Cotton Genomics and Genetics 2025, Vol.16, No.5, 241-248 242 2 Developmental Biology of Cotton Fiber Maturation 2.1 Developmental stages of cotton fiber and characteristics of the maturation phase Not all types of fibers mature at the same time. The length of this process can vary significantly among different genotypes. Generally speaking, the development of cotton fibers can be roughly divided into several overlapping stages: initiation, elongation, secondary cell wall (SCW) synthesis, and final maturation. What truly affects the quality is this last step. At the mature stage, cells have basically completed cellulose deposition and are no longer living cells but transform into hollow structures. At this time, the dehydration process is also underway, which has a significant impact on the strength, dyeability and yield of the fibers. Fibers that are not fully mature have weak strength and uneven dyeing, which will limit their textile applications. Mature fibers, however, exhibit excellent fineness and structural integrity (Prasad et al., 2022; Iqbal et al., 2023; Jareczek et al., 2023). 2.2 Dynamic changes in cell wall composition during maturation After the fibers stop stretching, the most obvious change is the rapid thickening of the cell wall. At this stage, the composition of the cell wall undergoes a drastic transformation, with cellulose almost becoming the sole protagonist, and its content can reach up to 98% at its peak. In fact, behind this is the synthetic transformation from primary wall to secondary wall, and the time point of this transformation varies among different types of cotton. Between 17 and 24 days after flowering, many studies have been able to observe this critical window through methods such as Fourier transform infrared spectroscopy (Abidi et al., 2010; Jareczek et al., 2023). The structure of the wall, such as its thickness and crystallinity, ultimately determines whether the fibers can meet the standards. Therefore, it is not the case that the more deposits there are, the better. It also depends on whether the organizational structure is reasonable. 2.3 Metabolic shifts and regulatory signals in the maturation process During the period of fiber maturation, metabolism within cells is not static. On the contrary, at this time, metabolic pathways actually undergo a "role transformation". At the beginning, energy was mainly used in aspects such as cell expansion and relaxation of wall structure. But once it enters the mature stage, the system will shift its focus to the synthesis of cellulose and the thickening of cell walls. This transformation is controlled by multiple signals, among which several plant hormones play a leading role, such as abalic acid, auxin, gibberellin, brassinolide, etc. Transcriptomic and proteomic studies have identified many gene modules and expression patterns related to these hormonal pathways (Gou et al., 2007; Xiao et al., 2019). Of course, hormones alone are not enough. What truly determines the final outcome is the coordination of multiple levels of regulation, including transcription factors, enzyme activity, and signal integration. If well regulated, the output and quality of fibers will be more stable (Jan et al., 2022; Grover et al., 2025). 3 Categories and Functions of Key Enzymes Related to Fiber Maturation 3.1 Cell wall biosynthetic enzymes Ultimately, the "building" process of cell walls plays a significant role in whether cotton fibers can grow strong or not. Enzymes like cellulose synthase exert the greatest force during the secondary wall formation stage, directly determining the strength and structure of the fiber. Rather than all the enzymes involved in the process being active at the same time period, such as β-1, 3-glucanase (GhGLU18), which mainly exerts its force in the late elongation stage and during secondary wall synthesis. Once the expression level of this enzyme increases, the accumulation of polysaccharides and the thickening of the cell wall will also be significantly enhanced. Conversely, if it is inhibited, a phenotype of short fibers and weak structure will be observed. GhGLU18 is also regulated by the NAC transcription factor GhFSN1. This hierarchical relationship indicates that it is not a supporting role in cell wall metabolism (Fang et al., 2024). As for cinnamyl alcohol dehydrogenase (GhCAD37A/D), its role is to participate in the synthesis of lignin monomers. When the lignin content changes, the mechanical properties of the fiber will naturally change accordingly (Li et al., 2024). 3.2 Oxidoreductases and energy metabolism enzymes: regulation of respiratory chain and antioxidant systems Oxidative stress has always been there, but not all enzymes can handle it. On the path of fiber maturation, ascorbic acid peroxidase (APX) becomes particularly important. Different APX subtypes come into play at different stages.
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