NEMATODE
RESISTANCE
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).
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