Peroxidase, polyphenoloxidase, and phenols in relation to resistance against Pseudomonas syringae pv. tomato in tomato plants

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Abstract
Peroxidase, polyphenoloxidase, and phenols were evaluated for their involvement in the resistance mechanism of tomato plants against Pseudomonas syringae pv. tomato (Okabe) Young, Dye and Wilkie. Healthy and infected leaf extracts of susceptible and resistant tomato cultivars were examined for their phenol content and activity of peroxidase and polyphenoloxidase. Peroxidase activity increased to a similar extent in infected mature and young leaves and in resistant and susceptible tomato cultivars. A positive correlation was found between the number of bacteria in the tissue and peroxidase activity in susceptible plants. Four different peroxidase isozymes were found in extracts from diseased plants, compared with only one in healthy plants. These facts indicated that peroxidase increases are not directly involved in the resistance mechanism of tomato plants. Significant positive correlations were found among resistance, polyphenoloxidase activity, and phenol accumulation in 20 different infected tomato cultivars, ranging from highly susceptible to highly resistant. Polyphenoloxidase activity and phenol accumulation were higher in inoculated, resistant cultivars than in inoculated susceptible ones. Inoculated resistant mature leaves showed both a higher polyphenoloxidase activity and a higher phenol content than younger susceptible leaves. A direct correlation was found between disease index, polyphenoloxidase activity, and phenol content in the tissue. Eight different polyphenoloxidase isozymes were found in susceptible diseased plants. Phenols extracted from inoculated resistant tomato plants inhibited pathogen growth. Our data suggest that the oxidative phenol system of tomato plants contributes to resistance to P. syringae pv. tomato.
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