GhMAX2 Contributes to Auxin-Mediated Fiber Elongation in Cotton (Gossypium hirsutum)

Strigolactones (SLs) represent a new group of phytohormones that play a pivotal role in the regulation of plant shoot branching and the development of adventitious roots. In cotton (Gossypium hirsutum, Gh), SLs play a crucial role in the regulation of fiber cell elongation and secondary cell wall thickness. However, the underlying molecular mechanisms of SL signaling involved in fiber cell development are largely unknown. In this study, we report two SL-signaling genes, GhMAX2-3 and GhMAX2-6, which positively regulate cotton fiber elongation. Further protein-protein interaction and degradation assays showed that the repressor of the auxin cascade GhIAA17 serves as a substrate for the F-box E3 ligase GhMAX2. The in vivo ubiquitination assay suggested that GhMAX2-3 and GhMAX2-6 ubiquitinate GhIAA17 and coordinately degrade GhIAA17 with GhTIR1. The findings of this investigation offer valuable insights into the roles of GhMAX2-mediated SL signaling in cotton and establish a solid foundation for future endeavors aimed at optimizing cotton plant cultivation.