Differential ethylene sensitivity of epidermal cells is involved in the establishment of cell pattern in the Arabidopsis root

Root hairs of Arabidopsis roots develop on trichoblasts located over the an ticlinal (radial) walls of underlying cortical cells. Non-hair cells, on th e other hand, develop on atrichoblasts overlying the periclinal (tangential ) walls of cortical cells. Dark-grown wild-type seedlings, which produce li ttle ethylene, are largely root hairless. Exogenous treatment of dark-grown plants with either ethylene or 1-aminocyclopropane-1-carboxylic acid (ACC) restores the development of root hairs in cells overlying the anticlinal c ortical cell walls, indicating that cells in this position are more sensiti ve to ethylene than atrichoblasts. We used mutations in genes that overprod uce ethylene (eto1, eto2, eto3 and eto4) to illustrate the positive regulat ory role of ethylene. The preferential development of root hairs on epiderm al cells overlying the cortical anticlinal cell walls in these mutants also illustrates that trichoblasts are more sensitive to ethylene than atrichob lasts. CTR1 is a negative regulator of the ethylene response and might, the refore, be a candidate regulator of differential sensitivity. CTR1 mRNA is expressed in all cell types in the root, suggesting that its transcriptiona l pattern alone cannot account for the differential sensitivity of epiderma l cells to ethylene. Cellular mapping of wild-type and mutant roots support s previous findings indicating that ethylene acts after, and perhaps indepe ndently, of TTG during the establishment of cell fate in the root epidermis .