Activities of oxidoreductase enzymes in tissue extracts and sterile root exudates of three crop plants, and some properties of the peroxidase component

Abstract

Aqueous extracts of homogenized shoot and root tissue of alfalfa (Medicago sativa L.), white mustard (Sinapis alba L.), and cress (Lepidium sativum L.), with the exudates of sterile roots of these crop plants, were examined spectrophotometrically for the activities of 20 oxidoreductase enzymes by standard procedures. In tissue extracts and root exudates, the reactions of laccase (EC 1.10.3.2), ascorbate oxidase (EC 1.10.3.3), monophenol monooxygenase (EC 1.14.18.1), and phenol 2-monooxygenase (EC 1.14.13.7) were readily detected. Of the aromatic-ring cleavage dioxygenases, those of the meta-cleavage pathway (EC 1.13.11.2 and 1.13.11.8) could also be detected. Guaiacol peroxidase (EC 1.11.1.7) was dominant in all samples. In sterile root exudates of alfalfa, this enzyme was represented by at least seven acidic isoforms. The formation of the ligninolytic Mn³⁺/malonate and Mn³⁺/citrate complexes from Mn²⁺ occurred in all tissue extracts and in root exudates of alfalfa. In root extracts of soybean (Glycinemax L.), the rate of Mn³⁺ generation correlated (P=0·993) with the activities of endogenous plant guaiacol peroxidase and horseradish peroxidase (HRP) supplements and also with the total phenol content in tissue extracts (P=0·984). Plant guaiacol peroxidase and purified HRP decolorized four aromatic dyes, an activity reported to be involved in ligninolysis. Although no enzymes capable of generating H₂O₂ as a consequence of the oxidation of simple sugars, amino acids, organic acids, and aldehydes were found, traces of peroxide were detected in tissue extracts and in the root exudate of alfalfa. It is concluded that the oxidoreductases found in plant tissues also occur in root exudates of aseptic whole plants. The significance of interrelations between oxidoreductase enzymes and enzymically generated higher-valency metal ions is discussed in the context of the oxidative conversion of phenolic compounds in soil and plant tissue.