The SCARECROW Gene Regulates an Asymmetric Cell Division That Is Essential for Generating the Radial Organization of the Arabidopsis Root (original) (raw)

stem cells or initials: the columella root cap initials; the *Department of Biology pericycle/vascular initials; the epidermal/lateral root cap New York University initials; and the cortex/endodermal initials (Dolan et al., New York, New York 10003 1993) (Figure 1A). It has been shown that at least some † The University of Georgia of these initials undergo asymmetric divisions (Dolan et Complex Carbohydrate Research Center al., 1993). The cortex/endodermal initial, for example, Athens, Georgia 30602-4712 first divides anticlinally (in a transverse orientation) (Fig-‡ Department of Plant Sciences ure 1B). This asymmetric division produces another ini-University of Arizona tial and a daughter cell. The daughter cell then divides Tucson, Arizona 85721 periclinally (in a longitudinal orientation) (Figure 1B). This second asymmetric division produces the progenitors of the endodermis and the cortex cell lineages (Fig-Summary ure 1B). We have identified and characterized mutations that In the Arabidopsis root meristem, initial cells undergo disrupt the asymmetric divisions of the cortex/endoderasymmetric divisions to generate the cell lineages of mal initial (Benfey et al., 1993; Scheres et al., 1995). The the root. The scarecrow mutation results in roots that short-root (shr) and scarecrow (scr) mutations result in are missing one cell layer owing to the disruption of the loss of a cell layer between the epidermis and pericyan asymmetric division that normally generates cortex cle. In both mutants, the cortex/endodermal initial diand endodermis. Tissue-specific markers indicate that vides anticlinally, but the subsequent periclinal division a heterogeneous cell type is formed in the mutant. The that increases the number of cell layers does not take deduced amino acid sequence of SCARECROW (SCR) place (Benfey et al., 1993; Scheres et al., 1995). The suggests that it is a member of a novel family of putadefect is first apparent in the embryo and extends the tive transcription factors. SCR is expressed in the corlength of the embryonic axis, which includes the embrytex/endodermal initial cells and in the endodermal cell onic root and hypocotyl (Scheres et al., 1995). This is lineage. Tissue-specific expression is regulated at the also true for the other radial organization mutants chartranscriptional level. These results indicate a key role acterized to date, suggesting that radial organization for SCR in regulating the radial organization of the that occurs during embryonic development may influroot. ence the postembryonic organization generated by the meristematic initials (Scheres et al., 1995).