A ubiquitin–GhUBX–GhCDC48 module regulates secondary cell wall thickening in cotton fiber

The secondary cell wall (SCW) is a critical supportive and protective structure in plant growth and development. In cotton fibers, it is established that SCW thickening directly determines fiber strength by enhancing mechanical support, but the molecular mechanisms governing this process remain poorly understood. Here, we elucidate a ubiquitin-GhUBX-GhCDC48 regulatory module in which GhUBX serves as a bridge between GhCDC48 and ubiquitin, promoting GhCDC48 degradation via the ubiquitin-26S proteasome pathway. Compared to the wild type receptor (W0), transgenic cotton lines in which GhUBX is silenced or GhCDC48 is overexpressed exhibited significantly increased SCW thickness and fiber strength. Conversely, GhCDC48 silencing resulted in stunted plant growth and impaired cell wall development, consistent with its role in regulating the cell division cycle. Furthermore, we revealed that GhCDC48 abundance positively correlates with ATPase activity in fiber cells and with the transcriptional activation of key SCW biosynthesis genes including GhCesA7, GhCesA8, GhSusA, GhFSN1, GhKNL1, and GhNAC21, thereby enhancing SCW deposition. Our findings reveal the ubiquitin-GhUBX-CDC48 regulatory module that influences cotton fiber strength and provide a genetic strategy for improving fiber quality through targeted modification of genes in the ubiquitin pathway.