Three cotton homeobox genes are preferentially expressed during early seedling development and in response to phytohormone signaling

Homeodomain-leucine zipper (HD-Zip) proteins are transcription factors unique to plants. In this study, three cDNAs (designated as GhHB2, GhHB3 and GhHB4) encoding HD-Zip proteins were isolated from cotton cDNA library. GhHB2 gene encodes a protein of 300 amino acids, GhHB3 gene encodes a peptide with 254 amino acids, and GhHB4 gene encodes a protein of 281 amino acids. The deduced proteins, which contain the homeodomain and leucine-rich zipper motif, share relatively high similarities with the other plant HD-Zip proteins. Quantitative RT-PCR analysis indicated that GhHB3 and GhHB4 were preferentially expressed in hypocotyls and cotyledons, whereas GhHB2 gene was predominantly expressed in young stems, at relatively high levels in hypocotyls. Expressions of all the three genes were up-regulated in roots, hypocotyls and cotyledons after GA(3) treatments. Additionally, GhHB4 expression was enhanced by 6-BA treatment. A GhHB2 promoter fragment was isolated from cotton by Genome-Walking PCR. Expression of GUS gene controlled under GhHB2 promoter was examined in the transgenic Arabidopsis plants. Strong GUS staining was detected in cotyledon, veins of the emerging leaves and shoot apices of 5- to 15-day-old transgenic seedlings, but GUS activity became more and more weak as the seedlings further developed. In addition, the promoter activity was induced by exogenous GA, indicating that GhHB2 promoter is very active during early seedling development, and may be GA-inducible. The results suggested that the three HB genes may function in early seedling development of cotton and in response to gibberellin signaling.