Incubation of oat coleoptile segments with 40 µm indoleacetic acid (IAA) induced a decrease of 35-60% in peroxidase activity at the cell wall compartment. Treatment with IAA also produced a similar decrease in the oxidation of NADH and IAA at the cell wall. Isoelectric focusing of ionic, covalent, and intercellular wall peroxidase fractions showed that acidic isoforms (pI 4.0-5.5) were reduced preferentially by IAA treatment. Marked differences were found between acidic and basic wall isoperoxidases in relation to their efficacy in the oxidation of IAA. A peroxidase fraction containing acidic isoforms oxidized IAA with a V(max)/s(0.5) value of 2.4 x 10(-2) min(-1). g fw(-1), 4.0 times higher than that obtained for basic peroxidase isoforms (0.6 x 10(-2) min(-1). g fw(-1)). In contrast, basic isoforms were more efficient than acidic isoperoxidases in the oxidation of coniferyl alcohol or ferulic acid with H(2)O(2) (5.6 and 2.1 times, respectively). The levels of diferulate and lignin in the walls of oat coleoptile segments were not altered by treatment with IAA. The decrease in cell wall peroxidase activity by IAA was related more to reduced oxidative degradation of the hormone than to covalent cell wall cross-linking.Key Words. IAA-Auxin-Wall peroxidases-Oat-Avena sativa-Diferulic acid-Ligninhttp://link.springer-ny.com/link/service/journals/00344/bibs/18n1p25.html