AUXIN CONTROL OF THE SENSITIVITY TO AUXIN OF THE PROTON TRANSLOCATIONCATALYZED BY THE TOBACCO PLASMA-MEMBRANE H-ATPASE()

The sensitivity to indole-3-acetic acid of the proton translocation ca talyzed by the plasma membrane proton pump from tobacco cells was dete rmined in vitro, on plasma membrane vesicles, according to the 2,4-dic hlorophenoxyacetic acid concentration during cell culture. The sensiti vity was shown to increase by 20-fold along with the 2,4-dichloropheno xyacetic acid concentration (between 0.05 mu M and 0.25 mu M). Treatme nt of cells with indole-3-acetic acid prior to membrane purification p romoted an increase in the sensitivity up to 100-fold. This increase w as observed after treatment with micromolar indole-3-acetic acid for c ells cultured in the presence of 0.05 mu M 2,4-dichlorophenoxyacetic a cid, but required sub-millimolar indole-3-acetic acid concentrations f or cells cultured in the presence of 0.25 mu M 2,4-dichlorophenoxyacet ic acid. On the other hand, the increase in sensitivity occured within ca 20 min irrespective of the 2,4-dichlorophenoxyacetic acid concentr ation during cell culture. It is proposed that such increase in the se nsitivity could constitute a progressive amplification process favouri ng the stimulation of proton translocation by limited changes in auxin concentration.