Cotton GhLAGE3-mediated GhMML3 transcription activates fiber initiation upon reception of cell cycle exit signals

Cotton ( Gossypium hirsutum L.) fiber initiation generally determines fiber yield, however, the mechanisms are still unclear. The MYB-MIXTA-Like transcription factor GhMML3 is a master regulator of fiber initiation upon receiving cell cycle exit signals by a yet unknown mechanism. Y1H screening identified GhLAGE3, a nuclear localized PCC1 domain protein, as a binding factor of the GhMML3 promoter. We show that binding of GhLAGE3 to the GhMML3 promoter strongly induces GhMML3 expression and that GhLAGE3 expression is strongly regulated in the ovule outer integument during the −3–3 DPA period critical for fiber cell differentiation. In addition, GhLAGE3 expression is induced by cell cycle arrest chemicals, but repressed by cell cycle acceleration inducers. Characterization of CRISPR-Cas9 knockout Ghlage3 mutants showed markedly reduced GhMML3 expression accompanied by decreased fiber density and lint percentage but showed little or no differences with WT controls on overall growth phenotype, seed size, seed index or fiber length. Our findings provide evidence that PCC1-like GhLAGE3 transmits cell cycle exit signals to GhMML3 for fiber initiation, offering new insights into cotton fiber development and providing a target for high-yield breeding. • GhLAGE3 (PCC1-like protein) binds GhMML3 promoter identified via yeast one-hybrid. • GhLAGE3 knockout impairs fiber initiation, downregulates GhMML3 . • GhLAGE3 bridges cell cycle exits to promote GhMML3-driven fiber initiation.