Endogenous cellulases in animals: isolation of beta-1, 4-endoglucanase genes from two species of plant-parasitic cyst nematodes

Molecular and biochemical characterization of an endo-beta-1,3-glucanase from the pinewood nematode Bursaphelenchus xylophilus acquired by horizontal gene transfer from bacteria

We report the cloning and functional characterization of an endo-beta-1,3-glucanase from the pinewood nematode Bursaphelenchus xylophilus acquired by horizontal gene transfer from bacteria. This is the first gene of this type from any nematode species. We show that a similar cDNA is also present in another closely related species B. mucronatus, but that similar sequences are not present in any other nematode studied to date. The B. xylophilus gene is expressed solely in the oesophageal gland cells of the nematode and the protein is present in the nematode's secretions. The deduced amino acid sequence of the gene is very similar to glycosyl hydrolase family 16 proteins. The recombinant protein, expressed in Escherichia coli, preferentially hydrolysed the beta-1,3-glucan laminarin, and had very low levels of activity on beta-1,3-1,4-glucan, lichenan and barley beta-glucan. Laminarin was degraded in an endoglucanase mode by the enzyme. The optimal temperature and pH for activity of the recombinant enzyme were 65 degrees C and pH 4.9. The protein is probably important in allowing the nematodes to feed on fungi. Sequence comparisons suggest that the gene encoding the endo-beta-1,3-glucanase was acquired by horizontal gene transfer from bacteria. B. xylophilus therefore contains genes that have been acquired by this process from both bacteria and fungi. These findings support the idea that multiple independent horizontal gene transfer events have helped in shaping the evolution of several different life strategies in nematodes.

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.

Phage display-selected single-chain antibodies confer high levels of resistance against Tomato spotted wilt virus

Rational design of antibodies targeting essential viral proteins can complement the palette of antiviral resistance strategies. Here, stable and high expression of single-chain monoclonal antibodies targeting the nucleoprotein of the economically important plant virus Tomato spotted wilt virus, a protein that is involved in multiple steps in the viral infection cycle, is reported. High cytoplasmic expression levels of three selected phage display-derived anti-viral single-chain antibodies were established. Of these antibodies, two led to high levels of resistance against this plant virus. Protoplast experiments provided evidence that the two resistance-conferring antibodies may have a different mode of action and could be combined for higher durability of resistance in the field.

Functional analysis of pathogenicity proteins of the potato cyst nematode Globodera rostochiensis using RNAi

RNA interference (RNAi) has been used widely as a tool for examining gene function and a method that allows its use with plant-parasitic nematodes recently has been described. Here, we use a modified method to analyze the function of secreted beta-1,4, endoglucanases of the potato cyst nematode Globodera rostochiensis, the first in vivo functional analysis of a pathogenicity protein of a plant-parasitic nematode. Knockout of the beta-1,4, endoglucanases reduced the ability of the nematodes to invade roots. We also use RNAi to show that gr-ams-1, a secreted protein of the main sense organs (the amphids), is essential for host location.

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.

Developmental expression and molecular analysis of two Meloidogyne incognita pectate lyase genes

Proteinaceous secretions from the oesophageal glands of plant-parasitic nematodes have crucial roles in nematode parasitism of plants. Two cDNAs (designated Mi-pel-1 and Mi-pel-2) encoding pectate lyases were isolated from the root-knot nematode, Meloidogyne incognita, oesophageal gland-cell subtractive cDNA libraries, and the corresponding genomic DNAs were subsequently cloned. Southern blot analyses revealed that homologues to these pectate lyase genes were broadly distributed in Meloidogyne species, and present as members of a small multigene family. Mi-pel-1 and Mi-pel-2 encoded, respectively, predicted proteins of 271 and 280 amino acids, each of which was preceded by a signal peptide for secretion. Interestingly, these pectate lyases showed diversity at the amino acid level, with only 31% identity and 49% similarity. These pectate lyases were classified into the same family of pectate lyases with those of other phytoparasitic nematodes that contain four conserved regions characteristic of the class III pectate lyases of microbes. In situ mRNA hybridisation analyses showed the transcripts of Mi-pel-1 and Mi-pel-2 accumulated exclusively within the subventral oesophageal gland cells of M. incognita. RT-PCR analysis confirmed that their transcriptions were strong at the pre-parasitic and early parasitic second-stage juveniles, and not detectable at the late parasitic stages of the nematodes. These results indicated that these pectate lyases, like cellulases, could be secreted into plant tissues to facilitate the penetration and intercellular migration of M. incognita during the early stages of plant parasitism.

Origin, distribution and 3D-modeling of Gr-EXPB1, an expansin from the potato cyst nematode Globodera rostochiensis

Southern analysis showed that Gr-EXPB1, a functional expansin from the potato cyst nematode Globodera rostochiensis, is member of a multigene family, and EST data suggest expansins to be present in other plant parasitic nematodes as well. Homology modeling predicted that Gr-EXPB1 domain 1 (D1) has a flat beta-barrel structure with surface-exposed aromatic rings, whereas the 3D structure of Gr-EXPB1-D2 was remarkably similar to plant expansins. Gr-EXPB1 shows highest sequence similarity to two extracellular proteins from saprophytic soil-inhabiting Actinobacteria, and includes a bacterial type II carbohydrate-binding module. These results support the hypothesis that a number of pathogenicity factors of cyst nematodes is of procaryotic origin and were acquired by horizontal gene transfer.

A method for double-stranded RNA-mediated transient gene silencing in Phytophthora infestans

SUMMARY Gene silencing, triggered by double-stranded RNA (dsRNA), has proved to be a valuable tool for determining and confirming the function of genes in many organisms. For Phytophthora infestans, the cause of late blight on potato and tomato, gene silencing strategies have relied on stable transformation followed by spontaneous silencing of both the endogenous gene and the transgene. Here we describe the first application of transient gene silencing in P. infestans, by delivering in vitro synthesized dsRNA into protoplasts to trigger silencing. A marker gene, gfp, and two P. infestans genes, inf1 and cdc14, both of which have been silenced previously, were selected to test this strategy. Green fluorescent protein (GFP) fluorescence was reduced in regenerating protoplasts up to 4 days after exposure to gfp dsRNA. A secondary reduction in expression of all genes tested was not fully activated until 12-17 days after introduction of the respective homologous dsRNAs. At this time after exposure to dsRNA, reduced GFP fluorescence in gfp dsRNA-treated lines, and reduced INF1 production in inf1 dsRNA-treated lines, was observed. Introduction of dsRNA for the stage-specific gene, cdc14, yielded the expected phenotype of reduced numbers of sporangia when cdc14 expression was significantly reduced compared with control lines. Silencing was shown to be sequence-specific, because analysis of inf1 expression in gfp-silenced lines yielded wild-type levels of gene expression. This report shows that transient gene silencing can be used to generate detectable phenotypes in P. infestans and should provide a high-throughput tool for P. infestans functional genomics.

Ancient Origin of Glycosyl Hydrolase Family 9 Cellulase Genes

While it is widely accepted that most animals (Metazoa) do not have endogenous cellulases, relying instead on intestinal symbionts for cellulose digestion, the glycosyl hydrolase family 9 (GHF9) cellulases found in the genomes of termites, abalone, and sea squirts could be an exception. Using information from expressed sequence tags, we show that GHF9 genes (subgroup E2) are widespread in Metazoa because at least 11 classes in five phyla have expressed GHF9 cellulases. We also demonstrate that eukaryotic GHF9 gene families are ancient, forming distinct monophyletic groups in plants and animals. As several intron positions are also conserved between four metazoan phyla then, contrary to the still widespread belief that cellulases were horizontally transferred to animals relatively recently, GHF9 genes must derive from an ancient ancestor. We also found that sequences isolated from the same animal phylum tend to group together, and in some deuterostomes, GHF9 genes are characterized by substitutions in catalytically important sites. Several paralogous subfamilies of GHF9 can be identified in plants, and genes from primitive species tend to arise basally to angiosperm representatives. In contrast, GHF9 subgroup E2 genes are relatively rare in bacteria.

Homologous soybean and Arabidopsis genes share responsiveness to cyst nematode infection

SUMMARY We previously isolated a partial soybean cDNA clone (D17.1) whose corresponding transcript increases in susceptible roots 1 day post inoculation (dpi) with the soybean cyst nematode, Heterodera glycines. Here we isolated the corresponding full-length cDNA from a soybean cDNA library and designated this gene of unknown function Gm17.1. Time course RNA gel blot analyses revealed that Gm17.1 mRNA steady-state levels were elevated in soybean roots following H. glycines infection up to at least 6 dpi. For further in-depth study we identified a homologous Arabidopsis thaliana gene and designated this gene At17.1. Arabidopsis is successfully infected by the sugar beet cyst nematode (H. schachtii), a close relative of H. glycines. We isolated the At17.1 promoter, fused it to the beta-glucuronidase (GUS) reporter gene, and transformed this construct into Arabidopsis plants as well as soybean hairy roots. Histochemical analysis of plant materials containing the At17.1::GUS construct revealed that the At17.1 promoter is functional in Arabidopsis as well as in soybean and that during normal plant development the At17.1 promoter directs GUS expression predominantly to the vascular tissues and root tips of both plant species. When At17.1::GUS Arabidopsis plants and soybean hairy roots were inoculated with cyst nematodes, strong GUS activity was detected within the cyst nematode-induced feeding structures. Further tests of At17.1 promoter activity in Arabidopsis revealed that this promoter was induced by auxin, jasmonic acid, mannitol and dehydration. Quantitative real-time reverse transcription-polymerase chain reaction assays of At17.1 expression confirmed the observed promoter characteristics. Based on our expression data and the observation that both the soybean and the Arabidopsis homologues behaved in a similar fashion following cyst nematode infection, it is likely that these genes are closely associated with cyst nematode parasitism of plants, potentially with hormone and osmotic changes occurring in the developing nematode feeding cells. Furthermore, these data provide additional insights into the strengths of the Arabidopsis-H. schachtii pathosystem to study cyst nematode-plant interactions in lieu of less tractable pathosystems. This finding is supported by the fact that the Arabidopsis promoter tested here produced similar results in Arabidopsis and soybean.

A survey of SL1-spliced transcripts from the root-lesion nematode Pratylenchus penetrans

Plant-parasitic nematodes are important and cosmopolitan pathogens of crops. Here, we describe the generation and analysis of 1928 expressed sequence tags (ESTs) of a splice-leader 1 (SL1) library from mixed life stages of the root-lesion nematode Pratylenchus penetrans. The ESTs were grouped into 420 clusters and classified by function using the Gene Ontology (GO) hierarchy and the Kyoto KEGG database. Approximately 80% of all translated clusters show homology to Caenorhabditis elegans proteins, and 37% of the C. elegans gene homologs had confirmed phenotypes as assessed by RNA interference tests. Use of an SL1-PCR approach, while ensuring the cloning of the 5' ends of mRNAs, has demonstrated bias toward short transcripts. Putative nematode-specific and Pratylenchus -specific genes were identified, and their implications for nematode control strategies are discussed.

Signaling between nematodes and plants

After hatching in the soil, root-knot nematodes must locate and penetrate a root, migrate into the vascular cylinder, and establish a permanent feeding site. Presumably, these events are accompanied by extensive signaling between the nematode parasite and the host. Hence, much emphasis has been placed on identifying proteins that are secreted by the nematode during the migratory phase. Further progress in understanding the signaling events has been made recently by studying the host response. Striking parallels can be drawn between the nematode-plant interaction and plant symbioses with other microorganisms, and evidence is emerging to suggest that nematodes acquired components of their parasitic armory from those microbes.

A new class of ubiquitin extension proteins secreted by the dorsal pharyngeal gland in plant parasitic cyst nematodes

By performing cDNA AFLP on pre- and early parasitic juveniles, we identified genes encoding a novel type of ubiquitin extension proteins secreted by the dorsal pharyngeal gland in the cyst nematode Heterodera schachtii. The proteins consist of three domains, a signal peptide for secretion, a mono-ubiquitin domain, and a short C-terminal positively charged domain. A gfp-fusion of this protein is targeted to the nucleolus in tobacco BY-2 cells. We hypothesize that the C-terminal peptide might have a regulatory function during syncytium formation in plant roots.

A family of glycosyl hydrolase family 45 cellulases from the pine wood nematodeBursaphelenchus xylophilus

We have characterized a family of GHF45 cellulases from the pine wood nematode Bursaphelenchus xylophilus. The absence of such genes from other nematodes and their similarity to fungal genes suggests that they may have been acquired by horizontal gene transfer (HGT) from fungi. The cell wall degrading enzymes of other plant parasitic nematodes may have been acquired by HGT from bacteria. B. xylophilus is not directly related to other plant parasites and our data therefore suggest that horizontal transfer of cell wall degrading enzymes has played a key role in evolution of plant parasitism by nematodes on more than one occasion.

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.

Developmental expression and biochemical properties of a beta-1,4-endoglucanase family in the soybean cyst nematode, Heterodera glycines

SUMMARY The soybean cyst nematode, Heterodera glycines, produces beta-1,4-endoglucanases (cellulases) that are secreted during infection of soybean. The gene structures of three, hg-eng-4, hg-eng-5 and hg-eng-6, of the six beta-1,4-endoglucanase genes, all family 5 glycosyl hydrolases previously identified from H. glycines, are presented here. Furthermore, we present the detailed expression analyses of beta-1,4-endoglucanase genes as well as the biochemical properties of four H. glycines endoglucanase enzymes. Two of the endoglucanases, HG-ENG-5 and HG-ENG-6, differed significantly in their amino acid sequence of the catalytic domains and their gene structure from that of the other four beta-1,4-endoglucanases. Quantitative real-time RT-PCR revealed distinct developmental expression differences among the hg-eng family members during the early stages of parasitism and relatively low expression levels in late parasitic stages, with the exception of the adult male stage for some eng genes. Recombinant HG-ENGs degraded carboxymethylcellulose and optimum enzyme activity ranged from pH 5.5 for HG-ENG-5 to pH 8 for HG-ENG-6. EDTA, Ca(2+), Co(2+), Mg(2+) and Fe(2+) did not affect enzyme activity of any ENG protein, whereas Zn(2+), Cu(2+) and Mn(2+) inhibited enzyme activity from 23% to 73% in some cases. In tests with 12 different polysaccharide substrates, enzyme activity was restricted to beta-1,4 linkages with all ENG proteins tested. Only HG-ENG-5 and HG-ENG-6 had relatively high activity on xylan and slightly degraded microcrystalline cellulose. Together, these data reveal distinct differences in expression and biochemistry of cyst nematode parasitism genes and proteins, respectively, and cast light on the intricate interactions between a parasitic animal and its plant host.

Evolution of plant parasitism among nematodes

Despite extraordinary diversity of free-living species, a comparatively small fraction of nematodes are parasites of plants. These parasites represent at least three disparate clades in the nematode tree of life, as inferred from rRNA sequences. Plant parasites share functional similarities regarding feeding, but many similarities in feeding structures result from convergent evolution and have fundamentally different developmental origins. Although Tylenchida rRNA phylogenies are not fully resolved, they strongly support convergent evolution of sedentary endoparasitism and plant nurse cells in cyst and root-knot nematodes. This result has critical implications for using model systems and genomics to identify and characterize parasitism genes for representatives of this clade. Phylogenetic studies reveal that plant parasites have rich and complex evolutionary histories that involve multiple transitions to plant parasitism and the possible use of genes obtained by horizontal transfer from prokaryotes. Developing a fuller understanding of plant parasitism will require integrating more comprehensive and resolved phylogenies with appropriate choices of model organisms and comparative evolutionary methods.

Interactions between bacteria and plant-parasitic nematodes: now and then

Based on genome-to-genome analyses of gene sequences obtained from plant-parasitic, root-knot nematodes (Meloidogyne spp.), it seems likely that certain genes have been derived from bacteria by horizontal gene transfer. Strikingly, a common theme underpinning the function of these genes is their apparent direct relationship to the nematodes' parasitic lifestyle. Phylogenetic analyses implicate rhizobacteria as the predominant group of 'gene donor' bacteria. Root-knot nematodes and rhizobia occupy similar niches in the soil and in roots, and thus the opportunity for genetic exchange may be omnipresent. Further, both organisms establish intimate developmental interactions with host plants, and mounting evidence suggests that the mechanisms for these interactions are shared too. We propose that the origin of parasitism in Meloidogyne may have been facilitated by acquisition of genetic material from soil bacteria through horizontal transfer, and that such events represented key steps in speciation of plant-parasitic nematodes. To further understand the mechanisms of horizontal gene transfer, and also to provide experimental tools to manipulate this promising bio-control agent, we have initiated a genomic sequence of the bacterial hyper-parasite of plant parasitic nematodes, Pasteuria penetrans. Initial data have established that P. penetrans is closely related to Bacillus spp., to the extent that considerable genome synteny is apparent. Hence, Bacillus serves as a model for Pasteuria, and vice versa.

Evidence for the presence of a cellulase gene in the last common ancestor of bilaterian animals

Until recently, the textbook view of cellulose hydrolysis in animals was that gut-resident symbiotic organisms such as bacteria or unicellular eukaryotes are responsible for the cellulases produced. This view has been challenged by the characterization and sequencing of endogenous cellulase genes from some invertebrate animals, including plant-parasitic nematodes, arthropods and a mollusc. Most of these genes are completely unrelated in terms of sequence, and their evolutionary origins remain unclear. In the case of plant-parasitic nematodes, it has been suggested that their ancestor obtained a cellulase gene via horizontal gene transfer from a prokaryote, and similar suggestions have been made about a cellulase gene recently discovered in a sea squirt. To improve understanding about the evolution of animal cellulases, we searched for all known types of these enzymes in GenBank, and performed phylogenetic comparisons. Low phylogenetic resolution was found among most of the sequences examined, however, positional identity in the introns of cellulase genes from a termite, a sea squirt and an abalone provided compelling evidence that a similar gene was present in the last common ancestor of protostomes and deuterostomes. In a different enzyme family, cellulases from beetles and plant-parasitic nematodes were found to cluster together. This result questions the idea of lateral gene transfer into the ancestors of the latter, although statistical tests did not allow this possibility to be ruled out. Overall, our results suggest that at least one family of endogenous cellulases may be more widespread in animals than previously thought.

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.

Purification, characterization, cDNA cloning and nucleotide sequencing of a cellulase from the yellow-spotted longicorn beetle, Psacothea hilaris

A cellulase (endo-beta-1,4-glucanase, EC 3.2.1.4) was purified from the gut of larvae of the yellow-spotted longicorn beetle Psacothea hilaris by acetone precipitation and elution from gels after native PAGE and SDS/PAGE with activity staining. The purified protein formed a single band, and the molecular mass was estimated to be 47 kDa. The purified cellulase degraded carboxymethylcellulose (CMC), insoluble cello-oligosaccharide (average degree of polymerization 34) and soluble cello-oligosaccharides longer than cellotriose, but not crystalline cellulose or cellobiose. The specific activity of the cellulase against CMC was 150 micro mol.min-1.(mg protein)-1. TLC analysis showed that the cellulase produces cellotriose and cellobiose from insoluble cello-oligosaccharides. However, a glucose assay linked with glucose oxidase detected a small amount of glucose, with a productivity of 0.072 micro mol.min-1.(mg protein)-1. The optimal pH of P. hilaris cellulase was 5.5, close to the pH in the midgut of P. hilaris larvae. The N-terminal amino-acid sequence of the purified P. hilaris cellulase was determined and a degenerate primer designed, which enabled a 975-bp cDNA clone containing a typical polyadenylation signal to be obtained by PCR and sequencing. The deduced amino-acid sequence of P. hilaris cellulase showed high homology to members of glycosyl hydrolase family 5 subfamily 2, and, in addition, a signature sequence for family 5 was found. Thus, this is the first report of a family 5 cellulase from arthropods.

Root-parasitic nematodes enhance soil microbial activities and nitrogen mineralization

Obligate root-parasitic nematodes can affect soil microbes positively by enhancing C and nutrient leakage from roots but negatively by restricting total root growth. However, it is unclear how the resulting changes in C availability affect soil microbial activities and N cycling. In a microplot experiment, effects of root-parasitic reniform nematodes ( Rotylenchulus reniformis) on soil microbial biomass and activities were examined in six different soils planted with cotton. Rotylenchulus reniformis was introduced at 900 nematodes kg(-1) soil in May 2000 prior to seeding cotton. In 2001, soil samples were collected in May before cotton was seeded and in November at the final harvest. Extractable C and N were consistently higher in the R. reniformis treatments than in the non-nematode controls across the six different soils. Nematode inoculation significantly reduced microbial biomass C, but increased microbial biomass N, leading to marked decreases in microbial biomass C:N ratios. Soil microbial respiration and net N mineralization rates were also consistently higher in the nematode treatments than in the controls. However, soil types did not have a significant impact on the effects of nematodes on these microbial parameters. These findings indicate that nematode infection of plant roots may enhance microbial activities and the turnover of soil microbial biomass, facilitating soil N cycling. The present study provides the first evidence about the direct role of root-feeding nematodes in enhancing soil N mineralization.

Root-knot nematode parasitism and host response: molecular basis of a sophisticated interaction

SUMMARY Taxonomy: Eukaryota; Metazoa; Nematoda; Chromadorea; order Tylenchida; Tylenchoidea; Heteroderidae; genus Meloidogyne. Physical properties: Microscopic-non-segmented worms. Meloidogyne species can reproduce by apomixis, facultative meiotic parthenogenesis or obligate mitotic parthenogenesis. Obligate biotrophic parasites inducing the re-differentiation of plant cells into specialized feeding cells. Hosts: Meloidogyne spp. can infest more than 3000 plant species including vegetables, fruit trees, cereals and ornamental flowers.

SYMPTOMS

Root swellings called galls. Alteration of the root vascular system. Disease control: Cultural control, chemical control, resistant cultivars. Agronomic importance: Major threat to agriculture in temperate and tropical regions.

The promoter of the nematode resistance gene Hs1pro-1 activates a nematode-responsive and feeding site-specific gene expression in sugar beet (Beta vulgaris L.) and Arabidopsis thaliana

The Hs1pro-1 gene confers resistance to the beet cyst nematode Heterodera schachtii in sugar beet (Beta vulgaris L.) on the basis of a gene-for-gene relationship. RNA-gel blot analysis revealed that the transcript of Hs1pro-1 was present in uninfected roots of resistant beet at low levels but increased by about fourfold one day after nematode infection. Treatments of plants with external stimuli including salicylic acid, jasmonic acid, gibberellic acid and abscisic acid as well as wounding or salt stress did not result in changes in the gene transcription, indicating de novo transcription of Hs1pro-1 upon nematode infection specifically. To study transcriptional regulation of Hs1pro-1 expression at the cellular level, a 3082 bp genomic fragment representing the Hs1pro-1 promoter, isolated from the YAC-DNA housing the Hs1pro-1 gene, was fused to the beta-glucuronidase reporter gene (1832prm1::GUS) and transformed into susceptible beet roots and Arabidopsis plants, respectively. Fluorometric and histochemical GUS assays on transgenic beet roots and Arabidopsis plants carrying the 1832prm1::GUS construct demonstrated that the Hs1pro-1 promoter is functional in both species and drives a nematode responsive and feeding site-specific GUS-expression. GUS activity was detected as early as at initiation of the nematode feeding sites and GUS staining was restricted to the nematode feeding sites. To delineate the regulatory domains of the Hs1pro-1 promoter, fusion genes with various 5' deletions of the Hs1pro-1 promoter and the GUS gene were constructed and analysed in transgenic beet roots as well. Cis elements responsible for feeding site-specific gene expression reside between -355 and +247 from the transcriptional initiation site of Hs1pro-1 whereas an enhancer region necessary for higher gene expression is located between -1199 and -705 of the promoter. The Hs1pro-1 promoter drives a nematode feeding site-specific GUS expression in both sugar beet and Arabidopsis suggesting a conserved mechanism of regulation of Hs1pro-1 expression in these two species.

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.

A profile of putative parasitism genes expressed in the esophageal gland cells of the root-knot nematode Meloidogyne incognita

Identifying parasitism genes encoding proteins secreted from a nematode's esophageal gland cells and injected through its stylet into plant tissue is the key to understanding the molecular basis of nematode parasitism of plants. Meloidogyne incognita parasitism genes were cloned by microaspirating the cytoplasm from the esophageal gland cells of different parasitic stages to provide mRNA to create a gland cell-specific cDNA library by long-distance reverse-transcriptase polymerase chain reaction. Of 2,452 cDNA clones sequenced, deduced protein sequences of 185 cDNAs had a signal peptide for secretion and, thus, could have a role in root-knot nematode parasitism of plants. High-throughput in situ hybridization with cDNA clones encoding signal peptides resulted in probes of 37 unique clones specifically hybridizing to transcripts accumulating within the subventral (13 clones) or dorsal (24 clones) esophageal gland cells of M. incognita. In BLASTP analyses, 73% of the predicted proteins were novel proteins. Those with similarities to known proteins included a pectate lyase, acid phosphatase, and hypothetical proteins from other organisms. Our cell-specific analysis of genes encoding secretory proteins provided, for the first time, a profile of putative parasitism genes expressed in the M. incognita esophageal gland cells throughout the parasitic cycle.

Perspectives on plant and soil nematology

During my career in Plant Pathology/Nematology, many major advancements have occurred in the study of nematodes-even with their being largely soilborne and thus often overlooked. These biotrophic organisms include the most widespread and important group of plant pathogens-the root-knot nematodes Meloidogyne species-which attack most major crops, as well as thousands of non-crop plant species. Landmark achievements that catalyzed research on these organisms included the discovery of effective nematicides, ectoparasitic forms, elucidation of disease complexes, nematodes as virus vectors, development of host resistance, and new technologies for research. Evolving research thrusts involve interfacing traditional and molecular systematics/diagnostics, adoption of the Caenorhabditis elegans-molecular genetics resource for general nematological research, focus on genetics of parasitism, use of molecular tools in developing host resistance, ecological and quantitative facets, and soil-biology-ecology based integrated management. Educational and international programs are encountering many changes and challenges, as is support for nematology in general.

Arabidopsis gene expression changes during cyst nematode parasitism revealed by statistical analyses of microarray expression profiles

With the availability of microarray technology, the expression profiles of thousands of genes can be monitored simultaneously to help determine the mechanisms of these biological processes. We conducted Affymetrix GeneChip microarray analyses of the Arabidopsis-cyst nematode interaction and employed a statistical procedure to analyze the resultant data, which allowed us to identify significant gene expression changes. Quantitative real-time RT-PCR assays were used to confirm the microarray analyses. The results of the expression profiling revealed 128 genes with altered steady-state mRNA levels following infection by the sugar beet cyst nematode (Heterodera schachtii; BCN), in contrast to only 12 genes that had altered expression following infection by the soybean cyst nematode (H. glycines; SCN). The expression of these 12 genes also changed following infection by BCN, i.e. we did not identify any genes regulated exclusively by SCN. The identification of 116 genes whose expression changes during successful cyst nematode parasitism by BCN suggests a potential involvement of these genes in the infection events starting with successful syncytium induction. Further characterization of these genes will permit the formulation of testable hypotheses to explain successful cyst nematode parasitism.

Purification and cDNA cloning of a cellulase from abalone Haliotis discus hannai

A cellulase [endo-beta-1,4-D-glucanase (EC 3.2.1.4)] was isolated from the hepatopancreas of abalone Haliotis discus hannai by successive chromatographies on TOYOPEARL CM-650M, hydroxyapatite and Sephacryl S-200 HR. The molecular mass of the cellulase was estimated to be 66 000 Da by SDS/PAGE, thus the enzyme was named HdEG66. The hydrolytic activity of HdEG66 toward carboxymethylcellulose showed optimal temperature and pH at 38 degrees C and 6.3, respectively. cDNAs encoding HdEG66 were amplified by the polymerase chain reaction from an abalone hepatopancreas cDNA library with primers synthesized on the basis of partial amino-acid sequences of HdEG66. By overlapping the nucleotide sequences of the cDNAs, a sequence of 1898 bp in total was determined. The coding region of 1785 bp located at nucleotide position 56-1840 gave an amino-acid sequence of 594 residues including the initiation methionine. The N-terminal region of 14 residues in the deduced sequence was regarded as the signal peptide as it was absent in HdEG66 protein and showed high similarity to the consensus sequence for signal peptides of eukaryote secretory proteins. Thus, matured HdEG66 was thought to consist of 579 residues. The C-terminal region of 453 residues in HdEG66, i.e. approximately the C-terminal three quarters of the protein, showed 42-44% identity to the catalytic domains of glycoside hydrolase family 9 (GHF9)-cellulases from arthropods and Thermomonospora fusca. While the N-terminal first quarter of HdEG66 showed 27% identity to the carbohydrate-binding module (CBM) of a Cellulomonas fimi cellulase, CenA. Thus, the HdEG66 was regarded as the GHF9-cellulase possessing a family II CBM in the N-terminal region. By genomic PCR using specific primers to the 3'-terminal coding sequences of HdEG66-cDNA, a DNA of 2186 bp including three introns was amplified. This strongly suggests that the origin of HdEG66 is not from symbiotic bacteria but abalone itself.

Meloidogyne javanica chorismate mutase 1 alters plant cell development

Root-knot nematodes are obligate plant parasites that alter plant cell growth and development by inducing the formation of giant cells for feeding. Nematodes inject secretions from their esophageal glands through their stylet and into plant cells to induce giant cell formation. Meloidogyne javanica chorismate mutase 1 (MjCM-1) is one such esophageal gland protein likely to be secreted from the nematode as giant cells form. MjCM-1 has two domains, an N-terminal chorismate mutase (CM) domain and a C-terminal region of unknown function. It is the N-terminal CM domain of the protein that is the predominant form produced in root-knot nematodes. Transgenic expression of MjCM-1 in soybean hairy roots results in a phenotype of reduced and aborted lateral roots. Histological studies demonstrate the absence of vascular tissue in hairy roots expressing MjCM-1. The phenotype of MjCM-1 expressed at low levels can be rescued by the addition of indole-3-acetic acid (IAA), indicating MjCM-1 overexpression reduces IAA biosynthesis. We propose MjCM-1 lowers IAA by causing a competition for chorismate, resulting in an alteration of chorismate-derived metabolites and, ultimately, in plant cell development. Therefore, we hypothesize that MjCM-1 is involved in allowing nematodes to establish a parasitic relationship with the host plant.

Horizontally transferred genes in plant-parasitic nematodes: a high-throughput genomic approach

BACKGROUND

Published accounts of horizontally acquired genes in plant-parasitic nematodes have not been the result of a specific search for gene transfer per se, but rather have emerged from characterization of individual genes. We present a method for a high-throughput genome screen for horizontally acquired genes, illustrated using expressed sequence tag (EST) data from three species of root-knot nematode, Meloidogyne species.

RESULTS

Our approach identified the previously postulated horizontally transferred genes and revealed six new candidates. Screening was partially dependent on sequence quality, with more candidates identified from clustered sequences than from raw EST data. Computational and experimental methods verified the horizontal gene transfer candidates as bona fide nematode genes. Phylogenetic analysis implicated rhizobial ancestors as donors of horizontally acquired genes in Meloidogyne.

CONCLUSIONS

High-throughput genomic screening is an effective way to identify horizontal gene transfer candidates. Transferred genes that have undergone amelioration of nucleotide composition and codon bias have been identified using this approach. Analysis of these horizontally transferred gene candidates suggests a link between horizontally transferred genes in Meloidogyne and parasitism.

The companion cell-specific Arabidopsis disaccharide carrier AtSUC2 is expressed in nematode-induced syncytia

Cyst nematodes induce a metabolically highly active syncytial cell complex in host roots. The syncytia are symplastically isolated. Because they form a strong sink, assimilates must be imported via the apoplast, thus suggesting that specific membrane-bound sugar transport proteins are expressed and activated. To identify possible candidate genes, transgenic Arabidopsis plants expressing different reporter genes under the control of different promoters from Arabidopsis sugar transporter genes were infected with the beet cyst nematode (Heterodera schachtii). With polymerase chain reaction, 13 additional sugar transporters were tested for their presence in the syncytia through the use of a syncytium-specific cDNA library. Analysis of the infected roots showed that the promoter of the sucrose (Suc) transporter AtSUC2 gene that codes for a companion cell-specific Suc transporter in noninfected plants was found to be expressed in syncytia. Its expression patterns in beta-glucuronidase and green fluorescent protein plants were monitored. Syncytium-specific gene expression was confirmed by reverse transcriptase-polymerase chain reaction. Results support the idea that AtSUC2 mediates the transmembrane transfer of Suc. AtSUC2 is the first disaccharide carrier described to be activated by pathogens.

Analysis and functional classification of transcripts from the nematode Meloidogyne incognita

BACKGROUND

Plant parasitic nematodes are major pathogens of most crops. Molecular characterization of these species as well as the development of new techniques for control can benefit from genomic approaches. As an entrée to characterizing plant parasitic nematode genomes, we analyzed 5,700 expressed sequence tags (ESTs) from second-stage larvae (L2) of the root-knot nematode Meloidogyne incognita.

RESULTS

From these, 1,625 EST clusters were formed and classified by function using the Gene Ontology (GO) hierarchy and the Kyoto KEGG database. L2 larvae, which represent the infective stage of the life cycle before plant invasion, express a diverse array of ligand-binding proteins and abundant cytoskeletal proteins. L2 are structurally similar to Caenorhabditis elegans dauer larva and the presence of transcripts encoding glyoxylate pathway enzymes in the M. incognita clusters suggests that root-knot nematode larvae metabolize lipid stores while in search of a host. Homology to other species was observed in 79% of translated cluster sequences, with the C. elegans genome providing more information than any other source. In addition to identifying putative nematode-specific and Tylenchida-specific genes, sequencing revealed previously uncharacterized horizontal gene transfer candidates in Meloidogyne with high identity to rhizobacterial genes including homologs of nodL acetyltransferase and novel cellulases.

CONCLUSIONS

With sequencing from plant parasitic nematodes accelerating, the approaches to transcript characterization described here can be applied to more extensive datasets and also provide a foundation for more complex genome analyses.

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.

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.

Secrets in secretions: genes that control nematode parasitism of plants

The most evolutionary advanced adaptations for plant parasitism by nematodes are the products of parasitism genes expressed in their esophageal gland cells and secreted through their stylet into host tissue to control the complex process of parasitism. Molecular analyses of nematode parasitism genes are revealing the complexity of the tools a nematode possesses that enable it to attack plants and paints a more elaborate picture of host cellular events under specific control by the parasite than previously hypothesized. Interestingly, the majority of the nematode parasitism genes discovered encodes proteins unique to plant parasites. Identifying the complete profile of parasitism genes expressed throughout the parasitic cycle of a nematode is the key to understanding the molecular basis of nematode parasitism of plants and identifying vulnerable points in the parasitic process that can be interfered with to achieve nematode control to limit nematode-induced yield losses in crops.

Characterisation and developmental expression of a chitinase gene in Heterodera glycines

A chitinase full-length cDNA (designated Hg-chi-1) was isolated from a Heterodera glycines oesophageal gland cell-specific long-distance PCR cDNA library. The cDNA hybridised to genomic DNA of H. glycines in Southern blots. The Hg-chi-1 cDNA contained an open reading frame encoding 350 amino acids with the first 23 amino acids being a putative signal peptide for secretion. Hg-CHI-1 contained a chitinase 18 family catalytic domain, and chitinolytic activity of recombinant Hg-CHI-1 was confirmed in glycol-chitin substrate gel electrophoresis. In situ mRNA hybridisation analyses showed that transcripts of Hg-chi-1 accumulated specifically in the subventral oesophageal gland cells of parasitic stages of H. glycines, but Hg-chi-1 expression was not detected in eggs or hatched pre-parasitic second-stage juveniles, suggesting that this chitinase does not have a role in egg hatching of H. glycines. The biological function of Hg-CHI-1 in H. glycines remains to be determined.

A polygalacturonase of animal origin isolated from the root-knot nematode Meloidogyne incognita

The first animal polygalacturonase (PG, EC 2.1.15) encoding cDNA, Mi-pg-1, was cloned from the plant parasitic nematode Meloidogyne incognita. The enzymatic activity of MI-PG-1 was confirmed after heterologous expression in Escherichia coli. The presence of a predicted signal peptide on the MI-PG-1 sequence together with the specific localization of the transcripts of the Mi-pg-1 gene in the oesophageal glands of infective juveniles imply that MI-PG-1 could be secreted into plant tissues. The potential role of MI-PG-1 in parasitism is discussed.

The use of DNA microarrays for the developmental expression analysis of cDNAs from the oesophageal gland cell region of Heterodera glycines

Summary A microarray was printed containing cDNAs from a library made from cytoplasm microaspirated from the oesophageal gland cell region of parasitic stages of the soybean cyst nematode, Heterodera glycines. The array contained both previously described clones (Wang et al. Mol. Plant-Microbe Interact. 2001, 14, 536-544) and uncharacterized cDNAs. Fluorescent probes for array hybridization were prepared using RNA polymerase amplification of nematode mRNA. Developmental expression profiles of the arrayed cDNAs were determined by hybridizing the microarray with probes from parasitic and non-parasitic H. glycines life stages. Distinct patterns of developmental expression were ascertained for the previously described gland expressed genes. In addition, four H. glycines cDNAs (SCN1018, SCN1020, SCN1028 and SCN1167) were identified that showed up-regulation in one or more parasitic life stages. Clone SCN1018 encodes a C-type lectin domain and is expressed in the hypodermis of females. Clone SCN1020 encodes a probable S-adenosylmethionine synthetase. Clone SCN1028 encodes a piwi protein with high similarity to the germ-line-specific protein R06C7.1 of Caenorhabditis elegans. The sequence of SCN1167 had no similarity to known genes. This paper describes the first use of cDNA microarrays to analyse genes of a plant-parasitic nematode and establishes a functional method to mine nematode cDNA libraries.

Identification of gene expression differences between Globodera pallida and G.'mexicana' by suppression subtractive hybridization

Summary Globodera pallida and G.'mexicana' are closely related nematode species that can mate and form viable hybrids on tomato but usually develop on different Solanaceous plants. Identification of nematode genes involved in parasitism is important for elucidation of disease resistance mechanisms in plants. In this study, we have used suppression subtractive hybridization (SSH) to investigate differences between the transcriptomes of G. pallida and G.'mexicana' J2s. This provides a basis for further studies characterizing pathogenicity factors in these nematodes. None of the cDNA fragments isolated in the SSH experiments appeared to be completely absent from the other transcriptome. Differences in expression levels of some of the isolated cDNAs between the two species were detected. Sequence analysis revealed that nearly 85% of the cloned sequences are nematode specific and a high proportion were pioneer genes for which no putative homologues were present in the databases. However, homologues of a cellulase and a putative pathogenicity factor previously described from G. rostochiensis were isolated. The putative roles of these sequences in parasitism are discussed.

Modification of polysaccharides and plant cell wall by endo-1,4-beta-glucanase and cellulose-binding domains

Cellulose is one of the most abundant polymers in nature. Different living systems evolved simultaneously, using structurally similar proteins to synthesize and metabolize polysaccharides. In the growing plant, cell wall loosening, together with cellulose biosynthesis, enables turgor-driven cell expansion. It has been postulated that endo-1,4-beta-glucanases (EGases) play a central role in these complex activities. Similarly, microorganisms use a consortium of lytic enzymes to convert cellulose into soluble sugars. Most, if not all, cellulases have a modular structure with two or more separate independent functional domains. Binding to cellulose is mediated by a cellulose-binding domain (CBD), whereas the catalytic domain mediates hydrolysis. Today, EGases and CBDs are known to exist in a wide range of species and it is evident that both possess immense potential in modifying polysaccharide materials in-vivo and in-vitro. The hydrolytic function is utilized for polysaccharide degradation in microbial systems and cell wall biogenesis in plants. The CBDs exerts activity that can be utilized for effective degradation of crystalline cellulose, plant cell wall relaxation, expansion and cell wall biosynthesis. Applications range from modulating the architecture of individual cells to an entire organism. These genes, when expressed under specific promoters and appropriate trafficking signals can be used to alter the nutritional value and texture of agricultural crop and their final products. EGases and CBDs may also find applications in the modification of physical and chemical properties of composite materials to create new materials possessing improved properties.

Cloning and characterization of an esophageal-gland-specific pectate lyase from the root-knot nematode Meloidogyne javanica

Root-knot nematodes (Meloidogynejavanica) are obligate sedentary endoparasites that must penetrate the host root to initiate their life cycle. Many enzymes are secreted by the nematode to facilitate host penetration; required enzymes may include pectate lyases and cellulases. Using differential screening, a class III pectate lyase, Mj-pel-1 (M. javanica pectate lyase 1), was cloned from a library enriched for esophageal gland genes. DNA gel blotting confirmed that the Mj-pel-1 gene was of nematode origin and a member of a small multigene family. In situ hybridization localized the expression of Mj-pel-1 to the basal cells of the esophageal glands, while immunolocalization detected the protein in the esophageal glands as well as on the exterior of the nematode, confirming that the protein is secreted. When MJ-PEL-1 was expressed in Pichia pastoris, the resulting protein was active. The pH optimum of MJ-PEL-1 was 10.0, and the enzyme was five times more active on pectate than on pectin. Like other class III pectate lyases, MJ-PEL-1 also displayed an absolute requirement for Ca2+. The root-knot nematode migrates through the middle lamella of the plant root; therefore, MJ-PEL-1 may be an important enzyme early in the infection process.

Cleavage of the xylosyl serine linkage between a core peptide and a glycosaminoglycan chain by cellulases

We previously found that endo-beta-xylosidase from Patinopecten is an endo-type glycosidase that cleaves the xylosyl serine linkage between a glycosaminoglycan chain and its core protein (Takagaki, K., Kon, A., Kawasaki, H., Nakamura, T., Tamura, S., and Endo, M. (1990) J. Biol. Chem. 265, 854-860). Screening for endo-beta-xylosidase activity in several cellulases detected this activity in the enzymes from Aspergillus niger, Penicillium funiculosum, Trichoderma reesei, Trichoderma viride, and Irpex lacteus. The cellulase derived from A. niger was purified, and its molecular weight was determined to be 26,000 by SDS-PAGE. Examination of the specificity of the cellulase revealed that 1) the enzyme acts on the linkage region (xylosyl serine) between a core peptide and a glycosaminoglycan chain; 2) enzymatic activity is greater with shorter glycosaminoglycan chains; 3) the enzyme readily hydrolyzes the linkage in glycosaminoglycan peptides, but intact proteoglycan is cleaved only slowly; and 4) the activity is unaffected by the glycosaminoglycan component (chondroitin sulfate, dermatan sulfate, and heparan sulfate). Judging from these enzymatic characteristics, this cellulase is different from the endo-beta-xylosidase of Patinopecten. We believe that this cellulase will become a useful tool in the further development of glycotechnology, because, like the endo-beta-xylosidase of Patinopecten, it enables the release of intact glycosaminoglycans from glycosaminoglycan peptides.

Direct identification of stylet secreted proteins from root-knot nematodes by a proteomic approach

The stylet secretions produced by plant parasitic root-knot nematodes are thought to be pathogenicity factors involved in the invasion of the root tissue and in the induction and maintenance of feeding cells. A new procedure was established that allowed the direct qualitative analysis of proteins secreted by Meloidogyne incognita infective juveniles. Purified proteins whose isoelectric point (pI) ranged from 5.0 to 7.5 were separated by two-dimensional (2D) electrophoresis and the seven most abundant proteins were identified by micro-sequencing. A calreticulin (CRT) was isolated and transcription of its gene in infective juveniles and adults was demonstrated. Moreover, evidence for expression of the CRT in the subventral oesophageal glands of infective juveniles was obtained. The potential roles of this secreted protein in pathogenesis and the advantages of developing this strategy to obtain new insights into plant-nematode interactions are discussed.

An Arabidopsis thaliana pectin acetylesterase gene is upregulated in nematode feeding sites induced by root-knot and cyst nematodes

By using differential display, gene expression was investigated in Arabidopsis thaliana roots shortly after nematode infection, and a putative pectin acetylesterase (PAE) homolog (DiDi 9C-12) was found to be upregulated. PAEs catalyze the deacetylation of pectin, a major compound of primary cell walls. mRNA in situ hybridization experiments showed that the expression of DiDi 9C-12 was enhanced very early after infection in initiating giant-cells and in cells surrounding the nematodes. Later on, the level of DiDi 9C-12 mRNA was lower in giant-cells and transcripts were mainly found in parenchyma, endodermis, and pericycle cells of the root gall. Twenty days after infection, DiDi 9C-12 transcripts could no longer be detected. DiDi 9C-12 transcripts were also found in young syncytia and in the cells surrounding the expanding syncytium. Our results suggest that plant parasitic nematodes can modulate the rapid growth of the feeding cells and the expansion of the root gall by triggering the expression of DiDi 9C-12. PAEs, which probably act together with a range of other pectin-degrading enzymes, could be involved in softening and loosening the primary cell wall in nematode-infected plant roots.

Characterisation of guanylyl cyclase genes in the soybean cyst nematode, Heterodera glycines

Parasitism by the soybean cyst nematode, Heterodera glycines, has become one of the major limiting factors in soybean production world-wide. A partial HG-gcy-1 cDNA clone was obtained by screening a H. glycines cDNA library with a probe derived from the HG-gcy1 genomic sequence, and HG-gcy-1 full-length cDNA was obtained by nested PCR and 5' rapid amplification of cDNA ends (5' RACE). Two additional, full-length guanylyl cyclase cDNA clones from H. glycines, named HG-gcy-2 and HG-gcy-3, were recovered directly by screening the H. glycines cDNA library with a probe derived from sequence of the HG-gcy-1 catalytic domain. The encoded proteins of all three HG-gcy genes had an extracellular ligand-binding domain, a single membrane-spanning domain, an intracellular protein kinase-like domain, and a guanylyl cyclase catalytic domain. The three HG-GCY proteins had conserved cysteine residues to form disulfide bridges within the extracellular domain similar to the predicted ligand-binding domains of other known membrane-bound guanylyl cyclases. mRNA in situ hybridisation detected the expression of HG-gcy-1 and HG-gcy-2 transcripts in specific and different sensory neurons within H. glycines specimens. HG-gcy-3 transcripts were not localised in H. glycines specimens by in situ hybridisation. The discovery of the three guanylyl cyclase genes in H. glycines is the first of its kind in a plant-parasitic nematode and may be representative of a conserved gene family used for chemosensory recognition in parasitic nematodes.

Gene expression in nematode feeding sites

The feeding sites induced by sedentary root-endoparasitic nematodes have long fascinated researchers. Nematode feeding sites are constructed from plant cells, modified by the nematode to feed itself. Powerful new techniques are allowing us to begin to elucidate the molecular mechanisms that produce the ultrastructural features in nematode feeding cells. Many plant genes that are expressed in feeding sites produced by different nematodes have been identified in several plant species. Nematode-responsive plant genes can now be grouped in categories related to plant developmental pathways and their roles in the making of a feeding site can be illuminated. The black box of how nematodes bring about such elaborate cell differentiation in the plant is also starting to open. Although the information is far from complete, the groundwork is set so that the functions of the plant and nematode genes in feeding site development can begin to be assessed.