Molecular Plant Breeding 2025, Vol.16, No.3, 191-201 http://genbreedpublisher.com/index.php/mpb 193 permeability, which makes the flesh tissue loose and juicy. The cell wall is composed of macromolecules such as pectin, cellulose, and hemicellulose, and its stability ensures the hardness of unripe fruits. During ripening, various cell wall hydrolases are triggered by signals such as ethylene, synthesized in large quantities and secreted into the cell wall, synergistically catalyzing the degradation of cell wall components. Polygalacturonase (PG) is one of the key enzymes for pectin degradation, which can hydrolyze the α-1,4-galacturonic acid bonds of the pectin backbone, leading to the disintegration of the intercellular mesoglea. In models such as tomatoes, PG has been shown to be expressed in large quantities from color change to maturity, and its activity is closely related to the softening rate of the fruit (Lobato-Gómez et al., 2021). Cellulase (also known as endo-β-1,4-glucanase) can degrade cellulose and hemicellulose networks, weaken the skeleton structure of the cell wall, and also play an important role in the softening of durian. The latest research shows that in tomatoes, the combined action of expansin and endoglucanase Cel can synergistically promote cell wall relaxation and fruit softening; it is speculated that a similar mechanism also exists in durian. β-galactosidase (β-Gal) accelerates cell wall decomposition by removing sugar residues from the side chains of pectin and hemicellulose, increasing the accessibility of other enzymes to the main chain of the cell wall (Pan et al., 2022). It is worth mentioning that there is also a synergistic or cascade effect between enzymes during cell wall degradation. For example, expansin first loosens the cell wall structure, exposing the substrate to further action by PG, Cel, etc.; β-galactosidase removes the side chains and promotes the hydrolysis of the pectin main chain by PG. 3.2 Key genes and regulatory pathways in pericarp softening Ethylene is one of the main hormones that control the softening of the durian peel. During ripening, the genes that help make ethylene, such as ACS (aminocyclopropane-1-carboxylic acid synthase), become more active. This leads to higher ethylene levels, which speeds up the softening process (Teh et al., 2017). Ethylene also increases the expression of genes for enzymes that break down the cell wall. In addition, it works together with other plant hormones, showing that the softening process is controlled by a complex network of signals (Osorio et al., 2013; Suntichaikamolkul et al., 2021). Several genes directly control the making of cell wall-degrading enzymes. These include PG, pectin lyase (PL), expansins (EXP), and pectin methylesterase (PME). Among these, PG and PME are especially important because their activity rises a lot as the fruit ripens, breaking down pectin and causing the peel to soften (Imsabai et al., 2002). Swelling protein genes, such as DzEXP1 and DzEXP2, are also involved in this process. The expression of these genes is regulated by ethylene (Palapol et al., 2015). They work synergistically with other cell wall degrading enzyme genes (such as PME, PL, EXP, and XTH) to degrade pectin and hemicellulose, rearrange microfibers, and cause softening of durian peel (Figure 1) (Palapol et al., 2015; Peng et al., 2022). 4 Postharvest Impacts on Durian 4.1 Impact of rapid pericarp softening When the skin of durian softens too quickly, the shelf life of the fruit will be greatly shortened, which not only reduces its market value but also increases the difficulty of export transportation. Many fruits face similar problems, as traditional post harvest treatments such as low temperature, controlled atmosphere, and radiation can delay aging and reduce losses, but often result in a decline in quality. Although early harvesting extends the shelf life, insufficient maturity affects flavor and nutrition, while mature harvesting faces the problem of short shelf life. According to a survey, about 33% of fruits and vegetables worldwide are wasted due to rapid decay (Figure 2) (Shipman et al., 2021), which brings huge economic losses to farmers and sellers. Fast softening also affects the texture and taste of durian, which are very important for consumer choice. As the cell wall breaks down, with cellulose and pectin being the main parts affected, the fruit becomes too soft and loses its good texture (Gao et al., 2024). When the texture changes too much, people may not like the fruit as much, which can lower demand.
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