Building a root: the control of patterning and morphogenesis during root development.

Roots are plant organs adapted to acquire water and nutri- ents from the environment. The rapid exploratory growth habit of roots makes them well suited for experimental stud- ies of organogenesis in plants. In this regard, one particu- larly advantageous feature of root development is that tissues and cell types arise from an apical meristem that is relatively simple in structure. Moreover, root development is not complicated by the formation of lateral appendages at the apex (Esau, 1965). The resulting regular arrangement of tissue and cell types simplifies the analysis of pattern formation and enables us to predict accurately the developmental history and fate of each of the tissues and cells from their location. Furthermore, cellular studies of root development are aided by the transparent nature of roots, and genetic analyses profit from the fact that roots of numerous seedlings growing on media of defined composition can be observed. In this review, we describe studies that are beginning to uncover the fundamental mechanisms controlling root de- velopment, with particular emphasis on the analysis of tis- sue and cell-type patterning. These studies point toward the general view that patterning in the root is largely guided by positional cues that are established during embryogenesis and maintained during postembryonic root development. Moreover, recent genetic and physical manipulations of de- veloping root cells have led to new insights into the plasticity of root development.