Mutational analysis of root initiation in the Arabidopsis embryo

In dicotyledonous plants, primordia of most seedling organs are laid down between fertilization and the formation of the heart-stage embryo. In Arabidopsis embryogenesis, highly regular cell divisions and cell expansions facilitate the characterization of mutant development. We have taken a genetic approach to identify genes involved in organising the hypocotyl/root axis. The initiation of this axis is marked by oriented expansions and longitudinal divisions of cells in the lower tier of the early globular embryo. These cells go on to form a defined number of parallel cell files that constitute the hypocotyl and most of the radicle. Mutants impaired in the initiation or elaboration of the hypocotyl/root axis were selected by their seedling phenotype and subsequently analysed at embryonic stages. Several conclusions are suggested by the phenotypes of these mutants. First, hypocotyl/root axis formation can be genetically separated from other aspects of embryonic pattern formation. Second, initiation of the hypocotyl/root axis appears to be genetically distinct from post-embryonic root initiation. Third, four loci were identified that appear to contribute to the elaboration of the axial pattern. Finally, an anatomical inspection of one of the mutants, amenable to an analysis at post-embryonic stages, suggests a genetic link between basal pattern formation in the embryo and aspects of vascular differentiation in the adult plant.