Two recessive mutant alleles at CAN OF WORMSI (COWI), a new locus involved in root hair morphogenesis, have been identified in Arabidopsis fhaliana L. Heynh. Root hairs on Cowl -mutants are short and wide and occasionally formed as pairs at a single site of hair formation. l h e COWI locus maps to chromosome 4. Root hairs on Cowl-plants form in the usual positions, suggesting that the phenotype is not the result of abnormal positional signals. Root hairs on Cowl-roots begin hair formation normally, forming a small bulge, or root hair initiation site, of normal size and shape and in the usual position on the hair-forming cell. However, when Cowl-root hairs start to elongate by tip growth, abnormalities in the shape and elongation rate of the hairs become apparent. Cenetic evidence from double-mutant analysis of cowl-1 and other loci involved in root hair development supports our conclusion that COW7 is required during root hair elongation.
~~The morphogenesis of multicellular organisms is the result of coordinated changes in the number, position, size, and shape of cells. Cell shape has two different roles: First, coordinated changes in cell shape are required during the elaboration of multicellular structures; for example, during anther maturation, coordinated changes in the elongation of a subset of cells produce the anther stalk. Second, cell shape is important for the function of a number of specialized cell types; for example, leaf trichomes have a polarized structure and shape critica1 to their function in protecting against grazing by insects. Molecular genetic approaches have revealed mechanisms governing cellular morphogenesis, the process of shape change, in Drosophila melanogaster (Adler, 1992; Wong and Adler, 1993) and yeast (Kron and Gow, 1995). These approaches are now being applied to