Rhys G. R. Copeland

Root lesion nematode (RLN; Pratylenchus spp.) infestations cause over $250 million in economic damage to Australian grain production annually. Two new species, P. quasitereoides and P. curvicauda, have been described recently in Western Australia (WA). After P. neglectus, P. quasitereoides is considered one of the most economically important Pratylenchus spp. in WA in terms of damage caused to cereal crops. The extent of economic losses caused by P. curvicauda has not yet been determined, but its ability to multiply on many cereal crops and other crop hosts makes it a potentially serious pest. Recent studies suggest that there could be mixed infestations by the two nematodes in the WA grainbelt, such infestations could be more damaging to cereal production than when only one species is present. One major aim of this research was to investigate how widespread the two species are in the southwest grainbelt of WA. A second major aim was to understand how RLNs find appropriate host roots.

Of the seven cereal fields sampled from a relatively wide area, morphological and molecular diagnostics indicated that P. quasitereoides was present in four fields, whereas P. curvicauda was present in two. They also existed as single or mixed infestations with P. penetrans and/or P. neglectus. However, the two species were not found co-infesting plants in the same field. The work confirmed the presence of P. curvicauda, which may be more widespread than previously thought, and indicates that commercial soil testing (‘Predicta B’) should be re-assessed to identify the presence of P. curvicauda in farmers’ fields, since management protocols need to be species-specific.

Metabolite profiles of root exudates of the wheat cv. Calingiri (susceptible to Pratylenchus neglectus) and triticale cv. 342 (resistant to P. neglectus) were used to evaluate if metabolites influence susceptibility or resistance to P. neglectus infection. GC-MS profiles of the two cultivars revealed that the concentrations of 51 metabolites differed between control plants and plants exposed to P. neglectus, 24, 48 and 72 hours after nematode exposure, suggesting that some metabolites may play a role in the response of the plants to nematode infection. Of these metabolites, eight were identified as potential attractants and thirteen as potential repellents. When the potential roles of five metabolites were assessed using chemotaxis assays, the concentrations used showed maltose and propanoic acid attracted the nematodes, but glycerol and fructose were potential repellents. Lower ribose concentrations repelled nematodes during the first two hours of the assay, but higher concentrations had the opposite effect.

To investigate how Pratylenchus spp. perceive host signals, such as the metabolites identified in this study, the response of homologues of chemosensory genes of C. elegans to specific metabolites were studied in P. neglectus. Transcripts of 24 of these genes were identified from a PacBio SMRT long-read transcriptome of P. neglectus. Using cellular localisation studies, four of the genes (arr-1, deg-3, klp-11 and gcy-7) were shown to express predominantly in regions associated with sensory organs; in the anterior and posterior regions of the nematode (arr-1 and deg-3), at the base of the stylet (gcy-7), and the region posterior to the nerve ring (klp-11). Following knockdown of the four genes using dsRNA in vitro, the nematodes’ attraction to root exudates and responses to propanoic acid and maltose were significantly reduced, suggesting the four genes were involved in the nematodes’ perception of root exudates, propanoic acid and maltose.

A minor aim, describing the neurons involved in metabolite perception by RLNs, was initiated, but time was lacking to complete this aspect.

The research findings broaden current knowledge on Pratylenchus spp. and their distribution in fields in the southwest grainbelt of WA, with implications on how the newly described species, P. quasitereoides and P. curvicauda should be identified and managed. The results of the metabolomics study highlight possible mechanisms by which susceptible and resistant cultivars interact with RLNs. RNA interference can be used to study genes that may be important to nematode-host interaction. This research contributes new information of value to plant breeders and helps contribute to understanding underlying RLN-crop interactions, so contributing to improving crop productivity.