A Study of Changes in Polyphenol Oxidase Activity in Interaction between Root-knot Nematode ( Meloidogyne javanica ) and Tomato Fusarium Wilt ( Fusarium oxysporum f.sp. lycopersici )

Polyphenol oxidase (PPO) activities were measured spectrophotometrically in tomato(Roma VF: resistant to Fusarium oxysporum f.sp. lycopersici and susceptible to Meloidogyne javanica) roots inoculated with nematode and fungi in four biological life stages of nematode. PPO specific activity showed a significant decrease in treatments (nematode + fungi) as compared with control plants. Results showed that nematode is able to change or suppress induction of this enzyme in spite of fungal induction. Nematode in all biological stages suppressed the induction of PPO activity in tomato roots but in young adult formation stage the suppression was higher than other life stages including giant cell formation, egg-laying and penetration. Native polyacrylamide gel revealed the presence of five isozymes of PPO with Rm values of 0.26, 0.31, 0.44, 0.50 and 0.63. Isoforms with Rm values of 0.26, 0.44, 0.50 and 0.53 showed significant changes in nematode + fungus inoculated tomato plants compared to plants inoculated with fungus alone. Based on the results obtained in this study, nematode infection resulted in suppression of tomato defenses against the fungi (as secondary pathogen) and intensified the fungal disease. Five PPO isoforms with Rm values of 0.26, 0.31, 0.44, 0.50 and 0.63 were shown in native polyacrylamide gel among which the isoform with Rm value of 0.31 had a thicker band in tomato plants inoculated with nematode or fungus alone. Two isoforms with Rm values of 0.44 and 0.53 in nematode-inoculated plants were similar to the corresponding bands in the healthy control. Isoforms with Rm values of 0.26, 0.44, 0.50 and 0.53 in nematode-fungus-inoculated tomato plants showed significant changes compared to plants inoculated with the fungus alone. Nematode infection of root tissue caused suppression of PPO specific activity and induced changes in PPO isoforms with Rm values of 0.26, 0.44, 0.50 and 0.63 more significantly than isoform 0.31.