A novel expression assay system for fiber-specific promoters in developing cotton fibers

Cotton fibers are unicellular trichomes that differentiate from epidermal cells of developing cotton ovules. Cotton fibers are not only an economically important commodity, but they also are a good experimental model for studying plant cell elongation and wall biogenesis because of their fast elongation rate and high cellulose content. During fiber development, many genes are known to be transcriptionally regulated; however, because of the long periods required to regenerate transgenic cottons and the lack of suitable transient assay systems, progress in characterizing cotton fiber-specific promoters has been slow. We report an expression assay system for determining promoter activities in cotton fibers by using cotton ovule culture and biolistic transformation techniques. The conditions for bombardment into cotton ovules were optimized to enhance the transformation efficiency of the beta-glucuronidase gene (GUS) into fiber cells. GUS gene expression patterns, which are regulated by a developmentally regulated fiber-specific promoter in cultured ovules, were consistent With the levels of the transcripts in planta. As a result, we propose that this newly developed expression assay system in cultured cotton fiber can be used to identify cis elements involved with the developmental and hormonal regulation of fiber development and fiber-specific expression.