Cotton Genomics and Genetics 2025, Vol.16, No.1, 1-11 http://cropscipublisher.com/index.php/cgg 2 estrolactone. But some hormones prevent cell elongation, such as cytokinin and abscisic acid (Xiao et al., 2019; Jan et al., 2022). For example, brassinolide (BR) can affect the synthesis of a substance called "very long chain fatty acids" (VLCFAs). VLCFAs are critical for cell wall formation and elongation (Yang et al., 2023). In addition, there is a small peptide hormone called PSK-α, which can regulate potassium ion efflux and plant respiration, thereby helping cells elongate (Han et al., 2014). 2.2 Crosstalk between hormonal pathways Cell elongation is not determined by just one hormone. It is the result of the combined action of multiple hormones. They cooperate with each other and sometimes affect each other's effects. For example, studies have found that glucose (Glc) can work with brassinolide (BR) to promote the growth of cotton fibers. Glucose activates some genes related to BR, making the fibers grow faster (Li et al., 2021). For another example, a protein called GhHOX3 and gibberellin (GA) signals also cooperate to enhance the expression of a protein in fiber cells that makes the cell wall "loosener", thereby helping the cells to grow longer (Shan et al., 2014). The "cooperation" of these hormones can enable plants to better cope with different environmental changes, and make cell lengthening more coordinated, which is beneficial to the development of the entire plant. 2.3 Significance in cotton fiber growth Cotton fibers are very special. They are the result of a single cell being elongated. Studying this cell elongation is very helpful in understanding how plants grow. Many hormones contribute to the growth of cotton fibers. For example, auxin, gibberellin and brassinolide (Xiao et al., 2019; Jan et al., 2022). Studies have found that a key gene in the BR signaling pathway is called GhBES1.4. When its expression is increased, it will regulate the genes that control VLCFA synthesis, thereby making the fibers grow longer (Liu et al., 2022; Yang et al., 2023). There is also a small peptide hormone called PSK-α, which can also help fibers elongate by regulating potassium ion outflow and reactive oxygen (Han et al., 2014). These studies show that hormone regulation is very important for the development of cotton fibers. Understanding these processes will help us improve the quality and yield of cotton. 3 Auxin-Mediated Regulation 3.1 Mechanisms of auxin signaling in fiber cells Auxin is particularly important in making cotton fiber cells grow longer. It works through a series of signals. There is a transcription factor called GhTCP14, which is a key regulator discovered in the study. It can affect the expression of auxin-related genes, such as AUX1 and PIN2. These genes determine how auxin is distributed and how it works in the cell (Wang et al., 2013). There is also an auxin response factor called GhARF18, which directly binds to the promoters of gibberellin-related genes, such as GhGA3OX4D and GhGA20OX1D-2. This increases the synthesis of gibberellins and makes the fibers grow faster (Zhu et al., 2021). In addition, GhERF108 interacts with GhARF7-1 and GhARF7-2. This effect connects the signals of ethylene and auxin, which is critical for the formation of cell walls in the later stages of fiber (Wang et al., 2023). 3.2 Auxin transport and distribution in cotton Auxin does not run around randomly. Its transport and distribution in cotton fibers are accomplished by PIN proteins. GhPIN3a is a special auxin transporter that can transport auxin outward and control its concentration in fiber cells. If these GhPIN genes are inhibited, cotton fibers will not easily begin to develop and their length will also be affected (Zhang et al., 2016). There is also a GTPase called GhROP6, which can control the location and degradation of GhPIN3a, thereby concentrating auxin in cells. This is very important for cell elongation (Xi et al., 2022). In addition, the study also found that most auxin is transported, not synthesized by fiber cells themselves, which also shows that the role of transport proteins is particularly large (Zhang et al., 2016). 3.3 Genetic evidence of auxin’s role in elongation Many genetic experiments have shown that auxin can indeed make cotton fibers longer. For example, if GhARF18 is expressed more, the fibers will become longer; and if GhARF16 is knocked out, the fibers will become shorter. This shows that they are indeed regulating fiber length (Figure 1) (Zhu et al., 2021).
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