THE TTG GENE IS REQUIRED TO SPECIFY EPIDERMAL-CELL FATE AND CELL PATTERNING IN THE ARABIDOPSIS ROOT

The control of cell fate was investigated in the root epidermis of Ara bidopsis thaliana. Two distinct types of differentiated epidermal cell s are normally present: root-hair-bearing cells and hairless cells. In wild-type Arabidopsis roots, epidermal cell fate was found to be corr elated with cell position, with root-hair cells located over radial wa lls between cortical cells, and with hairless cells located directly o ver cortical cells. This normal positional relationship was absent in ttg(transparent testa glabrous) mutants (lacking trichomes, anthocyani ns, and seed coat mucilage); epidermal cells in all positions differen tiate into root-hair cells. The opposite condition was generated in ro ots of transgenic Arabidopsis expressing the maize R (R-Lc) gene produ ct (a putative TTG homologue) under the control of a strong promoter ( CaMV35S), which produced hairless epidermal cells in all positions. In both the ttg and R-expressing roots, epidermal cell differentiation w as affected at an early stage, prior to the onset of cell elongation o r root-hair formation. The ttg mutations were also associated with abn ormalities in the morphology and organization of cells within and surr ounding the root apical meristem. The results indicate that alteration s in TTG activity cause developing epidermal cells to misinterpret the ir position and differentiate into inappropriate cell types. This sugg ests that, in wild-type roots, TTG provides, or responds to, positiona l signals to cause differentiating epidermal cells that lie over corti cal cells to adopt a hairless cell fate. (C) 1991 Academic Press, Inc.