Fasciclin-like arabinogalactan proteins (FLAs), a subclass of arabinogalactan proteins (AGPs), are usually involved in cell development in plants. To investigate the expression profiling as well as the role of FLA genes in fiber development, 19 GhFLA genes (cDNAs) were isolated from cotton (Gossypium hirsutum). Among them, 15 are predicted to be glycosylphosphatidylinositol anchored to the plasma membranes. The isolated cotton FLAs could be divided into four groups. Real-time quantitative reverse transcriptase polymerase chain reaction results indicated that the GhFLA genes are differentially expressed in cotton tissues. Three genes (GhFLA1/2/4) were specifically or predominantly expressed in 10 days post-anthesis fibers, and the transcripts of the other four genes (GhFLA6/14/15/18) were accumulated at relatively high levels in cotton fibers. Furthermore, expressions of the GhFLA genes are regulated in fiber development and in response to phytohormones and NaCl. The identification of cotton FLAs will facilitate the study of their roles in cotton fiber development and cell wall biogenesis.

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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.

Summary Aquaporins are thought to be associated with water transport and play important roles in cotton (Gossypium hirsutum) fibre elongation. Among aquaporins, plasma membrane intrinsic proteins (PIPs) constitute a plasma‐membrane‐specific subfamily and are further subdivided into PIP1 and PIP2 groups. In this study, four fibre‐preferential GhPIP2 genes were functionally characterized. The selective interactions among GhPIP2s and their interaction proteins were studied in detail to elucidate the molecular mechanism of cotton fibre development. GhPIP2;3 interacted with GhPIP2;4 and GhPIP2;6, but GhPIP2;6 did not interact with GhPIP2;4. Coexpression of GhPIP2;3/2;4 or GhPIP2;3/2;6 resulted in a positive cooperative effect which increased the permeability coefficient of oocytes, while GhPIP2;4/2;6 did not. GhBCP2 (a blue copper‐binding protein) inhibited GhPIP2;6 water channel activity through their interaction. Overexpression of GhPIP2 genes in yeast induced longitudinal growth of the host cells. By contrast, knockdown of expression of GhPIP2 genes in cotton by RNA interference markedly hindered fibre elongation. In conclusion, GhPIP2 proteins are the primary aquaporin isoforms in fibres. They selectively form hetero‐oligomers in order to regulate their activities to meet the requirements for rapid fibre elongation.

SummaryMicrosporogenesis, associated with the functional expression of many genes, is a highly programmed and regulated process in flowering plants. To elucidate the roles of genes during anther development, two antherspecific cDNAs (designated GhACS1 and GhACS2) encoding acyl-CoA synthetases (ACSs) were isolated from a cotton (Gossypium hirsutum) flower cDNA library. Subsequently, the corresponding GhACS1 gene was isolated from a cotton genomic DNA library. Real-time quantitative RT-PCR and northern blot analyses revealed that GhACS1 transcripts were predominantly accumulated in the developing anthers of cotton. The specificity of GhACS1 expression in primary sporogenous cells (PSCs), pollen mother cells (PMCs), microspores, and tapetal cells was demonstrated by in situ hybridization as well as histochemical assay of GUS expression controlled under the GhACS1 promoter. High levels of GhACS1 activity are crucial for fatty acid metabolism in PSCs, PMCs, microspores and particularly tapetal cells. Reduction of ACS enzymatic activity by suppressing GhACS1 expression severely affected the tapetal cells and consequently blocked normal microsporogenesis in early anther development. Aberrant and defective microspores were generated in the transgenic anthers. As a result, the transgenic plants failed to produce functional pollen grains and were male-sterile, suggesting that the GhACS1 gene is required for normal microsporogenesis in early anther development of cotton.