Plant development: pulled up by the roots.

Recent advances in the study of root development in Arabidopsis have begun to yield mechanistic insights into the processes that underpin morphogenesis, pattern formation and cell differentiation in plants. The most exciting feature of these advances is that, as a consequence of the simple and largely invariant cellular architecture of the root, the processes can be studied at a cellular level. Nevertheless, it is clear that although the cell lineages of the root are relatively invariant, we have no evidence that lineage per se is an important regulator of development. Instead, all the evidence indicates that the positional regulation of inductive cues is of primary importance. The availability of new root mutants is alerting us to the complexity of the contribution of cell size and cell expansion to plant development.