Influence of 2,3,5-triiodobenzoic acid on the transport and metabolism of IAA in lupin hypocotyls

Abstract

The influence of 2,3,5-triiodobenzoic acid (TIBA) on the transport and metabolism of indolyl-3-acetic acid (IAA) was studied in etiolated lupin (Lupinus albus L) hypocotyls. Double isotope-labeled IAA [(5-³H)-IAA plus (1-¹⁴C)-IAA] was applied to the cut surface of decapitated seedlings. This confirmed that the species mobilized was unaltered IAA and permitted us to measure the in vivo decarboxylation of applied IAA. A pretreatment with TIBA applied to the cut surface produced a partial or drastic inhibition in the basipetal IAA movement at 0.5 or 100 μM, respectively. Since TIBA inhibits auxin polar transport by interfering with the efflux carrier, the above results suggest that 100 μM TIBA is sufficient to saturate the binding sites in the transporting cells. Compared to the control plants, in vivo decarboxylation of IAA was enhanced in 0.5 μM TIBA-treated plants, while no decarboxylation was detected after treatment with 100 μM TIBA. The in vitro decarboxylation of (1-¹⁴C)-IAA catalyzed by purified peroxidase was moderately activated by 100 μM and unaffected by 0.5 μM TIBA. The paradoxical effect of TIBA in vivo vs in vitro assays suggests that the in vivo effect of TIBA on IAA oxidation might be the consequence of the action of TIBA on the auxin transport system. Thus, transport reduction by 0.5 μM TIBA caused a temporary accumulation of IAA in that apical region of the hypocotyl which has the highest capacity to decarboxylate IAA. In the presence of 100 μM TIBA, a concentration which presumably saturates the efflux carriers, most of the added IAA can be expected to be located in the transporting cells where, according to the present data, IAA decarboxylation cannot take place.