Root development: Signaling down and around

Because of its elegant simplicity, the Arabidopsis root has become a model for studying plant organogenesis. In this review we focus on recent results indicating the importance of signaling in root development. A role for positional information in root cell specification has been demonstrated by ablation analyses. Through mutational analysis, genes have been identified that play a role in radial pattern formation. The embryonic phenotypes of these mutants raised the possibility that division patterns in post‐embryonic roots are dependent on signaling that originates during embryonic development. Analysis of expression of the SCARECROW gene indicates that it may play a role in this ‘top‐down’ signaling process. Characterization of root epidermis development has led to the identification of negative regulators of root‐hair formation. These appear to set up a prepattern which is reinforced by signaling by plant hormones.