Metabolism of indole-3-acetic acid by orange (Citrus sinensis) flavedo tissue during fruit development

[5-H-3, 1'-C-14, C-13(6), C-12] Indole-3-acetic acid (IAA), was applied to the flavedo (epicarp) of intact orange fruits at different stages of develo pment, after incubation in the dark, at 25 degreesC, the tissue was extract ed with MeOH and the partially purified extracts were analyzed by reversed phase HPLC-RC. Six major metabolite peaks were detected and subsequently an alyzed by combined HPLC-frit-FaB-MS. The metabolite peak 6 contained oxindo le-3-acetic acid (OxIAA), indole-3-acetyl-N-aspartic acid (IAAsp) and also indole-3-acetyl-N-glutamic acid (IAGlu). The nature of metabolite 5 remains unknown. Metabolites 3 and 4 were diastereomers of oxindole-3-acetyl-N-asp artic acid (OxIAAsp). Metabolite 2 was identified as dioxindole-3-acetic ac id and metabolite 1 as a DiOx-IAA linked in position three to a hexose, whi ch is suggested to be 3-(-O-beta -glucosyl) dioxindole-3-acetic acid (DiOxI AGlc). Identification work as well as feeding experiments with the [5-3H]IA A labeled metabolites suggest that IAA is metabolized in flavedo tissue mai nly through two pathways, namely IAA-OxIAA-DiOxIAA-DiOxIAGlc and IAA-IAAsp- OxIAAsp. The flavedo of citrus fruit has a high capacity for IAA catabolism until the beginning of fruit senescence, with the major route having DiOxI AGlc as end product. This capacity is operative even at high IAA concentrat ions and is accelerated by pretreatment with the synthetic auxins 2,4-D, NA A and the gibberellin GA(3). (C) 2001 Elsevier Science Ltd. All rights rese rved.