Many fruit crops are attacked by nematodes of the species Meloidogyne spp., Xiphinema spp. and Longidorus spp. (Brown et al., 1993; Ploetz et al., 1994; Nyczepir and Halbrendt, 1993). Nematodes aredifficult to eradicate from infected soils and control is normally via nematocides, resistant cultivars and appropriate crop husbandrytechniques. However, resistant rootstocks are very rare (Roberts, 1992) and chemical treatments are expensive and not always effective since egg-containing cysts formed by some nematodes are very resistant to chemicals and can survive for years in the soil. Plants respond to infection with a variety of defence strategies including production of phytoalexins, deposition of lignin-like material and accumulation of hydroxyproline- rich glycoproteins, expression of PR-proteins and with an increase of lytic enzymes. Genes involved in nematode resistance have been identified in Beta procumbens and Solanumtuberosum (Hs1pro1and GPA2) and have been cloned (Stiekema et al., 1999).
Two strategies of genetic engineering for introducing resistance to nematodes have been suggested (Sijmons et al., 1994):
1. Introduction of an effector gene whose product is addressed to the parasite or its excretion,
2. Introduction of an effector gene whose product is addressed to the plant cells which feed the nematodes.
Potential anti-nematode genes have been reported and seem to be effective when they are constitutively expressed in plants. Usually these also are involved in the control of insects:
1. Genes over-expressing collagenases which damage the animal cuticle (Havstad et al., 1991).
2. Exotoxin of B.thuringiensis (Devidas and Rehberger, 1992) or other feeding inhibitor such as the cowpea trypsin inhibitor.This approach is based on the much localised expression of a phytotoxin gene responsible for the inhibition of development or maintenance of feeding structures of nematodes within the plant system. Genes encoding lipases, transcription factors, nucleases, proteases, and glucanases have been suggested (Sijmons et al., 1994).
Anti-nematode monoclonal antibodies (Schots et al., 1992). Molecular information on nematode resistance is limited, the availability of specific nematode-responsive regulatory plant sequences could represent an important goal and the durability of resistance is believed to depend on the combination of different chimeric constructs and strategies (Barthels et al., 1999).