Transcriptional changes of the root-knot nematode Meloidogyne incognita in response to Arabidopsis thaliana root signals.

Root-knot nematodes are obligate parasites that invade roots and induce the formation of specialized feeding structures. Although physiological and molecular changes inside the root leading to feeding site formation have been studied, very little is known about the molecular events preceding root penetration by nematodes. In order to investigate the influence of root exudates on nematode gene expression before plant invasion and to identify new genes potentially involved in parasitism, sterile root exudates from the model plant Arabidopsis thaliana were produced and used to treat Meloidogyne incognita pre-parasitic second-stage juveniles. After confirming the activity of A. thaliana root exudates (ARE) on M. incognita stylet thrusting, six new candidate genes identified by cDNA-AFLP were confirmed by qRT-PCR as being differentially expressed after incubation for one hour with ARE. Using an in vitro inoculation method that focuses on the events preceding the root penetration, we show that five of these genes are differentially expressed within hours of nematode exposure to A. thaliana roots. We also show that these genes are up-regulated post nematode penetration during migration and feeding site initiation. This study demonstrates that preceding root invasion plant-parasitic nematodes are able to perceive root signals and to respond by changing their behaviour and gene expression.

The Arabidopsis F-box/Kelch-repeat protein At2g44130 is upregulated in giant cells and promotes nematode susceptibility.

We report that the F-box/Kelch-repeat protein At2g44130 is specifically induced by the root-knot nematode Meloidogyne incognita during the initial stages of the initiation and maintenance of the feeding site. In addition, we show that the expression of this gene promotes susceptibility of infection because knocking down the F-box gene (At2g44130) drastically reduces nematode attraction to and infection of roots. In contrast, F-box overexpressing (OE) lines had a hypersusceptible phenotype, with an increase of 34% in nematode attraction and 67% in nematode infection when grown in soil. This hypersusceptibility might be the result of an increased attraction of the second-stage juveniles toward root exudates of the F-box OE, which would suggest that the blend of compounds in the root exudates of the OE line was somewhat different from the ones present in the root exudates of the wild type and the F-box knockout and tilling lines. Although the function of the F-box/Kelch-repeat protein (At2g44130) is not known, we postulate that its activation by nematode effectors released during the infection process leads to the formation of SCF((At2g44130)) (Skp1-Cullin1-F-box protein) complexes, which are involved in facilitating successful infection by the nematode through targeting specific proteins for degradation.

Biometrical, Biochemical, and Molecular Diagnosis of Portuguese Meloidogyne hispanica Isolates.

Meloidogyne hispanica infects many economically important crops worldwide. The accurate identification of this pathogen is essential for the establishment of efficient and sustainable integrated pest management programs. Portuguese M. hispanica isolates were studied by biometrical, biochemical, and molecular characteristics. Biometrical characteristics of M. hispanica females, males, and second-stage juveniles were similar to the original description. Biochemical studies revealed a unique enzyme pattern (Hi4) for M. hispanica esterases that allowed for species differentiation. Molecular analysis of the mtDNA region from COII and 16S rRNA genes resulted in amplification products (1,800 bp) similar to M. hispanica, M. ethiopica, and M. javanica, and the described HinfI was unable to discriminate M. hispanica from the other two species. Analysis of the mtDNA sequences revealed altered nucleotides among the isolates that created new restriction sites for AluI and DraIII. The resulting restriction patterns successfully discriminated between the three species, providing a new tool for Meloidogyne identification. Finally, the phylogenetic relationship between M. hispanica and several Meloidogyne spp. sequences was analyzed using mtDNA, confirming the divergence between meiotic and mitotic species and revealing the proximity of M. hispanica to closely related species. Based on the studies conducted, the application of isozyme or polymerase chain reaction restriction fragment length polymorphism analysis would be a useful and efficient methodology for M. hispanica identification.

Cuticle surface coat of plant-parasitic nematodes.

The surface coat (SC) of the plant-parasitic nematode cuticle is an understudied area of current research, even though it likely plays key roles in both nematode-plant and nematode-microbe interactions. Although in several ways Caenorhabditis elegans is a poor model for plant-parasitic nematodes, it is a useful starting point for investigations of the cuticle and its SC, especially in the light of recent work using this species as a model for innate immunity and the generic biology underpinning much host-parasite biology. We review the research focused on the involvement of the SC of plant-parasitic nematodes. Using the insights gained from animal-parasitic nematodes and other sequenced nematodes, we discuss the key roles that the SC may play.

Chemotaxis can take plant-parasitic nematodes to the source of a chemo-attractant via the shortest possible routes.

It has long been recognized that chemotaxis is the primary means by which nematodes locate host plants. Nonetheless, chemotaxis has received scant attention. We show that chemotaxis is predicted to take nematodes to a source of a chemo-attractant via the shortest possible routes through the labyrinth of air-filled or water-filled channels within a soil through which the attractant diffuses. There are just two provisos: (i) all of the channels through which the attractant diffuses are accessible to the nematodes and (ii) nematodes can resolve all chemical gradients no matter how small. Previously, this remarkable consequence of chemotaxis had gone unnoticed. The predictions are supported by experimental studies of the movement patterns of the root-knot nematodes Meloidogyne incognita and Meloidogyne graminicola in modified Y-chamber olfactometers filled with Pluronic gel. By providing two routes to a source of the attractant, one long and one short, our experiments, the first to demonstrate the routes taken by nematodes to plant roots, serve to test our predictions. Our data show that nematodes take the most direct route to their preferred hosts (as predicted) but often take the longest route towards poor hosts. We hypothesize that a complex of repellent and attractant chemicals influences the interaction between nematodes and their hosts.

Plant parasitic nematode proteins and the host parasite interaction.

This review focuses on the proteins and secretions of sedentary plant parasitic nematodes potentially important for plant-nematode interactions. These nematodes are well equipped for parasitism of plants. Having acquired the ability to manipulate fundamental aspects of plant biology, they are able to hijack host-cell development to make their feeding site. They feed exclusively from feeding sites as they complete their life cycle, satisfying their nutritional demands for development and reproduction. Biochemical and genomic approaches have been used successfully to identify a number of nematode parasitism genes. So far, 65 204 expressed sequence tags (ESTs) have been generated for six Meloidogyne species and sequencing projects, currently in progress, will underpin genomic comparisons of Meloidogyne spp. with sequences of other pathogens and generate genechip microarrays to undertake profiling studies of up- and down-regulated genes during the infection process. RNA interference provides a molecular genetic tool to study gene function in parasitism. These methods should provide new data to help our understanding of how parasitic nematodes infect their hosts, leading to the identification of novel pathogenicity genes.