Auxin carriers in Cucurbita vesicles : III. Specificity, with particular reference to 1-naphthylacetic acid.
Carrier-mediated uptake of indole-3-acetic acid (IAA) by microsomal vesicles from Cucurbita pepo L. hypocotyls was strongly inhibited by 2,4-dichlorophenoxyacetic acid (2,4-D; i 50= 0.3 μM) but only weakly by 1-naphthylacetic acid (NAA). The fully ionised auxin indol-3-yl methanesulphonic acid also inhibited (i 50=3 μM). The same affinity ranking of these auxins for the uptake carrier, an electroimpelled auxin anion-H(+) symport, is demonstrable in hypocotyl segments. The specificity of the auxin-anion eflux carrier was tested by the ability of different nonradioactive auxins to compete with [(3)H]IAA and reduce the stimulation of net radioactive uptake by N-1-naphthylphthalamic acid (NPA), a noncompetitive inhibitor of this carrier. By this criterion, NAA and IAA had comparable affinities, with 2,4-D interaction more weakly. Stimulation of [(3)H]IAA uptake by NAA, as a result of competition for the efflux carrier, could also be demonstrated when a suitable concentration of 2,4-D was used selectively to inhibit the uptake carrier. However, when [(3)H]NAA was used, no stimulation of its association with vesicles by NPA, 2,3,5-triiodobenzoic acid, or nonradioactive NAA was found. In hypocotyl segments, [(3)H]NAA net uptake was much less sensitive to NPA stimulation than was [(14)C]IAA uptake. The apparent contradictions concerning NAA could be explained by carrier-mediated auxin efflux making a smaller relative contribution to the overall transport of NAA than of IAA. The relationship between carrier specificity as manifested in vitro and the specificity of polar auxin transport is discussed.
Department of Biochemistry, University of Cambridge, CB2 1QW, Cambridge, UK.
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Rubery