Decarboxylative metabolism of [1′-14C]indole-3-acetic acid by tomato pericarp discs during ripening

Abstract

The rate of decarboxylation of [1′-14C]indole-3-acetic acid (IAA) infiltrated into tomato (Lycopersicon esculentum Mill.) pericarp discs was much more rapid in green than in breaker and pink tissues. Studies were carried out in order to determine whether the decarboxylative catabolism occurring in the green pericarp discs was associated with ripening or was a consequence of wound-induced peroxidase activity and/or ethylene production. After a 2-h lag, the decarboxylative capacity of the green pericarp discs increased exponentially during a 24-h incubation period. This increase was accompanied by increases in IAA-oxidase activity in cell-free preparations from the intercellular space and cut surface of the discs. Although higher IAA-oxidase activity was detected in extracts from the tissue residue, which comprises mainly intracellular peroxidases, this activity did not increase during the 24-h incubation period. Analysis of the cell-free preparations by isoelectric focusing revealed the major component in all samples was a highly anionic peroxidase (pI=3.5) the levels of which did not increase during incubation. However, the intercellular and cut-surface preparations contained additional anionic and cationic peroxidases which increased in parallel with the increases in both the IAA-oxidase activity of the preparations and the decarboxylative capacity of the green pericarp discs from which they were derived. Treatment of green discs with the ethylene-biosynthesis inhibitors aminooxyacetic acid and CoCl2, inhibited the development of an enhanced capacity to decarboxylate [1′-14C]IAA but the inhibition was not counteracted by exogenous ethylene. Another ethylene-biosynthesis inhibitor, aminoethoxyvinyl glycine, also reduced ethylene levels but did not affect IAA decarboxylation, indicating that the decarboxylation was not a consequence of wound-induced ethylene production. The data obtained thus demonstrate that the enhanced capacity to decarboxylate [1′-14C]IAA that develops in green tomato pericarp discs following excision is not associated with ripening but instead is attributable to a wound-induced increase in anionic and cationic peroxidase activity in the intercellular fluid and at the cut surface of the excised tissues.