Cotton Fiber Growth in Planta and in Vitro. Models for Plant Cell Elongation and Cell Wall Biogenesis

There are only a few cells in the plant kingdom that are as exaggerated in their size or composition as cotton fibers. It is precisely their highly elongated structure and exceptional chemical make-up that establishes cotton fiber as an ideal model for studies of plant cell elongation and cell wall biogenesis. Cotton fibers are unicellular, therefore cell elongation can be evaluated independently from cell division. Although commonly called fibers, the more botanically appropriate term is trichome, since these cells are not part of the vascular tissue and arise, instead, from the ovule epidermis. The economically important seed trichomes of Gossypium hirsutum, the cotton variety most commonly grown in the United States, typically range in length from 2.2 to 3.0 cm. Fibers from another commercially important species, Gossypium barbadense, may reach lengths of over 6 cm, or one-third the height of an Arabidopsis plant. Fiber cells from G. hirsutumrange in diameter from 11 to 22 μm and are, therefore, 1,000 to 3,000 times longer than they are wide. Cotton fibers are linear cells and are never branched like many leaf trichomes. In addition to being among the longest plant cells ever characterized, a single cell wall biopolymer, cellulose, accounts for more than 95% of the dry weight of mature cotton fiber. Unlike many plant secondary cell walls, the cotton fiber wall contains no lignin. Typical of many plant cells, cotton fibers have a large central vacuole that becomes prominent quite early in development.