Constructing a plant cell. The genetic control of root hair development.

Multicellular organisms possess a diverse array of cell types, which vary in size, shape, composition, and function. Therefore, a fundamental feature of development in multicellular organisms is the proper specification and differentiation of distinct cell types. In plants, the formation of hair cells in the root epidermis has been employed for more than a century as a simple model for studies of cell specification and differentiation (Cormack, 1935; Bunning, 1951; Cutter, 1978). Root hair cells are easy to observe and analyze because of their long tubular projections (root hairs), which are thought to aid plants in nutrient uptake, anchorage, and microbe interactions (Cutter, 1978). During the past several years, there has been a surge in research activity in root hair development, particularly in Arabidopsis. This heightened interest can be attributed to several features of root hair development that facilitate genetic analyses. First, root hairs are not essential for plant viability, which permits the recovery and analysis of all types of root hair mutants. Also, root hairs are visible on seedling roots shortly after seed germination, enabling highdensity genetic screens on defined media in petri dishes. Furthermore, several aspects of root hair development in Arabidopsis occur in a remarkably predictable fashion, including the position-dependent patterning of hair cells and the localization of hair outgrowth at a precise site along the epidermal cell. These features provide the opportunity for efficient detection of mutant phenotypes. This update article focuses on recent findings uncovered by the genetic analysis of root hair development in Arabidopsis. More broadly, the work outlined here provides a framework for understanding the genes and developmental processes that are necessary to construct a plant cell.