Effect And Mechanism Of Postharvest Hot Air Treatment Combined With Cryptococcus Laurentii On Induced Disease Resistance Of Cherry Tomato Fruit

Tomato is an important horticultural crop produced globally. Tomato fruit is low in fat and calories, but rich in vitamins, β-carotene, lycopene and other antioxidants. Cherry tomato is especially popular worldwide as fruit and vegetable due to its delicious taste and high nutrition. However, cherry tomato is highly perishable and susceptible to mechanical injury and fungal infection after harvest and during prolonged periods of storage. Hot air treatment is applied as one of the most promising non-fungicide methods for preventing postharvest decay. Cryptococcus laurentii is a well-known biocontrol yeast reported to reduce postharvest fungal diseases of fruits and vegetables such as peach, pear, apple and tomato. Therefore, in this research, we selected the ’Qianxi’ cultivar cherry tomato as experimental material, and our objective was to investigate the effect and mechanism of postharvest hot air treatment (HA) combined with C. laurentii on induced disease resistance of cherry tomato fruit.The contents and results are as follows:1. HA treatment (38℃, 12 h) and C. laurentii were investigated separately or in combination for controlling postharvest decay caused by artificially inoculated with Alternaria alternata and Botrytis cinerea, and natural infection in intact cherry tomato fruit.The research focused on the effects of different inoculation sequence and fruit maturity on the cherry tomato response to the combined treatment. Results indicated that the combined treatment significantly reduced disease incidence of black and gray molds in cherry tomato at both mature green and pink stages whether fruits were artificially inoculated with pathogens before or after treatment. Similar results were observed on testing fruits with natural infection. At the end of storage at 20℃, there was no natural decay in fruits harvested at mature green stage with the combined treatment, while the decay incidence of fruits at pink stage with the same treatment was 26%. Moreover, application of C. laurentii had no effect on fruit quality, while the combined treatment could delay fruit turning red and softening, inhibit respiration rate and ethylene production, and maintain fruit quality as well as HA treatment alone. These results suggest that HA combined with C. laurentii had curative and preventive effect to control postharvest disease in cherry tomato at both mature green and pink stages.2. The effect of HA treatment and C. laurentii on cell wall compositions, activities and gene expression of cell wall degrading enzymes of mature green cherry tomato fruit was investigated. In our study, the water-soluble pectin and cellulose contents in HA treated fruit were higher than that in control fruit, indicating that HA retarded pectin solubilization and cellulose degradation. Degradation of cell wall composition is mainly the result of activities of cell wall degrading enzymes. Pectin methylesterase (PME), polygalacturonase(PG), cellulase and β-galactosidase (β-GAL) are key enzymes involved in cell wall degradation. In our study, HA treatment inhibited PME, PG, β-GAL and cellulase activities.The qRT-PCR results showed that HA treatment down-regulated mRNA expression of six genes encoding cell wall degrading enzymes, such as LePGcat, LePME, LeTBG2, LeCell,LeEXP1 and LeXET, but in the meanwhile HA treatment up-regualted LeEXT gene expression. The softening of fruit will facilitate the infection of pathogen and increase postharvest decay. This indicates that HA treatment altered cell wall degradation and delayed softening would help to ward off the spread of pathogens, thus playing positive roles in enhancing disease resistance. Application of C. laurentii had no influence on cell wall metabolism, and the combination treatment of HA and C. laurentii had similar effect with HA treatment.3. We investigated the effect of HA combined with C. laurentii on the activities and gene expression of defence related enzymes in mature green cherry tomato fruit. The results showed that HA combined with C laurentii induced higher activities of peroxidase (POD),catalase (CAT), superoxide dismutase (SOD), ascorbic acid peroxidase (APX), glutathione peroxidase (GPX), chitinase (CHT) and β-1, 3-glucanases (GLU) than the control. The relative quantitative analysis results by qRT-PCR showed that HA combined with C.laurentii induced up-regualtion of gene expression of antioxidant enzymes such as LePOD,LeAPX and LeGPX, and pathogenesis-related (PR) proteins LePR1, LeCHI, LeGLU and LePR5. Up-regulation gene expression of antioxidant enzymes and PR proteins and higher activities of the defence realted enzymes would be one of the mechanisms involved induced disease resistance of cherry tomato by the combined treatment. The result partly explained why HA combined with C. laurentii improved the efficacy of single treatment against B.cinerea and A.alternata of cherry tomato fruit.4. In this study, we investigated the effect of HA combined with C. laurentii on the activities and gene expression of key enzymes, and metabolites in phenylpropanoid metabolic pathway in mature green cherry tomato fruit. The results showed that HA combined with C. laurentii increased the activities of phenylalanine ammonia-lyase (PAL)and cinnamate-4-hydroxylase (C4H) than single treatment. HA treatment inhibited 4-coumarate-CoA ligase (4CL) activity, while C. laurentii treatment increased 4CL activity,and 4CL activity of the combination treated fruit was inhibited by HA treatment, and then increased by C. laurentii. The qRT-PCR results showed that HA treatment up-regulated mRNA expression of four genes encoding enzymes of phenylpropanoid metabolism, such as LePAL, LeCHI, LeFLS and LeF3H, but in the meanwhile HA treatment down-regualted Le4CL,LeCHS1 and LeCHS2 gene expression. Application of C. laurentii induced the up-regulation of the seven genes encoding enzymes of phenylpropanoid metabolism. The gene expression of LePAL, LeCHI, LeFLS and LeF3H was increased by the combined treatment. While Le4CL, LeCHSl and LeCHS2 gene expression were inhibited by HA treatment firstly and then increased by C. laurentii. The phenylpropanoid metabolites were anlaysed by LC-MS/MS and we identified 44 phenylpropanoid metabolites in cherry tomato fruit. HA combined with C. laurentii induced the accumulation of phenylpropanoid metabolites, increased the content of total phenolic, total flavonoids and lignin, and increased the content of p-coumaric acid, chlorogenic acid and caffeic acid, quercetin,naringenin and rutin in cherry tomato fruit. HA combined with C. laurentii improved the induced disease resistance against B.cinerea and A.alternata by enhancing phenylpropanoid metabolic pathway of cherry tomato fruit.