Natural variation in the GhTTL promoter modulates cotton fiber elongation through the GhTALE-GhTTL-GhBIN2 module

Cotton fiber length, a key determinant of its industrial utility, is one of the most important agricultural traits subjected to domestication. However, the genetic determinants and molecular mechanisms underlying natural variation in fiber length remain inadequately characterized. We identified GhTTL as a critical positive regulator of fiber elongation through genome-wide association analysis (GWAS). Natural polymorphisms in the promoter region of GhTTL were shown to significantly influence the binding affinity of the transcription factor GhTALE to its cis-regulatory element, thereby modulating GhTTL expression levels across diverse cotton populations. Mechanistic investigations revealed that GhTTL interacts with GhBIN2, a central negative regulator of the brassinosteroid (BR) signaling pathway, and anchors it to the cell membrane. This interaction reduces cytoplasmic levels of GhBIN2, preventing it from associating with GhBES1, the master transcription factor of BR signaling. Consequently, this facilitates the nuclear localization of GhBES1, which promotes fiber cell elongation. These findings elucidate the molecular basis of natural variation in cotton fiber length, providing deeper insights into the regulation of fiber elongation. Additionally, this study highlights potential targets for genetic improvement to enhance cotton fiber quality and yield.