The primary walls of cotton fibers contain an ensheathing pectin layer

Cotton fiber walls (1-2 days post anthesis) are distinctly bilayered compar ed to those of nonfiber epidermal cells, with a more electron-opaque outer layer and a less electron-opaque, more finely fibrillar inner layer. When p robed with antibodies and affinity probes to various saccharides, xylogluca ns and cellulose are found exclusively in the inner layer and de-esterified pectins and extensin exclusively in the outer layer. Ovular epidermal cell s that do not differentiate into fibers have no pectin sheath, but are labe lled throughout with antixyloglucan and cellulase-gold probes. Middle lamel lae between adjacent cells were clearly labelled with the antibodies to de- esterified pectins, however. Similarly, cell walls of leaf trichomes have a bilayered wall strongly enriched in pectin, whereas other epidermal cells are not bilayered and are pectin poor. These data indicate that one of the early markers of fiber and trichome cells from other epidermal cells involv es the production of a pectin layer. The de-esterified pectins present in t he ensheathing layer may allow for expansion and elongation of the fiber ce lls that does not occur in the other epidermal cells without such a sheath or may even be a consequence of the elongation process.