Meloidogyne javanica chorismate mutase 1 alters plant cell development

Engineered resistance and hypersusceptibility through functional metabolic studies of 100 genes in soybean to its major pathogen, the soybean cyst nematode

During pathogen attack, the host plant induces genes to ward off the pathogen while the pathogen often produces effector proteins to increase susceptibility of the host. Gene expression studies of syncytia formed in soybean root by soybean cyst nematode (Heterodera glycines) identified many genes altered in expression in resistant and susceptible roots. However, it is difficult to assess the role and impact of these genes on resistance using gene expression patterns alone. We selected 100 soybean genes from published microarray studies and individually overexpressed them in soybean roots to determine their impact on cyst nematode development. Nine genes reduced the number of mature females by more than 50 % when overexpressed, including genes encoding ascorbate peroxidase, β-1,4-endoglucanase, short chain dehydrogenase, lipase, DREPP membrane protein, calmodulin, and three proteins of unknown function. One gene encoding a serine hydroxymethyltransferase decreased the number of mature cyst nematode females by 45 % and is located at the Rhg4 locus. Four genes increased the number of mature cyst nematode females by more than 200 %, while thirteen others increased the number of mature cyst nematode females by more than 150 %. Our data support a role for auxin and ethylene in susceptibility of soybean to cyst nematodes. These studies highlight the contrasting gene sets induced by host and nematode during infection and provide new insights into the interactions between host and pathogen at the molecular level. Overexpression of some of these genes result in a greater decrease in the number of cysts formed than recognized soybean cyst nematode resistance loci.

The 8D05 parasitism gene of Meloidogyne incognita is required for successful infection of host roots

Parasitism genes encode effector proteins that are secreted through the stylet of root-knot nematodes to dramatically modify selected plant cells into giant-cells for feeding. The Mi8D05 parasitism gene previously identified was confirmed to encode a novel protein of 382 amino acids that had only one database homolog identified on contig 2374 within the Meloidogyne hapla genome. Mi8D05 expression peaked in M. incognita parasitic second-stage juveniles within host roots and its encoded protein was limited to the subventral esophageal gland cells that produce proteins secreted from the stylet. Constitutive expression of Mi8D05 in transformed Arabidopsis thaliana plants induced accelerated shoot growth and early flowering but had no visible effects on root growth. Independent lines of transgenic Arabidopsis that expressed a double-stranded RNA complementary to Mi8D05 in host-derived RNA interference (RNAi) tests had up to 90% reduction in infection by M. incognita compared with wild-type control plants, suggesting that Mi8D05 plays a critical role in parasitism by the root-knot nematode. Yeast two-hybrid experiments confirmed the specific interaction of the Mi8D05 protein with plant aquaporin tonoplast intrinsic protein 2 (TIP2) and provided evidence that the Mi8D05 effector may help regulate solute and water transport within giant-cells to promote the parasitic interaction.

The root-knot nematode calreticulin Mi-CRT is a key effector in plant defense suppression

Root-knot nematodes (RKN) are obligate biotrophic parasites that settle close to the vascular tissues in roots, where they induce the differentiation of specialized feeding cells and maintain a compatible interaction for 3 to 8 weeks. Transcriptome analyses of the plant response to parasitic infection have shown that plant defenses are strictly controlled during the interaction. This suggests that, similar to other pathogens, RKN secrete effectors that suppress host defenses. We show here that Mi-CRT, a calreticulin (CRT) secreted by the nematode into the apoplasm of infected tissues, plays an important role in infection success, because Mi-CRT knockdown by RNA interference affected the ability of the nematodes to infect plants. Stably transformed Arabidopsis thaliana plants producing the secreted form of Mi-CRT were more susceptible to nematode infection than wild-type plants. They were also more susceptible to infection with another root pathogen, the oomycete Phytophthora parasitica. Mi-CRT overexpression in A. thaliana suppressed the induction of defense marker genes and callose deposition after treatment with the pathogen-associated molecular pattern elf18. Our results show that Mi-CRT secreted in the apoplasm by the nematode has a role in the suppression of plant basal defenses during the interaction.

Manipulation of plant cells by cyst and root-knot nematode effectors

A key feature of sedentary plant-parasitic nematodes is the release of effector proteins from their esophageal gland cells through their stylets into host roots. These proteinaceous stylet secretions have been shown to be crucial for successful parasitism by mediating the transition of normal root cells into specialized feeding sites and by negating plant defenses. Recent technical advances of purifying mRNA from esophageal gland cells of plant-parasitic nematodes coupled with emerging sequencing technologies is steadily expanding our knowledge of nematode effector repertoires. Host targets and biological activities of a number of nematode effectors are continuously being reported and, by now, a first picture of the complexity of sedentary nematode parasitism at the molecular level is starting to take shape. In this review, we highlight effector mechanisms that recently have been uncovered by studying the host-pathogen interaction. These mechanisms range from mediating susceptibility of host plants to the actual triggering of defense responses. In particular, we portray and discuss the mechanisms by which nematode effectors modify plant cell walls, negate host defense responses, alter auxin and polyamine signaling, mimic plant molecules, regulate stress signaling, and activate hypersensitive responses. Continuous molecular characterization of newly discovered nematode effectors will be needed to determine how these effectors orchestrate host signaling pathways and biological processes leading to successful parasitism.

de novo analysis and functional classification of the transcriptome of the root lesion nematode, Pratylenchus thornei, after 454 GS FLX sequencing

The migratory endoparasitic root lesion nematode Pratylenchus thornei is a major pest of the cereals wheat and barley. In what we believe to be the first global transcriptome analysis for P. thornei, using Roche GS FLX sequencing, 787,275 reads were assembled into 34,312 contigs using two assembly programs, to yield 6,989 contigs common to both. These contigs were annotated, resulting in functional assignments for 3,048. Specific transcripts studied in more detail included carbohydrate active enzymes potentially involved in cell wall degradation, neuropeptides, putative plant nematode parasitism genes, and transcripts that could be secreted by the nematode. Transcripts for cell wall degrading enzymes were similar to bacterial genes, suggesting that they were acquired by horizontal gene transfer. Contigs matching 14 parasitism genes found in sedentary endoparasitic nematodes were identified. These genes are thought to function in suppression of host defenses and in feeding site development, but their function in P. thornei may differ. Comparison of the common contigs from P. thornei with other nematodes showed that 2,039 were common to sequences of the Heteroderidae, 1,947 to the Meloidogynidae, 1,218 to Radopholus similis, 1,209 matched expressed sequence tags (ESTs) of Pratylenchus penetrans and Pratylenchus vulnus, and 2,940 to contigs of Pratylenchus coffeae. There were 2,014 contigs common to Caenarhabditis elegans, with 15.9% being common to all three groups. Twelve percent of contigs with matches to the Heteroderidae and the Meloidogynidae had no homology to any C. elegans protein. Fifty-seven percent of the contigs did not match known sequences and some could be unique to P. thornei. These data provide substantial new information on the transcriptome of P. thornei, those genes common to migratory and sedentary endoparasitic nematodes, and provide additional understanding of genes required for different forms of parasitism. The data can also be used to identify potential genes to study host interactions and for crop protection.

Functional roles of effectors of plant-parasitic nematodes

Plant pathogens have evolved a variety of different strategies that allow them to successfully infect their hosts. Plant-parasitic nematodes secrete numerous proteins into their hosts. These proteins, called effectors, have various functions in the plant cell. The most studied effectors to date are the plant cell wall degrading enzymes, which have an interesting evolutionary history since they are believed to have been acquired from bacteria or fungi by horizontal gene transfer. Extensive genome, transcriptome and proteome studies have shown that plant-parasitic nematodes secrete many additional effectors. The function of many of these is less clear although during the last decade, several research groups have determined the function of some of these effectors. Even though many effectors remain to be investigated, it has already become clear that they can have very diverse functions. Some are involved in suppression of plant defences, while others can specifically interact with plant signalling or hormone pathways to promote the formation of nematode feeding sites. In this review, the most recent progress in the understanding of the function of plant-parasitic nematode effectors is discussed.

Metabolic priming by a secreted fungal effector

Maize smut caused by the fungus Ustilago maydis is a widespread disease characterized by the development of large plant tumours. U. maydis is a biotrophic pathogen that requires living plant tissue for its development and establishes an intimate interaction zone between fungal hyphae and the plant plasma membrane. U. maydis actively suppresses plant defence responses by secreted protein effectors. Its effector repertoire comprises at least 386 genes mostly encoding proteins of unknown function and expressed exclusively during the biotrophic stage. The U. maydis secretome also contains about 150 proteins with probable roles in fungal nutrition, fungal cell wall modification and host penetration as well as proteins unlikely to act in the fungal-host interface like a chorismate mutase. Chorismate mutases are key enzymes of the shikimate pathway and catalyse the conversion of chorismate to prephenate, the precursor for tyrosine and phenylalanine synthesis. Root-knot nematodes inject a secreted chorismate mutase into plant cells likely to affect development. Here we show that the chorismate mutase Cmu1 secreted by U. maydis is a virulence factor. The enzyme is taken up by plant cells, can spread to neighbouring cells and changes the metabolic status of these cells through metabolic priming. Secreted chorismate mutases are found in many plant-associated microbes and might serve as general tools for host manipulation.

How nematodes manipulate plant development pathways for infection

Sedentary plant-parasitic nematodes establish long term relationships with their hosts. Root vascular cells are transformed into large multinucleate feeding cells from which the nematodes feed for more than one month. Recent transcriptome analyses suggest that feeding cells are different from other plant cell types. Their development, however, remains poorly understood, despite new evidence that appears to confirm previously proposed models, such as the important role of auxin. From the analysis of nematode effector proteins that interact with plant proteins, it has become clear that nematodes manipulate many aspects of plant development, including auxin transport and plant cell differentiation pathways. These studies are also revealing roles for effectors in the inhibition of plant stress and defense responses to establish feeding cells. In the coming years breakthroughs can be expected in our understanding of plant-nematode interactions from the functional analysis of nematode effector genes as well as the involved plant genes.

Proteins secreted by root-knot nematodes accumulate in the extracellular compartment during root infection

Root-knot nematodes are biotrophic parasites that invade the root apex of host plants and migrate towards the vascular cylinder where they induce the differentiation of root cells into hypertrophied multinucleated giant cells. Giant cells are part of the permanent feeding site required for nematode development into the adult stage. To date, a repertoire of candidate effectors potentially secreted by the nematode into the plant tissues to promote infection has been identified. However, the precise role of these candidate effectors during root invasion or during giant cell induction and maintenance remains largely unknown. Primarily, the identification of the destination of nematode effectors within plant cell compartment(s) is crucial to decipher their actual functions. We analysed the fine localization in root tissues of five nematode effectors throughout the migratory and sedentary phases of parasitism using an adapted immunocytochemical method that preserves host and pathogen tissues.  We showed that secretion of effectors from the amphids or the oesophageal glands is tightly regulated during the course of infection. The analysed effectors accumulated in the root tissues along the nematode migratory path and along the cell wall of giant cells, showing the apoplasm as an important destination compartment for these effectors during migration and feeding cell formation.

Horizontal gene transfer in nematodes: a catalyst for plant parasitism?

The origin of plant parasitism within the phylum Nematoda is intriguing. The ability to parasitize plants has originated independently at least three times during nematode evolution and, as more molecular data has emerged, it has become clear that multiple instances of horizontal gene transfer (HGT) from bacteria and fungi have played a crucial role in the nematode's adaptation to this new lifestyle. The first reported HGT cases in plant-parasitic nematodes were genes encoding plant cell wall-degrading enzymes. Other putative examples of HGT were subsequently described, including genes that may be involved in the modulation of the plant's defense system, the establishment of a nematode feeding site, and the synthesis or processing of nutrients. Although, in many cases, it is difficult to pinpoint the donor organism, candidate donors are usually soil dwelling and are either plant-pathogenic or plant-associated microorganisms, hence occupying the same ecological niche as the nematodes. The exact mechanisms of transfer are unknown, although close contacts with donor microorganisms, such as symbiotic or trophic interactions, are a possibility. The widespread occurrence of horizontally transferred genes in evolutionarily independent plant-parasitic nematode lineages suggests that HGT may be a prerequisite for successful plant parasitism in nematodes.

Ectopic expression of a Meloidogyne incognita dorsal gland protein in tobacco accelerates the formation of the nematode feeding site

Meloidogyne spp., plant-parasitic nematodes present worldwide, are intensively studied because of the damage caused to a large variety of agronomically important crops. Several reports indicate that proteins from the Meloidogyne spp. dorsal gland might play an important role to allow proper establishment of a functional nematode feeding site. The precise role of these proteins in the process of feeding cell development is unknown. To gain insights into the function of these secreted M. incognita proteins, we constitutively (ectopically) expressed the nematodes dorsal gland protein 7E12 in tobacco plants. It was found that the number of galls at 8 and 16 days after nematode infection was significantly higher in transgenic plants compared to control plants. Eggs from nematodes in transgenic plants hatched faster than those in control plants. Histological analysis of nematode induced galls in transgenic plants clearly shows a different morphology. Giant feeding cells harbor more vacuoles and an increased amount of cell wall invaginations, while neighboring cells surrounding feeding cells are more numerous. These results suggest that the presence of the 7E12 protein in tobacco accelerates gall formation. This assumption is supported by our data illustrating faster gall formation and egg eclosion in transgenic plants.

The plant apoplasm is an important recipient compartment for nematode secreted proteins

Similarly to microbial pathogens, plant-parasitic nematodes secrete into their host plants proteins that are essential to establish a functional interaction. Identifying the destination of nematode secreted proteins within plant cell compartment(s) will provide compelling clues on their molecular functions. Here the fine localization of five nematode secreted proteins was analysed throughout parasitism in Arabidopsis thaliana. An immunocytochemical method was developed that preserves both the host and the pathogen tissues, allowing the localization of nematode secreted proteins within both organisms. One secreted protein from the amphids and three secreted proteins from the subventral oesophageal glands involved in protein degradation and cell wall modification were secreted in the apoplasm during intercellular migration and to a lower extent by early sedentary stages during giant cell formation. Conversely, another protein produced by both subventral and dorsal oesophageal glands in parasitic stages accumulated profusely at the cell wall of young and mature giant cells. In addition, secretion of cell wall-modifying proteins by the vulva of adult females suggested a role in egg laying. The study shows that the plant apoplasm acts as an important destination compartment for proteins secreted during migration and during sedentary stages of the nematode.

New insights into the shikimate and aromatic amino acids biosynthesis pathways in plants

The aromatic amino acids phenylalanine, tyrosine, and tryptophan in plants are not only essential components of protein synthesis, but also serve as precursors for a wide range of secondary metabolites that are important for plant growth as well as for human nutrition and health. The aromatic amino acids are synthesized via the shikimate pathway followed by the branched aromatic amino acids biosynthesis pathway, with chorismate serving as a major intermediate branch point metabolite. Yet, the regulation and coordination of synthesis of these amino acids are still far from being understood. Recent studies on these pathways identified a number of alternative cross-regulated biosynthesis routes with unique evolutionary origins. Although the major route of Phe and Tyr biosynthesis in plants occurs via the intermediate metabolite arogenate, recent studies suggest that plants can also synthesize phenylalanine via the intermediate metabolite phenylpyruvate (PPY), similarly to many microorganisms. Recent studies also identified a number of transcription factors regulating the expression of genes encoding enzymes of the shikimate and aromatic amino acids pathways as well as of multiple secondary metabolites derived from them in Arabidopsis and in other plant species.

Additive effects of plant expressed double-stranded RNAs on root-knot nematode development

Ectopically expressed double-stranded RNAs (dsRNAs) have recently been shown to suppress parasitic success of Meloidogyne spp. in plants. We have targeted two genes from the root-knot nematode Meloidogyne incognita; a dual oxidase gene implicated in the tyrosine cross-linking of the developing cuticle and a subunit of signal peptidase, a protein complex required for the processing of secreted proteins. While these genes are involved in different aspects of nematode development, the phenotypic consequences of RNA interference (RNAi) were similar with >or=50% reduction in nematode numbers in the roots and retardation of development to the egg-producing saccate females. Expression of processed dsRNA was observed, but no evidence of detectable levels of small interfering RNAs (siRNAs) was found in the transgenic plants. We show, to our knowledge for the first time, that combining expression of these dsRNAs by crossing appropriate Arabidopsis thaliana lines resulted in an additive effect that further reduced nematode numbers and developmental capacity. Combining RNAi target genes has the potential to enhance the efficacy of RNAi and may allow control of different nematode species or genera in the crop of interest.

Xanthomonas oryzae pv. oryzae XKK.12 contains an AroQgamma chorismate mutase that is involved in rice virulence

Chorismate mutase (CM) is a key enzyme in the shikimate pathway which is responsible for the synthesis of aromatic amino acids. There are two classes of CMs, AroQ and AroH, and several pathogenic bacteria have been reported to possess a subgroup of CMs designated AroQ(gamma). These CMs are usually exported to the periplasm or outside the cell; in a few cases, they have been reported to be involved in virulence and their precise role is currently unknown. Here, we report that the important rice pathogen Xanthomonas oryzae pv. oryzae XKK.12 produces an AroQ(gamma) CM which we have purified and characterized from spent supernatants. This enzyme is synthesized in planta and X. oryzae pv. oryzae knock-out mutants are hypervirulent to rice. The role of this enzyme in X. oryzae pv. oryzae rice virulence is discussed.

Arabidopsis spermidine synthase is targeted by an effector protein of the cyst nematode Heterodera schachtii

Cyst nematodes are sedentary plant parasites that cause dramatic cellular changes in the plant root to form feeding cells, so-called syncytia. 10A06 is a cyst nematode secretory protein that is most likely secreted as an effector into the developing syncytia during early plant parasitism. A homolog of the uncharacterized soybean cyst nematode (Heterodera glycines), 10A06 gene was cloned from the sugar beet cyst nematode (Heterodera schachtii), which is able to infect Arabidopsis (Arabidopsis thaliana). Constitutive expression of 10A06 in Arabidopsis affected plant morphology and increased susceptibility to H. schachtii as well as to other plant pathogens. Using yeast two-hybrid assays, we identified Spermidine Synthase2 (SPDS2), a key enzyme involved in polyamine biosynthesis, as a specific 10A06 interactor. In support of this protein-protein interaction, transgenic plants expressing 10A06 exhibited elevated SPDS2 mRNA abundance, significantly higher spermidine content, and increased polyamine oxidase (PAO) activity. Furthermore, the SPDS2 promoter was strongly activated in the nematode-induced syncytia, and transgenic plants overexpressing SPDS2 showed enhanced plant susceptibility to H. schachtii. In addition, in planta expression of 10A06 or SPDS2 increased mRNA abundance of a set of antioxidant genes upon nematode infection. These data lend strong support to a model in which the cyst nematode effector 10A06 exerts its function through the interaction with SPDS2, thereby increasing spermidine content and subsequently PAO activity. Increasing PAO activity results in stimulating the induction of the cellular antioxidant machinery in syncytia. Furthermore, we observed an apparent disruption of salicylic acid defense signaling as a function of 10A06. Most likely, increased antioxidant protection and interruption of salicylic acid signaling are key aspects of 10A06 function in addition to other physiological and morphological changes caused by altered polyamines, which are potent plant signaling molecules.

Early transcriptomic events in microdissected Arabidopsis nematode-induced giant cells

Root-knot nematodes differentiate highly specialized feeding cells in roots (giant cells, GCs), through poorly characterized mechanisms that include extensive transcriptional changes. While global transcriptome analyses have used galls, which are complex root structures that include GCs and surrounding tissues, no global gene expression changes specific to GCs have been described. We report on the differential transcriptome of GCs versus root vascular cells, induced in Arabidopsis by Meloidogyne javanica at a very early stage of their development, 3 days after infection (d.p.i.). Laser microdissection was used to capture GCs and root vascular cells for microarray analysis, which was validated through qPCR and by a promoter-GUS fusion study. Results show that by 3 d.p.i., GCs exhibit major gene repression. Although some genes showed similar regulation in both galls and GCs, the majority had different expression patterns, confirming the molecular distinctiveness of the GCs within the gall. Most of the differentially regulated genes in GCs have no previously assigned function. Comparisons with other transcriptome analyses revealed similarities between GCs and cell suspensions differentiating into xylem cells. This suggests a molecular link between GCs and developing vascular cells, which represent putative GC stem cells. Gene expression in GCs at 3 d.p.i. was also found to be similar to crown galls induced by Agrobacterium tumefaciens, a specialized root biotroph.

Phenotypic and molecular evaluation of cotton hairy roots as a model system for studying nematode resistance

Agrobacterium rhizogenes-induced cotton (Gossypium hirsutum L.) hairy roots were evaluated as a model system for studying molecular cotton-nematode interactions. Hairy root cultures were developed from the root-knot nematode (RKN) (Meloidogyne incognita [Kofoid and White] Chitwood, race 3)-resistant breeding line M315 and from the reniform nematode (RN) (Rotylenchulus reniformis Linford & Oliveira)-resistant accession GB713 (G. barbadense L.) and compared to a nematode-susceptible culture derived from the obsolete cultivar DPL90. M315, GB713, and DPL90 hairy roots differed significantly in their appearance and growth potential; however, these differences were not correlated with transcript levels of the A. rhizogenes T-DNA genes rolB and aux2 which help regulate hairy root initiation and proliferation. DPL90 hairy roots were found to support both RKN and RN reproduction in tissue culture, whereas M315 and GB713 hairy roots were resistant to RKN and RN, respectively. M315 hairy roots showed constitutive up-regulation of the defense gene MIC3 (Meloidogyne Induced Cotton3) compared to M315 whole-plant roots and DPL90 hairy roots. Our data show the potential use of cotton hairy roots in maintaining monoxenic RKN and RN cultures and suggest hairy roots may be useful in evaluating the effect of manipulated host gene expression on nematode resistance in cotton.

Structural and functional investigation of a secreted chorismate mutase from the plant-parasitic nematode Heterodera schachtii in the context of related enzymes from diverse origins

In this article, we present the cloning of Hscm1, a gene for chorismate mutase (CM) from the beet cyst nematode Heterodera schachtii. CM is a key branch-point enzyme of the shikimate pathway, and secondary metabolites that arise from this pathway control developmental programmes and defence responses of the plant. By manipulating the plant's endogenous shikimate pathway, the nematode can influence the plant physiology for its own benefit. Hscm1 is a member of the CM gene family and is expressed during the pre-parasitic and parasitic stages of the nematode's life cycle. In situ mRNA hybridization reveals an expression pattern specific to the subventral and dorsal pharyngeal glands. The predicted protein has a signal peptide for secretion in addition to two domains. The N-terminal domain of the mature protein, which is only found in cyst nematodes, contains six conserved cysteine residues, which may reflect the importance of disulphide bond formation for protein stabilization. The C-terminal domain holds a single catalytic site and has similarity to secreted CMs of pathogenic bacteria, classifying HsCM1 as an AroQ(gamma) enzyme. The presumed catalytic residues are discussed in detail, and genetic complementation experiments indicate that the C-terminal domain is essential for enzyme activity. Finally, we show how the modular design of the protein is mirrored in the genomic sequence by the intron/exon organization, suggesting exon shuffling as a mechanism for the evolutionary assembly of this protein.

Common and contrasting themes in host cell-targeted effectors from bacterial, fungal, oomycete and nematode plant symbionts described using the Gene Ontology

A wide diversity of plant-associated symbionts, including microbes, produce proteins that can enter host cells, or are injected into host cells in order to modify the physiology of the host to promote colonization. These molecules, termed effectors, commonly target the host defense signaling pathways in order to suppress the defense response. Others target the gene expression machinery or trigger specific modifications to host morphology or physiology that promote the nutrition and proliferation of the symbiont. When recognized by the host's surveillance machinery, which includes cognate resistance (R) gene products, defense responses are engaged to restrict pathogen proliferation. Effectors from diverse symbionts may be delivered into plant cells via varied mechanisms, including whole organism cellular entry (viruses, some bacteria and fungi), type III and IV secretion (in bacteria), physical injection (nematodes and insects) and protein translocation signal sequences (oomycetes and fungi). This mini-review will summarize both similarities and differences in effectors and effector delivery systems found in diverse plant-associated symbionts as well as how these are described with Plant-Associated Microbe Gene Ontology (PAMGO) terms.

RNAi and functional genomics in plant parasitic nematodes

Plant nematology is currently undergoing a revolution with the availability of the first genome sequences as well as comprehensive expressed sequence tag (EST) libraries from a range of nematode species. Several strategies are being used to exploit this wealth of information. Comparative genomics is being used to explore the acquisition of novel genes associated with parasitic lifestyles. Functional analyses of nematode genes are moving toward larger scale studies including global transcriptome profiling. RNA interference (RNAi) has been shown to reduce expression of a range of plant parasitic nematode genes and is a powerful tool for functional analysis of nematode genes. RNAi-mediated suppression of genes essential for nematode development, survival, or parasitism is revealing new targets for nematode control. Plant nematology in the genomics era is now facing the challenge to develop RNAi screens adequate for high-throughput functional analyses.

Effective and specific in planta RNAi in cyst nematodes: expression interference of four parasitism genes reduces parasitic success

Cyst nematodes are highly evolved sedentary plant endoparasites that use parasitism proteins injected through the stylet into host tissues to successfully parasitize plants. These secretory proteins likely are essential for parasitism as they are involved in a variety of parasitic events leading to the establishment of specialized feeding cells required by the nematode to obtain nourishment. With the advent of RNA interference (RNAi) technology and the demonstration of host-induced gene silencing in parasites, a new strategy to control pests and pathogens has become available, particularly in root-knot nematodes. Plant host-induced silencing of cyst nematode genes so far has had only limited success but similarly should disrupt the parasitic cycle and render the host plant resistant. Additional in planta RNAi data for cyst nematodes are being provided by targeting four parasitism genes through host-induced RNAi gene silencing in transgenic Arabidopsis thaliana, which is a host for the sugar beet cyst nematode Heterodera schachtii. Here it is reported that mRNA abundances of targeted nematode genes were specifically reduced in nematodes feeding on plants expressing corresponding RNAi constructs. Furthermore, this host-induced RNAi of all four nematode parasitism genes led to a reduction in the number of mature nematode females. Although no complete resistance was observed, the reduction of developing females ranged from 23% to 64% in different RNAi lines. These observations demonstrate the relevance of the targeted parasitism genes during the nematode life cycle and, potentially more importantly, suggest that a viable level of resistance in crop plants may be accomplished in the future using this technology against cyst nematodes.

Parasitism proteins in nematode-plant interactions

The current battery of candidate parasitism proteins secreted by nematodes to modify plant tissues for parasitism includes cell-wall-modifying enzymes of potential prokaryotic origin, multiple regulators of host cell cycle and metabolism, proteins that can localize to the plant cell nucleus, potential suppressors of host defense, mimics of plant molecules, and a relatively large cadre of predicted novel nematode parasitism proteins. Phenotypic effects of expressing nematode parasitism proteins in transformed plant tissues, protein-protein interaction assays, and RNA-mediated interference (RNAi) analyses are currently providing exciting evidence of the biological role of candidate nematode secreted parasitism proteins and identifying potential novel means of developing transgenic resistance to nematodes in crops.

An easy method using glutaraldehyde-introduced fluorescence for the microscopic analysis of plant biotrophic interactions

The method introduced in this article makes use of the glutaraldehyde-induced auto-fluorescence of proteins after cross-linking with glutaraldehyde for the analysis of cellular and sub-cellular structures. Because the interface of biotrophic interactions is rich in proteins, the method presented is particularly suitable for the analysis of such interactions; we have exemplified its usefulness by analyzing (1) the root feeding sites induced in roots from Arabidopsis thaliana by the root-knot nematode Meloidogyne incognita; (2) leaves from Cucurbita pepo infected by powdery mildew and (3) roots from Nicotiana tabacum colonized by the arbuscular mycorrhizal fungus Glomus intraradices. The use of confocal and multi-photon laser scanning microscopy allows three-dimensional reconstructions from optical sections of complex biotrophic interactions. In the case of root-knot nematode feeding sites, our method enabled us to simultaneously study the development of the plant xylem elements (using lignin auto-fluorescence), the nematode feeding site and the nematode itself.

Root-knot nematodes manipulate plant cell functions during a compatible interaction

Sedentary endoparasitic nematodes are root parasites that interact with their hosts in a remarkable way. These obligate biotrophic pathogens establish an intimate relationship with their host plants, inducing the redifferentiation of root cells into specialized feeding cells. The successful establishment of feeding cells is essential for nematode development. Root-knot nematodes, of the genus Meloidogyne, have evolved strategies enabling them to induce feeding cell formation in thousands of plant species, probably by manipulating fundamental elements of plant cell development. Feeding cells enlarge and are converted into multinucleate giant cells through synchronous nuclear divisions without cell division. Fully differentiated giant cells may contain more than a hundred polyploid nuclei that may have undergone extensive endoreduplication. Hyperplasia and hypertrophy of the surrounding cells lead to the formation of the typical root gall. Giant cell formation requires extensive changes to gene expression. The induction of feeding cells remains poorly understood, but it is thought that effectors secreted by the nematode play a key role in parasitism, with potential direct effects on recipient host cells. In this review, we focus on the most recent investigations of the molecular basis of the plant-root-knot nematode interaction. Recently, microarray technology has been used to study the plant response to Meloidogyne spp. infection. Such a genome-wide expression profiling provides a global view of transcriptional changes, especially for genes involved in cell wall, transport processes and plant defense responses during giant cell formation. The identification of nematode-responsive plant genes constitutes a major step toward understanding how root-knot nematodes dramatically alter root development to induce and maintain giant cells. The characterization of nematode secretions as parasitism effectors and the development of RNAi technology should improve our understanding of the molecular events and regulatory mechanisms involved in plant parasitism. Finally, Meloidogyne genome sequences should provide further insight into plant-root-knot nematode interactions.

Gene expression profiles for cell wall-modifying proteins associated with soybean cyst nematode infection, petiole abscission, root tips, flowers, apical buds, and leaves

Changes in transcript accumulation for cell wall-modifying proteins were examined in excised soybean root pieces colonized by soybean cyst nematodes (SCN), Heterodera glycines, using RT-PCR and soybean Affymetrix GeneChips. Sequence-specific PCR primer pairs were prepared from sequence data for core sequences in the GenBank soybean database and consensus sequences derived from the assembly of soybean ESTs. In addition, to identify previously uncharacterized soybean transcripts, degenerate primers were prepared for conserved motifs in cellulases (endo-1,4-beta-glucanases, EGases) and polygalacturonases (PGs) and these were used to amplify segments of transcripts that were then extended with 3' and 5' RACE. Several novel EGase and PG transcripts were identified. Gene expression patterns were determined by real-time RT-PCR for 11 EGases, three expansins (EXPs), 14 PGs, two pectate lyases (PLs), and two xyloglucan endotransglucosylase/hydrolases (XTHs) in soybean roots inoculated with SCN, non-inoculated roots, serial dissections of root tips, leaf abscission zones, flowers, apical buds, and expanding leaves. A large number of genes associated with cell wall modifications are strongly up-regulated in root pieces colonized by SCN. However, in contrast to most of the transcripts for cell wall proteins, two XTH transcripts were specifically down-regulated in the colonized root pieces. Gene expression in serial dissections of root tips (0-2 mm, and 2-7 mm) and whole roots indicate that the SCN up-regulated genes are associated with a wide range of developmental processes in roots. Also of interest, many of the cDNAs examined were up-regulated in petiole abscission zones induced to abscise with ethylene.

QPCR analysis and RNAi define pharyngeal gland cell-expressed genes of Heterodera glycines required for initial interactions with the host

Changes in transcript abundance of genes expressed in the three pharyngeal gland cells of Heterodera glycines after host invasion were monitored by quantitative polymerase chain reaction (qPCR) and the consequences of disrupting their expression studied by RNAi treatment prior to invasion. Two transcripts were known to be expressed in the two subventral gland cells (hg-pel and hg-eng-1), a further two in the single dorsal gland cell only (hg-gp and hg-syv46), and a fifth transcript (hg-cm) was expressed by both gland cell types. The qPCR study established that transcripts of hg-syv46 and hg-gp increased in abundance by 2 days postinfection (dpi), with the former remaining the most abundant. The hg-cm transcript level showed minor changes from 0 to 14 dpi but did fall by 21 dpi. In contrast, hg-eng-1 and hg-eng-2 messenger (m)RNA declined by 7 dpi and hg-pel by 14 dpi before it increased at 21 dpi. RNAi-targeting of hg-eng-1 reduced the number of females present on the plants at 10 days. Targeting of hg-gp, hg-cm, and hg-pel caused a change in sexual fate favoring male development on roots. Both effects were evident after targeting hg-syv46. Suppression of hg-eng-1 mRNA levels in second-stage juveniles (J2i) by RNAi was transient, with a recovery by 15 days of incubation in water after treatment. Presoaking H. glycines J2 with double-stranded RNA has value for studying gene function during the nematode's early interaction with a plant.

Transcriptome analysis of root-knot nematode functions induced in the early stages of parasitism

Root-knot nematodes of the genus Meloidogyne are obligate biotrophic parasites able to infest > 2000 plant species. The nematode effectors responsible for disease development are involved in the adaptation of the parasite to its host environment and host response modulation. Here, the differences between the transcriptomes of preparasitic exophytic second-stage juveniles (J2) and parasitic endophytic third-stage juveniles (J3) of Meloidogyne incognita were investigated. Genes up-regulated at the endophytic stage were isolated by suppression subtractive hybridization and validated by dot blots and real-time quantitative polymerase chain reaction (PCR). Up-regulation was demonstrated for genes involved in detoxification and protein degradation, for a gene encoding a putative secreted protein and for genes of unknown function. Transcripts of the glutathione S-transferase gene Mi-gsts-1 were 27 times more abundant in J3 than in J2. The observed Mi-gsts-1 expression in the oesophageal secretory glands and the results of functional analyses based on RNA interference suggest that glutathione S-transferases are secreted during parasitism and are required for completion of the nematode life cycle in its host. Secreted glutathione S-transferases may protect the parasite against reactive oxygen species or modulate the plant responses triggered by pathogen attack.

GeneChip profiling of transcriptional responses to soybean cyst nematode, Heterodera glycines, colonization of soybean roots

Soybean cyst nematode (SCN) is currently the most devastating pathogen of soybean. SCN penetrates the root and migrates toward the central vascular bundle where it establishes a complex multinucleated feeding structure that provides plant-derived nutrients to support the development and growth of the nematode. To identify host genes that play significant roles in SCN development in susceptible roots, RNA from SCN-inoculated and non-inoculated root pieces were hybridized to the Affymetrix soybean genome GeneChips. RNA was collected at 8, 12, and 16 d post-inoculation from root pieces that displayed multiple swollen female SCN and similar root pieces from non-inoculated roots. Branch roots and root tips were trimmed from the root pieces to minimize the amount of RNA contributed by these organs. Of the 35 593 transcripts represented on the GeneChip, approximately 26,500 were expressed in the SCN-colonized root pieces. ANOVA followed by False Discovery Rate analysis indicated that the expression levels of 4616 transcripts changed significantly (Q-value < or =0.05) in response to SCN. In this set of 4616 transcripts, 1404 transcripts increased >2-fold and 739 decreased >2-fold. Of the transcripts to which a function could be assigned, a large proportion was associated with cell wall structure. Other functional categories that included a large number of up-regulated transcripts were defence, metabolism, and histones, and a smaller group of transcripts associated with signal transduction and transcription.

1.6 A crystal structure of the secreted chorismate mutase from Mycobacterium tuberculosis: novel fold topology revealed

The presence of exported chorismate mutases produced by certain organisms such as Mycobacterium tuberculosis has been shown to correlate with their pathogenicity. As such, these proteins comprise a new group of promising selective drug targets. Here, we report the high-resolution crystal structure of the secreted dimeric chorismate mutase from M. tuberculosis (*MtCM; encoded by Rv1885c), which represents the first 3D-structure of a member of this chorismate mutase family, termed the AroQ(gamma) subclass. Structures are presented both for the unliganded enzyme and for a complex with a transition state analog. The protomer fold resembles the structurally characterized (dimeric) Escherichia coli chorismate mutase domain, but exhibits a new topology, with helix H4 of *MtCM carrying the catalytic site residue missing in the shortened helix H1. Furthermore, the structure of each *MtCM protomer is significantly more compact and only harbors one active site pocket, which is formed entirely by one polypeptide chain. Apart from the structural model, we present evidence as to how the substrate may enter the active site.

Isolation and characterization of another cDNA encoding a chorismate mutase from the phytoparasitic nematode Meloidogyne arenaria

A new cDNA, named Ma-cm-2, encoding a chorismate mutase (CM), has been isolated from Meloidogyne arenaria. The full-length cDNA, carrying the trans-spliced SL1 leader sequence, was 753-bp long with an open reading frame of 576 bp. The deduced protein MA-CM-2 including amino-terminal signal peptide shows significant similarity to CMs of Meloidogyne incognita, Meloidogyne javanica, and also bacteria. Secondary structure prediction of MA-CM-2 indicates the presence of the three conserved alpha-helix domains present in the Escherichia coli CMs. Reverse transcription and polymerase chain reaction analysis showed that its transcript abundance is high in the early developmental stages and low in later ones. In situ mRNA hybridization revealed that the transcripts of Ma-cm-2 accumulated specifically in the two subventral oesophageal gland cells of M. arenaria. The widespread existence of CMs in the sedentary endoparasitic nematodes implicates that this enzyme plays an important role in the host-parasite interaction.

A model plant pathogen from the kingdom Animalia: Heterodera glycines, the soybean cyst nematode

The soybean cyst nematode, Heterodera glycines, adversely affects the production of soybean, Glycine max, in many areas of the world, particularly in the United States, where it is the most economically important soybean pathogen. Despite the availability of hundreds of H. glycines-resistant soybean cultivars, the nematode continues to be a major limiting factor in soybean production. The use of nonhost rotation and resistance are the primary means of reducing losses caused by the nematode, but each of these options has disadvantages. As a subject for study of nematode parasitism and virulence, H. glycines provides a useful model despite its obligately parasitic nature. Its obligately sexual reproduction and ready adaptation to resistant cultivars, formerly referred to as "race shift," presents an excellent opportunity for the study of virulence in nematodes. Recent advances in H. glycines genomics have helped identify putative nematode parasitism genes, which, in turn, will aid in the understanding of nematode pathogenicity and virulence and may provide new targets for engineering nematode resistance.

In planta secretion of a calreticulin by migratory and sedentary stages of root-knot nematode

Esophageal secretions from endoparasitic sedentary nematodes are thought to play key roles throughout plant parasitism, in particular during the invasion of the root tissue and the initiation and maintenance of the nematode feeding site (NFS) essential for nematode development. The secretion in planta of esophageal cell-wall-degrading enzymes by migratory juveniles has been shown, suggesting a role for these enzymes in the invasion phase. Nevertheless, the secretion of an esophageal gland protein into the NFS by nematode sedentary stages has never been demonstrated. The calreticulin Mi-CRT is a protein synthesized in the esophageal glands of the root-knot nematode Meloidogyne incognita. After three-dimensional modeling of the Mi-CRT protein, a surface peptide was selected to raise specific antibodies. In planta immunolocalization showed that Mi-CRT is secreted by migratory and sedentary stage nematodes, suggesting a role for Mi-CRT throughout parasitism. During the maintenance of the NFS, the secreted Mi-CRT was localized outside the nematode at the tip of the stylet. In addition, Mi-CRT accumulation was observed along the cell wall of the giant cells that compose the feeding site, providing evidence for a nematode esophageal protein secretion into the NFS.

Molecular aspects of cyst nematodes

SUMMARY Taxonomy: Superkingdom Eukaryota; kingdom Metazoa; phylum Nematoda; class Chromadorea; order Tylenchida; suborder Tylenchina; superfamily Tylenchoidea; family Heteroderidae; subfamily Heteroderinae; main genera Heterodera and Globodera.

HOST RANGE

Cyst nematodes comprise approximately 100 known species in six genera. They are pathogens of temperate, subtropical and tropical plant species and the host range of many species is narrow. The most economically important species are within the Globodera and Heterodera genera. Globodera pallida and G. rostochiensis are important pathogens of potato crops. There are many economic species in the Heterodera genus, including Heterodera glycines (soybean cyst nematode), H. avenae (cereal cyst nematode) and H. schachtii (sugar beet cyst nematode), the last of which attacks a range of Chenopodiaceae and Cruciferae, including Arabidopsis thaliana. Disease symptoms: Field symptoms of severe cyst nematode infection are often stunting, wilting and chlorosis, but considerable yield loss can occur without obvious symptoms. The only unique indicator of cyst nematode infection is the presence of adult female nematodes attached to host roots after several weeks of parasitism. Disease control: This is usually achieved by using integrated pest management involving cultural practices such as crop rotation, resistant cultivars if available and chemical control when economically justified.

Selection of Heterodera glycines chorismate mutase-1 alleles on nematode-resistant soybean

The soybean cyst nematode Heterodera glycines is the most destructive pathogen of soybean in the Unites States. Diversity in the parasitic ability of the nematode allows it to reproduce on nematode-resistant soybean. H. glycines chorismate mutase-1 (Hg-CM-1) is a nematode enzyme with the potential to suppress host plant defense compounds; therefore, it has the potential to enhance the parasitic ability of nematodes expressing the gene. Hg-cm-1 is a member of a gene family where two alleles, Hg-cm-1A and Hg-cm-1B, have been identified. Analysis of the Hg-cm-1 gene copy number revealed that there are multiple copies of Hg-cm-1 alleles in the H. glycines genome. H. glycines inbred lines were crossed to ultimately generate three F2 populations of second-stage juveniles (J2s) segregating for Hg-cm-1A and Hg-cm-1B. Segregation of Hg-cm-1A and 1B approximated a 1:2:1 ratio, which suggested that Hg-cm-1 is organized in a cluster of genes that segregate roughly as a single locus. The F2 H. glycines J2 populations were used to infect nematode-resistant (Hartwig, PI88788, and PI90763) and susceptible (Lee 74) soybean plants. H. glycines grown on Hartwig, Lee 74, and PI90763 showed allelic frequencies similar to Hg-cm-1A/B, but nematodes grown on PI88788 contained predominately Hg-cm-1A allele as a result of a statistically significant drop of Hg-cm-1B in the population. This result suggests that specific Hg-cm-1 alleles, or a closely linked gene, may aid H. glycines in adapting to particular soybean hosts.

Characterization of the secreted chorismate mutase from the pathogen Mycobacterium tuberculosis

The gene encompassing ORF Rv1885c with weak sequence similarity to AroQ chorismate mutases (CMs) was cloned from the genome of Mycobacterium tuberculosis and expressed in Escherichia coli. The gene product (*MtCM) complements a CM-deficient E. coli strain, but only if produced without the predicted N-terminal signal sequence typical of M. tuberculosis. The mature *MtCM, which was purified by exploiting its resistance to irreversible thermal denaturation, possesses high CM activity in vitro. The enzyme follows simple Michaelis-Menten kinetics, having a k(cat) of 50 s(-1) and a K(m) of 180 microM (at 30 degrees C and pH 7.5). *MtCM was shown to be a dimer by analytical ultracentrifugation and size-exclusion chromatography. Secondary-structure prediction and CD spectroscopy confirmed that *MtCM is a member of the all-alpha-helical AroQ class of CMs, but it seems to have a topologically rearranged AroQ fold. Because CMs are normally intracellular metabolic enzymes required for the biosynthesis of phenylalanine and tyrosine, the existence of an exported CM in Gram-positive M. tuberculosis is puzzling. The observation that homologs of *MtCM with a predicted export sequence are generally only present in parasitic or pathogenic organisms suggests that secreted CMs may have evolved to participate in some aspect of parasitism or pathogenesis yet to be unraveled.

Two chorismate mutase genes from the root-knot nematode Meloidogyne incognita

SUMMARY Parasitism genes encoding secretory proteins expressed in the oesophageal glands of phytoparasitic nematodes play critical roles in nematode invasion of host plants, establishment of feeding sites and suppression of host defences. Two chorismate mutase (CM) genes potentially having a role in one or more of these processes were identified from a Meloidogyne incognita oesophageal gland-cell subtractive cDNA library. These M. incognita enzymes (designated as MI-CM-1 and MI-CM-2) with amino-terminal signal peptides, were significantly similar to chorismate mutases in M. javanica and bacteria. The complementation of an Escherichia coli CM-deficient mutant by the expression of Mi-cm-1 or Mi-cm-2 confirmed their CM activity. In-situ mRNA hybridization showed that the transcripts of Mi-cm-1 and Mi-cm-2 accumulated specifically in the two subventral oesophageal gland cells of M. incognita. RT-PCR analysis confirmed that their transcript abundances were high in the early parasitic juvenile stages, and low (Mi-cm-1) or undetectable (Mi-cm-2) in later parasitic stages of the nematode. Southern blot analysis revealed that these CM genes were members of a small multigene family in Meloidogyne species. The widespread presence of CMs in the specialized sedentary endoparasitic nematode species suggests that this multifunctional enzyme may be a key factor in modulating plant parasitism.

Feeding cell development by cyst and root-knot nematodes involves a similar early, local and transient activation of a specific auxin-inducible promoter element

SUMMARY To study the role of the phytohormone auxin in nematode feeding cell induction and early development, the transcriptional regulation of the artificial auxin-responsive promoter element DR5 was monitored in Arabidopsis thaliana roots infected with the cyst nematode Heterodera schachtii or the root-knot nematode Meloidogyne incognita. For both nematode species, a specific and strong activation of DR5::gusA was observed inside the initial feeding cells at 18 h post inoculation, pointing to an increase in the perceived auxin concentration. This high expression was maintained until 3-5 days post inoculation and subsequently the GUS staining was reduced. Cyst and root-knot nematodes are distantly related and the feeding sites they induce are highly dissimilar. In this respect, the similarities between the two nematode-induced DR5 activation patterns in A. thaliana roots are remarkable. A transient and local increase in auxin perception could be due to an accumulation or to an increased sensitivity. Based on previously published data, a local auxin accumulation seems to be the more probable explanation. The observed early and localized increase of the perceived IAA concentration in the initial feeding structure underlines that this phytohormone could be an important clue in feeding cell induction by plant parasitic nematodes.

Secretions of plant-parasitic nematodes: a molecular update

The interaction between sedentary endoparasitic nematodes and plants is fascinating, because these animals have developed an ingenious way to manipulate the plant's gene regulation and metabolism to their own advantage. They are able to form highly specialized feeding structures in the plant root to satisfy their nutritional demands for development and reproduction. This ability makes them extremely successful parasites with severe consequences for agriculture. Triggered by these economical losses, detailed studies of the parasitic interaction have been performed, which resulted in an extensive descriptive knowledge. However, the underlying biochemical and molecular events of this intimate relationship have still not been elucidated. It is generally accepted that secretions produced by the nematode are responsible for the dramatic alteration of specific cells in the host plant. In the past few years, the identification of genes coding for secreted proteins was a breakthrough in plant nematode research. However, the available information is still too limited to allow the formulation of a comprehensive model, mainly because the sequences of many of these genes are novel with no similar sequence found in the existing databases. A new challenge in the coming years will be the functional analysis of these putative parasitism genes.

Characterization of two genes expressed in the salivary glands of the Hessian fly, Mayetiola destructor (Say)

Two genes, SSGP-11A1 and SSGP-12A1, have been isolated that encodes proteins with a secretion signal peptide at theN-terminals from the Hessian fly (Mayetiola destructor (Say)). The SSGP-11A1 gene contains one small intron (89 bp) and encodes a putative protein with 79 amino acids. The first 18 amino acids constitute a putative secretion signal peptide. The SSGP-12A1 gene contains three small introns and encodes a putative protein with 234 amino acids. The first 19 amino acids constitute a putative secretion signal peptide. Northern blot analysis revealed that both of the genes are primarily expressed in the salivary glands of Hessian fly larvae, the feeding stage of the insect. These observations are consistent with the possibility that the proteins encoded by them are secreted into host plants during feeding. Even though both genes are exclusively expressed in Hessian fly larvae, the expression profiles between them were quite different in insects at different instars. The SSGP-11A1 gene was expressed in all instars of larvae while the SSGP-12A1 gene was almost exclusively expressed in the first instar larvae. The differential expression suggests that the proteins encoded by these two genes may perform different functions. In situ hybridization revealed that the SSGP-11A1 gene is located on the short arm of chromosome A1 while SSGP-12A1 gene is on the long arm of chromosome A2.

The parasitome of the phytonematode Heterodera glycines

Parasitism genes expressed in the esophageal gland cells of phytonematodes encode secretions that control the complex process of plant parasitism. In the soybean cyst nematode, Heterodera glycines, the parasitome, i.e., the secreted products of parasitism genes, facilitate nematode migration in soybean roots and mediate the modification of root cells into elaborate feeding cells required to support the growth and development of the nematode. With very few exceptions, the identities of these secretions are unknown, and the mechanisms of cyst nematode parasitism, therefore, remain obscure. The most direct and efficient approach for cloning parasitism genes and rapidly advancing our understanding of the molecular interactions during nematode parasitism of plants is to create gland cell-specific cDNA libraries using cytoplasm microaspirated from the esophageal gland cells of various parasitic stages. By combining expressed sequence tag analysis of a gland cell cDNA library with high throughput in situ expression localization of clones encoding secretory proteins, we obtained the first comprehensive parasitome profile for a parasitic nematode. We identified 51 new H. glycines gland-expressed candidate parasitism genes, of which 38 genes constitute completely novel sequences. Individual parasitome members showed distinct gland cell expression patterns throughout the parasitic cycle. The parasitome complexity discovered paints a more elaborate picture of host cellular events under specific control by the nematode parasite than previously hypothesized.

Plant-nematode interactions

Root-knot nematodes and cyst nematodes are obligate, biotrophic pathogens of numerous plant species. These organisms cause dramatic changes in the morphology and physiology of their hosts. The molecular characterization of induced plant genes has provided insight into the plant processes that are usurped by nematodes as they establish their specialized feeding cells. Recently, several gene products have been identified that are secreted by the nematode during parasitism. The corresponding genes have strong similarity to microbial genes or to genes that are found in nematodes that parasitize animals. New information on host resistance genes and nematode virulence genes provides additional insight into this complex interaction.

A chorismate mutase from the soybean cyst nematode Heterodera glycines shows polymorphisms that correlate with virulence

Parasitism genes from phytoparasitic nematodes are thought to be essential for nematode invasion of the host plant, to help the nematode establish feeding sites, and to aid nematodes in the suppression of host plant defenses. One gene that may play several roles in nematode parasitism is chorismate mutase (CM). This secreted enzyme is produced in the nematode's esophageal glands and appears to function within the plant cell to manipulate the plant's shikimate pathway, which controls plant cell growth, development, structure, and pathogen defense. Using degenerate polymerase chain reaction primers, we amplified and cloned a chorismate mutase (Hg-cm-1) from Heterodera glycines, the soybean cyst nematode (SCN), and showed it had CM activity. RNA in situ hybridization of Hg-cm-1 cDNA to SCN sections confirms that it is specifically expressed in the nematodes' esophageal glands. DNA gel blots of genomic DNA isolated from SCN inbred lines that have differing virulence on SCN resistant soybean show Hg-cm-1 is a member of a polymorphic gene family. Some Hg-cm family members predominate in SCN inbred lines that are virulent on certain SCN resistant soybean cultivars. The same polymorphisms and correlation with virulence are seen in the Hg-cm-1 expressed in the SCN second-stage juveniles. Based on the enzymatic activity of Hg-cm-1 and the observation that different forms of the mutase are expressed in virulent nematodes, we hypothesize that the Hg-cm-1 is a virulence gene, some forms of which allow SCN to parasitize certain resistant soybean plants.

Parasitic nematode interactions with mammals and plants

Parasitic nematodes that infect humans, animals, and plants cause serious diseases that are deleterious to human health and agricultural productivity. Chemical and biological control methods have reduced the impact of these parasites. However, surviving environmental stages lead to persistent reinfection of host species. In addition, development of resistance to nematicides and anthelmintics by these parasites and reduced availability of some nematicides, for environmental protection, pose significant obstacles for current and future prospects of effective parasite control. Due to marked differences in host species, research on animal and plant parasitic nematodes often proceeds independently. Despite the differences between animals and plants, basic cellular properties are shared among these host organisms. Some common properties may be important for mechanisms [homologous or convergent (homoplastic)] by which nematodes successfully infect these diverse hosts or by which animal and plant hosts resist infections by these pathogens. Here we compare host/parasite interactions between plant parasitic nematodes (PPN) and animal parasitic nematodes, with an emphasis on mammalian hosts (MPN). Similarities and differences are considered in the context of progress on molecular dissection of these interactions. A comprehensive coverage is not possible in the space allotted. Instead, an illustrative approach is used to establish examples that, it is hoped, exemplify the value of the comparative approach.