Cotton Genomics and Genetics 2025, Vol.16, No.4, 192-201 http://cropscipublisher.com/index.php/cgg 193 hormones are also at work, such as auxin, gibberellins, brassinolide and ethylene. Through single-cell and spatial transcriptome technology, researchers have discovered some key genes and metabolites that are very important for the formation and early development of fiber cells. For example, genes such as DOX2, KCS19.4, BEE3, HOS3.7, SVB and SVBL, as well as substances such as linoleic acid, spermine and spermidine. At this step, the number of initial fiber cells will be determined, and their number will directly affect how many fibers can be grown in the end (Qin et al., 2022; Zhai et al., 2023; Bai and Scheffler, 2024; Sun et al., 2025; Zhang et al., 2025). 2.2 Cellular characteristics during elongation After formation, fiber cells will grow rapidly, and some can even grow to several centimeters. This stage has several characteristics: the internal turgor pressure of the cell is very high, the cell wall will continue to loosen, and the cytoskeleton will also change, mainly the rearrangement of the actin and microtubule structures. At this time, the auxin signal becomes stronger, the enzymes related to cell wall relaxation are also very active, and lipid metabolism is also accelerated. These changes are conducive to cell expansion. Transcriptome and metabolome analysis found that the expression levels of many genes and metabolites have changed, among which aquaporins and regulatory factors related to gibberellins are particularly important. This stage mainly determines how long the fiber can grow, and the length of the fiber is a key criterion for evaluating fiber quality (Gou et al., 2007; Zhang et al., 2017; Prasad et al., 2022; Iqbal et al., 2023). 2.3 Secondary cell wall synthesis and maturation When cotton fibers stop lengthening, they enter the next important phase: the synthesis of secondary cell walls. This phase generally occurs between 16 and 24 days after flowering, but may vary for different cotton varieties. At this point, fiber cells begin to synthesize cellulose in large quantities, while other metabolic activities gradually decrease, allowing the cells to focus on one thing: building a cell wall that is almost entirely made of cellulose. Proteins such as GhTCP4 and GhFIM2 control when this process begins and how fast it progresses. Studies of protein and gene expression have found that many genes related to carbohydrate metabolism, the cytoskeleton, and cellulose synthase become particularly active during this phase. Other genes involved in general metabolism decrease in expression. Next comes the maturation phase. The walls of fiber cells continue to thicken, and the cells themselves slowly move toward programmed cell death. This phase is like the final step in cotton fiber development, as the cells complete their mission and exit, leaving only mature fibers with complete structures. This whole set of processes together determines the final quality of cotton fiber (Abidi et al., 2010; Wang et al., 2010; Zhou et al., 2019; Cao et al., 2020; Jareczek et al., 2023; Grover et al., 2024; Meera et al., 2024). 3 Major Types of Post-Translational Modifications 3.1 Phosphorylation Phosphorylation is a protein modification that can occur repeatedly. It is a process in which an enzyme called "kinase" adds a phosphate group to the protein. This action changes the function of the protein, such as increasing or decreasing its activity, and may also change its stability or affect the way it cooperates with other molecules. In plants, phosphorylation is very important. Especially in some signaling pathways such as MAPK, it can regulate cell differentiation and development. These processes are critical to the growth of cotton fibers. For example, phosphorylation can regulate the activity of certain enzymes and transcription factors, and also affect how plant hormones such as auxin transmit signals. And these changes will affect how the fiber elongates and when it matures (Aguilar-Hernández et al., 2020). 3.2 Ubiquitination Ubiquitination is another common modification. It is to attach ubiquitin molecules to proteins. It's like marking a protein. With this mark, the protein may be degraded or its function may change. This process requires three enzymes to complete together, called E1, E2 and E3. In plants, ubiquitination is often combined with other modifications, such as phosphorylation and acetylation. Together, they regulate protein stability and help cells transmit signals. This type of regulation is very helpful for the rapid growth of fiber cells and allows cells to change their morphology in a timely manner (Cui et al., 2023; Zhang and Zeng, 2020; Lacoursiere et al., 2022).
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