A NOVEL METABOLIC PATHWAY FOR INDOLE-3-ACETIC-ACID IN APICAL SHOOTS OF POPULUS-TREMULA (L) X POPULUS-TREMULOIDES (MICHX)

Metabolism of indole-3-acetic acid (IAA) in apical shoots of Populus t remula (L.) x Populus tremuloides (Michx.) was investigated by feeding a mixture of [C-12]IAA, [C-13(6)]IAA, and [1'-C-14]IAA through the ba se of the excised stem. HPLC of methanolic plant extracts revealed eig ht major radiolabeled metabolites after a 24-h incubation period. Comp arison between feeds with [5-H-3]IAA and [1'-C-14]IAA showed that all detectable metabolites were nondecarboxylative products. The purified radiolabeled HPLC fractions were screened by frit-fast atom bombardmen t liquid chromatography-mass spectrometry for compounds with character istic fragment pairs originating from the application with C-12 and C- 13 isotopes. Samples of interest were further characterized by gas chr omatography-mass spectrometry. Using this procedure, oxindole-3-acetic acid (OxIAA), indole-3-acetyl-N-aspartic acid (IAAsp), oxindole-3-ace tyl-N-aspartic acid (OxIAAsp), and ring-hydroxylated oxindole-3-acetic acid were all identified as IAA metabolites. Furthermore, a novel met abolic pathway from IAA via IAAsp and OxIAAsp to OxIAA was established on the basis of refeeding experiments with the different IAA metaboli tes.