Manganese superoxide dismutase in Meloidogyne incognita isolates selected for virulence on Mi-1-carrying tomato: gene expression and enzyme activity

Root knot nematodes (Meloidogyne spp.) are important pests of a wide range of crops, including tomato. Resistance of tomato to root-knot nematodes is conferred by the single dominant gene Mi-1, which currently is present in all commercially available resistant tomato cultivars. However, several resistance-breaking populations are being collected worldwide. Two isolates coming from the same standard population of Meloidogyne incognita, one selected for virulence against Mi-1 (SM1) and the other left avirulent (avr1), have been used in this study. qRT-PCR was used to detect transcript levels of the manganese superoxide dismutase (Mn-SOD) gene from (a)virulent pre-parasitic second-stage juveniles (J2). Over-expression of the Mn-SOD gene was found in the virulent isolate compared with the avirulent counterpart. The enzyme activity of membrane-bound mitochondrial Mn-SOD was assayed in J2 and adult females as the fraction of total SOD activity insensitive to hydrogen peroxide (H2O2). J2 from SM1 showed about a two-fold higher enzyme activity than J2 from avr1; conversely, no difference was found when adult females were tested. Proteins of J2 extracts were separated by n-PAGE on special mini-gels and stained for SOD. One slow migrating and three fast migrating bands were stained. SOD activity of the slow migrating band was H2O2-insensitive and enriched by treatment with the detergent Triton X-100. J2 survival was monitored in suspensions provided or not provided with the cell oxygen radical generator paraquat, at high concentration. Virulent J2 responded to paraquat treatment by increasing life extension with respect to control conditions; by contrast, avirulent J2 suffered major mortality in the presence of paraquat. Mn-SOD gene expression is discussed in relation to nematode fitness in oxidative stresses and vir phenotype.