Mature cotton fiber secondary cell wall contains over 90%cellulose with little xylan and lignin.However,little is known about the regulation of secondary wall biosynthesis in cotton fiber.In this report,we have characterized a R2R3-MYB transcription factor,GhMYB7,previously proposed to function in the transcriptional regulation of the Ltp3 gene and participate in the GA signaling pathway during fiber development.GhMYB7 is predominantly expressed in developing fibers,most strongly in 0 dpa fibers and to a lower extent in 20 dpa fibers undergoing secondary wall thickening.Its encoded protein is targeted to the nucleus and has transcriptional activation activity suggesting GhMYB7 is likely to be transcriptional activator functioning in cotton fiber.We showed that ectopic expression of GhMYB7 in Arabidopsis resulted in small up-curly dark green leaves,shorter and thinner inflorescence stems.In addition,cross sections of basal stems revealed cell wall thickness of vessels and interfascicular fibers is increased in overexpression lines compared with the wild type.We also demonstrated that constitutive expression of GhMYB7 in Arabidopsis induced ectopic deposition of cellulose and lignin and activated expression of a suite of secondary wall biosynthetic genes and secondary wall associated transcriptional factors.The ectopic deposition of secondary walls may have started early before cessation of cell expansion.Moreover,GhMYB7 was capable of binding to the promoters of AtSND1 and AtCesA4 which implies GhMYB7 may function upstream of NAC transcription factors.Collectively,these findings suggest GhMYB7 may be a potential transcriptional activator and participate in the regulation of the whole secondary wall biosynthesis program in cotton fiber.