Research progress on postharvest fruit softening and pectin degradation in climacteric fruits

Abstract: As an important part of the daily diet, fruits are an important source of vitamin a, vitamin c and dietary fiber. However, fresh fruits are difficult to preserve and will spoil and rot quickly, especially the fruits with respiratory climacteric. This kind of fruits rapidly ripen and soften after the peak of respiration, which not only affects their texture, taste and nutritional value, but also causes the loss of commercial value. The softening process is believed to be related to the degradation of cell wall pectin. Pectin in the cell wall is a kind of complex acidic polysaccharide with D-galacturonic acid as the main component connected by 1, 4-glycosidic bonds, contributing to the adhesion between cells and the mechanical strength of cells. Currently the smooth and hairy region model is the most recognized pectin structure, which consists of four structural domains-homogalacturonans, rhamnogalacturonan Ⅰ, rhamnogalacturonan Ⅱ and xylogalacturonan. Studies have found that the process of fruit ripening and softening is often related to changes in pectin composition. The degree of esterification of pectin and polymerization will decrease. The insoluble protopectin degrades to form soluble pectin and pectic acid, and intercellular adhesion decreases, resulting in softening of the fruit. For example, alkaline soluble pectin is more in the early stage of maturity while water soluble pectin is more in the later stage of maturity in tomato. One of the main reasons for fruit softening is the degradation of pectin, which is catalyzed by specific pectinase including pectin methylesterase (PME), polygalacturonase (PG), pectate lyases (PL) and β-galactosidase (β-GAL). The main function of PME is to act on the methyl esterified carboxyl group to form methanol and polygalacturonic acid. PG is a key enzyme for the softening of most fruits with respiratory climacteric. Its role is to hydrolyze the glycosidic bond of demethylated polygalacturonic acid residues to produce galacturonic acid. PL degrades pectin polymers directly by β- elimination mechanism that results in the formation of 4 ,5-unsaturated oligogalacturonides, promoting the process of fruit softening. There are two kinds of β-GAL. One acts on the β-(1, 3) and β-(1, 6) glycosidic bonds of arabinogalactan, and the other acts exclusively on the β-(1, 4) galactosidic bonds of pectin branched chains. Both of them are related to the strength of cell wall structure. The activities of the four pectinases will change when fruits mature, and generally show a trend of first rising and then falling. Among them, the activity of β-GAL changes in the early post-harvest period, which is earlier than the change of PG and PME, and it may have a greater impact on fruit softening than the other enzymes. Activities of all kinds of enzymes are regulated by genes. It is helpful to clarify the molecular mechanism of fruit softening by studying the expression of related genes and the activity changes of various pectinases. Several genes related to pectinase have been reported. For PME, 71 genes have been found in peach. The expression of Solyc03g083360, Solyc07g071600 and Solyc12g098340 increased at maturity, which may be related to softening of tomato. PG genes are classified into 3 major clades by phylogenetic analysis and each clade contains PG genes that involved in fruit softening. Exogenous ethylene and 1-MCP mainly regulate the enzyme activity by affecting the expression of PG genes, which in turn affects the ripening and softening of fruits. The SlPL is involved in the ripening and softening of tomato and silencing it can inhibit the expression of genes related to cell wall degradation and slow down the fruit softening. There are many expression patterns of β- GAL gene, some of which have high expression level at maturity, some have high expression level at fruit development, while others always maintain a low level. In general, PME genes are up- regulated during cold storage and highly expressed when the fruit is ripe. PG genes are mainly expressed at maturity, and the gene expression level increases first and then decreases with fruit maturation. The change in gene expression level is consistent with the change in PG enzyme activity. PL genes are mainly expressed before fruit ripening and softening, and some members show high levels of expression at the end of ripening. Most of the β-GAL genes are expressed at the beginning of ripening and early fruit softening. Studying the relationship between fruit softening and cell wall pectin degradation and revealing its mechanism will contribute to the formulation of postharvest fruit preservation strategies so as to prolong fruit shelf life and maintain good fruit quality and commerciality. In this paper, the composition and molecular structure of pectin in plants are summarized according to the latest research progress at home and abroad, and the degradation of pectin in cell wall during the softening process is introduced. At the same time, the mechanisms of pectin degradation and fruit softening are revealed from pectin degradation, pectinase activity and the expression level of related genes.