The morphological development of cotton fibers has been documented (Basra and Malik, 1984; Graves and Stewart, 1988; Mauney, 1984; Ramsey and Berlin, 1976; Ruan and Chourey, 1998; Ruan et al., 2000; Stewart, 1975). Cotton fibers undergo four overlapping developmental stages: fiber cell initiation, elongation, secondary wall biosynthesis, and maturation. Fiber initiation is a rapid process. White fuzzy fibers begin to develop immediately after anthesis and continue up to 3 days post-anthesis (DPA), which is followed by fiber cell elongation (until 10 DPA). Secondary wall biosynthesis initiates and continues to 25 DPA, followed by a maturation process until 45-60 DPA. Cotton fibers are derived from ovular epidermal cells (maternal tissues). However, only ~25-30% of the epidermal cells differentiate into the commercially important lint fibers (Kim and Triplett, 2001). The majority of cells do not differentiate into fibers or develop into short fibers or fuzz. For the cells committed to fiber development, cell initiation and elongation are nearly synchronous on each ovule, indicating that changes in gene expression are orchestrated during fiber differentiation and development through intercellular signaling and/or timing mechanisms.
Cotton fibers are seed trichomes. Unlike leaf trichomes (Hulskamp et al., 1994; Hulskamp and Schnittger, 1998; Marks, 1997), cotton fibers are unicellular and never branch. Trichome initiation in Arabidopsis leaves is mediated through the action of GLABROUS1 (GL1) and TRANSPARENT TESTA GLABRA (TTG), whereas GL2 is required for cell expansion, branching and maturation (Szymanski et al., 2000). The GL1 and GL2 genes encode Myb and homeodomain transcription factors, respectively. Genetic tests indicate that GL2 acts downstream of TTG and GL1 since gl2/gl1 and gl2/tg mutants lack trichomes whereas those with gl2 mutations initiate trichomes normally (Hulskamp et al., 1994; Szymanski and Marks, 1998). The molecular basis for cottonseed trichomes is unknown.
During fiber elongation and secondary wall metabolism, the fiber cells elongate rapidly, synthesize secondary wall components, and show dramatic cellular, molecular and physiological changes. Fiber elongation is coupled with rapid cell growth and expansion (Seagull, 1991) and constant synthesis of a large amount of cell metabolites and cell wall components such as cellulose. Indeed, ~95% of the dry-weight in mature cotton fibers is cellulose (Pfluger and Zambryski, 2001; Ruan et al., 2001). Non-cellulosic components are also important to fiber cell development (Hayashi and Delmer, 1988; Huwyler et al., 1979; Meinert and Delmer, 1977; Peng et al., 2002). Compared to other plant cells, cotton fibers do not contain lignin in secondary walls but have large vacuoles that are presumably related to rapid cell growth and expansion (Basra and Malik, 1984; Kim and Triplett, 2001; Mauney, 1984; Ruan and Chourey, 1998; Ruan et al., 2000). Moreover, endoreduplication observed in Arabidopsis leaves (Galbraith et al., 1991; Szymanski and Marks, 1998) also occurs in cotton fibers (van.t Hof, 1999), although the role of endopolyploidy in fiber cell development is unclear. Changes in gene expression have been observed during fiber development (Ferguson et al., 1996; Graves and Stewart, 1988; John and Crow, 1992; Liu et al., 2000) and many genes have been studied (Cui et al., 2001; Delmer et al., 1995; John, 1996; John and Crow, 1992; John and Keller, 1995; Kawai et al., 1998; Loguercio et al., 1999; Ma et al., 1997; Orford and Timmis, 2000; Pear et al., 1996; Reinhart et al., 1996; Shimizu et al., 1997; Smart et al., 1998; Song and Allen, 1997; Whittaker and Triplett, 1999).
