GhbHLH18 Negatively Regulates Fiber Strength And Elongation By Enhancing Lignin Biosynthesis In Cotton

Cotton fibers are developed epidermal cells of the seed coat and contain high amounts of cellulose and minor lignin components.Lignin in the cell walls of cotton fibers effectively provides mechanical strength and is also presumed to restrict fiber elongation and secondary cell wall synthesis.To analyze the effect of lignin and lignin-like phenolics on fiber quality and the transcriptional regulation of lignin synthesis in cotton fibers,we characterized the function of a bHLH transcription factor,GhbHLH18,during the fiber elongation stage.GhbHLH18 knock-down plants had longer and stronger fibers,and accumulated less lignin-like phenolics in both developing and mature cotton fibers.By mining public transcriptomic data for developing fibers and other cotton tissues,we discovered that GhbHLH18 is coexpressed with most lignin synthesis pathway genes.Furthermore,yeast one hybrid assay and EMSA assay showed that GhbHLH18 directly binds to the E-box in the peroxidase8(GhPER8)promoter region and activates its expression.Transient expression of GhPER8 protein in tobacco leaves significantly decreased the content of ferulic alcohol and sinapic alcohol-the substrate respectively for G-lignin and S-lignin biosynthesis.These results demonstrate that GhbHLH18 is negatively associated with fiber quality and activates peroxidase-mediated lignin metabolism,thus the paper represents an alternative strategy to improve fiber quality.