TARGETING OF AUXIN CARRIERS TO THE PLASMA-MEMBRANE - EFFECTS OF MONENSIN ON TRANSMEMBRANE AUXIN TRANSPORT IN CUCURBITA-PEPO L TISSUE

Treatment of etiolated zucchini (Cucurbita pepo L.) hypocotyl tissue w ith sub-micromolar concentrations of the cationophore monensin rapidly ( < 20 min) inhibited the transport catalytic activity of the specifi c auxin-anion efflux carrier and reduced the inhibition of this carrie r by the phytotropin N-1-naphthylphthalamic acid (NPA). Monensin inhib ited the basipetal polar transport of indol-3yl-acetic acid (IAA) in l ong (30 mm) zucchini segments. At concentrations lower than 10(-5) mol . dm-3 monensin did not affect uptake of the DELTApH probe [2-C-14]5, 5-dimethyloxazolidine-2,4-dione (DMO) or that of the membrane-potentia l probe tetra[C-14-phenyl]phosphonium bromide (TPP+), did not affect t he response of IAA net uptake to external Ca2+ concentration and did n ot alter the metabolism of IAA. It was concluded that low concentratio ns of monensin inhibit transport through the Golgi apparatus of auxin efflux carrier protein and that the efflux carriers turn over very rap idly in the plasma membrane. Monensin pretreatment did not affect the saturable binding of [H-3]NPA to microsomal membranes, indicating that the auxin-efflux catalytic sites and the NPA-binding sites are locate d on separate proteins. At higher concentrations (less-than-or-equal-t o 10(-5) mol . dm-3) monensin inhibited both mediated uptake and media ted efflux components of IAA transport. This effect was at least in pa rt attributable to perturbation by monensin of the driving forces for mediated uptake since high concentrations of monensin also reduced the uptake of DMO and TPP+.