Molecular characterization of cotton GhTUA9 gene specifically expressed in fibre and involved in cell elongation

The microtubule cytoskeleton may play an important role in the polarized growth of fibre cells that are single-cell trichomes on the surface of cotton ovules. To investigate whether the high expression levels of alpha-tubulin genes are correlated with fibre elongation, nine GhTUA genes (cDNAs) encoding alpha-tubulins with 449-451 amino acid residues were isolated and characterized in cotton. The GhTUA genes share high sequence homology at the nucleotide level (62-93% identity) in the coding region and at the amino acid level (89-99% identity), and can be classified into two subgroups. Real-time quantitative RT-PCR analysis revealed that seven out of the nine GhTUA genes are predominantly expressed in developing fibres. Among them, GhTUA9 displays the highest level of expression, revealing its fibre specificity. The GhTUA9 transcripts in fibres reached its peak value between 5-10 DPA, and dramatically declined to undetectable levels as the ovule matured further, suggesting that its expression is developmentally-regulated in fibres. The GhTUA9 gene including the promoter region was isolated from the cotton genome. To demonstrate the specificity of the GhTUA9 promoter, the 5 '-flanking region, including the promoter and 5 '- untranslated region, was fused with the GUS gene. Histochemical assays demonstrated that the GhTUA9: GUS gene was specifically expressed in elongating fibres. Overexpression of GhTUA9 in fission yeast (Schizosaccharomyces pombe) promoted atypical longitudinal growth of the host cells by 1.4-1.7-fold, indicating that the GhTUA9 gene is involved in cell elongation. Given all the above results, it is proposed that the GhTUA9 gene may play an important role in fibre elongation.