Conversion of indole-3-acetaldehyde to indole-3-acetic acid in cell-wall fraction of barley (Hordeum vulgare) seedlings

The cell-wall fraction of barley seedlings was able to oxidize indole-3-acetaldehyde (IAAld) to form IAA, whereas the fraction did not catalyze the conversion of indole3-acetonitrile or indole-3-acetamide to IAA. The activity was lower in a semi-dwarf mutant that had an endogenous IAA level lower than that of the normal isogenic strain [Inouhe ct al. (1982) Plant Cell Physiol. 23: 689]. The soluble fraction also contained some activity; the activity was similar in the normal and mutant strains. The optimal pH for the conversion of IAAld to IAA in the cell-wall fraction was 7; that of soluble fraction was 6. The K-m value of the cell-wall fraction for IAAld was 5 mu M; that of soluble fraction was 31 mu M. The activity was not solubilized by treatments with 1% Nonidet P-40, 1 M NaCl, 3 M LiCl, or 50 mM MgCl2. The oxidation activity was increased by the addition of NAD(+). These results suggest that IAAld oxidation activity is bound to cell-wall components and that the lower level of IAA in the mutant probably results from reduced activity of oxidation enzyme bound to cell-wall components.