Research progress in sugar and acid in pear fruit

Abstract：Pears are the third temperate fruit crop in the world and are widely popular due to their unique taste. The contents of soluble sugars and organic acids in pear fruits are very important to fruit quality. The differences in the content and composition of soluble sugars and organic acids in the fruit contribute to the different flavors of different pear varieties. During the growth, development, maturation and aging of pear fruits, a series of physiological and biochemical reactions occur through sugar and acid metabolism, and there are some patterns in the sugar and acid content and component changes in the fruit. Research on the contents of soluble sugars and organic acids in pear fruits has emerged endlessly, and a significant progress has been made. This article will provide an overview of the research on the sugar and acid contents in pear fruits. It mainly consists of four aspects: research progress in sugar and acid contents during fruit growth and development, sugar and acid content in post harvest pear fruits, factors affecting sugar and acid content and molecular mechanisms of sugar and acid change. During fruit growth and development, organic acids are formed in the early stages of fruit development and gradually decrease as the fruits mature. The organic acid components in pear fruits mainly include malic acid, citric acid, quinic acid, oxalic acid, shikimic acid and tartaric acid. Pear varieties are divided into malic acid dominant and citric acid dominant varieties based on the content of each organic acid component in the fruit. With Asian pear varieties, the main organic acid found in most pear varieties ismalic acid, while with European pear varieties, the main organic acid found in most pear varieties is citric acid. Malic acid shows a first increasing and then decreasing trend throughout the entire growth and development process, while citric acid dominant varieties will undergo a transition of malic acid decreasing and citric acid increasing in the middle stage of fruit development. Malic acid is the main factor causing significant differences between varieties. The total soluble sugar content is relatively low in the early stage of fruit development and gradually increases with fruit maturity. The soluble sugar components present in pear fruits mainly include sorbitol, fructose, glucose and sucrose. Most pear varieties have sorbitol as the main sugar in the young fruit, while fructose is the main sugar in the mature fruit. There are significant differences in the proportion of sugar components at different developmental stages, with an increase in fructose content and a significant decrease in sorbitol content during maturity. Sucrose is an important factor causing differences among varieties. Pears belong to the respiratory climacteric type of fruit. During storage, fructose and glucose contents in the fruit gradually increase, sorbitol and malic acid contents gradually decrease, and sucrose first increase and then decrease. Under low temperature storage conditions, the contents of fructose, glucose and sucrose increase until they gradually decrease after 90 days. The organic acid contents show a trend of first decreasing and then stabilizing. Although the total soluble sugar content of refrigerated fruits is higher than that of unfrozen fruits, longterm refrigeration can lead to a deterioration and decrease in sucrose content in the fruits. Bagging treatment could reduce the content of total soluble sugar and reduce the content of malic acid. The application of different plant hormones or growth regulators could affect the sugar and acid contents in pear fruits. Ethylene treatment could increase the soluble sugar content and reduce the titratable acid content. GA treatment could increase the contents of sucrose and malic acid. IAA and ABA treatment could promote the accumulation of sorbitol, while MT treatment could increase the content of sucrose and sorbitol. Applying potassium and calcium fertilizers could increase the accumulation of glucose, fructose, sorbitol and sucrose in fruits. 1-MCP treatment could delay and inhibit the downward trend of organic acids, but also delay the increase of total soluble sugar content. The sugar and acid contents in pear fruits are a quantitative trait controlled by multiple genes. With the rapid development of genomics, many key gene functions has been verified. PbCPK28 promotes the phosphorylation of PbTST4 and PbVHA-A1, promoting sugar transport and storage in vacuoles. PbrAc-Inv1 and PbrII5 are involved in the degradation of sucrose. Overexpression of PbPH5 can significantly increase the malic acid content. The sugar transporter family genes involved in sugar transport in pear fruits include PbSWEETs, PbSOTs, PbTMTs, PbSUTs, PbPLTs and PbTSTs. Overexpression of PuSWEET15 increases sucrose content, while silencing PuSWEET15 reduces sucrose content. PbTMT4 is an important contributor to the accumulation of fructose, glucose and sucrose in pear fruits. By developing sugar related QTL markers, two individual sugar content regions associated with acid invertase genes have been identified in linkage groups LG1 and LG7. This article has mainly summarized and reviewed the research papers related to the sugar and acid contents of pear fruit in recent years around these four parts, in order to provide reference for the study on sugar and acid contents in the pear fruit, component evaluation, functional gene mining and new variety breeding, and to propose prospects for future research directions.