Resistance to Nematode Pathogens

The major nematode pathogens of crop plants are the sedentary endoparasites root-knot (Meloidogyne spp.) and cyst-nematodes (Hetero-dera/Globodera spp.). Root-knot nematodes are more prevalent in subtropical and tropical regions, whereas cyst-nematodes are more of a problem in temperate regions. There are also semi-endoparasites and ectoparasites.
The endoparasites develop an intimate association with hosts, with the formation of specific feeding cells (giant cells for Meloidogyne spp. and syncytia for Heterodera and Globodera spp.), which take the form of multinucleate transfer cells.4 The nematodes are dependent on these cells for nutrients and will not complete their life-cycles if the feeding cells are damaged. The feeding cells therefore provide a major target for engineered resistance.This can take the form of genes that prevent feeding cell induction, genes that prevent or inhibit feeding cell function or toxic compounds delivered to the nematode via the feeding cells. For endo-, semi-endo- and ectoparasitic nematodes, it is also possible to attack the body wall of the nematode itself, by expression of specific cuticle-degrading enzymes Some resistance to nematode attack has been achieved by expression of protease inhibitors in feeding cells, and by damaging feeding cells by feeding cell specific expression of a ribonuclease (barnase). Because of the clear requirement for feeding cell formation and function and the differences between the nematode and its host plant, it is probable that effective broadly applicable synthetic resistance genes to nematodes will soon be available.
It is also of interest that cloned nematode R-genes (to sugar beet and cereal cyst-nematodes and to root-knot nematodes) have been characterised. The Mi R-gene also confers resistance to aphids, and this is the first example of an R-gene which gives resistance to two different classes of pests (i.e. nematodes and aphids).