TORNADO1 and TORNADO2 are required for the specification of radial and circumferential pattern in the Arabidopsis root

The cell layers of the Arabidopsis primary root are arranged in a simple ra dial pattern. The outermost layer is the lateral root cap and lies outside the epidermis that surrounds the ground tissue. The files of epidermal and lateral root cap cells converge on a ring of initials (lateral root cap/epi dermis initial) from which the epidermal and lateral root cap tissues of th e seedling are derived, once root growth is initiated after germination. Ea ch initial gives rise to a clone of epidermal cells and a clone of lateral root cap cells. These initial divisions in the epidermal/lateral root cap i nitial are defective in tornado1 (trn1) and trn2 plants indicating a requir ement for TRN1 and TRN2 for initial cell function. Furthermore, lateral roo t cap cells develop in the epidermal position in trn1 and trn2 roots indica ting that TRN1 and TRN2 are required for the maintenance of the radial patt ern of cell specification in the root. The death of these ectopic lateral r oot cap cells in the elongation zone (where lateral root cap cells normally die) results in the development of gaps in the epidermis. These observatio ns indicate that TRN1 and TRN2 are required to maintain the distinction bet ween the lateral root cap and epidermis and suggest that lateral root cap f ate is the default state. It also suggests that TRN1 and TRN2 repress later al root cap fate in cells in the epidermal location. Furthermore, the posit ion-dependent pattern of root hair and non-root hair cell differentiation i n the epidermis is defective in trn1 and trn2 mutants. Together these resul ts indicate that TRN1 and TRN2 are required for the maintenance of both the radial pattern of tissue differentiation in the root and for the subsequen t circumferential pattern within the epidermis.