The COW1 Locus of Arabidopsis Acts after RHD2, and in Parallel with RHD3 and TIP1, to Determine the Shape, Rate of Elongation, and Number of Root Hairs Produced from Each Site of Hair Formation

Abstract

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

AXR3andSHY2interact to regulate root hair development

“…Divisions continue immediately behind the initials in the division zone. Following the cessation of cell division, the cells continue to elongate, in the elongation zone, after which they differentiate into either trichoblasts (root hair cells) or atrichoblasts (hair-less cells) (Grierson et al, 1997). Trichoblasts are always located over the junction between two underlying cortical cells, resulting in a pattern of alternating files of trichoblasts and actrichoblasts around the root.…”

“…Tip growth is a rapid form of directional elongation, which involves precise targeting of vesicles carrying cell wall precursors to the growing tip (Benfey and Schiefelbein, 1994;Grierson et al, 1997).…”

“…Root hair outgrowth itself can be split into three developmental stages: bulge formation, initiation and tipgrowth (Grierson et al, 1997). Tip growth is a rapid form of directional elongation, which involves precise targeting of vesicles carrying cell wall precursors to the growing tip (Benfey and Schiefelbein, 1994;Grierson et al, 1997).…”

Genetic Interactions during Root Hair Morphogenesis in Arabidopsis

“…Plants homozygous for mutations in COW1 and TIP1 produce short, wide root hairs, and sometimes multiple hairs form at a single site of hair formation (Schiefelbein et al, 1993;Grierson et al, 1997;Ryan et al, 1998). Plants homozygous for mutations in RHD3 and RHD4 have short, wavy root hairs (Schiefelbein and Somerville, 1990;Galway et al, 1997Galway et al, , 1999Wang et al, 1997).…”

“…Each hair is a long, narrow tube produced from a single cell by tip growth (the deposition of new membrane and cell wall material at a growing tip). Large collections of viable and fertile Arabidopsis root hair mutants are available (Schiefelbein and Somerville, 1990;Masucci and Schiefelbein, 1994;Grierson et al, 1997;Pitts et al, 1998) with all of the advantages of the Arabidopsis genome project. In Arabidopsis, root hairs form in a predictable pattern, so developing hair cells can be observed from the beginning of their life in the root meristem throughout the developmental process.…”