Auxin is a positive regulator for ethylene-mediated response in the growthof arabidopsis roots

The requirement of auxin for the ethylene-mediated growth response in the r oot of Arabidopsis thaliana seedlings was investigated using two ethylene-r esistant mutants, aux1-7 and eir1-1, whose roots have been shown to have a defect in the auxin influx and efflux carriers, respectively. A 50% inhibit ion of growth (I-50) was achieved with 0.84 mul liter(-1) ethylene in wild- type roots, but 71.3 mul liter(-1) ethylene was required to induce I-50 in eir1-1 roots. In aux1-7 roots, I-50 was not obtained even at 1,000 mul lite r-1 ethylene. By contrast, in the presence of 10 nM l-naphthaleneacetic aci d (NAA), the concentrations of ethylene required to induce I-50 in eir1-1 a nd aux1-7 roots were greatly reduced nearly to the level required in wild-t ype roots, Since the action of NAA to restore the ethylene response in aux1 -7 roots was not replaced by IAA, an increase in the intracellular level of auxin is likely to be the cause for the restoration of ethylene response. NAA at 10 nM did not inhibit root growth when applied solely, but it was th e optimum concentration to recover the ethylene response in the mutant root s. These results suggest that auxin is a positive regulator for ethylene-in duced inhibition in root elongation.