Glucose regulates cotton fiber elongation by interacting with brassinosteroid.

In plants, glucose (Glc) plays important roles, as a nutrient and signal molecule, in the regulation of growth and development. However, the function of Glc in fiber development of Upland cotton (Gossypium hirsutum) is unclear. Here, using gas chromatography-mass spectrometry (GC-MS), we found that the Glc content in fibers was higher than that in ovules during the fiber elongation stage. In vitro ovule cultures revealed that lower Glc concentrations promoted cotton fiber elongation, while higher concentrations had inhibitory effects. The hexokinase inhibitor N-acetyl-glucosamine (NAG) inhibited cotton fiber elongation in the cultured ovules, indicating that Glc-mediated fiber elongation depends on the Glc signal transduced by hexokinase. RNA sequencing (RNA-seq) analysis and hormone content detection showed that 150 mM Glc significantly activated brassinosteroid (BR) biosynthesis, and the expression of signaling-related genes have also increased, which promoted fiber elongation. And in vitro ovule cultures clarified that BR induced cotton fiber elongation in a dose-dependent manner. In hormone recovery experiments, only BR compensated for the inhibitory effects of NAG on fiber elongation in a Glc-containing medium. However, the ovules cultured with a BR biosynthetic inhibitor brassinazole (BRZ) and from the BR-deficient cotton mutant pag1 had greatly reduced fiber elongation levels at all the tested Glc concentrations, demonstrating that Glc does not compensate for the inhibition of fiber elongation caused by BR biosynthetic defects, which suggested that BR signaling pathway works downstream of Glc during cotton fiber elongation. Altogether, our study showed that Glc occupies an enviable place and crosstalk occurs between Glc and BR signaling during modulation of fiber elongation.