Harnessing the acid growth theory to optimize apoplastic acidification for enhancing cotton fiber elongation

Cotton (Gossypium spp.), a major global fiber crop, serves as an ideal model for research on plant cell development. According to the acid growth theory, plasma membrane (PM) H⁺-ATPase (HA) regulates cell wall acidification, thereby promoting cell elongation and providing a mechanistic framework for understanding this process. However, its application to cotton fiber cells has remained limited. In this study, the acid growth theory was utilized to investigate the elongation of cotton fibers. Comparative genomics revealed an expansion in the number of gene family members associated with acid growth, including PM HA and transmembrane kinase (TMK) genes, in tetraploid cotton. Transcriptomic analysis highlighted the co-expression of these genes during fiber elongation. Functional validation using chemical modulators and CRISPR-Cas9-mediated knockout mutants demonstrated that PM HA activity is essential for apoplastic acidification and fiber elongation. Specifically, GhHA4A and GhTMK3A were identified as potential regulators of proton extrusion; their loss-of-function mutants exhibited elevated apoplastic pH and reduced fiber length. Furthermore, the results indicated that an optimal apoplastic pH is required for fiber elongation, whereas insufficient or excessive acidification inhibits growth. Spatiotemporal modulation of PM HA activity in transgenic cotton plants enhanced fiber length without affecting other fiber- and seed-related traits, demonstrating the potential of the acid growth theory for fiber improvement. These findings not only extend the acid growth theory beyond conventional model systems but also provide an innovative strategy for increasing fiber length in cotton breeding.