Cotton fibers (Gossypium hirsutum)are unicellular trichomes and consist of almost pure cellulose. During the transition from elongation growth to secondary wall thickening during cotton fiber development, the rate of cellulose biosynthesis rises nearly 100-fold. Although the first two cellulose synthase catalytic subunits (CesAs) were isolated from developing cotton fibers, it is not clear how many CesAs are involved in secondary wall synthesis in fibers and how they are regulated during cell wall biogenesis.

Using quantitative RT-PCR, we have shown that at least six different secondary wall CesAs are involved in secondary wall biosynthesis in developing cotton fiber. Expression levels of secondary wall CesAs in cultured fibers could be up-regulated with exogenous auxin and/or abscisic acid at a time prior to initiation of secondary cell wall synthesis, and were down-regulated with the addition of exogenous gibberellic acid. To study transcriptional regulation of secondary wall CesAs, we have characterized a secondary wall CesA promoter by in silico analyses and functional analyses of a promoter deletion series in cotton ovule cultures and transgenic Arabidopsis seedlings. The promoter analyses show that several putative cis-acting auxin responsive elements may be involved in basal-level expression in Arabidopsis root vascular tissue and cotton fibers, and two other cis-acting elements may be involved in repression of CesA expression by auxin. For further analyses, additional secondary wall CesA promoters in G. hirstutum have been isolated from BIBAC libraries and are being characterized.