International Journal of Horticulture, 2025, Vol.15, No.5, 218-233 http://hortherbpublisher.com/index.php/ijh 222 3.3 Cell wall loosening enzymes: expansins, XTHs, pectin methylesterases During the growth and maturation process, plants adjust the structure of the cell wall through a series of cell wall modification enzymes, thereby affecting the hardness and texture of the tissue. Expansins are a class of non-hydrolyzable proteins located in the cell wall. They can interrupt the hydrogen bonding between cellulose microfibrils and hemicellulose, thereby relaxing the cell wall without significantly degrading polysaccharides. Expansins are highly expressed in tissues with rapid cell expansion (such as young stems, buds, and fruit swelling). Their activation reduces the Young's modulus of the wall, promotes cell elongation and tissue softening. In sugarcane, the introduction of exogenous expansin genes also produces a cell wall relaxation effect. A study overexpressed the expansin EaEXPA1 in high-biomass hybrid sugarcane (Erianthus arundinaceus × sugarcane) in cultivated sugarcane. As a result, the transgenic plants showed better stem elongation and biomass accumulation under drought, and the flexibility of the stem cell wall increased. Although this study focused on drought resistance, it also indirectly proved that swelling proteins can change the physical properties of sugarcane cell walls (Narayan et al., 2021). Another important type of wall-modifying enzyme is xylan endotransglycosylase/hydrolase (XTH). XTH can cut and reconnect the main chain of hemicellulose (such as xylan), thereby reorganizing the architecture of the cellulose-hemicellulose network (Santiago et al., 2018). During fruit ripening and softening, XTH often works synergistically with swelling proteins to change the cell wall structure from tight to loose, which manifests as a softening texture (Mira et al., 2024). It is speculated that in fresh-eating sugarcane varieties, moderate expression of XTH may help reduce the tightness of the fiber, making the stem meat easier to chew and not as tough as sugar cane. However, there are not many studies on the role of XTH in sugarcane stem development, and it is necessary to draw on the research of other crops to infer its potential impact. The third type of enzymes worth noting are pectin-modifying enzymes, including pectin methylesterase (PME) and polygalacturonase (PG). PME removes the methoxyl groups on the pectin side chains, changing pectin from a high esterification state to a low esterification state, thereby creating action sites for hydrolases such as PG. During the softening process of fruits, the continuous action of PME and PG will cause the disintegration of pectin gel in the middle layer, the decrease of cell adhesion, and the softening of tissue texture (Tipu and Sherif, 2024). Although the pectin content in sugarcane stalks is not as high as that in juicy fruits, it is relatively high in young and tender parts. Therefore, it can be inferred that during the ripening process of fresh sugarcane, the activity of PME may gradually increase to moderately soften the cell wall and improve edibility. If the PME activity is too strong and causes excessive degradation of pectin, the intercellular binding force will decrease too much, which may cause the tissue to loosen and lose its crispness; on the contrary, if the PME activity is too low, the cell wall will remain stiff and tight, and the chewing will be hard and not crisp enough. Ideally, PME and related enzymes are moderately expressed during the ripening period of fresh sugarcane, making the stem meat tender and crisp. The mechanism in this regard is currently less studied and is a potential direction for optimizing the taste of fresh sugarcane in the future. 4 Transcriptional Regulation and Gene Expression Patterns 4.1 Spatiotemporal expression of sugar metabolism - and wall-related genes Sugarcane is a complex polyploid crop with multiple cell types and multi-stage development, and there are significant differences in gene expression in different tissues and developmental stages. As a source organ, leaves highly express genes encoding photosynthesis and sucrose synthesis, such as photosynthetic enzymes Rubisco, PEPC, SPS, SuSy, etc., so that mesophyll cells can quickly synthesize and export sucrose. In contrast, as the main sink organ, the stem specifically expresses a group of genes related to sugar storage and cell wall thickening. For example, in mature sugarcane stem nodes, sucrose transporter SUT and vacuolar sugar transporter TST genes are highly expressed to ensure that exogenous sucrose is continuously introduced and stored in the vacuole (Mehdi et al., 2024b); at the same time, some secondary cell wall synthase genes (such as cellulose synthase and lignin synthase) are upregulated in the mechanical tissue of the stem to thicken the cell wall and improve the strength of the stem (Wang et al., 2020; Lu et al., 2024). This differentiation of gene expression profiles between source leaves and sink stems reflects the functional division of labor: source tissues focus on assimilation and output,
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