Loss of susceptibility as an alternative tool in tomato defence to Meloidogyne incognita.

Root-knot nematodes of the genus Meloidogyne are obligate biotrophs able to infest almost all cultivated plants and responsible for the highest economic damage world wide. They modify host root tissue by using effector proteins to create a feeding site (giant cells) that is their sole source of nutrition. A deeper knowledge of plant defence pathways during compatible interaction would help in developing alternative strategies to counteract nematode infection. Consequently, loss of host susceptibility could result in resistance to the pathogens. In this study, we examined the role of an analogue of SA, benzo (1,2,3) thiodiazole-7-carbothioic acid S-methyl ester (BTH) in enhancing tomato defence to Meloidogyne incognita infection. Exogenous treatments of BTH on susceptible tomato roots resulted in 60% decrease of the number of galls and egg-masses produced after M. incognita infection. We found some changes in the morphology of galls and nematode feeding sites: galls of BTH treated roots remained smaller and light microscope observations of stained sections revealed some differences in the organisation, structure and number of giant cells of treated vs untreated infected tissues. The effect on cell wall strengthening was studied by measuring lignin content and peroxidase activity in root tissues 2, 3 and 7 days after nematode infection in both treated and untreated root galls. An increase in the content of lignin was detectable from 2 days after the inoculation. Activities of peroxidase significantly increasing in treated infected tissues since 2 days after infection correlated with expression of two anionic peroxidases, TAP1 and TAP2, which were more expressed in galls of treated tissues. These data suggest that BTH likely seems to limit the development of nematodes by strengthening cell walls and hence inducing a loss of susceptibility in tomato.