Comparative cellular responses in susceptible and resistant soybean cultivars infected by Meloidogyne incognita
Meloidogyne incognita , a predominant nematode parasite of soybean in South Africa, increasingly threatens production of the crop as it is expanding to maize-producing areas infected by this nematode. The parasitic relationship between M. incognita and soybean were compared on a susceptible and a resistant cultivar in terms of nematode penetration, development, reproduction and fecundity as well as histopathology studies. Second-stage juveniles (J2) of M. incognita were inoculated on roots of a resistant (LS5995) and a susceptible (Prima2000) cultivar in three concurrent but separate glasshouse trials. For pre-infectional studies, root systems of plants were harvested 2, 4, 10, 16 and 20 DAI. Sampling times for the post-infectional experiment were 4, 10, 20 and 30 DAI, whilst those for the histopathology experiment were 2, 4, 10, 20 and 30 DAI. J2 penetrated roots of both cultivars in comparable numbers 2 DAI but vermiform J2 numbers were significantly lower in roots of LS5995 at 4, 16 and 20 DAI. Final (Pf) J2 population density (vermiform plus swollen individuals) in roots of Prima2000 was significantly higher at all sampling times than those in roots of LS5995. Development of M. incognita J2 to third- (J3) and fourth-stage juveniles (J4) was significantly affected by cultivar susceptibility and time, being slower in LS5995. Development of J2 to J3 and J4 or into mature females was also consistently slower in LS5995 for the duration of this experiment. Adult females in roots of Prima2000 produced significantly more (98%) eggs per egg mass and also maintained significantly more egg and J2 numbers (98.5%) per root system 30 DAI than those in roots of LS5995. Histopathological observations showed that J2 penetrated roots of both soybean cultivars and migrated intercellularly to undifferentiated provascular tissue 2 and 4 DAI, with pronounced cellular changes taking place. A hypersensitive reaction was observed 2 DAI in roots of the resistant cultivar. From 10 to 30 DAI giant cell formation in the differentiated vascular tissue in the roots of LS5995 differed substantially from those in roots of Prima2000. Giant cells that developed in roots of the LS5995 were smaller and fewer compared to those in Prima2000. Giant cells in roots of LS5995 also contained empty as well as sub-optimal giant cells with thicker cell walls than those reported for resistant soybean cultivars in earlier studies. The induction, development and maintenance of giant cells in LS5995 proved to be typically retarded.