GhTTLL12 Coordinates With Transcriptional Regulators GhMML3 and GhMYB86 to Orchestrate Cotton Fibre Development Through Modulating Microtubule Dynamics.

Microtubules (MTs) are crucial for cell division, growth, development and morphogenesis in plants. Cotton fibres are single-celled trichomes that originate from the epidermal cells of the ovule, making them an excellent model for studying plant cell differentiation and rapid elongation. However, the roles of MTs in cotton fibre development remain incompletely understood. In this study, we identified GhTTLL12, a tubulin-tyrosine ligase-like protein 12, as a positive regulator of fibre initiation and elongation via Gh-Gb introgression analysis. GhTTLL12 was preferentially expressed during the rapid elongation stage of cotton fibres. Overexpression of GhTTLL12 enhanced plant height, root length, fibre cell protrusion number and fibre length in cotton. Conversely, CRISPR/Cas9-mediated knockout of GhTTLL12 led to opposite phenotypes, thereby significantly reducing fibre quality. MT co-sedimentation and immunofluorescence assays demonstrated that GhTTLL12 binds directly to MTs and promotes their assembly while facilitating the formation of transverse MT arrays in elongating fibres. Further investigation of the molecular mechanisms revealed that after GhTTLL12 is recruited into the nucleus by GhTUB8, it promotes mitosis in ovule epidermal cells upon activation by GhMML3, increasing the number of fibre cell protrusions. GhMYB86, a negative regulator of cotton fibre elongation, represses GhTTLL12 transcription in the nucleus, leading to attenuated mitotic activity. Cytoplasmic GhTTLL12 modulates fibre cell elongation by regulating MT assembly and ordered arrangement. Collectively, our findings define a GhMML3/GhMYB86-GhTTLL12-GhTUB8 regulatory module that links stage-specific transcriptional regulation to MT remodelling during cotton fibre development, providing new insights into the improvement of fibre quality and yield.