Involvement of induced resistance by methyl jasmonate to blue mold and reactive oxygen species metabolism in navel orange fruit

Abstract:【Objective】Navel orange [Citrus sinensis (Linn.) Osbeck ], one of the most widely planted citrus fruit trees in the world, is mainly planted in Chongqing, Hubei, Jiangxi, Fujian and other places in southern China. However, navel orange fruits are susceptible to diseases caused by pathogens during postharvest transportation and storage, seriously affecting the quality and economic value of the fruits. The blue mold caused by Penicillium italicum is one of the most important postharvest diseases in navel orange fruits, which is currently mainly controlled by chemicals. Currently there is an urgent need to develop safer and more efficient preservation techniques to control post-harvest diseases of navel orange fruits. Methyl jasmonate (MeJA) is an important natural plant growth regulator, which can be used as aninducer of resistance to pathogen infestation to produce a defense response in plants. In recent years, the aspect of MeJA-induced disease resistance in plants has become a hot issue, but there are fewer reports on MeJA-induced resistance to P. italicum in navel orange fruits. In this study, we investigated the relationship between exogenous MeJA-induced postharvest resistance to the blue mold and reactive oxygen metabolism in navel orange fruits to provide a theoretical basis for the potential of the application of MeJA to control postharvest fruit diseases.【Methods】In this study, Newhall and Longhuihong navel orange fruits were used as test materials. Firstly, the fruits were cleaned with 0.05% sodium hypochlorite and water, then after being dried at room temperature, one group fruits were treated with 50 μmol· L^-1 MeJA for 24 h at (26±1) ℃ and 85%-95% relative humidity, another group fruits were sprayed with 100 μmol·L^-1 SHAM, and the third group treated with sterile water was used as control. The conidia of P. italicum that had been cultured on PDA medium for one week were collected and made into a spore suspension with a concentration of 1.25×106 spores·mL^-1 . The surface of the fruits was sterilized with 75% ethanol and inoculation was carried out by puncturing the equatorial part of the fruit at equal distances using an inoculation needle with a wound size of 2 mm in diameter and 3 mm in depth and injecting 20 µL of spore suspension of P. italicum. Ten fruits were selected from each treatment for 7 consecutive days, and 1 cm of peel tissue was taken from the disease-health junction of navel oranges and quickly frozen with liquid nitrogen for further analysis. The activities of glutathione reductase (GR), ascorbate peroxidase (APX), superoxide dismutase (SOD), catalase (CAT), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), and the relative expression levels of related genes including CsGR, CsAPX, Cu-ZnSOD, CsCAT, CsMDHAR and CsDHAR3 were measured. All data were statistically organized and analyzed by one-way analysis of variance (ANOVA) using Excel 2018 and SPSS 20.0 software. The origin 2018 software was used for graphing, and all data in the graphs were expressed as mean ± standard error (SE).【Results】MeJA fumigation treatment significantly enhanced the activities of GR, APX, SOD, CAT, MDHAR, DHAR and the relative expression levels of CsGR, CsAPX, Cu-ZnSOD, CsCAT, CsMDHAR and CsDHAR3 in two navel orange varieties fruits. In the MeJA treatment of Newhall, they were 1.10, 1.16, 1.12, 1.69, 1.21, 1.43, 1.16, 1.42, 1.56, 1.36, 1.58 and 2.32 times as much as the control, respectively. In the MeJA treatment of Longhuihong, they were 1.13, 1.49, 1.02, 1.41, 1.26, 1.79, 2.98, 1.59, 1.10, 1.79, 1.57 and 1.31 times as much as the control, respectively. At the same time, the content of reduced glutathione (GSH), ascorbic acid (AsA) and DPPH free radical scavenging rate were significantly increased, the content of hydrogen peroxide (H2O2) and the production rate of superoxide anion (O2 .- ) were decreased, and the accumulation of malondialdehyde (MDA) content was delayed.【Conclusion】In summary, MeJA fumigation of navel orange fruits for 24 h before inoculation could significantly enhance the resistance of fruits to P. italicum, increase the activity of enzymes related to active oxygen metabolism and the expression levels of key enzyme genes. The above results suggested that the enhancement of the resistance to the blue mold in navel orange fruits induced by the MeJA treatment might be related to its regulation of the reactive oxygen species metabolism. This study could provide theoretical basis and technical reference for the prevention and control of the blue mold disease during postharvest storage of the navel orange fruits.