Mapping the Phosphorylation Regulatory Network of Single‐Celled Fibre Elongation Mediated by <scp> <i>GhBIN2</i> </scp> in Cotton

Protein phosphorylation plays a pivotal role in cellular signal transduction and plant development. The plant steroid hormone Brassinosteroids (BRs) signal transduction relies primarily on protein kinase-mediated phosphorylation cascades. However, the specific mechanisms of phosphorylation regulation in BR signalling remain to be fully elucidated. This study focuses on BIN2, an indispensable protein kinase in the BR signalling pathway, utilising single-celled cotton fibre to investigate the mechanisms by which phosphorylation regulates cell elongation. Firstly, we confirmed the inhibitory role of GhBIN2 in fibre elongation through its overexpression. Subsequently, we employed 4D-fastDIA quantitative phosphoproteomics and proteomics analysis to map the GhBIN2-mediated phosphorylation regulatory network. Through a comprehensive analysis of this network, we identified six credible substrates of GhBIN2. Further investigation revealed that GhBIN2 interacts with substrate GhIQD14 and increases its abundance through phosphorylation to negatively regulate fibre elongation. This study deepens the understanding of BR signalling in cotton fibre elongation and provides experimental evidence and new insights for comprehending the regulatory role of protein phosphorylation in plant cell elongation processes.