Metabolism of [14C]-indole-3-acetic acid by soybean callus and hypocotyl sections

Metabolism of indole-3-acetic acid in soybean [ Glycine max (L.) Merr.] was investigated with [1-¹⁴C]- and [2-¹⁴C]-indole-3-acetic acid (IAA) applied by injection into soybean hypocotyl sections and by incubation with soybean callus. Free IAA and its metabolites were extracted with 80% methanol and separated by high performance liquid chromatography with [³H]-IAA as an internal standard. Metabolism of IAA in soybean callus was much greater than that in tobacco ( Nicotiana tabacum L.) callus used for comparison. High performance liquid chromatography of soybean extracts showed at least 10 metabolite peaks including both decarboxylated and undecarboxylated products. A major unstable decarboxylated metabolite was purified. [¹⁴C]-indole-3-methanol (IM) was three times more efficient than [2-¹⁴C]-IAA as substrate for producing this metabolite. It was hydrolyzable by β-glucosidase (EC 3.2.1.21), yielding an indole and D-glucose. The indole possessed characteristics of authentic IM. Thus, the metabolite is tentatively identified as indole-3-methanol-β-D-glucopyranoside. The results suggest that soybean tissues are capable of oxidizing IAA via the decarboxylative pathway with indole-3-methanol-glucoside as a major product. The high rate of metabolism of IAA may be related to the observed growth of soybean callus with high concentrations of IAA in the culture medium.