Pattern in the Root Epidermis: An Interplay of Diffusible Signals and Cellular Geometry

The epidermis of roots is composed of hair and non-hair cells. Patterning of this epidermis results from spatially regulated differentiation of these cell types. Root epidermal development in vascular plants may be divided into three broad groups based on the mode of hair development; Type 1: any cell in the epidermis can form a root hair; Type 2: the smaller product of an asymmetric cell division forms a root hair; Type 3: the epidermis is organized into discrete files of hair and non-hair cells. The Arabidopsis root epidermis is composed of discrete files of hair and non-hair cells (Type 3). Genetic and physiological evidence indicates that ethylene is a positive regulator of hair cell development. Genes with opposite roles in the development of hair cells in the shoot (trichomes) and hair cells in the root have been identified. Plants with presumptive loss of function alleles in the Transparent Testa Glabra (TTG) or Glabra2 (GL2) genes are devoid of trichomes indicating that these genes are positive regulators of trichome development. The development of supernumerary root hair cells in these mutant backgrounds illustrates that these genes are also negative regulators of root hair cell development. A model that explains the spatial pattern of epidermal cell differentiation implicates ethylene or its precursor 1-amino-l-cyclopropane carboxylate as a diffusible signal. Possible roles for the TTG and GL2 genes in relation to the ethylene signal are discussed.