Biology and molecular characterisation of the root lesion nematode, Pratylenchus curvicauda

Australia is the driest inhabited continent with about 70% of the land arid or semi-arid, and soils which are geologically old, weathered, and many are infertile. This is a challenging environment for agricultural production, which is further impacted by biotic constraints such as root lesion nematodes (RLNs), Pratylenchus spp. These soil-borne nematodes cause significant economic losses in yields of winter cereals, and in other crops, particularly under conditions of moisture and nutrient stress. RLNs are widely distributed in Australian broadacre cropping soils, and losses in cereal production are greater when more than one RLN species is present, a situation which often occurs in Western Australia (WA). Hence, to develop appropriate management regimes, accurate identification of RLN species is needed, combined with understanding the biology of host-nematode interactions. The initial aim of this research was to extend the molecular and biological characterisation of P. quasitereoides, a recently described species of root lesion nematode from WA. Morphological measurements of two important characters, tail shape and the per cent distance of the vulva from the anterior end of the nematode body, were made from nematodes collected from the four locations of WA. These measurements did not match with the published data on P. quasitereoides. As a result, nematode samples from Pingelly were sent to Dr. M. R. Siddiqi in the UK for detailed morphometric measurements: he concluded that the nematodes from Pingelly differed from other recognised species of Pratylenchus in Australia. The longer conus (more than half of the spear length), three annules in the lip region, large rounded basal knobs, phasmids in the anterior region of the tail, usually about one-fourth the tail length behind the anus, and the absence of males were good characters for species comparisons. The morphometric measurements suggested that the species under study was in fact P. curvicauda, a species which had been reported previously and described from WA (Siddiqi et al., 1991). Analyses of SEM images also supported the morphological description of the nematode samples from Pingelly as being P. curvicauda. Study of the genetic variation of this species with other known species of Pratylenchus was undertaken based on analysis of sequences of both the ribosomal DNA (rDNA) ITS and D2-D3 regions. Sequence analyses showed that there was considerable variation in P. curvicauda rDNA in samples from four wheatbelt regions, and this was also the case for P. neglectus, P. penetrans and P. thornei. The ITS sequences of P. curvicauda differed by 39% from those of P. neglectus and P. thornei, but were more similar to those of P. penetrans. Nucleotide variations between sequences of P. curvicauda and those of P. neglectus, P. penetrans and P. thornei were mainly in the ITS1 and ITS2 regions, which differed by 44% and 49% in nucleotide sequences of these regions with P. neglectus and P. thornei, whereas with only 10% in both regions compared to P. penetrans sequences. For the D2-D3 region, sequences from P. curvicauda, P. neglectus, P. penetrans and P. thornei differed by up to 30% at the nucleotide level. The D2-D3 sequences of P. curvicauda were very different from the published sequences for P. quasitereoides, with up to a 39% variation in nucleotide sequence. These results support the morphological data that Pratylenchus sp. collected from the four WA locations did not appear to be any of the species of Pratylenchus currently described as being in WA (i.e. P. neglectus, P. penetrans, P. thornei or P. quasitereoides). This result prompted a more detailed study of the biology of the nematode. The ability of different plant taxa/genotypes to support the reproduction of P. curvicauda was assessed using soil collected from Pingelly under glasshouse conditions. Crop and pasture species challenged included wheat, barley, pasture species, brassicas, and model plants, both by manual inoculation and grown in naturally infested field soil. Wheat, barley and sorghum were good hosts; canola, tomato and other brassicas were poor hosts; most pastures species and tobacco were non-hosts. A part of this research focused on how P. curvicauda was attracted to and penetrated roots of host plants identified as resistant or susceptible. Since attraction to and entry into roots of hosts classified as resistant or susceptible was essentially the same, host resistance is probably expressed later in the interaction. The life cycle of this species was also studied. It took 35-42 d to complete one generation in susceptible wheat roots under glasshouse conditions. Histological studies of roots infested by P. curvicauda were also undertaken. Cellular changes in infested roots included deformation of cell wall, loss of cytoplasmic contents and death of invaded cells, and there was some evidence of changes in cells further from the invading nematodes. There were no clear differences in the damage caused in susceptible and resistance hosts, at least up to 7 d after infection. This study is the first to investigate the biology and molecular characteristics of P. curvicauda