Cotton Genomics and Genetics 2025, Vol.16, No.1, 1-11 http://cropscipublisher.com/index.php/cgg 4 There is also a gene called GhLTP4, which is related to both lipid transport and auxin response. If its expression is upregulated, the fibers will be longer; if it is downregulated, the fibers will be shorter (Duan et al., 2023). In addition, GhMYB212 is a transcription factor that can regulate the process of sucrose transport into cells, which is critical for elongation. If the expression of this gene is reduced, the fibers will become shorter and sucrose will also be reduced (Sun et al., 2019). This also shows that auxin not only acts directly, but also helps the fibers to lengthen by affecting sucrose transport. 4 Gibberellins and Fiber Growth 4.1. Role of gibberellins in cell expansion Gibberellins (GA) can make cells larger, which is important for the elongation of cotton fibers. There is a gene called GhGA20ox1 that is involved in the synthesis of GA. If it is expressed more, the GA in the cotton body, especially GA₄, will increase, resulting in longer fibers and more fibers growing on the ovules (Xiao et al., 2010). GA can also increase the expression of some cell wall-related genes, such as CesA (cellulose synthase), which help the cell wall grow so that the cell can continue to elongate (Xiao et al., 2015). In addition, GA will work with some transcription factors (such as GhHOX3) to transmit GA signals to genes that regulate cell wall loosening, thereby promoting the continued elongation of fiber cells (Xiao et al., 2015). 4.2 GA synthesis and sensitivity in cotton In cotton, the synthesis of GA and the plant's response to it are both controlled by specific genes. For example, GhGA20ox1, GhGA20ox2 and GhGA20ox3 are all involved in the synthesis of GA. Among them, GhGA20ox1 is mainly expressed in elongated fibers, while the other two are more active in ovules (Xiao et al., 2010). This different expression pattern shows that the production of GA is "regionally regulated" so that the fibers can be elongated in the right place. Another point is that GA response elements (called GARE) can be found in the promoters of many genes involved in cell wall synthesis, such as KCS and CesA. This means that these genes can directly receive GA signals and then regulate fiber growth and cell wall thickness (Xiao et al., 2015). 4.3. Interaction with Other Hormones GA does not "work alone". It often cooperates with other hormones to regulate fiber growth. For example, trimethylolate (SL) works after the GA signal. It can promote the synthesis of some very long chain fatty acids (VLCFAs) and cellulose, which is important for cell elongation and cell wall thickening (Zhang et al., 2022a). Studies have also found that GA can activate a transcription factor called GhGRF4, which in turn promotes the expression of SL-related genes, so that the amount of SL in fiber cells increases (Figure 2) (Zhang et al., 2022b). GA also cooperates with jasmonic acid (JA) to promote the initiation and elongation of fibers. Proteins such as GhJAZ3 and GhSLR1 are responsible for "messaging" between them (Xia et al., 2018). These complex hormone combinations indicate that the development of cotton fiber is not accomplished by a single hormone, but by multiple hormones working together. Figure 2 Schematic diagram depicting the mechanistic framework of SL-GA crosstalk signaling in the regulation of cotton fiber cell elongation and cell wall thickness (Adopted from Zhang et al., 2022b)
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