1 Introduction

As a high-sugar crop with both edible value and economic benefits, fresh sugarcane (Saccharum spp.) is widely cultivated in many provinces and regions in southern my country (Chen et al., 2022). Its main characteristics include high sucrose content, rich juice, crisp taste, and easy peeling. It is a special economic crop that combines fruit use and ornamental (Liu et al., 2024). With the improvement of residents' consumption level and the enhancement of health awareness, the demand for fresh sugarcane in the domestic and foreign markets continues to expand, and the added value of the industry has increased significantly, which has positive significance for optimizing the planting structure and promoting farmers' income.

In the quality evaluation system of fresh sugarcane, sweetness and texture are the two most critical indicators. Sweetness directly determines the edible quality and consumer preference, and its level is mainly regulated by the content of sucrose, glucose and fructose in the stem; while texture is related to the chewing experience, which is jointly determined by factors such as cell wall structure, lignin content and cell wall enzyme activity (Liu et al., 2024). However, in actual breeding, high sweetness is often accompanied by an increase in the degree of tissue lignification, which reduces the palatability of texture. Therefore, in-depth analysis of the synergistic regulation mechanism of sweetness and texture is of great scientific and practical significance for breaking the negative correlation between quality traits and achieving the breeding goal of "sweet and crisp" high-quality sugarcane (Huang and Li, 2024).

This study will systematically sort out the physiological and molecular mechanisms of sweetness and texture formation in fresh sugarcane stalks, and analyze the core contents such as sugar metabolism-related enzymes (such as SuSy, SPS, invertase), sugar transport systems (such as SWEET, SUT), cell wall polysaccharide structure and modification enzymes (such as expansin, XTH, pectinase), etc., focusing on the gene expression regulation mechanism affecting sweetness and texture formation, including the spatiotemporal expression pattern of key genes, the role of transcription factors such as NAC/MYB/bZIP, and the regulatory role of plant hormones and environmental factors (such as light and temperature). Finally, through comparative studies among typical varieties, attempts are made to reveal the molecular basis of the formation of different types of sugarcane quality traits from the perspective of transcriptome, in order to provide theoretical support for molecular breeding and precise improvement of high-quality fresh sugarcane.

2 Sugar Metabolism and Accumulation

2.1 Key enzymes: roles of SuSy, SPS, and invertases in sucrose synthesis and breakdown

Sugarcane is famous for its rich sucrose content in its stems, and its sweetness essentially depends on the high concentration of sucrose accumulated in the pith cells of the stems. This process is strictly regulated by a series of enzymatic reactions, among which sucrose phosphate synthase (SPS), sucrose synthase (SuSy), and invertase are the key enzymes for sucrose synthesis and decomposition. SPS catalyzes the synthesis of uridine diphosphate glucose (UDP-Glc) and fructose-6-phosphate produced by photosynthesis into sucrose-6-phosphate, which is then dephosphorylated by sucrose-6-phosphate phosphatase to produce sucrose. It is the rate-limiting enzyme in the sucrose biosynthesis pathway (Khan et al., 2023). Studies have shown that the SPS enzyme activity level in different sugarcane varieties is significantly positively correlated with their final sucrose content, and high-sucrose varieties tend to show stronger SPS activity. SuSy can reversibly catalyze between sucrose and UDP-Glc. On the one hand, it works with SPS in the direction of sucrose synthesis, and on the other hand, it provides substrates for cellular respiration and the construction of polysaccharides such as cellulose in the direction of sucrose decomposition. There are usually multiple SuSy isoenzymes in crops such as sugarcane, with spatiotemporal expression patterns to adapt to the needs of sucrose synthesis or decomposition at different developmental stages. In contrast, invertases are responsible for irreversibly hydrolyzing sucrose into glucose and fructose, including acid invertases in the vacuole and neutral invertases in the cytoplasm.

High invertase activity often means that sucrose is continuously broken down into reducing sugars, which provides energy and carbon skeletons in rapidly growing young tissues, but excessive invertase activity reduces the net accumulation rate of sucrose in the stem (Mehdi et al., 2024b). Comparative tests of high-sugar and low-sugar sugarcane varieties confirmed this difference in key enzyme activity: the SPS activity in the stems of high-sugar varieties (such as GT35) was significantly higher than that of low-sugar varieties, while the invertase activity was relatively low, so the sucrose content in the stem cells was significantly higher than that of low-sugar varieties. On the contrary, in low-sugar varieties, neutral and acid invertase activities were stronger, resulting in more sucrose being decomposed into glucose and fructose, and sucrose accumulation in the stems was limited. This shows that the efficient expression of synthases such as SuSy and SPS and the moderate decrease in invertase activity are conducive to the accumulation of sucrose in sugarcane stems (Niu et al., 2019). On the other hand, the sucrose decomposition enzyme system cannot be too low, otherwise the stem cells will lack sufficient respiratory substrates and metabolic regulation, which will inhibit plant growth. Therefore, sugarcane achieves a carbon distribution balance between the source (leaf photosynthetic products) and the sink (stem sugar storage) by finely regulating the expression of sucrose synthesis and decomposition enzymes, so as to meet growth needs and maximize sugar storage (Mehdi et al., 2024a).

2.2 Sugar transport and storage: function of SWEET, SUT, and tonoplast transporters

The sucrose produced by photosynthesis needs to be transported through sieve tube assimilates to reach "sink" organs such as stems and stored in intracellular vacuoles, which is an important mechanism for the accumulation of high sugar content in sugarcane. In the long-distance transport of sucrose, SWEET and SUT family transporters play a key role. SWEET is a class of sucrose transmembrane export proteins that mediate the release of sucrose from source tissue cells into the intercellular space, and then is actively loaded by sucrose transporters (SUTs) in phloem companion cells and sieve tubes, thereby realizing the transport of sucrose from leaves to stems (Chen et al., 2022). A variety of SUT and SWEET genes have been identified in sugarcane, among which SWEET13c is mainly highly expressed in mature mesophyll cells, promoting the export of photosynthetic product sucrose to the phloem, while SWEET4a/4b is mainly expressed in stem tissues, participating in the unloading of sucrose from sieve tubes to surrounding storage cells. On the "sink" side, sugarcane proteins such as SUT1 and SUT4 are located on the sieve tube companion cells and parenchyma cell membranes of stem tissues, responsible for absorbing sucrose from the intercellular space into the cells.

In addition to plasma membrane transport, stem medullary parenchyma cells also pump sucrose into the vacuole for storage through the sucrose/H+ antiporter on the vacuole membrane. For example, Arabidopsis vacuolar sugar transporter TMT1/2 can actively transport glucose and sucrose into the vacuole, and there is a highly homologous Tonoplast Sugar Transporter (TST) family in sugarcane that performs similar functions. These transport processes increase the sucrose concentration in the vacuole of sugarcane stem cells, which can be as high as more than 50% of the dry weight, which is the source of the high sweetness of sugarcane (Mehdi et al., 2024b). It should be pointed out that the accumulation of sucrose in the vacuole will increase the osmotic pressure of the cell. In order to maintain balance, sugarcane often increases the storage of reducing sugars such as glucose and fructose in the vacuole. Studies have found that transgenic sugarcane with increased sucrose synthase SPS activity also increases the acid invertase activity in its leaves, and the glucose and fructose content increases. This may be a compensatory mechanism: plants adjust excessive sucrose concentrations by moderately increasing sucrose decomposition to avoid feedback inhibition of photosynthesis and osmotic stress. Therefore, the long-distance transport and intracellular storage of sugars is a dynamic equilibrium process, and the sugar signal sensing and transporter expression of both the source and the sink are precisely regulated to achieve effective redistribution and efficient accumulation of sucrose in sugarcane.

2.3 Dynamic balance of sucrose, glucose, and fructose in different maturation stages

The ratio of sucrose, glucose and fructose in sugarcane stalks changes significantly from the elongation stage to the maturity stage. In tender stems and elongation tissues, the invertase activity is high, and sucrose is quickly decomposed and utilized, so the content of reducing sugars (glucose, fructose) is relatively higher and the sucrose concentration is lower. As internodes mature, photosynthate input increases and stem tissue growth slows down, the activity of synthases such as SPS and SuSy reaches a peak at the mature stage, while the activity of acidic and neutral invertases decreases significantly, causing sucrose to accumulate in large quantities in mature stem cells. Especially in the late stage of sugarcane growth, the decrease in the activity of neutral invertase (NI) in the stem is considered to be one of the main reasons for the high accumulation of sucrose (Khan et al., 2021; Martins et al., 2024). The distribution pattern of neutral invertase activity in mature sugarcane internodes is closely related to the sugar gradient - in the mature zone at the base of the internode, lower NI activity is accompanied by higher sucrose concentration, while in the tender zone at the top of the internode, higher NI activity leads to an increase in the proportion of reducing sugar.

Recent experimental data also support this dynamic: the high-sugar variety GT35 has a much higher sucrose content in mature stem segments than the low-sugar variety B8, while the glucose and fructose contents are lower. Correspondingly, the activities of SPS and cell wall invertase (CIN) in mature stem segments of GT35 are significantly higher than those of B8, while the activities of invertase and SuSy decomposition are lower. This indicates that in the late stage of sugarcane stalk development, the enzymes that synthesize sucrose maintain high activity, while the enzymes that decompose sucrose are feedback inhibited, allowing sucrose to accumulate continuously in the cells. In addition, the source-sink signal regulation of different varieties is also different: heat-resistant varieties can maintain high SPS and SuSy expression at high temperatures, so that sucrose can still accumulate in the late growth period, while sensitive varieties have a sharp drop in enzyme activity under heat stress, resulting in blocked sugar accumulation (Mehdi et al., 2023). It can be seen that the dynamic balance of sucrose, glucose and fructose in sugarcane stems depends on the coordinated changes of multiple enzymes under developmental stages and environmental conditions. The large accumulation of sucrose in the mature stage is the result of the combined effects of multiple metabolic regulations.

3 Cell Wall Structure and Texture Formation

3.1 Composition of cell wall polysaccharides: cellulose, hemicellulose, pectin

Cellulose, hemicellulose and pectin each play their own role in the sugarcane cell wall, and together determine the mechanical properties and degradability of the cell wall. The cellulose content directly affects the stiffness of the stalk - the more and denser the cellulose microfibrils, the greater the tensile and shear strength of the cell wall, and the tougher and harder the tissue is to chew (Liu et al., 2024). Hemicellulose (mainly arabinoxylan) fills the pores between cellulose microfibrils and cross-links with them. Its presence increases the rigidity of the cell wall, but also reduces the degradation efficiency of the fiber in chemical pretreatment and enzymatic hydrolysis. Lignin is deposited on the cellulose-hemicellulose network, making the cell wall hydrophobic and stronger, and plays a "gluing" and reinforcement role on the cell wall (Buckeridge et al., 2019). The degree of lignification of sugarcane stalks gradually increases during maturity, and the wall thickness increases. Although this improves the ability to resist lodging, it increases the chewing resistance of the stem meat fiber.

Therefore, sugarcane is usually harvested at the right time of maturity when lignification is not too serious, so as to take into account both sugar content and texture (Chen et al., 2022). Pectin in the primary wall also affects the softness of the texture. Pectin is composed of polygalacturonic acid as the main chain, and its degree of esterification and cross-linking determines the hardness of the middle layer of the cell wall: highly methylated pectin chains are soft and sticky, which is conducive to intercellular adhesion; after demethylation by pectin methylesterase, pectin is easily cross-linked with calcium ions to form an "egg box" structure, making the middle layer hard and brittle. Some fresh sugarcane varieties may have a higher degree of pectin methylation, so the intercellular binding force is moderate, and it is neither too loose nor too hard and brittle when chewing. These hypotheses still need further experimental verification, but from the research on other fruit tissues, pectin modification does play an important role in the softness and hardness of the taste (Tipu and Sherif, 2024).

3.2 Lignin biosynthesis and its influence on chewing resistance

Lignin content is considered to be one of the key factors affecting the chewing difficulty of sugarcane stalks. High-fiber and high-lignin stalks are often thick and hard, making them difficult to chew and break; while low-lignin tender stalks are easy to chew into dregs. Plants enhance mechanical strength to resist adverse environments by regulating the content of cellulose, hemicellulose and lignin in cell walls. Comparisons between sugarcane varieties show that fiber content (mainly representing the total amount of cellulose and lignin) is significantly positively correlated with stalk toughness, that is, varieties with high fiber content have harder stalks, more chewy and less likely to be chewed. Therefore, reducing lignin synthesis is a potential way to improve the texture of sugarcane. Genetic engineering intervention has proved this: using site-directed mutagenesis technology to knock out the key gene caffeic acid-O-methyltransferase (COMT) in sugarcane lignin synthesis can reduce the lignin content of transgenic sugarcane by about 20%, while increasing the sugar accumulation level, and even increasing the enzymatic saccharification rate of fiber raw materials by more than 40%.

More importantly, this lignin-reduced sugarcane mutant did not show obvious growth defects or yield reduction in the field, indicating that moderate reduction of lignin can soften the stem without compromising the mechanical strength of the plant, providing a genetic target for improving the texture of sugarcane (Kannan et al., 2018). In addition to regulating lignin content through genetic means, agricultural measures can also affect the degree of stem lignification. For example, silicon can be deposited in the cell wall of sugarcane stems to form siliceous bodies, thereby improving the lodging resistance and insect resistance of the stems. Adding silicon fertilizer often makes the stem wall thicker and harder, which is beneficial for sugarcane but not for the texture of fresh sugarcane, and needs to be weighed in cultivation (Wang et al., 2020). Reducing the relative content of lignin and cellulose and optimizing the composition of cell walls are one of the important ideas for improving the chewability of sugarcane. In actual breeding, by screening materials with weaker lignin synthesis or using molecular means to inhibit the expression of some lignin pathway genes, new sugarcane varieties with softer fibers and easier chewing may be obtained.

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, while sink tissues focus on storage and support. Even at different stages of the same root and stem, gene expression changes dynamically. Transcriptome analysis found that the expression of some gene modules in different developmental stages of sugarcane was inversely correlated with the fiber and sugar content of the stem: for example, a group of genes related to fiber accumulation was highly expressed in the later stage, while genes related to sugar accumulation were more active in the early stage (Figure 1).

Figure 1 (a) Field trial of TALEN-mediated COMT mutants. (b) Field performance of COMT mutant lines and wild type (WT). CB6 and CB7 are mutant lines derived through direct embryogenesis and biolistic transformation. (c) Stems of field-grown TALEN-mediated COMT mutant (CB6) in comparison with wild type (WT), immediately after juice extraction with roller mills (Adopted from Kannan et al., 2018)

This pattern of increase and decrease means that plants switch metabolic priorities at different growth stages: more carbon is allocated to building structures (cellulose, lignin) in the early stage, while more is used to store energy (sucrose) in the later stage. Perlo et al. (2022) used weighted gene co-expression network analysis (WGCNA) to reveal for the first time the changing pattern of sugarcane gene expression with developmental stage and variety, and identified two gene co-expression modules associated with high sugar content and high fiber content, respectively. Among them, the high sugar module is enriched with genes in pathways such as sucrose synthesis and transport, while the high fiber module is enriched with genes related to cell wall biosynthesis and defense, and the expression of the two modules is significantly negatively correlated. This result indicates that there is a genetic network in sugarcane that coordinates sweetness and fiber formation, and the spatiotemporal expression of different gene clusters jointly shapes the final quality traits. By analyzing the expression patterns of these key genes, researchers can further understand when and where regulation can achieve the optimization of sugarcane quality. For example, if sugar accumulation genes can be specifically upregulated and some lignin synthesis genes can be downregulated in the late stage of sugarcane stalk development, the dual purpose of increasing sweetness and reducing fiber content may be achieved. This fine regulation needs to be based on a full grasp of the spatiotemporal expression patterns of genes.

4.2 Transcription factors involved in trait coordination: NAC, MYB, bZIP

Transcription factors (TFs) are upstream molecular switches that regulate gene expression and play a pivotal role in plant secondary metabolism and cell wall development. Some key TFs can simultaneously affect sugar metabolism and cell wall synthesis pathways, and are therefore regarded as molecular targets for the coordinated improvement of sweetness and texture traits (Khan et al., 2023). In crops such as sugarcane, transcription factors of the NAC and MYB families have been shown to be involved in the regulation of secondary cell walls. NAC transcription factors are a large class of regulators unique to plants, among which the master switch genes for secondary wall thickness regulation often belong to the NST subfamily. The sugarcane genome contains a large number of NAC genes. Wang et al. (2023) identified NAC genes of the ATAF subfamily in Saccharum spontaneum and functionally analyzed the homologous ScNAC2 of cultivated sugarcane, suggesting that it plays an important role in biological stress response and other aspects. Although there is insufficient evidence that NAC directly regulates sugarcane cell wall synthesis, given that Arabidopsis NST1/2 can initiate the transcription of cellulose and lignin synthesis genes, it can be speculated that some NACs in sugarcane may play a similar "master switch" role in the process of stem lignification. When these NACs are highly expressed, they may promote cell wall reinforcement and make the stems harder; conversely, inhibiting the activity of some NACs may help reduce lignin deposition and thus soften the texture.

This provides ideas for future molecular breeding. MYB transcription factors are also widely involved in metabolic regulation, especially phenolic and lignin pathways. Studies have shown that AtMYB58/63 and others can directly activate the expression of lignin synthase genes, thereby controlling the degree of fiber lignification. The corresponding MYB regulatory network in sugarcane has not yet been clearly identified, but some MYB gene expression changes have been observed in sugarcane with genetic engineering to reduce lignin, suggesting that MYB may be involved in the feedback regulation of lignin synthesis. The bZIP family transcription factors play a special role in the connection between sugar signals and sugar metabolism genes. Mehdi et al. (2024b) reported that a bZIP transcription factor ScbZIP44 in sugarcane is controlled by sucrose levels: there is a sucrose-responsive upstream open reading frame (SC-uORF) on the 5'UTR of the gene, and the uORF is activated to inhibit the translation of the main ORF when sucrose is high. When the sweetness is high, the protein level of ScbZIP44 decreases, resulting in the release of its inhibitory effect on downstream sucrose metabolism genes, thereby promoting more sucrose accumulation. Conversely, under low sugar conditions, the increase in bZIP44 protein will inhibit sucrose synthesis-related genes, forming a negative feedback.

This discovery reveals the ingenious self-regulatory mechanism in the sugar metabolism pathway, and also shows that it is possible to improve the sweetness of sugarcane through transcription factors (such as modifying the uORF element of ScbZIP44 so that it is no longer inhibited by sucrose). Transcription factors such as AP2/ERF, WRKY, and bHLH are also involved in multiple pathways related to sugarcane quality. For example, ERF factors mediate ethylene signals, which can indirectly affect the expression of cell wall softening enzymes and defense enzymes; WRKY and bHLH regulate carbon allocation in response to adverse stress, which may change the accumulation pattern of sucrose and fiber (Khan et al., 2023). Transcription factors provide a powerful molecular tool for improving the sweetness and texture of sugarcane at the same time. By adjusting the activity of these factors through genetic engineering or molecular design breeding, it is theoretically possible to achieve synchronous regulation of multiple downstream structural genes, thereby synergistically optimizing complex traits.

4.3 Differential gene expression between sweet-crisp and fibrous cultivars

The differences in sweetness and texture between fresh sugarcane varieties ultimately stem from differences in metabolic pathways and regulatory networks caused by genotype differences. High-sugar varieties often carry favorable allele combinations, which enable them to assimilate and accumulate sugar more efficiently under the same environment; accordingly, some genes of these varieties (such as SPS, SuSy, and sucrose transporter genes) may be more strongly expressed than ordinary varieties, allowing more photosynthetic carbon to flow to the sucrose pool (Niu et al., 2019). Conversely, some varieties with coarse and hard fibers may show higher basal expression levels in genes of the cell wall synthesis pathway, or have genetic mutations that make cell walls difficult to degrade, such as more active promoters of lignin monomer synthesis genes. Modern omics technologies provide a means to reveal these varietal differences. Transcriptome comparison can screen genes that are specifically upregulated in varieties with soft taste and genes that are highly expressed in varieties with coarse and hard fibers. Although there are not many reports on comparative transcriptome studies on the taste of fresh sugarcane, we can infer some possible conclusions from relevant studies: for example, sugarcane varieties with a tender and crisp taste may up-regulate the expression of some cell wall relaxants (such as expansins, XTH) and sucrose transporters, and down-regulate some lignin and fiber synthesis-related genes; on the contrary, varieties with coarser fibers have the opposite expression trend of these genes.

Perlo et al. (2022) conducted a large-scale transcriptome analysis of 24 sugarcane varieties and found that the variation in gene expression patterns was closely related to the genotype, even exceeding the influence of the developmental stage. This shows that the differences in gene regulation of different varieties will cause them to present different metabolic states at the same developmental stage. This has been confirmed in the comparison of high-sugar and low-sugar varieties: high-sugar varieties maintain higher sucrose synthase activity and lower invertase activity throughout the growth period, making their sugar accumulation rate always faster than that of low-sugar varieties. Similarly, we have reason to believe that high-quality fresh sugarcane varieties may be born with certain favorable gene regulation characteristics, such as weaker lignification or stronger sugar redistribution ability, which makes them both sweet and soft. These differences can be explored through genomic and transcriptomic methods. For example, transcriptome comparison of several representative sugarcane varieties and common sugarcane varieties, or the use of genome-wide association analysis (GWAS) to find gene loci associated with sweetness and texture in natural germplasm will help to clarify the molecular basis of differences between varieties and provide targets for molecular breeding (Mehdi et al., 2024a).

5 Hormonal and Environmental Regulation

5.1 Impact of auxin and gibberellin on sucrose metabolism

Auxin (IAA) and gibberellin (GA) are two key hormones that regulate plant growth and development. They not only determine the height and internode elongation of sugarcane, but also affect the distribution of carbohydrates in the body through complex signaling pathways (Khan et al., 2023). Auxin is present in high levels in young tissues and can promote cell elongation and division. It also induces source-sink conversion, causing more photosynthetic products to be consumed by fast-growing parts, which may reduce the allocation of sugar storage to the stem to a certain extent. This is considered to be one of the reasons why plants accumulate sugar slowly during the peak growth period. However, there is a two-way "dialogue" relationship between auxin and sucrose metabolism: sucrose itself can act as a signal to regulate the expression of genes related to the auxin pathway, such as affecting the distribution of auxin polar transport proteins and the transcription of IAA synthase genes. Studies have found that excessive auxin in sugarcane promotes the elongation of stem nodes but reduces the sucrose accumulation rate, while moderately reducing IAA levels (such as through IAA antagonist treatment) can increase the sugar content in the stem without significantly affecting growth. It can be seen that IAA has an indirect inhibitory effect on sugarcane sugar accumulation, and a balance needs to be achieved between growth and sugar storage.

In contrast, the effect of gibberellins on sugarcane sugar metabolism is more complicated. Traditionally, it is believed that exogenous GA treatment will promote sugarcane internode elongation and increase biomass, but may cause a decrease in sugar content per stem segment due to the "dilution effect". However, recent studies have shown that the application of GA at a specific developmental stage can increase the final sucrose accumulation. Roopendra et al. (2018) reported that the application of GA3 to late-maturing sugarcane varieties in the middle period can significantly increase the size of the sink organ (cell volume increased by 42%, internode lengthened by 39%), and strongly induced the gene expression of enzymes related to sucrose metabolism: such as acid invertase activity increased by 7.5 times, cell wall invertase by 4.5 times, and SPS activity increased by 6 times. As a result, at maturity, the sucrose concentration of the GA-treated group increased by about 2-3 percentage points compared with the control (from about 38% to more than 41%).

This indicates that GA delays the saturation of the reservoir through the dual effects of "enlarging the water tank" (increasing the storage capacity) and "accelerating the water flow" (increasing the source supply and conversion), thereby accumulating more sucrose (Roopendra et al., 2024). However, the sugar-enhancing effect of similar treatment on another early-maturing variety did not last until maturity, probably because the invertase activity induced by GA caused some sucrose to be decomposed again in the later stage. The effect of GA on the sweetness of sugarcane depends on the application period and variety characteristics: timely and appropriate GA promotes the growth of stems and the temporary storage capacity of sucrose. In the subsequent maturation process, if the photosynthetic supply is sufficient and the invertase activity is controlled, more sucrose will remain in the stem; on the contrary, if GA is excessive or the effect lasts too long, it may cause premature conversion and consumption of sucrose, which will reduce the sugar content.

This explains the cautious use of GA in production from a mechanistic perspective: applying GA during the elongation period of sugarcane can increase yield, but it must be avoided near the maturity period to avoid reducing sugar. In addition to directly acting on metabolic enzyme genes, GA and auxin also participate in the response to sugar signals through transcriptional regulatory networks. For example, some auxin response factors (ARFs) and DELLA proteins (GA signal inhibitors) in sugarcane were found to have expression correlations with sucrose content. These findings suggest that we can indirectly regulate sugar distribution in sugarcane by regulating key genes in the IAA and GA pathways, such as increasing the expression of inhibitors sensitive to sucrose signals, so that the plants "mistakenly believe" that sucrose is insufficient, thereby continuously strengthening assimilation and sink strength, and ultimately increasing the sugar content of the stem (Mehdi et al., 2024b).

5.2 Ethylene signaling and cell wall softening

Ethylene is a gaseous plant hormone known for promoting fruit ripening and tissue senescence. In many juicy fruits, ethylene is the dominant signal that induces softening, and it controls texture by regulating the gene expression of a series of cell wall degrading enzymes. Typical examples are peak respiratory fruits such as tomatoes and peaches: when ethylene is released in large quantities, the transcription of various cell wall hydrolases such as polygalacturonase (PG), cellulase, and β-galactosidase is aggregated, leading to pectin decomposition, hemicellulose reduction, and cellulose depolymerization, which ultimately causes the fruit to soften rapidly (Tipu and Sherif, 2024). For non-climacteric stem tissues such as sugarcane, the role of ethylene in softening is relatively unobvious, but it is still worth paying attention to.

On the one hand, if sugarcane is mildewed or mechanically damaged after harvest, a certain amount of ethylene will be produced, which may trigger a softening process similar to that of fruits, making the sugarcane stem pith tissue loose and easy to collaps. This needs to be avoided during storage and transportation. On the other hand, moderate ethylene signals may help fresh sugarcane maintain its juiciness and palatable texture during maturity. Studies have shown that the fruits of peach mutants (hd mutations) that do not produce ethylene remain hard and crisp throughout the fruit, but soften after exogenous ethylene is applied. By analogy, the slight increase in endogenous ethylene levels during the ripening of fresh sugarcane may promote the action of some wall-relaxing enzymes (such as PG and β-glucanase), making the fiber tissue slightly softer from extremely hard, thereby improving the chewing experience. Of course, this speculation has not yet been directly supported by experiments, and further research is needed on the role of ethylene in sugarcane storage and ripening. It is worth mentioning that ethylene has extensive crosstalk with other hormones.

For example, ethylene and auxin interact with each other in organ abscission and cell elongation; and ABA cooperates in maturation and senescence (Tipu and Sherif, 2024). In the regulation of sugarcane quality, the combined effects of multiple signals should also be considered. In general, although sugarcane is not a typical ethylene-driven softening tissue, key factors in the ethylene signaling pathway may implicitly affect cell wall metabolism. Through exogenous 1-MCP (ethylene receptor inhibitor) or adjusting the expression of ethylene synthesis genes, it may be possible to achieve a certain regulation of sugarcane texture in the future.

5.3 Light and temperature effects on quality-related gene networks

The effects of environmental factors on the sweetness and texture of sugarcane have long been proven in production practice: good light and suitable temperature can significantly increase the sugar content and stem fullness of sugarcane, while unfavorable environments (such as weak light, heat/cold stress) often lead to reduced sugarcane yield and quality (Mehdi et al., 2024a). Light directly determines the source intensity by affecting photosynthesis, which in turn affects the accumulation of sugar in the reservoir. Sufficient sunlight can increase the photosynthetic rate of sugarcane leaves and provide more assimilated products for the stems. It is reported that from the end of sugarcane tillering to the elongation stage, if there is sufficient sunlight (more than 7~9 hours of sunshine per day), the growth and sugar accumulation of the plants are significantly accelerated; on the contrary, if there is continuous rain or short-day season, the stems become thinner and softer, and the sugar accumulation is significantly reduced. This is related to the fact that the expression of sucrose synthesis-related enzymes under light is regulated by circadian rhythm: sufficient light can upregulate the activity of enzymes such as SPS in leaves during the day and promote the output of sucrose. At the same time, it may regulate the activity of SuSy and invertase in stems at night through sugar signals, so that more sucrose can be stored in time (Zhao and Li, 2015). If there is a long-term lack of light, the plant will mobilize the stored sugar in the stem for growth, which is manifested as a decrease in sugar content and poor mechanical strength. Therefore, in sugarcane cultivation, reasonable dense planting and avoiding shading are particularly important to ensure the sugar content and quality of a single stem.

Temperature affects the formation of sugarcane quality by affecting the activity of metabolic enzymes and membrane stability. Sugarcane is a C4 crop, and its photosynthesis and growth are most vigorous at around 30 ℃. High temperature or low temperature stress will interfere with its normal metabolism. High temperature (for example, daily temperature continues to be above 35 °C~40 °C) will accelerate respiration and may cause an increase in the sucrose cycle decomposition rate, resulting in impaired net sugar accumulation. At the same time, high temperature can also induce the expression of heat shock proteins and stress resistance genes, which often adjust carbon flow to defense pathways (such as proline synthesis, soluble sugar as osmotic protectants, etc.) rather than storage pathways. Mehdi et al. (2023) showed that under heat stress gradually increased to 45 °C, the sucrose content and pure sugar yield of the two sugarcane varieties decreased significantly, and high temperature treatment caused the expression of sucrose metabolic enzymes (including SPS and multiple invertases) to be significantly reduced.

This shows that high temperature inhibits the supply of photosynthetic products at the source and directly inhibits the activity of enzymes in the sink. The dual effects lead to poor sugar accumulation. In relatively heat-resistant varieties, some sucrose synthase and antioxidant enzyme genes maintain high expression, thus showing better sugar production stability. The impact of low temperature on sugarcane cannot be ignored. Most sugarcane varieties grow slowly when the average daily temperature is below 20 °C. If the temperature drops further to below 10 °C, the photosynthetic efficiency is significantly reduced and the sucrose transport rate slows down. Sometimes metabolic byproducts such as starch accumulate in the stems and leaves, affecting sugar conversion. Studies have found that the activity of neutral invertase in the stems of sugarcane increases after being cold, which may be an attempt by the plant to increase the concentration of cell sap by increasing sucrose decomposition to prevent frost damage, but this also reduces the net accumulation of sucrose. Therefore, it is one of the traditional experiences to harvest in time before the arrival of winter to avoid the sugar content of sugarcane being reduced by low temperature.

In addition to light and temperature, water conditions also significantly affect the sweetness and texture of sugarcane. Adequate water is conducive to photosynthesis and the dissolution and transportation of sugar, but excessive water supply or drought will have negative effects. Especially under dry farming conditions, if there is a lack of effective irrigation, the stomata of the plants will close, resulting in reduced photosynthesis and reduced sucrose accumulation in the stems; at the same time, drought will accelerate the lignification process to enhance drought resistance, and the stems will become harder and more fibrous, which is not conducive to fresh eating. An experiment in Guangdong compared the quality of sugarcane grown in dryland sprinkler irrigation and traditional dryland.

The results showed that sprinkler irrigation significantly increased the sugar content of sugarcane stems by 1.42 Bx and reduced the fiber content by about 0.47 percentage points. It can be seen that improving water conditions can not only help to increase sweetness, but also help to make the texture tender. This suggests that in the cultivation and management of fresh sugarcane, we should provide crops with a suitable water and nutrient environment through irrigation and other measures, so that they can maintain good metabolic functions even in high-light and high-temperature seasons, thereby achieving efficient accumulation of sugars and moderate reinforcement of cell walls. Environmental factors such as light, temperature and moisture profoundly regulate the network of stalk sweetness and texture formation by affecting the supply of photosynthetic products and the expression of metabolic enzyme genes in sugarcane. In production practice and breeding strategies, it is necessary to fully consider the impact of the environment on quality traits, and select and promote high-quality sugarcane varieties with higher resistance to adverse environments (Mehdi et al., 2024a).

6 Case Studies in High-Quality Fresh-Eating Cultivars

6.1 Sweetness accumulation in purple vs. green rind varieties

Fresh sugarcane can be roughly divided into two categories according to the color of the stem skin: purple-skinned (black-skinned) and green-skinned (yellow-green-skinned) varieties. For example, typical purple-skinned sugarcane varieties include Black Skin No. 3 in Guangxi and Puzhe Black Skin in Yunnan, while green-skinned varieties include Baiyuzhe (Bamboo Sugarcane) in Guangdong and Green-skinned Sugarcane in Taiwan (Figure 2) (Ni et al., 2021). It is generally believed that the skin color is determined by the deposition of anthocyanins in the epidermis and does not directly affect the sucrose synthesis pathway. Therefore, there is no obvious difference in the potential sugar content between purple-skinned and green-skinned sugarcanes. However, in actual planting, different color varieties are often accompanied by other genetic background differences, resulting in different sweetness performance. Common varieties of purple-skinned sugarcane have thick stems and strong growth potential. At the same maturity, the sucrose content of their stem juice is often slightly higher than that of some green-skinned varieties (Chen et al., 2022). This may be because many purple-skinned sugarcanes are selected from the offspring of traditional inferior sugarcane varieties (tropical sugarcane S. officinarum), retaining the high-sugar gene; while some green-skinned bamboo sugarcane varieties have medicinal or processing purposes and are not specifically enhanced for high-sugar traits.

Figure 2 Three sugarcane varieties of different colors. Badila (a), ROC22 (b), and FN15 (c) are shown. The rind and pith are shown on the left and right, respectively (Adopted from Ni et al., 2021)

Of course, there are also high-sugar green-skinned varieties and low-sugar purple-skinned varieties, which mainly depends on their respective breeding parents and breeding goals. Taking the yellow-skinned sugarcane (also known as bamboo sugarcane) in Guangdong and the black-skinned sugarcane in Guangxi as examples, the field refractive sugar content can reach more than 16-18°Bx during maturity, which is comparable. However, the internodes of yellow-skinned sugarcane are longer, the fibers are looser, and the sweet juice is easy to release when chewing, while the internodes of black-skinned sugarcane are slightly shorter and the fibers are tighter, so the sweetness is released slightly slower but the aftertaste is stronger. There is no essential difference in the sugar accumulation pathways of the two, but the difference in fiber structure leads to different subjective sweetness perception when tasting: the green-skinned yellow sugarcane is more crisp and tender, and the juice quickly overflows the tip of the tongue, and the initial sweetness is strong; the purple-skinned sugarcane has harder fibers and needs to be chewed a few more times before the juice overflows, but the sugar concentration is high and the aftertaste is sweet and lasting. In general, there is little difference in the intrinsic sugar content between purple-skinned and green-skinned fresh sugarcanes, and the difference in sweetness between varieties is more due to the difference in sugar metabolism efficiency in their respective genetic backgrounds. For example, it has been reported that by comparing a pair of purple-skinned and green-skinned closely related lines, they have differences in the expression levels of sucrose transporter and invertase genes, resulting in different soluble sugar accumulation rates in the stems (Yuan et al., 2022). Therefore, in breeding, the sweetness should not be judged by skin color, but the sugar accumulation capacity of the variety should be evaluated by actual measurement through metabolic and molecular marker methods.

6.2 Texture variation among tender, juicy, and coarse genotypes

Fresh sugarcane can be further divided into tender and crisp type, juicy type and coarse fiber type according to texture. Tender and crisp varieties, such as Guangdong's Baiyuzhu sugarcane, are characterized by thin and easy-to-peel skin, crispy stems, soft fibers, and easy-to-break residues when chewed; juicy varieties, such as Guangxi's Guitang Xinyuan series, have slightly soft stems but a lot of juice, and a mouthful of sweet juice when bitten; coarse fiber varieties, such as some tall sugarcanes or dual-purpose sugarcanes, have thick stems but high fiber content, and the residues are not easy to break after chewing and need to be spit out. These texture differences first come from the differences in cell wall composition and structure. Tender and crisp sugarcanes often have low fiber and lignin content, thin cell walls, and underdeveloped mechanical tissues, so the stems are tender and easy to break. For example, experimental data show that the fiber content of some tender and crisp sugarcane varieties is only about 5% (fresh weight basis), which is significantly lower than the 8%-10% level of ordinary sugarcane (Chen et al., 2022), which confirms that low fiber content is conducive to improving the crisp taste. In addition to moderate fiber content, juicy varieties often have large stem pith cells, well-developed vacuoles, and high water content. Although the cell walls of this type of variety are not necessarily particularly thin, due to the fullness of juice, the cells are easily broken and release juice when chewed, and the fiber bundles are also dispersed by the juice, so it feels "more water and less residue".

In contrast, the anatomical structure of coarse-fiber varieties shows that their vascular sheaths and thick-walled tissues are well developed, the cell walls of xylem fibers are thick, lignin is deposited a lot, the intercellular spaces are small, and the water content is low (Wang et al., 2020). When chewing this type of sugarcane, the tough fibers are not easily cut by the teeth, and the juice is not easily squeezed out completely. The residue clumps into filaments and has a poor taste. Therefore, the formation of fresh sugarcane with different texture types is directly related to the quantity and quality of cell walls. In terms of molecular mechanism, it is speculated that the expression of genes in the cell wall synthesis pathway of tender and crisp sugarcane is weakened, while the expression of wall relaxation enzymes such as swelling protein and XTH may be relatively high, resulting in relatively thin and soft cell walls; the coarse fiber type is the opposite, and the genes related to lignin synthesis and secondary wall development may be highly expressed, making the cell wall thicker and harder. These inferences need to be verified by transcriptome and metabolome experiments. Existing studies such as Chen et al. (2022) reported that sugarcane generally has "low fiber content and sweet and delicious sugarcane juice", suggesting that high-quality fresh sugarcane tends to accumulate more sugar and water in metabolism and allocate less carbon to secondary wall substances. These characteristics are closely related to specific gene expression regulation and are the targets for the next step of quality improvement.

6.3 Transcriptomic comparison of elite vs. traditional germplasm

In order to understand the quality formation mechanism of fresh sugarcane from a global perspective, researchers have begun to use multi-omics methods to compare the differences between high-quality varieties and ordinary varieties and wild species. In particular, transcriptome sequencing can reveal the full picture of gene expression of different genotypes at key developmental stages. For example, a study compared the transcriptomes of high-sugar and low-fiber sugarcane varieties with those of a common sugarcane variety and found significant expression differences in hundreds of genes between the two, involving multiple functional categories such as carbohydrate synthesis and degradation, cell wall metabolism, hormone signaling, and transcriptional regulation (Li et al., 2023). Genes specifically upregulated in high-quality sugarcane varieties include some sucrose transport and synthesis enzymes (such as SWEET4, SPS genes) and cell wall modification enzymes (such as EXP, XTH genes), which are consistent with their high-sugar and soft phenotypes; while a group of genes highly expressed in common varieties are related to xylem development and defense response (such as PAL, DIR genes), which may explain the characteristics of their harder fibers but stronger stress resistance.

Similarly, similar trends have been observed between wild relatives and cultivated species of sugarcane. About 80% of the genome of modern sugarcane varieties is derived from high-sugar cultivated species (tropical sugarcane), 10%-20% is derived from high-fiber wild species (cut-hand dense), and a small part is a recombination of the two. This genomic hybridization has resulted in modern varieties that have both high sugar content and toughness. However, comparisons of extreme materials, such as the analysis of hybrid offspring between the wild species of Scutellaria baicalensis and the noble species, found that the sucrose content and fiber content of the stems were genetically negatively correlated. Some alleles from wild species tend to increase fiber yield but reduce sugar content, while alleles from cultivated species are the opposite. The average hammer of the wild species of Scutellaria baicalensis was only 8.75°Bx, and the hammer of the F1 offspring hybridized with cultivated sugarcane increased to 13.75°Bx, increased to 18.2°Bx in the first backcross generation, and reached 20.2°Bx in the third backcross generation. This shows that the sweetness of sugarcane can be greatly improved by aggregating the high-sugar genes of cultivated species and removing the high-fiber genes of wild species through several generations of backcrossing (Deng et al., 2019). The differences in gene expression behind this are also obvious: the expression levels of some key sugar accumulation genes such as SPS and SuSy in the cut hand secret are much lower than those in the cultivated species, while the expression of genes related to lignin synthesis is higher than that in the cultivated species (Zhang et al., 2018). Due to multiple generations of breeding, the transcriptome characteristics of modern sugarcane varieties tend to be more "high sugar and low fiber" gene expression profiles, that is, more similar to the ancestors of cultivated species.

By constructing sugarcane whole genome databases and transcriptome databases, researchers are gradually mapping gene network maps related to quality. This will provide a basis for the precise improvement of fresh sugarcane. For example, if it is known that the transcription factor A specifically expressed in a high-quality variety can simultaneously upregulate sucrose synthesis genes and downregulate lignin synthesis genes, then by molecular means to increase the expression of A in other varieties, it is expected to replicate its quality traits. Using multi-omics comparative analysis of the differences between high-quality fresh sugarcane varieties and ordinary varieties and wild species, a group of key genes that determine sweetness and texture can be identified, which will point out the direction for future molecular design breeding.

7 Concluding Remarks

The formation of sweetness and texture quality of fresh sugarcane stems is the result of the comprehensive effect of multi-level regulation. On the one hand, sucrose synthase, decomposition enzyme and transporter in the sugar metabolism pathway jointly determine the accumulation rate and upper limit of soluble sugar in stem cells; on the other hand, cell wall biosynthesis and modification enzyme system determine the amount and toughness of fiber. The two are both independent and mutually influential during the development process, and achieve dynamic balance through a complex signal network. High sweetness often means that the plant diverts more carbohydrates to the sucrose pool instead of building new cell walls, while a softer texture means appropriate adjustment of cell wall components and structures, such as reducing lignin and optimizing the fiber-colloid ratio. Fresh sugarcane varieties that can take into account both sweetness and softness are masterpieces of the above-mentioned mechanism of balanced regulation: they have an efficient sugar synthesis and transportation system and moderately "relax" the degree of cell wall reinforcement. In this balance, a series of signal and transcription regulatory factors play a key role, including feedback mechanisms mediated by sugar signals, the effects of hormones such as auxin/ethylene, and the overall coordination of transcription factors such as NAC and bZIP. Research in recent years has begun to reveal some unique regulatory mechanisms in sugarcane, such as sucrose affecting its own accumulation through uORF regulation of transcription factor ScbZIP44, and different co-expressed gene modules corresponding to two opposite traits of high sugar and high fiber. These findings provide a theoretical basis for a deeper understanding of the intrinsic relationship between sweetness and texture formation.

The cultivation of high-quality fresh sugarcane needs to be guided by the above-mentioned mechanism research. In traditional breeding, it is often necessary to balance many traits. For example, increasing the sugar content may cause the stem to become fragile and the yield to decrease. Therefore, only by deeply understanding the biological basis of sweetness and texture can we break the constraint of negative correlation of traits and realize the breeding of "double-excellent" varieties. Sugar metabolism enzymes, cell wall components and regulatory genes provide rich targets for future molecular breeding. For example, it is possible to consider using genetic engineering to overexpress SPS or inhibit invertase to increase sucrose accumulation; at the same time, moderately downregulate lignin synthesis genes (such as COMT) through gene editing to soften the fiber. Such a combination of measures is expected to breed new "sweeter and softer" varieties. In practical applications, marker-assisted selection should also be combined to aggregate high-sugar and low-fiber gene alleles into the same variety. Breakthroughs in sugarcane genome sequencing and map construction in recent years have created conditions for this. In particular, the multi-omics data of sugarcane has gradually become rich, and the analysis of traits aggregated by multiple genes has become possible. By integrating genomic, transcriptomic, and metabolomic data, researchers can accurately locate the key gene loci that affect sweetness and texture, and make precise improvements through technologies such as gene editing.

Looking to the future, the breeding of fresh sugarcane will shift from relying mainly on field trait selection to precise breeding of the "trait-gene-molecule" trinity: using multi-omics to analyze trait mechanisms, using genomics to lock in favorable genes, and finally using molecular technology to efficiently introduce these genes into new strains. It can be foreseen that in the near future, a batch of new fresh sugarcane varieties with both high sugar content and good taste will be born with the support of multi-omics and molecular breeding, further promoting the quality upgrade of the sugarcane industry.

Acknowledgments

We thank Mr Y. Ding from the Institute of Life Science of Jiyang College of Zhejiang A&F University for his reading and revising suggestion.

Conflict of Interest Disclosure

The authors affirm that this research was conducted without any commercial or financial relationships that could be construed as a potential conflict of interest.

References

Buckeridge M., Grandis A., and Tavares E. Q.P., 2019, Disassembling the glycomic code of sugarcane cell walls to improve second-generation bioethanol production, in Bioethanol Production from Food Crops, pp.17-35.

https://doi.org/10.1016/B978-0-12-813766-6.00002-3

Chen P., Bao H., Chen P., and Shen W., 2022, Field effect evaluation of healthy seed canes of chewing cane, Chinese Agricultural Science Bulletin, 38(16): 1-5.

Deng Q., Dou Z., Chen J., Wang K., and Shen W., 2019, Drought tolerance evaluation of intergeneric hybrids of BC3F1 lines of Saccharum officinarum × Erianthus arundinaceus, Euphytica, 215: 85.

https://doi.org/10.1007/s10681-019-2513-3

Huang D. D., and Li Z., 2024, Advances in molecular breeding techniques for pitaya (Hylocereus), Tree Genetics and Molecular Breeding, 14(5): 239-246.

https://doi.org/10.5376/tgmb.2024.14.0023

Kannan B., Jung J.-H., Moxley G. W., Lee S., and Altpeter F., 2018, TALEN-mediated targeted mutagenesis of more than 100 COMT copies/alleles in highly polyploid sugarcane improves saccharification efficiency without compromising biomass yield, Plant Biotechnology Journal, 16(4): 856-866.

https://doi.org/10.1111/pbi.12833

Khan Q., Qin Y., Guo D., Yang L., Song X., Xing Y., and Li Y., 2023, A review of the diverse genes and molecules involved in sucrose metabolism and innovative approaches to improve sucrose content in sugarcane, Agronomy, 13(12): 2957.

https://doi.org/10.3390/agronomy13122957

Khan Q., Chen J.Y., Zeng X.P., Qin Y.Z., Guo D.J., Mahmood A., Yang L. T., Liang Q., Song X.P., Xing Y., and Li Y.R., 2021, Transcriptomic exploration of a high sucrose mutant in comparison with the low sucrose mother genotype in sugarcane during sugar accumulating stage, GCB Bioenergy, 13(9): 1448-1465.

https://doi.org/10.1111/gcbb.12868

Li H., Chen Y., Zhang C., Zhang S., Hu L., Huang C., Wu H., Liu X., Wang H., and Li Y., 2023, Assessing differential gene expression and metabolic regulation underlying sugar and fiber accumulation in sugarcane genotypes, Plants, 12(5): 1041.

https://doi.org/10.3390/plants12051041

Liu Q., Hu P., Zheng M., Lin Z., Li Z., and Li T., 2024, Research on crop tissue structure and mechanical properties, Journal of South China Agricultural University, 45(3): 446-456.

Lu Y.Q., He Y.X., Wang Z.X., and Tshibunga F.M., 2024, Molecular mechanisms of cambium formation and activity maintenance: a systematic review of the collaborative regulation of tree stem cells in growth, development, and environmental adaptation, Tree Genetics and Molecular Breeding, 14(5): 218-228.

https://doi.org/10.5376/tgmb.2024.14.0021

Martins T.D.S., Magalhães Filho J.R., Cruz L., Machado D.F.S.P., Erismann N. M., Gondim-Tomaz R.M.A., Marchiori P.E.R., Silva A.L.B.O., Machado E.C., and Ribeiro R.V., 2024, Physiological and biochemical processes underlying the differential sucrose yield and biomass production in sugarcane varieties, Experimental Agriculture, 60: e13.

https://doi.org/10.1017/S0014479724000061

Mehdi F., Cao Z., Zhang S., Gan Y., Cai W., Peng L., Wu Y., Wang W., and Yang B., 2024a, Factors affecting the production of sugarcane yield and sucrose accumulation: suggested potential biological solutions, Frontiers in Plant Science, 15: 1374228.

https://doi.org/10.3389/fpls.2024.1374228

Mehdi F., Javed T., Liu X., Aman J., Riaz A., Galani S., Shaheen T., Nawaz M.A., and Peng L., 2024b, Current perspectives on the regulatory mechanisms of sucrose accumulation in sugarcane, Current Plant Biology, 27: 100240.

https://doi.org/10.1016/j.cpb.2024.100240

Mehdi F., Liu X., Riaz A., Javed T., Aman J., and Galani S., 2023, Expression of sucrose metabolizing enzymes in different sugarcane varieties under progressive heat stress, Frontiers in Plant Science, 14: 1221301.

https://doi.org/10.3389/FPLS.2023.1269521

Mira W., Heinz O., Gonçalvez A., Crema L., Vicentini R., Cardoso S., Berto G. L., Dias I. K. R., Arantes V., Romanel E., and Ferraz A., 2024, SacEXP32 sugarcane expansin gene expression increases cell size and improves biomass digestibility, Journal of Plant Biochemistry and Biotechnology. 33(3): 509-518.

https://doi.org/10.1007/s13562-024-00891-3

Narayan J., Chakravarthi M., Nerkar G., Manoj V.M., Dharshini S., Subramonian N., Premachandran M.N., Kumar R., Surendar K., Hemaprabha G., Ram B., and Appunu C., 2021, Overexpression of expansin EaEXPA1, a cell wall loosening protein, enhances drought tolerance in sugarcane, Industrial Crops and Products, 159: 113035.

https://doi.org/10.1016/j.indcrop.2020.113035

Ni Y., Chen H., Liu D., Zeng L., Chen P., and Liu C., 2021, Discovery of genes involved in anthocyanin biosynthesis from the rind and pith of three sugarcane varieties using integrated metabolic profiling and RNA-seq analysis, BMC Plant Biology, 21: 2986.

https://doi.org/10.1186/s12870-021-02986-8

Niu J., Miao X., Wang D., Huang W., Yang L., and Li Y., 2019, Analysis of sugar accumulation characteristics and metabolism-related enzymes activities in the high and low sugar sugarcane at elongation stage, Jiangsu Journal of Agricultural Sciences, 35(3): 530-538.

Perlo V., Margarido G., Jaffé F., Garcia A., Lakshmanan P., Ming R., and Henry R., 2022, Transcriptome changes in the developing sugarcane culm associated with high yield and early-season high sugar content, Theoretical and Applied Genetics, 135(5): 1619-1633.

https://doi.org/10.1007/s00122-022-04058-3

Roopendra K., Priyanka A., Chandra A., Akhter Y., and Saxena S., 2024, Transcriptome scale analysis to decode the differential sucrose accumulation mechanisms in sugarcane under the effect of gibberellin, Physiologia Plantarum, 176(2): e14290.

https://doi.org/10.1111/ppl.14290

Roopendra K., Sharma A., Chandra A., and Saxena S., 2018, Gibberellin-induced perturbation of source-sink communication promotes sucrose accumulation in sugarcane, 3 Biotech, 8: 1-13.

https://doi.org/10.1007/s13205-018-1429-2

Santiago T., Pereira V.M., de Souza W. R., Steindorff A., Cunha B.A., Gaspar M., Fávaro L. C.L., Formighieri E. F., Kobayashi A. K., and Molinari H. B.C., 2018, Genome-wide identification, characterization and expression profile analysis of expansins gene family in sugarcane (Saccharum spp.), PLoS ONE, 13(1): e0191081.

https://doi.org/10.1371/journal.pone.0191081

Tipu M. M.H., and Sherif S.M., 2024, Ethylene and its crosstalk with hormonal pathways in fruit ripening: mechanisms, modulation, and commercial exploitation, Frontiers in Plant Science, 15: 1475496.

https://doi.org/10.3389/fpls.2024.1475496

Wang F., Lin D., Wang Y., and Wang J., 2020, Research progress on insect resistance mechanism of sugarcane, Journal of Agriculture, 10(6): 32-39.

https://doi.org/10.3390/agriculture10060032

Wang H., Zhang C., Wu M., Li X., Jiang Z., Lin R., Guo J., and Que Y., 2023, Identification of the ATAF subfamily of NAC transcription factors in sugarcane cultivars and functional analysis of the ScNAC2 gene in cultivated varieties, Acta Agronomica Sinica, 49(1): 46-61.

https://doi.org/10.3724/SP.J.1006.2023.00046

Yuan Z., Dong F., Pang Z., Fallah N., Zhou Y., Li Z., and Hu C., 2022, Integrated metabolomics and transcriptome analyses unveil pathways involved in sugar content and rind color of two sugarcane varieties, Frontiers in Plant Science, 13: 921536.

https://doi.org/10.3389/fpls.2022.921536

Zhang J., Gao S., Zhang Y., Wang L., Li H., Wang H., Wang X., Liu X., Yang X., Chen K., and Xu Y., 2018, Identification of expression variation and candidate genes for sugar content and lignin synthesis in sugarcane using RNA-seq, BMC Genomics, 19: 51.

https://doi.org/10.1186/s12864-018-4436-3

Zhao D., and Li Y., 2015, Climate change and sugarcane production: potential impact and mitigation strategies, International Sugar Journal, 117(1399): 1443-1452.

https://doi.org/10.1155/2015/547386