Box 2 | Comparison of trichome and root-hair patterning

Root hairs seem to be specified by position-dependent induction as root hairs are only formed over the cleft between two underlying CORTEX CELLS (see figure, part a). Epidermal cells that are positioned on a cortex cell do not form root hairs. Most of the genes (or their homologues) that are involved in root-hair patterning are also involved in trichome development (see figure, part b). The proposed models for trichome and root-hair patterning are similar, but the resulting phenotypes are different. Whereas those cells that eventually express GLABRA2 (GL2) in the shoot become trichomes, GL2-expressing root epidermal cells adopt a non-root-hair cell fate. For both systems it is postulated that in those cells that express the GL2 gene, a trimeric active complex is formed that consists of a MYB-related transcription factor (GLABRA1 (GL1) in trichomes or WEREWOLF (WER) in roots), a basic helix–loop–helix protein (GLABRA3 (GL3) and the homologous ENHANCER OF GL3 (EGL3)) and a WD40 protein (TRANSPARENT TESTA GLABRA1 (TTG1)). This active complex is thought to activate GL2 and the expression of the negative regulators TRIPTYCHON (TRY), CAPRICE (CPC) and CAPRICE TRIPTYCHON1 (ETC1). These travel into the neighbouring cells where they compete with the MYB-related transcrption factor for binding to the complex, which causes the complex to become inactivated. Identical or homologous proteins are shown in the same colour.