Cotton Genomics and Genetics 2025, Vol.16, No.1, 1-11 http://cropscipublisher.com/index.php/cgg 5 5 Ethylene Signaling in Fiber Elongation 5.1 Ethylene’s contribution to fiber cell wall softening Ethylene can make cotton fiber cell walls softer, which is a key step in cell elongation. It promotes the production of reactive oxygen species (such as hydrogen peroxide H2O2), which affect the activity of some enzymes, such as ascorbate peroxidase (APX). This enzyme helps stabilize the level of reactive oxygen, thereby loosening the cell wall and making the cell larger (Qin et al., 2008). In addition, ethylene can also increase the expression of some cell wall-related genes, such as expansin. This protein can directly soften the cell wall and make the fiber elongate more smoothly (Shi et al., 2006). 5.2. Regulatory genes involved in ethylene synthesis Ethylene synthesis does not occur randomly. It is controlled by some enzymes, especially ACS (1-aminocyclopropane-1-carboxylic acid synthase) and ACO (1-aminocyclopropane-1-carboxylic acid oxidase). For example, genes such as ACO1-3 are activated in large quantities when the fiber is elongated, indicating that they are directly related to the increase of ethylene (Shi et al., 2006). There is also an E3 ubiquitin ligase called GhXB38D, which inhibits the synthesis of ethylene. It marks ACS4 and ACO1 for degradation, thereby reducing the amount of ethylene and affecting the elongation of the fiber (Song et al., 2023). 5.3. Temporal dynamics of ethylene in fiber development The role of ethylene is also different at different time stages. Studies have found that the level of ethylene is the highest during the stage of rapid fiber elongation. At this time, ethylene can help the cell swell and is also conducive to its continued elongation (Shi et al., 2006). The production of ethylene is also regulated by the reactive oxygen species (ROS) feedback mechanism. Simply put, ethylene promotes the production of H2O2, and H2O2 can in turn stimulate the synthesis of more ethylene. In this way, the level of ethylene can be continuously maintained at a state that is conducive to growth (Qin et al., 2008). In addition, during this rapid growth period, some genes related to ethylene response, such as GhVTC1, will also be activated. It is involved in the synthesis of ascorbic acid, which also shows that the role of ethylene in this stage is very critical (Song et al., 2019). 6 Cytokinin and Abscisic Acid (ABA) Interplay 6.1 Impact of cytokinin on fiber development Cytokinins are important for plants, as they help cells divide and differentiate. However, they behave differently in cotton fiber development. Studies have found that cytokinins actually inhibit fiber growth. For example, after external addition of cytokinins, the initiation and elongation of cotton fibers are weakened (Xiao et al., 2019; Wu, 2024). This inhibitory effect may be because it changes the expression of some genes related to cell wall synthesis. And the cell wall is very important for fiber elongation. 6.2 ABA’s role in stress response and elongation Abscisic acid (ABA) is the main hormone used by plants to cope with stress such as drought and salt. Its "main job" is to cope with stress, but in cotton fiber development, it also plays a certain inhibitory role. Studies have shown that when ABA levels increase, cotton fibers tend to become shorter. This shows that ABA is not conducive to fiber elongation (Xiao et al., 2019). This may be because ABA affects the expression of some genes related to elongation and stress. In this way, plants will shift resources from "growth" to "self-protection". 6.3 Balance between cytokinin and aba for fiber growth Both cytokinin and ABA hormones inhibit the elongation of cotton fibers. However, there is a "degree" to their interaction. If the two can maintain a balance, it will help the fiber grow normally. If one of them is too strong, it will easily break the balance and make the fiber development worse. Therefore, plants need to precisely regulate the levels of these two hormones. On the one hand, the fiber can continue to elongate, and on the other hand, it must be prepared to deal with external pressure (Xiao et al., 2019). Understanding how they interact will help us use hormone regulation methods to improve cotton fiber quality and yield.
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