Cotton Genomics and Genetics 2025, Vol.16, No.1, 1-11 http://cropscipublisher.com/index.php/cgg 6 7 Emerging Roles of Brassinosteroids 7.1 Brassinosteroid pathways in fiber elongation Brassinosteroids (BR) are a plant hormone that is particularly important for cotton fiber elongation. It can help fiber growth by regulating some metabolic pathways. One of the key pathways is the synthesis of very long chain fatty acids (VLCFA). For example, when cotton lacks BR, the expression of GhKCSs (3-ketoacyl-CoA synthase) required for VLCFA synthesis will decrease, which will affect fiber elongation (Yang et al., 2023). In addition, BR also cooperates with the glucose signaling pathway. Glucose can increase the synthesis of BR and enhance the signaling function of BR, further helping fiber elongation (Li et al., 2021). The BR signaling process is not simple. It includes a series of "phosphorus addition" and "phosphorus removal" processes. GSK3 kinase is one of the "brake molecules" that inhibits BR signals (Wang et al., 2020). 7.2 Functional studies in cotton Many experiments have shown that BR is indeed critical to the growth of cotton fibers. For example, researchers expressed GhBES1.4, a BR-related transcription factor, more in cotton and found that the fibers became significantly longer; but if this gene was "turned off", the fibers became shorter (Liu et al., 2022; Yang et al., 2023) (Figure 3). For another example, adding external BR (such as brassinolide BL) to cotton will also make the fibers longer. On the contrary, if a drug called Brz is used to inhibit the synthesis of BR, the fibers will not grow (Sun et al., 2005). There is also a BR-deficient cotton mutant called pagda1 (or pag1), whose fibers cannot grow. But if BL is added to it, the fibers can resume growth. This also shows that BR is necessary for the development of cotton fibers (Yang et al., 2014). Figure 3 A model for the regulation of cotton fiber elongation through the crosstalk between BR and VLCFA. In the presence of BR, the key TF in the BR signaling pathway, GhBES1, moves to the nucleus and directly binds to the BRRE of GhKCSs to activate its expression. High expression of GhKCSs, encoding the rate-limiting enzyme of VLCFA biosynthesis, increases endogenous VLCFAs contents, which promotes fiber elongation (Adopted from Yang et al., 2023) 7.3 Interaction with auxin and gibberellins BR does not work alone. It often cooperates with auxin and gibberellins to affect fiber elongation. For example, some studies have added auxin and gibberellin to cultured cotton ovules and found that they can promote the initial development and elongation of fibers, which is very similar to the effect of BR (Sun et al., 2005). The relationship between these hormones is very complex. Studies have found that BR can enhance the expression of some genes in the auxin and gibberellin pathways, that is, it "amplifies" their effects (Xiao et al., 2019). In addition, BR and gibberellin can also regulate some of the same target genes together, which shows that they actually have a "cooperative mechanism" when regulating fiber elongation (Xiao et al., 2010).
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