International Journal of Horticulture, 2025, Vol.15, No.5, 218-233 http://hortherbpublisher.com/index.php/ijh 226 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.
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