Proteomic mechanism of sugar and organic acid metabolism during Korla fragrant pear (Pyrus sinkiangensis Yü) fruit development

Abstract

Korla fragrant pear (Pyrus sinkiangensis Yü) fruit development involves complex physiological and biochemical processes; however, few data are available at the proteomic and metabolomic levels, which would be helpful for understanding the molecular mechanisms of fruit and quality development. In our study, Korla fragrant pear was investigated across three important stages, namely, early development (5 weeks after flower blooming, WAF), middle development (10WAF), and near ripening (15WAF), via tandem mass tag (TMT) labeling technology. Ultra-performance liquid chromatography‒tandem mass spectrometry (UPLC‒MS/MS) was subsequently performed to detect changes in the concentrations of sugars and organic acids in pear fruits at different developmental stages. Through proteomic and metabolomic analyses, we identified a total of 8487 proteins expressed during pear fruit development, and 3762 differentially expressed proteins (DEPs) were characterized in Korla fragrant pear at three fruit development stages. Moreover, 27 soluble sugars and 43 organic acids were found to accumulate differentially in the fruit at different developmental stages. The functional annotation of the DEPs revealed that many were involved in flavonoid biosynthesis, starch and sucrose metabolism, fructose and mannose metabolism, galactose metabolism, galactose metabolism and organic acid metabolism. The expression of proteins related to sugar metabolism and accumulation increased with increasing fruit development stage, which was consistent with the trend of soluble sugar content during fruit development. All 6 disaccharides, including cellobiose (Cel), lactose (Lac), maltose (Mal), trehalose (Tre), phenylglucoside (Phe) and sucrose (Suc), detected in this study were present at low levels in the early stages of fruit development but accumulated in large amounts from 15 WAF to 20 WAF, which may explain the high sweetness of ripe Korla fragrant pear. In addition, the expression levels of proteins related to organic acid metabolism were consistent with the trends of organic acid contents detected by UPLC‒ESI‒MS during fruit development. The contents of 34 organic acids are low in ripe Korla fragrant pear, and the ripe pear fruits used for consumption contain mainly nine organic acids, which are 2-hydroxyphenylacetic acid, homovanillic acid, pyruvic acid, cis-aconitic acid, 3-methyladipic acid, hydroxyphenyllactic acid, taurine, fumaric acid and tartaric acid. The low levels of organic acids detected during fruit development may explain the low organic acid content of Korla fragrant pear. Thus, our proteomic and metabolomic analyses reveal the molecular basis for the characteristics of high sweetness and low organic acidity in Korla fragrant pear.