Cotton Classical Extensin GhCEXT1-A Regulates Fiber Elongation by Modulating Cell Wall Structure and Composition.

Classical extensins (CEXTs) are hydroxyproline-rich glycoproteins that belong to the extensin superfamily and constitute critical structural components of the plant cell wall. However, the genome-wide characterization and functional role of CEXTs in cotton fiber development remain very limited. Here, we conducted a comprehensive comparative genomic analysis across four Gossypium species and identified a total of 30 CEXT genes, which were systematically classified into four evolutionary lineages (Groups A-D) based on analyses of phylogenetic relationship and gene structural organization. Sequence alignment analysis suggested the presence of two ancestral CEXTs in angiosperms, which subsequently expanded in representative dicots, indicating their potential functional diversity. Transcriptomic profiling and quantitative real-time PCR validation showed that GhCEXT1-A was predominantly expressed during rapid fiber elongation stages in G. hirsutum. Virus-induced gene silencing of GhCEXT1-A in cotton led to significantly shortened fiber length, confirming its role as a positive regulator of fiber elongation. Further cell wall component detection indicated that in GhCEXT1-A-VIGS cotton plants, soluble pectin content was significantly reduced, while the contents of cellulose, hemicellulose, and lignin were significantly accumulated, coupled with a marked increase in cell wall thickness. These results suggest that GhCEXT1-A affects cotton fiber elongation by regulating the dynamics of major cell wall component and structure. This work establishes a genomic framework for the cotton CEXT family and identifies GhCEXT1-A as an important positive regulator linking pectin-driven cell wall extensibility to fiber elongation, providing a molecular basis for precision breeding of cotton.