Different levels of anti-oxidant enzyme activities in tomato genotypes susceptible and resistant to root-knot nematodes

Seven tomato cultivars and 9 hybrids coming from a breeding programme, susceptible or resistant to root-knot nematode by carrying the resistance gene Mi-1, were compared by determining free phenol content and catalase (CAT) activity in their leaves. A host suitability test based on the determination of the number of egg masses per root system (EM), after inoculation with an avirulent field population of Meloidogyne incognita, confirmed the response to root-knot nematodes of the positive and negative controls, and revealed that all the hybrids except one were highly resistant to the infection. Catalase activity and free phenols extracted from leaves were detected on 3 different groups of tomato plants: 1) known susceptible cultivars; 2) known resistant cultivars; 3) accessions from the breeding programme. The level of catalase activity of the groups 2 and 3 was similar and approximately 2-fold higher than that of the group 1. Conversely, no consistent difference was found in free phenol content among the 3 groups. Moreover, one pair of susceptible and resistant tomato cultivars was used to check changes of anti-oxidant activities due to nematode infection. Catalase and peroxidase activities were measured in roots 5 days after inoculation with 300 and 600 juveniles (J2) of a M. incognita population, and in uninoculated roots used as controls. Catalase was found to be inhibited and enhanced, respectively, in infected resistant and susceptible roots, with respect to controls. The degree of inhibition was higher in the resistant roots inoculated with 600 J2. Guaiacol peroxidase activity did not change after inoculation of susceptible plants, whereas it was enhanced in infected resistant roots compared with controls. The use of catalase activity as a biochemical marker of Mi-1-mediated resistance in tomato is proposed.