Sphingolipids promote fiber elongation via the GhSIL- <i>GhBR6OX2</i> module in upland cotton

Fibers are the main economic product of cotton (Gossypium hirsutum). As an ideal model for studying plant cell development, many factors that influence cotton fiber have been revealed. Previous studies have demonstrated the important role of sphingolipids in fiber development through chemical and genetic methods, but the regulatory module by which sphingolipids regulate fiber development remains unclear. Here, we used the sphingolipid biosynthesis inhibitor fumonisin B1 (FB1) and found that it effectively suppressed the expression of an orthologue of the trihelix transcription factor SIP1 clade-like (GhSIL) in upland cotton. Lipid-protein binding assays also revealed that ceramide (t18:0/24:0) and 24-epibrassinolide (BL) bind to GhSIL protein in vitro. GhSIL was preferentially expressed in elongating fibers. The transcriptome and reverse transcription quantitative PCR analysis revealed that the expression of brassinosteroid synthesis genes was significantly decreased in GhSIL downregulated cotton fibers during the fiber elongation stage. Yeast 1-hybrid assay, dual-luciferase (LUC) assay, and chromatin immunoprecipitation (ChIP) assays showed that GhSIL can directly bind to the GT1-box element in the brassinosteroid-6-oxidase 2 (GhBR6OX2) promoter region to enhance its expression. Overexpression of GhSIL and GhBR6OX2 promoted fiber elongation, consistent with the results of exogenous application of BL in cotton ovule culture in vitro. By contrast, suppressing GhSIL and GhBR6OX2 expression inhibited fiber elongation, which was similar to the exogenous application of brassinazole (BRZ). These results illuminated that sphingolipids regulate fiber cell elongation through the GhSIL-GhBR6OX2-brassinolide module, which reveals a node in the molecular mechanism by which sphingolipids regulate plant growth and development.