Mapping the epigenetic regulatory network of cotton single‐cell fiber elongation mediated by H3K4me3

Summary Histone modifications are central to plant development, yet how they coordinate cellular morphogenesis remains poorly understood. Cotton fibers, as single‐celled trichomes derived from seed epidermal cells, provide an ideal model to address this question. Multi‐omics approaches were used, including cleavage under targets and tagmentation for genome‐wide H3K4me3 profiling and transcriptome sequencing. These were integrated with molecular and genetic validation experiments such as virus‐induced gene silencing, overexpression, CRISPR/Cas9, yeast one/two‐hybrid, electrophoretic mobility shift assay, co‐immunoprecipitation, and split‐luciferase complementation assays. We identify GhMYBL as a core transcriptional repressor that negatively regulates fiber elongation. GhMYBL is subject to dual control: H3K4me3 governs its transcription, whereas GhHBI1 posttranslationally attenuates its DNA‐binding activity. GhMYBL directly represses the positive regulators GhXTH22 and GhALP1 , limiting elongation; GhHBI1 relieves this repression, enabling their normal expression and synergistic promotion of fiber elongation. Our results demonstrate that differential H3K4me3 modification orchestrates GhMYBL‐mediated transcriptional repression, which is fine‐tuned by GhHBI1 interaction to coordinate fiber cell elongation. This study provides mechanistic insights into the epigenetic control of cell elongation and highlights promising targets for cotton fiber improvement.