Antagonistic bHLH Transcription Factors CsIBH1 and CsPRE5 Fine-Tune <i>CsPMEI14</i> to Enhance Fluoride Tolerance via CW Pectin Remodeling in Tea Plants

Fluoride (F) abounds in acidic tea plantation soils, and excessive F accumulation threatens tea plant growth and safe tea production. Cell wall (CW) pectin metabolism orchestrates plant stress adaptation. As key regulators, pectin methylesterase inhibitors (PMEIs) modulate pectin characteristics by regulating pectin methylesterase (PME) activity, yet their roles in F tolerance remain largely unclear. Here, we identified CsPMEI14 as a key F-responsive gene: its overexpression enhanced F tolerance by reducing F accumulation and remodeling CW pectin to increase pectin content and strengthen CW F fixation. Furthermore, two bHLH transcription factors (TFs), CsIBH1 (activator) and CsPRE5 (repressor), antagonistically regulated CsPMEI14 transcription. CsIBH1 coordinated broader CW remodeling, whereas CsPRE5 acted as a subtle modulator. CsWRKY75 functioned as a coregulator by interacting with both TFs, enhancing CsIBH1-mediated activation and alleviating CsPRE5-mediated repression. Collectively, this CsWRKY75-CsIBH1/CsPRE5- CsPMEI14 regulatory module provides new insights into PMEI-mediated CW remodeling and potential targets for improving plant stress resistance.