Endoparasitic Nematodes Research Articles

Endophyte infection in perennial ryegrass reduces the susceptibility of black cutworm to an entomopathogenic nematode

Abstract The perennial ryegrass, Lolium perenne, forms a symbiotic relationship with Neotyphodium lolii, a fungus that produces alkaloids. This relationship provides a competitive advantage to the host plant in grassland communities by increasing drought tolerance, and disease and herbivore resistance. Black cutworm, Agrotis ipsilon, is among the few insect species that are able to feed and develop on endophytic perennial ryegrass. Some insects can use plant secondary compounds to defend themselves against predators, therefore we hypothesized that the cutworms fed on endophytic grasses would exhibit greater defense against a lethal endoparasitic nematode, Steinernema carpocapsae. Laboratory experiments involving 4–5th instars support the hypothesis that A. ipsilon feeding on grass clippings from field plots with high (> 90%) incidence of endophyte infected perennial ryegrass are less susceptible to entomopathogenic nematodes than larvae fed grass clippings from plants with little or no incidence of endophyte. Laboratory studies resulted in similar overall mortality after 48 h. Field studies, however, show decreased susceptibility to S. carpocapsae when larvae were confined to areas of endophytic grass (> 75% infected). Early instars (2–3rd) fed on endophyte free grass suffered greater overall mortality at all nematode concentrations than 4–5th instars fed similarly. Early (2–3rd) instars were equally susceptible to nematode attack regardless of food source. Our results indicate that the fungal endosymbionts of grasses can influence the biology of natural enemies of an herbivorous insect.

Relative mycorrhizal dependency and mycorrhiza-nematode interaction in banana cultivars ( Musa spp.) differing in nematode susceptibility

Four Musa cultivars, differing in nematode susceptibility, were selected to study their relative mycorrhizal dependency and to study the interaction between the arbuscular mycorrhizal fungus (AMF), Glomus mosseae, and two migratory endoparasitic nematodes, Radopholus similis and Pratylenchus coffeae. Mycorrhization with G. mosseae resulted in significantly better plant growth, even in the presence of R. similis and P. coffeae. No differences in relative mycorrhizal dependency (RMD) were observed among the four cultivars. G. mosseae suppressed nematode population build-up in Grande Naine and Pisang Jari Buaya. Only in the case of R. similis (Indonesian population) in Pisang Jari Buaya, no significant suppression was observed. In the case of P. coffeae, the AMF reduced the damage in the roots, caused by the nematodes. For R. similis, no reduction of damage was observed. In all, except one experiment, the frequency of the mycorrhizal colonisation was negatively affected by the nematodes.

Host resistance and tolerance to migratory plant-parasitic nematodes

AbstractPlant-parasitic nematodes are divided according to their feeding strategy into three major groups: sedentary endoparasites, migratory endoparasites and ectoparasites. Compared to what is known about sedentary endoparasitic nematode species, resistant and tolerant relationships between the nematodes from the latter two groups and their hosts are much less documented. However, methods for screening and evaluation of the resistance and tolerance of plants to migratory plant-parasitic nematodes have been well developed and sources of resistance and tolerance to these nematodes have been found. Advances have been made in breeding resistance to migratory plant-parasitic nematodes in rice, alfalfa, banana, pine trees, grape, woody fruits and other crops. Although accessions immune to stem, leaf and bud nematodes are found quite frequently, host resistance to migratory root-parasitic nematodes has been detected less frequently and generally only partly reduces nematode multiplication. Host tolerance to migratory nematodes is important even for resistant varieties and therefore is gaining attention. An insufficient degree of resistance and tolerance, their variability with the environment, and their linkage to undesired agricultural or horticultural characters are commonly observed. Polygenic bases for plant resistance and tolerance to migratory nematodes have been demonstrated by genetic and biochemical observations and make breeding even more complicated than that for resistance to sedentary plant-parasitic nematodes. These factors, with the presence of different nematode species in the field and community and population differences in pathogenicity, hinder the availability of host resistance and tolerance and offer a big challenge.

Molecular phylogenetic analysis and characterization of secretory enzyme genes of the plant parasitic nematodes

This article is an abstract of the paper presented by a winner of the Young Scientist Award at the 2003 Annual Meeting of the Phytopathological Society of Japan in Tokyo Root lesion nematodes, Pratylenchus spp., are cosmopolitan and economically important migratory endoparasitic nematodes. More than 20 Pratylenchus species are found in Japan. The taxonomic distinction of Pratylenchus species is difficult because they have little morphological diversity. Taxonomic difficulties often arise from underestimating the intraspecific variability of certain morphological characteristics presently used for distinguishing species. The success of management programs depends on rapid, reliable identification of the nematodes. Pratylenchus spp. also have a wide host range and invade roots at any developmental stage. In nearly all cases, parasitism is confined to cortical cells. However, few molecular studies have been conducted on the role of secretory enzymes in the relation between Pratylenchus and plants.

Characterization of a chorismate mutase from the potato cyst nematode Globodera pallida

SUMMARY Some plant endoparasitic nematodes are biotrophic and induce remarkable changes in their hosts in order to ensure a continuous supply of food. Proteins secreted from oesophageal gland cells have been implicated in this pathogenic process. A potentially secreted chorismate mutase has been isolated from the potato cyst nematode Globodera pallida. The gene encoding this protein is expressed in the subventral oesophageal gland cells of the nematode, and the mRNA derived from this gene is only present in the early parasitic stages. Sequence analysis of this gene shows that, like other genes involved in the host-parasite interaction of plant parasitic nematodes, it is likely to have been acquired by horizontal gene transfer from bacteria. The presence of a signal peptide in the deduced amino acid sequence of the G. pallida chorismate mutase and its expression in the subventral oesophageal gland cells suggest that it is secreted from the nematode, pointing to a role for the protein in the host-parasite interaction. The shikimate pathway, of which chorismate mutase is normally a part, is not found in animals but is present in plants and bacteria. In plants it gives rise to a variety of compounds which are important in amino acid synthesis and defence signalling pathways, as well as auxins, which have been implicated in the early development of nematode feeding sites. The potential roles of a nematode chorismate mutase are discussed.

Development of a negative plant–soil feedback in the expansion zone of the clonal grass Ammophila arenaria following root formation and nematode colonization

Summary The feedback between individual plants and their soil communities is a major driver of plant community processes. We analyse the rate at which plant–soil feedback develops in the root zone of the clonal dune grass Ammophila arenaria. Ammophila arenaria grows vigorously when it can form new roots in newly deposited windblown beach sand. The colonization zone is hypothesized to provide an enemy‐free space, with root pathogens and parasites possibly contributing to degeneration of A. arenaria when deposition stops. We quantified root biomass and plant parasitic nematode densities in new and 1‐year‐old root zones, as well as in degenerate stands, in the field at monthly intervals. Each month, we studied the biomass production of Ammophila seedlings in controlled conditions in sterilized and non‐sterilized soil from the different sampling sites, with and without nematicides. Colonization of newly deposited sand by the endoparasitic nematodes Heterodera arenaria and Pratylenchus spp. within 1 month of root formation coincided with a negative feedback in the bioassays. Nematicides counteracted growth reduction significantly, but their effectiveness decreased in soil samples collected later in the growing season. In older layers, roots and nematodes were already present before the sampling started. Growth reduction in unsterilized sand was observed at all sampling events, but was not counteracted by nematicides. In the field, root biomass of A. arenaria in the newly colonized sand increased throughout the growing season, despite the development of a negative plant‐soil feedback. We conclude that the development of negative plant–soil feedback in the root zone of A. arenaria closely follows the colonization of newly deposited sand by roots and then by the endoparasitic nematode species Heterodera arenaria. There seems to be a shift in organisms causing this feedback during the first growing season, but root biomass nevertheless increases. However, in older root layers and in the degenerate stand, ongoing negative plant–soil feedback may contribute to a gradual decrease in root biomass.

Metazoan parasites and food composition of juvenile Etmopterus spinax (L., 1758) (Dalatiidae, Squaliformes) from the Norwegian Deep.

A total of 37 juvenile Etmopterus spinax from the Norwegian Deep were examined for stomach contents and metazoan ecto- and endoparasites. These squaliform elasmobranchs were caught by benthopelagic net in May 2001 at a depth of 194-214 m. The euphausiid Meganyctiphanes norvegica and the teleost Maurolicus muelleri were the principal prey organisms. With increasing total length of E. spinax, the frequency of Meganyctiphanes norvegica prey decreased and that of M. muelleri increased. Seven different metazoan parasite species were found: adult Monogenea (2), larval and adult Cestoda (3), and larval Nematoda (2). The predominant parasites were an unidentified monocotylid monogenean and the cestode Aporhynchus norvegicus, with respective prevalences of infestation of 83.8% and 81.1%. The sites of infestation were the gills ( Squalonchocotyle spinacis, Monogenea), nasal cavities (Monocotylidae indet.), body cavity ( Lacistorhynchus tenuis, Cestoda), stomach wall and organs of the body cavity ( Anisakis simplex, Nematoda), and stomach and spiral valve ( A. norvegicus and Pseudophyllidea indet., Cestoda; Hysterothylacium aduncum, Nematoda). No other metazoan parasite taxa were found, and the musculature was free of parasites. Five new host and three new locality records were established. M. muelleri seems to be an important intermediate host for the endoparasitic nematodes which were found, with E. spinax serving as a paratenic host. E. spinax also serves as an intermediate host for the trypanorhynch cestode L. tenuis, and as the definitive host for the two monogeneans and the trypanorhynch A. norvegicus. The latter uses Meganyctiphanes norvegica as the second intermediate host in the Norwegian Deep. The relationship between the feeding ecology, habitat, and vagility of E. spinax and the resulting parasite fauna is discussed.

Alarm responses of the green sea urchin, Strongylocentrotus droebachiensis , induced by chemically labelled durophagous predators and simulated acts of predation

The green sea urchin, Strongylocentrotusdroebachiensis, exhibited immediate behavioural responses to waterborne chemosensory cues from two durophagous predators, the Atlantic wolffish, Anarhichaslupus, and the edible crab, Cancerpagurus; as well as from crushed conspecifics and crushed blue mussels, Mytilusedulis. The response patterns were dose dependent and diet dependent. Strong responses were elicited by water conditioned by echinivorous wolffish (97.5%), undiluted urchin extract (73%), undiluted mussel extract (52.5%), and by water conditioned by echinivorous crab (45%). In contrast, urchin extract diluted to 1%, mussel extract diluted to 10%, and water conditioned by predators on a mussel diet elicited weak responses (~20%). Infection by the endoparasitic nematode Echinomermellamatsi had no significant effect on the response pattern of S. droebachiensis. There was no conclusive evidence of an alarm response to the predators per se, as the weak response to stimuli from non-echinivorous wolffish and crab, as well as from the seastar Asteriasrubens, may have been caused by chemical cues transmitted from their mussel diet. The diet-dependent response to predators suggests that active predators were labelled by chemical cues from their echinoid prey. The chemical cue from echinivorous wolffish acted as both an arrestant and as a repellent, whereas the response to other cues was predominantly or exclusively repellent. The response to echinivorous wolffish was quantitatively stronger than, and qualitatively different from, the response to other stimuli, including undiluted urchin extract. The wolffish is apparently being labelled by a latent chemical cue which derives its potency from activation by, or interaction with, substances in the digestive system of the wolffish. We interpret this novel phenomenon as evidence of alarm signal magnification. The induced behavioural modifications demonstrate the green sea urchins' ability to detect chemical cues associated with active durophagous predators, and would therefore seem to have adaptive potential as a predator defence mechanism.

Endoparasitic nematodes in cropping soils of Western Australia

Soil samples were collected in Western Australia from cereal fields at about harvest time to determine levels of the endoparasitic nematodes, Pratylenchus and Heterodera. A systematic survey in 1997 and 1998 centred on the 40 shires with the highest proportion of cereal cropping. One hundred and eighty samples were collected in 1997 and 227 in 1998 at 10-km intervals along north–south transects 35 km apart. A targeted survey in 1997 included 98 soil samples from fields selected by growers as having poor productivity without an evident cause. No Heterodera cysts were found in any survey sample but these could be extracted from soils previously known to be infested. Pratylenchus neglectus was most commonly detected followed by P. thornei and P. zeae. Populations identified as P. brachyurus, P. penetrans, P. scribneri, and an undescribed species similar to P. thornei were also found. Overall Pratylenchus was extracted from 63% of samples (mean 1.1, median 0.3, max. 22.0/mL of soil). Maps of the data indicate that Pratylenchus populations were aggregated with some areas having relatively light infestations. There was a positive relationship with the intensity of cereal cropping and a negative relationship with pulse cropping. The findings indicate that potentially damaging Pratylenchus numbers occur in a significant proportion of fields and highlight the need to develop and implement strategies to lower population densities.

DO ARBUSCULAR MYCORRHIZAL FUNGI ALTER PLANT–PATHOGEN RELATIONS?

Arbuscular mycorrhizal (AM) fungi colonize roots of terrestrial plants and have been hypothesized to reduce susceptibility or to improve the vigor of hosts challenged by root pathogens. Meta-analysis was used to test whether a broad pattern exists in which AM fungi not only enhance plant growth but fundamentally alter plant-pathogen interac- tions. Data were gathered from studies published between 1970 and early 1998. In these studies nonmycorrhizal or mycorrhizal plants were untreated or challenged with root-in- fecting fungal pathogens or nematodes. Effect sizes for AM fungus treatment, pathogen treatment, and their interaction were calculated based on measures of average plant growth, pathogen growth, or AM colonization as reported by authors. AM fungus had a very large positive effect, and pathogens had a very large negative effect on plant growth. However, the interaction of AM fungi and pathogen depended upon the class of pathogen. AM fungi tended to decrease the harmful effects of fungal pathogens but to exacerbate the harmful effects of nematodes. Overall, inoculation with AM fungi had a large negative effect on growth of pathogens, but the outcome for nematodes depended upon mode of feeding. AM fungi harmed sedentary endoparasitic nematodes. AM fungi improved growth of migratory endoparasitic nematodes, but this outcome was not significant in an analysis limited to independent experiments. Reduced growth of pathogens in mycorrhizal plants may indicate increased host resistance. Alternatively, reduced AM colonization in pathogen-treated plants may suggest that root- infecting organisms compete for resources. Some experiments that did not meet criteria for meta-analysis were analyzed with x2 tests by calculating the proportion of experiments in which authors reported a significant pathogen effect in the absence vs. presence of AM fungi. AM fungi did not alter the effect of fungal pathogens but reduced the proportion of experiments in which nematodes de- pressed plant growth in a test limited to independent observations. Comparison of frequency distributions of effect sizes suggests that the difference in results for nematodes in the meta-analysis and the x2 tests stems from differences in the AM fungus-plant-pathogen systems examined in each approach rather than a statistical artifact. AM fungi reduced pathogen growth in -50% of studies included in x2 tests, and this effect was similar for fungal vs. nematode pathogens and sedentary vs. migratory nema- todes. Fungal pathogens reduced AM fungus growth more frequently than did nematodes. Both AM fungi and pathogens suffered reduced growth in 16% of experiments that assessed reciprocal effects. Reciprocal suppression was more common when the pathogen was a fungus, which may indicate that fungal pathogens are more likely than nematodes to compete with AM fungi. Almost all research on the effects of AM fungi on plant-pathogen relations has examined economically important species, in low-phosphorus soil, in greenhouse or microplot culture. Although these analyses revealed general patterns of the effects of AM fungi on plant- pathogen relations, research on a broader range of species over a wider range of environ- mental conditions is required to determine the domain for which these patterns apply.

Ecologia das comunidades de metazoários parasitos, do xaréu, Caranx hippos (Linnaeus) e do xerelete, Caranx latus Agassiz (Osteichthyes, Carangidae) do litoral do estado do Rio de Janeiro, Brasil

Sixty specimens of Caranx hippos (Linnaeus, 1766) and fifty-five specimens of Caranx latus Agassiz, 1831 collected from the coastal zone of the State of Rio de Janeiro (21-23oS, 41-45oW), Brazil, from October 1998 to October 1999, were necropsied to study their metazoan parasites. All specimens of C. hippos were parasitized, and the majority of specimens of C latus (96.4%) were parasitized by one or more metazoan species. Nineteen species of parasites were collected in C. hippos: 5 digeneans, 5 monogeneans, 2 cestodes, 5 nematodes, and 2 copepods. Seventeen species of parasites were collected in C. latus: 6 digeneans, 2 monogeneans, 3 cestodes, 4 nematodes, and 2 copepods. The endoparasites (digeneans, cestodes, and nematodes) were the majoriry, 76.4% and 63.3%, of the total number of parasite specimens collected in C. hippos and C. latus, respectively. The monogeneans Allopyragraphorus hippos (Hargis, 1956) and Cemocotyle carangis (MacCallum, 1913) were the most dominant species with the highest parasitic prevalence in the parasite community of C. hippos and C. latus, respectively. The metazoan parasites of the two host species showed the typical overdispersed pattern of distribution. Bucephalus varicus Manter, 1940. A. hippos, Protomicrocotyle mirabilis (MacCallum, 1918), Cucullanus pulcherrimus Barreto, 1918, and Lemanthropus giganteus KrΦyer, 1863 had a positive correlation only between the host's total length and abundance andlor prevalence in C. hippos. Bucephalus varicus, Tergestia pectinata (Linton, 1905), C. carangis, and Pseudoterranovo sp. had a positive correlation between the host's total length and abundance and/or prevalence in C. latus. In C. hippos, the copepod Caligus robustus Bassett-Smith, 1898 had the highest values of prevalence and abundance in the female hosts. No parasite species showed influence of the host's sex on their prevalence and abundance in C. latus. The mean diversity and the parasite species richness of the parasite infracommunities of C. hippos and C. latus were not significantly different. Only the parasite species diversity of C. hippos was correlated with the host's total length; in both host species the parasite diversity did not showed differences in relation to the sex of the host. Only one pair of ectoparasite species, A. hippos - P. mirabilis, showed significant positive co-occurrence and covariation in the parasite infracommunities of C. hippos. Two endoparasite species, B. varicus - Parahemiurus merus (Linton, 1910), showed negative co-occurrence and positive covariation; and the pair B. varicus - Pseudoterranova sp. had positive co-occurrence and covariation in the infracommunities of C. latus. The values of qualitative and quantitative simila rity coefficients between the parasite communities of C. hippos and C. latus were 55.5 and 30.4, respectively. The parasite communities of C. hippos and C. latus were defined as closest to isolationist type because there are few evidences of interspecific associations or covariations. Additional parasitological studies on other species of carangid fishes from the South American Atlantic Ocean are needed to evaluate the structure of carangid parasite communities in the Neotropical region.

Biotic and abiotic effects on endoparasites infecting Dipodomys and Perognathus species.

Between 1989 and 1998, 3,504 rodents of the genera Dipodomys and Perognathus were collected from 4 permanent collecting sites on the University of New Mexico's Long Term Ecological Research station, located on the Sevilleta National Wildlife Refuge (SNWR), Socorro County. New Mexico. All animals were killed and examined for endoparasites (acanthocephalans, cestodes, coccidia, and nematodes). The present report focuses on 3 endoparasite groups, cestodes, coccidia, and nematodes. Specific analyses address how prevalence changes were related to abiotic factors such as habitat, season, or precipitation, and how prevalence of each parasite species in each host species differed in relation to host age, host sex, host reproductive status, host body mass, host density, parasite-parasite interactions, and host specificity. A logistic regression was used to determine which host characters and which abiotic factors are correlated with a parasite infection. Significant variables for at least half of the parasites include season, site, and winter precipitation. However, no parasite prevalences were correlated, and significant variables were not identical between parasites, indicating that each parasite species varied independently and that no generalizations can be drawn. The parasite prevalences in these rodents on the SNWR vary in independent and complex ways.

Arabidopsis thaliana genes expressed in the early compatible interaction with root-knot nematodes.

In the compatible interaction between Arabidopsis thaliana and the endoparasitic nematode Meloidogyne incognita, galls are formed on the roots of the host plant. Differential display was used to identify alterations of gene expression in young A. thaliana root galls caused by M. incognita. Six genes were confirmed as plant genes by DNA gel blot hybridizations. Significant homology was found with a trypsin inhibitor, peroxidase, mitochondrial uncoupling protein, endomembrane protein, 20S proteasome alpha-subunit, and diaminopimelate decarboxylase. The cellular and temporal expression of each of the six genes was analyzed by mRNA in situ hybridizations.

Plant-parasitic nematodes in field crops in South Africa. 6. Soybean

AbstractTwelve plant-parasitic nematode genera and 25 species were identified in soil and root samples collected from 17 localities in the soybean production areas of South Africa during the 1995/96 season. Predominant endoparasites found included Meloidogyne incognita, M. javanica, M. hapla, M. ethiopica, Pratylenchus zeae and P. brachyurus. Meloidogyne species occurred in 91% of all root samples, P. zeae in 87% and P. brachyurus in 33%. The occurrence of the three predominant endoparasites was not restricted to sandy soil, but they were also prevalent in soils with high clay contents. Other endoparasitic nematode species were Pratylenchus crenatus, P. teres, P. neglectus, P. thornei, Rotylenchulus parvus and Ditylenchus africanus. Nematodes found in soil included Helicotylenchus dihystera and Scutellonema brachyurus, which occurred in 78 and 71%, respectively, of all soil samples. Other nematodes found in soil samples were Helicotylenchus pseudorobustus, H. digonicus, H. microcephalus, Scutellonema truncatum, Rotylenchus unisexus, Mesocriconema sphaerocephalum, Paratrichodorus minor, Longidorus pisi, Xiphinema vanderlindei, X. elongatum and Tylenchorhynchus goffarti. Highest prominence values (PV) for Meloidogyne species were recorded on cultivars Prima, Bakgat, Tamboti, Hutcheson and Knap and the lowest on cultivars PAN812, SCS1, Nyala, Gazelle and A7119. Eight nematode genera and 19 species extracted during this study are reported for the first time in association with soybean in South Africa.

Identification of mariner-like elements from the root-knot nematode Meloidogyne spp.

The Meloidogyne species are agriculturally important pests widespread in the world. These polyphagous endoparasitic nematodes possess an astonishing ability to bypass the plant resistance genes in few generations. However, the genes and mechanisms involved in this molecular determinism are not yet known. Except cytogenetic and cytotaxonomic studies, few data are available concerning their genome. There is therefore an important need of molecular tools for genetic investigation of their virulence character and other aspects of host–pathogen interactions. In that respect, the presence of mariner-like-elements (MLEs) was assessed in these endoparasitic nematodes by a polymerase chain reaction (PCR) assay using degenerate primers designed from two conserved regions of the mariner transposase open reading frame (ORF). Four Meloidogyne species of the five tested revealed the presence of MLEs in their genome. Southern blot analysis indicated that sequences hybridizing to the mariner transposase-like PCR clones occur at a moderate to low copy number in the different Meloidogyne spp. genomes. The phylogenetic analysis show that the Meloidogyne MLEs may form new subfamilies of mariner. Moreover, five PCR clones were shown to possess a continuous ORF suggesting the presence of putative transposase-like coding regions.

Host plant response of Eumusa and Australimusa bananas (Musa spp.) to migratory endoparasitic and root-knot nematodes

AbstractTwenty-five banana varieties of section Eumusa (AA-group) and seven of the section Australimusa (Fe'i-group) from Papua New Guinea were evaluated for resistance to Radopholus similis, Pratylenchus coffeae and Meloidogyne spp. The host plant responses were compared with the susceptible reference cvs Grande Naine and Cavendish 901. In vitro propagated plants were transferred to the glasshouse in loamy sand and inoculated with approximately 1000 migratory endoparasitic nematodes at 4 weeks after planting. Reproduction of R. similis and P.coffeae in the roots was determined at 8 or 10 weeks, respectively, after inoculation. Reproduction of Meloidogyne spp. was determined 8 weeks after inoculation with 3300 to 5000 eggs. No resistance to R. similis was found in the diploid varieties. The Fe'i variety Rimina and possibly Menei were resistant to R. similis. All varieties tested were susceptible to P.coffeae and Meloidogyne spp. Tests de résistance de bananiers Eumusa et Australimusa (Musa spp.) envers les nématodes endoparasites migrateurs et galligènes - Vingt-cinq variétés de bananier de la section Eumusa (groupe AA) et sept de la section Australimusa (group Fe'i) provenant de Papouasie-Nouvelle Guinée ont été testées pour leur résistance envers Radopholus similis, Pratylenchus coffeae et Meloidogyne spp. Les résponses de ces variétés ont été comparées à celles des cultivars sensibles de référence Grande Naine et Cavendish 901. Des vitroplants ont été mis en place en serre sur un sol argilo-sableux et inoculés 4 semaines après plantation avec environ 1000 R. similis ou P.coffeae dont la reproduction a été déterminée 8 et 10 semaines, respectivement, après inoculation. La reproduction de Meloidogyne spp. l'a été 8 semaines après inoculation avec 3300 à 5000 oeufs. Aucune résistance à R. similis n'a été observée chez les variétés diploïdes. Les variétés du groupe Fe'i Rimina et Menei se sont montrées résistantes à R. similis, avec un certain doute dans le cas de la dernière. Toutes les variétés testées sont sensibles à P.coffeae et Meloidogyne spp.

Sedentary endoparasitic nematodes as a model for other plant parasitic nematodes

AbstractPlant parasitic nematodes are known to cause a severe reduction in crop yield. Recently much effort is being put to engineering new nematode-resistant crop cultivars. Plant parasitic nematodes occur in three widely separated orders: Triplonchida, Dorylaimida and Tylenchida. All triplonchid and dorylaimid plant parasitic nematodes are migratory ectoparasites of roots. Within the Tylenchida, several different types of plant parasitism can be recognised. The sedentary endoparasites have the most complex interaction with their host, and are responsible for the vast majority of the agricultural damage. This causes most research to be concentrated on two groups of the sedentary endoparasitic nematodes: cyst- and root-knot nematodes. Both induce specialised feeding structures in the vascular cylinder of the plant root. The mechanism of phytoparasitism of the cyst- and root-knot nematodes is reviewed, of which some aspects will be applicable to the study of the other plant parasitic nematodes. Les nématodes parasites de plantes sont connus pour provoquer de sévères réductions dans les rendements des cultures. Actuellement, un effort se développe pour créer de nouveaux cultivars résistants aux nématodes. Les nématodes parasite de plantes appartiennent à trois ordres très éloignés: Triplonchida, Dorylaimida et Tylenchida. Tous les nématodes parasites de plantes chez les Triplonchida et Dorylaimida sont des ectoparasites migrateurs. Chez les Tylenchida, plusieurs types différents de parasitisme peuvent être identifiés. Les endoparasites sédentaires ont l’interaction la plus complexe avec leur hôte et sont responsables de la plus grande part des dégâts agricoles. C’est la raison pour laquelle la plupart des recherches sont concentrées sur deux groupes de nématodes endoparasites sédentaires, les nématodes à kystes et les nématodes galligènes. Ces deux groupes induisent des structures d’alimentation spécialisées dans les tissus vasculaires de la racine végétale. Le mécanisme parasitaire des nématodes à kystes et galligènes est revu, certaines de leurs caractéristiques pouvant être applicables à l’étude des autres nématodes phytoparasites.

PLANT GENE EXPRESSION IN NEMATODE-INDUCED FEEDING SITES AND STRATEGIES FOR ENGINEERING NEMATODE CONTROL

Efforts by several research teams to gather information on how feeding cells of sedentary endoparasitic nematodes are molecularly supported evolve more and more toward a comprehension of the involved plant gene expression and regulation. Effective genetically engineered nematode control can be achieved by using a combination of a specific nematode-inducible promoter and the destructive effect of a selected gene product targeted toward the nematode or the feeding cell. Feeding site-expressed genes are currently being catalogued. The expression of many plant genes with known function and pattern is being monitored by using reporter gene systems (e.g. beta-glucuronidase) to study the potential role of these genes in the interaction between the nematode and its host. Specific searches including the screening of cDNA libraries and, more recently, the differential display technique allowed the identification of differentially expressed sequences. Promoter traps have been successfully used to isolate nematode-responsive regulatory regions and deletion studies are aimed to obtain more refined nematode-responsive elements. So far, linkage of a tagged promoter with a native gene could not often be proven, but such a correspondence might not be necessary for engineering nematode control. Because the plant's gene regulation was not developed as such to serve nematodes, strict confinement of gene activity to feeding structures is not always feasible and therefore the so-called two-component system might be useful for its counterbalancing effect on eventual undesired leakiness. The more the catalogue broadens, the more possibilities it will create to find the most suitable and promising promoter-gene combination to fight nematode attack.

Host status and reaction of Arabidopsis thaliana ecotypes to infection by the northern root‐knot nematode (Meloidogyne hapla)

AbstractA phenotypically and geographically diverse collection of 45 Arabidopsis thaliana ecotypes was evaluated for its reaction to the northern root‐knot nematode (Meloidogyne hapla) under greenhouse conditions. A significant ecotype effect was observed for both criteria used for host reaction assessment: namely, root‐gall rating (RGR) and nematode reproductive index (RI). A non‐host (non‐galling) response was not detected in this germplasm collection. The majority of the ecotypes showed high degrees of RGR and RI. However, some medium‐late maturing ecotypes such as ‘CS‐1540’ and ‘CS‐6028’(both collected in the UK) displayed high RGR but in association with very reduced RI. The confirmation of A. thaliana as an experimental host of M. hapla and the identification of differential levels of response to infection by this endoparasitic nematode provides the opportunity for genetic and molecular characterization of a new set of plant genes expressed during plant‐nematode interaction. A. thaliana is the smallest and the best characterized genome among plant species, which may facilitate gene isolation and cloning. This may hasten transgenic transfer to economically important host plants of the genetic factors controlling low RI levels identified in some A. thaliana ecotypes.

Plant parasitic nematodes and spatio-temporal variation in natural vegetation

In this paper, the possible role of plant parasitic nematodes as a driving force of change in natural vegetation is analysed using the succession at coastal foredunes as a model. In pot trials, when grown in natural substrates from mature stands, seedlings of dominant foredune plants showed poor growth when compared to sterilised substrates. In natural substrates from preceding stages, growth reduction was far less, or even non-significant. Chemical soil treatment with nematicides resulted in enhanced growth of the seedlings. Inoculation of ectoparasitic nematodes reduced plant growth, but only at densities much higher than those occurring in control pots with unsterilised field soil. Identification at species level showed that endoparasitic nematodes are specifically associated with particular plant species. Therefore, it is hypothesised that the distribution of endoparasitic nematodes may be an important factor leading to the specific nature of soil pathogen complexes in coastal foredunes. We discuss the role of endoparasitic nematodes as possible key species in soil communities and compare the possible effects of specific vs. generalistic plant parasitic nematodes in soil communities on vegetation processes.