Biochemical Bases for the Loss of Basipetal IAA Transport with Advancing Physiological Age in Etiolated Helianthus Hypocotyls: Changes in IAA Movement, Net IAA Uptake, and Phytotropin Binding

Basipetal transport of ¹⁴C]IAA in hypocotyl segments isolated from various regions of etiolated Helianthus annuus L. cv NK 265 seedlings declines with increasing physiological age. This decline was the result of a reduction in both transport capacity and apparent velocity. Net IAA uptake was greater and the abilities of auxin transport inhibitors to stimulate net IAA uptake were reduced in older tissues. Net IAA accumulation by microsomal vesicles exhibited a similar behavior with respect to age. Specific binding of ³H]N-1-naphthylphthalamic acid (NPA) to microsomes prepared from young and older hypocotyl regions was saturable and consistent with a single class of binding sites. The apparent affinity constants for NPA binding in microsomes prepared from young versus older tissues were 6.4 and 10.8 nanomolar, respectively, and the binding site densities for young versus old tissues were 7.44 and 3.29 picomoles/milligram protein, respectively. Specific binding of ³H]NPA in microsomes prepared from both tissues displayed similar sensitivities toward unlabeled flurenol and exhibited only slight differences in sensitivity toward 2,3,5-triiodobenzoic acid. These results demonstrate that the progressive loss of basipetal IAA transport capacity in etiolated Helianthus hypocotyls with advancing age is associated with substantial alterations in the phytotropin-sensitive, IAA efflux system and they suggest that these changes are, at least partially, responsible for the observed reduction of polar IAA transport with advancing tissue age.