A COMMON POSITION-DEPENDENT MECHANISM CONTROLS CELL-TYPE PATTERNING AND GLABRA2 REGULATION IN THE ROOT AND HYPOCOTYL EPIDERMIS OF ARABIDOPSIS

A position-dependent pattern of epidermal cell types is produced durin g root development in Arabidopsis thaliana. This pattern is reflected in the expression pattern of GLABRA2 (GL2), a homeobox gene that regul ates cell differentiation in the root epidermis. GL2 promoter:GUS fusi ons were used to show that the TTG gene, a regulator of root epidermis development, is necessary for maximal GL2 activity but is not require d for the pattern of GL2 expression. Furthermore, GL2-promoter activit y is influenced by expression of the myc-like maize R gene (35S::R) in Arabidopsis but is not affected by gl2 mutations. A position-dependen t pattern of cell differentiation and GL2-promoter activity was also d iscovered in the hypocotyl epidermis that was analogous to the pattern in the root. Non-GL2-expressing cell Files in the hypocotyl epidermis located outside anticlinal cortical cell walls exhibit reduced cell l ength and form stomata. Like the root, the hypocotyl GL2 activity was shown to be influenced by ttg and 35S::R but not by gl2. The parallel pattern of cell differentiation in the root and hypocotyl indicates th at TTG and GL2 participate in a common position-dependent mechanism to control cell-type patterning throughout the apical-basal axis of the Arabidopsis seedling.