ACTIVITIES OF OXIDOREDUCTASE ENZYMES IN TISSUE-EXTRACTS AND STERILE ROOT EXUDATES OF 3 CROP PLANTS, AND SOME PROPERTIES OF THE PEROXIDASE COMPONENT

Aqueous extracts of homogenized shoot and root tissue of alfalfa (Medi cago 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 oxidored uctase enzymes by standard procedures. In tissue extracts and root exu dates, 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-monoox ygenase (EC 1.14.13.7) were readily detected. Of the aromatic-ring cle avage dioxygenases, those of the meta-cleavage pathway (EC 1.13.11.2 a nd 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, th is enzyme was represented by at least seven acidic isoforms. The forma tion 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 (Glycine max L.), the rate of Mn3+ genera tion correlated (P = 0.993) with the activities of endogenous plant gu aiacol peroxidase and horseradish peroxidase (HRP) supplements and als o with the total phenol content in tissue extracts (P = 0.984). Plant guaiacol peroxidase and purified HRP decolorized four aromatic dyes, a n activity reported to be involved in ligninolysis. Although no enzyme s capable of generating H2O2 as a consequence of the oxidation of simp le sugars, amino acids, organic acids, and aldehydes were found, trace s of peroxide were detected in tissue extracts and in the root exudate of alfalfa. It is concluded that the oxidoreductases found in plant t issues also occur in root exudates of aseptic whole plants. The signif icance of interrelations between oxidoreductase enzymes and enzymicall y generated higher-valency metal ions is discussed in the context of t he oxidative conversion of phenolic compounds in soil and plant tissue .