Histological characterization of root-knot nematode resistance in cowpea and its relation to reactive oxygen species modulation.

Root-knot nematodes (Meloidogyne spp.) are sedentary endoparasites with a broad host range which includes economically important crop species. Cowpea (Vigna unguiculata L. Walp) is an important food and fodder legume grown in many regions where root-knot nematodes are a major problem in production fields. Several sources of resistance to root-knot nematode have been identified in cowpea, including the widely used Rk gene. As part of a study to elucidate the mechanism of Rk-mediated resistance, the histological response to avirulent M. incognita feeding of a resistant cowpea cultivar CB46 was compared with a susceptible near-isogenic line (in CB46 background). Most root-knot nematode resistance mechanisms in host plants that have been examined induced a hypersensitive response (HR). However, there was no typical HR in resistant cowpea roots and nematodes were able to develop normal feeding sites similar to those in susceptible roots up to 9-14 d post inoculation (dpi). From 14-21 dpi giant cell deterioration was observed and the female nematodes showed arrested development and deterioration. Nematodes failed to reach maturity and did not initiate egg laying in resistant roots. These results confirmed that the induction of resistance is relatively late in this system. Typically in pathogen resistance HR is closely associated with an oxidative burst (OB) in infected tissue. The level of reactive oxygen species release in both compatible and incompatible reactions during early and late stages of infection was also quantified. Following a basal OB during early infection in both susceptible and resistant roots, which was also observed in mechanically wounded root tissues, no significant OB was detected up to 14 dpi, a profile consistent with the histological observations of a delayed resistance response. These results will be useful to design gene expression experiments to dissect Rk-mediated resistance at the molecular level.