The best-established strategy for developing nematode resistant plants targets feeding nematodes in plant roots. All plant parasitic nematode species must digest protein using cysteine proteinases, so we have developed a cysteine proteinase inhibitor (cystatin)-based defenses that can counter several species in one crop in contrast to what can be achieved using a natural resistance genes. Cystatins have a proven wide value against a range of nematodes with differing modes of parasitism, including results from field trials of transgenic potatoes challenged by species of Globodera (potato cyst nematode). Work in the lab has deployed cystatins from rice and maize to confer nematode resistance to the range of crops we work on. Other work is developing proteinase inhibitors to other nematode digestive proteinases.
A novel defence that disrupts location and invasion of host roots by plant parasitic nematodes has also been developed and deployed into the field in the last few years. Nematodes must sense and respond appropriately to a range of chemical signals in order to achieve a successful parasitic invasion. Two synthetic peptides were identified in the lab that interfere with nematode chemoreception by binding to receptors in the nematodes nervous system. Transgenic plants were subsequently developed that secrete the peptides from their roots. One of the peptides suppressed the number of female Heterodera schachtii (beet cyst nematode) that developed on Arabidopsis thaliana by more than 80%, whilst expression in the root tips of potato plants resulted in almost 95% resistance to Globodera pallida. Potato plants secreting this peptide from their root tips provided effective resistance against potato cyst nematode in both containment glasshouse and field trials. This resistance strategy may effectively target other nematode species as the target receptors are common to all nematodes studied. The infective stages of sedentary endoparasitic nematodes are vulnerable to sensory intervention before feeding cell initiation whilst migratory ecto-parasites and endo-parasites remain motile and may be affected throughout their lifecycle.
RNA interference (RNAi) is the process in which double-stranded RNA (dsRNA) triggers the silencing of specific target genes through mRNA degradation. It has been widely used as a tool for functional analysis of nematode genes and is being developed as a defence against plant parasitic nematodes. Transgenic plants that produce dsRNA that trigger an RNAi effect in feeding nematodes have shown potential against both cyst and root-knot nematodes and over the past few years, screening methods have been developed to allow the evaluation of many gene targets.
Work in the lab has demonstrated suppression of Heterodera glycines on soybean plantlets from targeting genes required for parasitism. One strength of the RNAi approach is that on-going and completed genome sequencing for plant parasitic nematodes provides a large range of potential targets that can be screened in vitro to select those for plant transformation constructs.
Cyst and root-knot nematodes are the two most important economic nematode groups but some crops such as banana are also severely damaged by migratory endoparasites including Radopholus similis which causes rotting of banana roots. It is susceptible to RNAi although the extent of silencing can vary according to the region of the nematode gene targeted and the experimental occasion. Work continues in the lab to develop the potential of this strategy for the crops we work on.