Uptake and metabolism of indole-3-butyric acid and indole-3-acetic acid by Petunia cell suspension culture

The uptake and metabolism of indole-3-acetic acid (IAA) and indole-3-butyric acid (IBA) were studied in suspension cell cultures of Petunia hybrida. The initial uptake of ³H-IBA was much higher than that of ³H-IAA, and after 10 min of incubation with labeled IBA and IAA, 4.6 pM vs 0.35 (39% vs 12% of total applied radioactivity) respectively, were found in the cell extracts. The uptake of IBA reached a plateau of 6.0 pM (62%) after 2 h while that of IAA increased continuously up to 1.5 pM (46%) after 24 h. Following the addition of 40 µM of unlabeled auxin more IBA was taken in initially than IAA (39% vs 12%), but the level almost equalized after 24 h of incubation when IBA uptake reached 890 nM (55%) and IAA 840 nM (46%).IBA was metabolized very rapidly by Petunia cell suspension to new compounds. HPLC of the cell extracts demonstrated a new metabolite after only 2 min of incubation, and after 30 min 60% of the radioactivity was in the new metabolite vs 10% in the IBA. The new compound was resolved by autofluorography to two metabolites but after 24 h only one metabolite was present. The IBA metabolites were identified tentatively as IBA aspartic acid (IBAasp) and IBA glucose (IBAglu). In the medium IBA disappeared at a fast rate and after 24h most of the radioactivity was present in the new metabolite, probably IBAasp. IAA was also converted rapidly to two new metabolites and both were still present after 24 h. No attempt was made to identify the metabolites of IAA. IAA metabolism proceeded at a slower rate, and autofluorography showed that while free IBA disappeared after 0.5 h, free IAA was still present after 1 h of incubation. We postulate that Petunia cells conjugate IBA rapidly to IBAglu which in turn is converted to form IBAasp which probably acts as a slow release hormone. Only intact cells were able to metabolize IBA and the reaction was affected by low temperature and anaerobic conditions. The fast rate of IBA uptake, the need for whole cells for the metabolism to proceed, and the fast change of IBA to a new metabolite in the medium, all suggest that both uptake and metabolism of IBA in Petunia cells occur on the cell surface.