Characteristics Of Sorbitol Accumulation In Sand Pear (Pyruspyrifolia) Fruits And Identification Of Critical Genes Involved

Sorbitol is the main component of soluble sugar in pear fruit, and has been reported to be related with the occurrence of watercore disease. In this study, the change rules of sorbitol and other soluble sugar content on fruit bagging treatment, greenhouses cultivation, and postharvest storage were conducted with two sand pear (Pyrus pyrifolia Nakai) cultivars,’Cuiguan’ and ’Cuiyu’. On that basis, the characteristics of sorbitol dehydrogenase (SDH) and sorbitol transporter (SOT) gene families were carried out and their roles in pear fruit quality formation were analyzed. The transcriptional differences between watercored and non-watercored flesh of susceptible cultivar,’Zhongli 4’, were analyzed by using of RNA-seq technology. The main results were as follows:1. The sorbitol content accounted for more than 50% of total soluble sugar in young fruit flesh of ’Cuiguan’ & ’Cuiyu’, and over 30% in mature fruit. The sorbitol content of ’Cuiguan’ pear fruit was decreased remarkably in black double-layer bag treatment, but with no effect on ’Cuiyu’. The sorbitol content of both varieties were increased in black non-woven bag treatment, the glucose and fructose content increased at the same time, while the sucrose content decreased. The sorbitol content decreased remarkably in ’Cuiyu’ pear fruit under white nonwoven bag treatment, but with no effects in ’Cuiguan’ pear fruit. Under greenhouse cultivation, the content of glucose and fructose were getting higher than control throughout the fruit development with no difference of peak time, but the peak time of sorbitol content under greenhouse cultivation was later than control. The content of glucose and fructose increased quickly during 25℃ storage, while the content of sorbitol decreased distinctly. The sucrose content decreased in ’Cuiguan’ during 25℃ storage, but increased in ’Cuiyu’. Both glucose and fructose content were increased during 4℃ storage, but sorbitol content had no obvious change. Based on another quality index, white non-woven bag was recommended to be used in ’Cuiguan’ pear fruit production, and black non-woven bag to ’Cuiyu’; the longest storage period of ’Cuiguan’ and ’Cuiyu’ was recommended as about 30 d and 60 d, respectively.2. Fourteen expressed PpySDH genes and twenty two expressed PpySOT genes have been identified from sand pear (Pyrus pyrifolia Nakai cv.’Cuiyu’). PpySDH2, PpySDH4, PpySDH8, PpySDH12, PpySDH13 and PpySDH14 were the top six strongest expressed members, which accounted for 96% of total PpySDH transcript abundance. PpySOT2, PpySOT3, PpySOT4, PpySOT8, PpySOT 10, PpySOT25, PpySOT26, PpySOT28, PpySOT32 and PpySOT33 were top ten strongest expressed members, which accounted for about 92% of total PpySOT abundance. Greatest abundance of these six PpySDHs were found in functional leaf petioles. In ripening fruit, PpySDH2, PpySDH4 and PpySDH14 were the main functional members. The expression level of PpySDHs was much higher in’Cuiguan’than in’Cuiyu’, which might be the reason of divergence in content and composition of sugar accumulated. PpySOT8 expressed everywhere, while PpySOT26 only expressed somewhat in young leaves. Expression of PpySOT2, PpySOT8, PpySOT 10/28 and PpySOT33 increased during fruit development and showed well positive correlation with sorbitol accumulation pattern. Expression of six members, PpySOT3, PpySOT4, PpySOT8, PpySOT25, PpySOT32 and PpySOT33, were induced significantly under 4℃, which might be closely related to chilling response and resistance.3. The symptom of watercore disease in’Zhongli 4’fruit occurred at 100 days after full bloom. In GA3+7 treated fruit, the symptom occurred at 75 days after full bloom. The sugar accumulation and transcriptional differences between watercored and non-watercored flesh of sand pear were analyzed. Results showed that sucrose content in non-watercored flesh was higher than in watercored flesh, while glucose and fructose content were significantly lower. Sorbitol content has no difference between two groups. The transcript abundance of all PpySOTs in watercored flesh had no difference with non-watercored flesh, while the transcript abundance of 3 PpySDH members were observed lower dramatically in watercored flesh than in non-watercored flesh. By using RNA-Seq technology,1,016 differentially expressed genes were found between non-watercored and watercored flesh, in which 848 up-regulated, and 168 down-regulated. Another 934 differentially expressed genes were found betweenGA-treated non-watercored & watercored flesh, in which 828 up-regulated and 106 down-regulated. KEGG pathway enrichment analysis results showed that these genes were distributed on about 60 different pathways, in which Glycolysis/ Gluconeogenesis, Tyrosine metabolism, and Fatty acid metabolism were the top 3 most significantly concentrated pathways. Protein interaction analyze results indicated that the different expressed genes were mainly come from Glycolysis/ Gluconeogenesis pathway.