Nematodes

Plant parasitic nematodes are obligate parasites that cause billions of dollars in losses annually to the world’s farmers (Williamson 1998). They have been divided into ectoparasites and endoparasites. The sedentary endoparasites of the family Heteroderidae cause the most economic damage and include two groups: the cyst nematodes, which include the groups Heterodera and Globodera, and the root-knot nematodes (genus Meloidogyne). Root-knot nematodes infect a broad range of plant species whereas cyst nematodes have a narrower host range.

With increasing restrictions on the use of chemical pesticides, the use of host resistance for nematode control has grown in importance. A number of genes that mediate nematode resistance have now been or soon will be cloned from a variety of plant species. Nematode resistance genes are present in several crop species and are an important component of many breeding programmes including those for tomato, potato, soybeans and cereals (Trudgill 1991). Several resistance genes have been mapped to chromosomal locations or linkage groups and some of them have been cloned. The first nematode resistance gene to be cloned was Hs1^pro-1, a gene from a wild relative of sugar beet conferring resistance to Heterodera schachtii (Cai et al. 1997). The cDNA, under the control of the CaMV 35S promoter, was able to confer nematode resistance to sugar beets transformed with Agrobacterium rizhogenes in an in vitro assay (Cai et al. 1997). The Mi gene from tomato conferred resistance against a root-knot nematode and an aphid in transgenic potato (Rossi et al. 1998). The gene Mi is a true R gene, characterised by the presence of NBS and LRR domains (see Plant R genes). Recently, Gpa2, a gene that confers resistance against some isolates of the potato cyst nematode Globodera pallida was identified (Van Der Vossen et al. 2000).

This gene shares extensive homology with the Rx1 gene that confers resistance to potato virus X suggesting a similarity in function (Van Der Vossen et al. 2000).

Although an important constituent of current nematode management strategies is the incorporation of natural resistance, one must be aware of the fact that there may not be appropriate resistance loci available for many crops. In addition, it is not a given fact that a particular gene will function effectively in heterologous hosts. Attempts to transfer Mi-mediated resistance into tobacco have not so far been successful (Williamson 1998). Furthermore, nematodes can eventually develop virulence, which may limit the effectiveness of this approach. Clearly, novel strategies to control nematode infestation are required combining existing with new approaches (Gheysen et al. 1996).