Effects of 1-MCP and low temperature treatments on the expression of endo-PG family genes in peach during post harvest storage

Abstract: 【Objective】As a common hydrolase, polygalacturonase (PG) can be divided into endo-polygalacturonase (endo-PG, EC3.2.1.15) and exo-polygalacturonase (exo-PG, EC3.2.1.67) . Endo-PG can randomly hydrolyze α-1-4 polygalacturonic acid, take part in pectin degradation, and cause the structure disintegration of cell wall, leading to the fruit ripening and softening. Particularly, the endo-PG family genes can be involved in fruit softening, and the expression patterns are distinct among different fleshy varieties with different treatments during fruit softening. However, molecular mechanism towards endoPG family genes in relation to peach fruit ripening remains unclear. To study the differential responsive effects of endo-PG family genes in different texture peaches and under different storage conditions during peach softening, we carried out characterization of endo-PG family genes from peach genome, and analysed the gene expression levels of endo-PG family genes in different peach texture cultivars under 1-MCP and low temperature treatments.【Methods】Different peach cultivars of‘Xiahui 5'with softmelting flesh, ‘Zijinhong 3'with hard-melting flesh, ‘Xiacui'with stonyhard flesh, and‘Babygold 7'with non-melting flesh in 80% maturity were collected as test materials. The changes of flesh firmness under different temperatures storage (room temperature 25 ℃ and cold 4 ℃) and 1-MCP treatments were analysed. Total RNA of the flesh was extracted and c DNA was synthesized as a template by reverse transcription. Through the bioinformatics analysis, the peach endo-PG gene family was excavated and predicted, the genes of the endo-PG family were screened and identified from the peach genome, and quantitative real-time PCR (qRT-PCR) was carried out to determine the expression profiles of each gene in different tissues and different fleshy peaches under different storage time treatments. Correlation analysis between fruit flesh firmness and endo-PG family gene expression were determined using SPSS 19.0 software (SPSS, Chicago, IL, USA) .【Results】1-MCP treatment significantly inhibited fruit softening of‘Xiahui 5'with soft-melting flesh and‘Babygold 7'with non-melting flesh in the initial storage, while it did not significantly effected fruit softening of‘Xiacui'with stonyhard flesh and‘Zijinhong 3'with hard-melting flesh. Under low temperature treatment, the flesh firmness of‘Zijinhong3'‘Xiacui'and‘Babygold 7'maintained at a high level in the long cold storage, whereas flesh firmness of‘Xiahui 5'quickly softened during the second day of the cold storage. In addition, 13 endo-PG genes were identified in peach genomes and distributed on 5 different chromosomes. All peach endoPG genes possess at least 2 introns, Prupe.7 G040000 and Prupe.3 G081700 have the most amount of introns (8) . QRT-PCR analysis showed that the expression levels of endo-PG family genes were differencially observed in different peach organs, and the expression levels of each gene was significantly distinct among peaches with different flesh. Dramatically, the expression level of Prupe.4 G261900 gene in‘Xiahui 5'flesh was the highest and maintained in higher expression levels under different treatments, while expression level of the other genes were relatively lower or not detected. Compared with the control (25 ℃) , both 1-MCP and low temperature treatments significantly reduced the expression level of Prupe.4 G261900 of‘Xiahui 5'with soft-melting flesh during the first 2 and 4 days of the cold storage period. 1-MCP treatment significantly decreased the expression levels of 4 genes of‘Babygold 7'with non-melting flesh, but had no such effect on‘Xiacui’with stonyhard flesh. Results show that 1-MCP treatment induced the expression levels of Prupe.7 G005500 and Prupe.3 G081700 genes of‘Xiahui 5'.Moreover, low temperature treatment significantly reduced expression of 4 genes of‘Xiacui'and‘Babygold 7'or made them to be kept in a relatively steady level, but induced the expression levels of Prupe.7 G269200, Prupe.7 G005500 and Prupe.3 G081700 genes of‘Xiahui 5'with soft-melting flesh.Notably, this study showed that expressions of these genes were not detected in‘Zijinhong 3'with hard-melting flesh. Correlation analysis showed that fruit flesh firmness was negatively correlated with Prupe.4 G261900 under the control treatment and with Prupe.7 G005500 of both soft-melting or nonmelting peach cultivars under low temperature treatment, respectively.【Conclusion】13 of endo-PG family genes were determined in peach genome. Genes of Prupe.4 G261900 and Prupe.7 G005500 were negatively correlated with fruit flesh firmness during postharvest storage. 1-MCP treatment may regulate Prupe.4 G261900 expression, while low temperature treatment regulate Prupe.7 G005500 expression, and further delay the decrease of flesh firmness during earlier shelf-stage after harvest in both soft-melting and non-melting peach cultivars. Favorably, this study provides theoretical basis for postharvest fruit softening during storage.