIL1, SlEIL3, and SlERF2, was enhanced by 50 µM 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.

Keywords: Ethylene; gene expression; melatonin; postharvest; ripening; tomato..

Fig. 1.

Melatonin’s effect on tomato pigment accumulation.CK: fruits pre-treated with water; M50: fruits pre-treated with 50 µmol l^–1 melatonin; M50: fruits pre-treated with 50 µmol l^–1 melatonin. (A) Lycopene content at 17 d after treatment. (B) Real-time PCR analysis of PSY1 and CRTISO expression levels in fruit at 17d after treatment. Vertical bars at each time point represent the least significant difference (LSD) test when significant at P=0.05. (C) The colour of tomato at 17 d after treatment. The scale bar indicates 1cm.

Fig. 2.

Melatonin content in tomato fruits after treatment. Tomatoes were treated without (water) or with melatonin in concentrations: 1, 50, 100, and 500 µM. The melatonin content of tomatoes after treatment at (A): mature green stage and (B): 17 DAT. Vertical bars at each time point represent the LSD when significant at P=0.05.

Fig. 3.

Melatonin affects softening and firmness of tomato fruit. CK: fruits pre-treated with water; M50: fruits pre-treated with 50 µmol l^–1 melatonin. (A) Firmness. (B) Pectin level. (C) Real-time PCR analysis of cell wall structure-related gene expression in tomato fruits at 17 d after treatment. Vertical bars at each time point represent the LSD when significant at P=0.05. (This figure is available in colour at JXB online.)

Fig. 4.

Melatonin has an effect on water loss in tomato fruit. CK: sample pre-treated with water; M50: sample pre-treated with 50 µmol l^–1 melatonin. (A) The rate of water loss of tomato during postharvest storage. (B) Real-time PCR analysis of aquaporin genes expression in tomato fruits at 17 d after treatment. Vertical bars at each time point represent the LSD when significant at P=0.05.

Fig. 5.

Melatonin has an effect on ethylene production, signalling pathway, and relative gene expression. CK: sample pre-treated with water; M50: sample pre-treated with 50 µmol l^–1 melatonin. (A) Ethylene production of tomato fruits during postharvest storage. (B) Real-time PCR analysis of ethylene biosynthesis-related genes expression in tomato fruits at 17 d after treatment. (C) Real-time PCR analysis of ethylene signalling-related gene expression in tomato fruits at 17 d after treatment. Vertical bars at each time point represent the LSD when significant at P=0.05.

Fig. 6.

Melatonin has an effect on tomato flavour components. CK: samples pre-treated with water; M50: samples pre-treated with 50 μmol/L melatonin. (A) The volatiles content of hexanal, and (B) related gene expression. (C) Water-soluble sugar of tomato fruit at 17 d after treatment. Vertical bars at each time point represent the LSD when significant at P=0.05. (This figure is available in colour at JXB online.)

Fig. 7.

A model of the role of melatonin in postharvest ripening tomato fruits. Melatonin may promote tomato fruit ripening by affecting ethylene production and signalling. Arrows represent positive regulation, the dotted lines represent regulatory steps in which a direct physical link between upstream and downstream components has yet to be demonstrated.