Characterization of intestinally active proteinases of cyst-nematodes

Expression interference of a number of Heterodera avenae conserved genes perturbs nematode parasitic success in Triticum aestivum

The cereal cyst nematode, Heterodera avenae is distributed worldwide and causes substantial damage in bread wheat, Triticum aestivum. This nematode is extremely difficult to manage because of its prolonged persistence as unhatched eggs encased in cysts. Due to its sustainable and target-specific nature, RNA interference (RNAi)-based strategy has gained unprecedented importance for pest control. To date, RNAi strategy has not been exploited to manage H. avenae in wheat. In the present study, 40 H. avenae target genes with different molecular function were rationally selected for in vitro soaking analysis in order to assess their susceptibility to RNAi. In contrast to target-specific downregulation of 18 genes, 7 genes were upregulated and 15 genes showed unaltered expression (although combinatorial soaking showed some of these genes are RNAi susceptible), suggesting that a few of the target genes were refractory or recalcitrant to RNAi. However, RNAi of 37 of these genes negatively altered nematode behavior in terms of reduced penetration, development and reproduction in wheat. Subsequently, wheat plants were transformed with seven H. avenae target genes (that showed greatest abrogation of nematode parasitic success) for host-induced gene silencing (HIGS) analysis. Transformed plants were molecularly characterized by PCR, RT-qPCR and Southern hybridization. Production of target gene-specific double- and single-stranded RNA (dsRNA/siRNA) was detected in transformed plants. Transgenic expression of galectin, cathepsin L, vap1, serpin, flp12, RanBPM and chitinase genes conferred 33.24-72.4 % reduction in H. avenae multiplication in T₁ events with single copy ones exhibiting greatest reduction. A similar degree of resistance observed in T₂ plants indicated the consistent HIGS effect in the subsequent generations. Intriguingly, cysts isolated from RNAi plants were of smaller size with translucent cuticle compared to normal size, dark brown control cysts, suggesting H. avenae developmental retardation due to HIGS. Our study reinforces the potential of HIGS to manage nematode problems in crop plant.

Genome-Wide Identification and Expression Analysis of the Protease Inhibitor Gene Families in Tomato

The protease inhibitors (PIs) in plants are involved primarily in defense against pathogens and pests and in response to abiotic stresses. However, information about the PI gene families in tomato (Solanumlycopersicum), one of the most important model plant for crop species, is limited. In this study, in silico analysis identified 55 PI genes and their conserved domains, phylogenetic relationships, and chromosome locations were characterized. According to genetic structure and evolutionary relationships, the PI gene families were divided into seven families. Genome-wide microarray transcription analysis indicated that the expression of SlPI genes can be induced by abiotic (heat, drought, and salt) and biotic (Botrytiscinerea and tomato spotted wilt virus (TSWV)) stresses. In addition, expression analysis using RNA-seq in various tissues and developmental stages revealed that some SlPI genes were highly or preferentially expressed, showing tissue- and developmental stage-specific expression profiles. The expressions of four representative SlPI genes in response to abscisic acid (ABA), salicylic acid (SA), ethylene (Eth), gibberellic acid (GA). and methyl viologen (MV) were determined. Our findings indicated that PI genes may mediate the response of tomato plants to environmental stresses to balance hormone signals. The data obtained here will improve the understanding of the potential function of PI gene and lay a foundation for tomato breeding and transgenic resistance to stresses.

Molecular identification and functional characterization of the cathepsin B gene (Ab-cb-1) in the plant parasitic nematode Aphelenchoides besseyi

The rice white tip nematode, Aphelenchoides besseyi, is widely distributed in rice planting areas worldwide and causes serious economic losses. Cathepsin genes have been demonstrated to have importance in studying the reproduction, development, pathogenicity, and control methods of plant nematodes. In this paper, a novel cathepsin B gene, Ab-cb-1, was found and cloned. The Ab-cb-1 gene was 1347 bp in length and encodes 369 amino acids. The Ab-CB-1 protein contains characteristic occluding loops but no signal peptide. A homology analysis showed that Ab-CB-1 had the highest identity value (64%) to the known amino acid sequence of cathepsin B-like cysteine protease 6 from Toxocara canis. When Ab-cb-1 was expressed in a prokaryotic system, the protein massed approximately 45 kDa and could decompose carrot callus. Ab-cb-1 mRNA was localized in the nematode intestine. The relative expression level of Ab-cb-1 in the A. besseyi Ab-S24 population, which had high reproductivity, was approximately 6.9 times that in the Ab-N10 population, which had low reproductivity, and the difference was significant (p<0.05). The Ab-cb-1 expression level was highest in females; the expression levels in males, juveniles and eggs were 30%, 12.2% and 5% of that in females, respectively, and the differences were significant among all four stages (p<0.05). Nematodes of the Ab-S24 population were treated with Ab-cb-1 dsRNA for 12 h, 24 h, 36 h and 48 h, and their reproduction decreased with increasing time. These results demonstrated that Ab-CB-1 was a digestive enzyme with hydrolytic protease properties and that Ab-cb-1 played an important role in the reproduction of A. besseyi.

Tomato transgenic plants expressing hairpin construct of a nematode protease gene conferred enhanced resistance to root-knot nematodes

Root-knot nematodes (Meloidogyne incognita) cause substantial yield losses in vegetables worldwide, and are difficult to manage. Continuous withdrawal of environmentally-harmful nematicides from the global market warrants the need for novel nematode management strategies. Utility of host-delivered RNAi has been demonstrated in several plants (Arabidopsis, tobacco, and soybean) that exhibited resistance against root-knot and cyst nematodes. Herein, a M. incognita-specific protease gene, cathepsin L cysteine proteinase (Mi-cpl-1), was targeted to generate tomato transgenic lines to evaluate the genetically modified nematode resistance. In vitro knockdown of Mi-cpl-1 gene led to the reduced attraction and penetration of M. incognita in tomato, suggesting the involvement of Mi-cpl-1 in nematode parasitism. Transgenic expression of the RNAi construct of Mi-cpl-1 gene resulted in 60-80% reduction in infection and multiplication of M. incognita in tomato. Evidence for in vitro and in vivo silencing of Mi-cpl-1 was confirmed by expression analysis using quantitative PCR. Our study demonstrates that Mi-cpl-1 plays crucial role during plant-nematode interaction and plant-mediated downregulation of this gene elicits detrimental effect on M. incognita development, reinforcing the potential of RNAi technology for management of phytonematodes in crop plants.

Molecular characterization of putative parasitism genes in the plant-parasitic nematode Meloidogyne hispanica

Meloidogyne hispanica (Mhi) is a difficult-to-control polyphagous root-knot nematode (RKN) species of emerging importance for economically valuable crops. Nematode secretions are likely to be the first signals perceived by the plant and are thought to be involved in various aspects of the plant-nematode interaction. The aims of this work were to identify and characterize M. hispanica parasitism genes: cathepsin L cysteine protease (cpl-1), calreticulin (crt-1), β-1,4-endoglucanase-1 (eng-1) and manganese superoxide dismutase (mnsod). As there are no genomic data available for M. hispanica, primers were designed from the conserved regions of the putative parasitism genes in M. incognita and M. hapla and used to amplify the genes in M. hispanica, which led to the successful amplification of these genes in M. hispanica. Partial gene sequences were also obtained for M. arenaria, M. hapla, M. hispanica, M. incognita and M. javanica cpl-1, crt-1, eng-1 and mnsod genes, and their phylogenetic relationship analysed. In order to determine whether these genes are differentially expressed during M. hispanica development, cDNA was amplified from mRNA isolated from eggs, second-stage juveniles (J2) and females. Amplification products were observed from cDNA of all developmental stages for the Mhi-cpl-1 and Mhi-crt-1 genes. However, the gene Mhi-crt-1 exhibited intense amplification bands in females, while the Mhi-eng-1 gene was equally amplified in eggs and J2 and the Mhi-mnsod gene was only expressed in eggs. In comparison to the other RKN species, the genes Mhi-eng-1 and Mhi-mnsod showed transcription in different nematode developmental stages.

Plant cystatins

Plant cystatins have been the object of intense research since the publication of a first paper reporting their existence more than 20 years ago. These ubiquitous inhibitors of Cys proteases play several important roles in plants, from the control of various physiological and cellular processes in planta to the inhibition of exogenous Cys proteases secreted by herbivorous arthropods and pathogens to digest or colonize plant tissues. After an overview of current knowledge about the evolution, structure and inhibitory mechanism of plant cystatins, we review the different roles attributed to these proteins in plants. The potential of recombinant plant cystatins as effective pesticidal proteins in crop protection is also considered, as well as protein engineering approaches adopted over the years to improve their inhibitory potency and specificity towards Cys proteases of biotechnological interest.

Protective effects of a cysteine proteinase propeptide expressed in transgenic soybean roots

Sedentary endoparasitic nematodes cause extensive damage to a large number of ornamental plants and food crops, with estimated economical losses over 100 billion US$ worldwide. Various efforts have put forth in order to minimize nematode damage, which typically involve the use of nematicides that have high cost and enhanced toxicity to humans and the environment. Additionally, different strategies have been applied in order to develop genetically modified plants with improved nematode resistance. Among the strategies are anti-invasion and migration, feeding-cell attenuation, and anti-nematode feeding. In the present study, we focus on anti-nematode feeding, which involves the evaluation and potential use of the cysteine proteinase (CPs) propeptide as a control alternative. The cysteine proteinase prodomain, isolated from Heterodera glycines (HGCP prodomain), is a natural inhibitory peptide used to transform soybean cotyledons using Agrobacterium rhizogenes. Genetically modified soybean roots expressing the propeptide were detected by Western blot and expression levels were measured by ELISA (around 0.3%). The transgenic roots expressing the propeptide were inoculated with a thousand H. glycines at the second juvenile stage, and a remarkable reduction in the number of females and eggs was observed. A reduction of female length and diameter was also observed after 35 days post-inoculation. Furthermore, the H. glycines mature protein was detected in females fed on soybean transformed root expressing or not expressing the propeptide. The data presented here indicate that the HGCP propeptide can reduce soybean cyst nematode infection and this strategy could be applied in the near future to generate resistant crop cultivars.

Meloidogyne incognita: molecular cloning and characterization of a cDNA encoding a cathepsin D-like aspartic proteinase

Herein we describe the cloning and characterization of a cDNA encoding an aspartic proteinase from the root-knot nematode Meloidogyne incognita. Using PCR techniques, a 1471-bp cDNA fragment encoding a cathepsin D-like (Mi-asp1) transcript was isolated from second-stage larvae mRNA. Its predicted amino acid sequence comprises a pro-region of 71 amino acid residues and a mature protease of 378 amino acid residues with a predicted molecular mass of 41.502kDa. Protein sequence comparisons of Mi-asp1 with GenBank (DQ360827) sequences showed 59-71% identity with nematode-specific cathepsin D-like aspartic proteinases. Southern blot analysis, RT-PCR amplification and EST mining suggest the existence of a developmentally expressed gene family encoding aspartic proteinases in M. incognita. Mi-asp1 may represent a potential target for molecular intervention for the purposes of plant-parasitic nematode control.

Nematode resistance

Plant-parasitic nematodes are major pests of both temperate and tropical agriculture. Many of the most damaging species employ an advanced parasitic strategy in which they induce redifferentiation of root cells to form specialized feeding structures able to support nematode growth and reproduction over several weeks. Current control measures, particularly in intensive agriculture systems, rely heavily on nematicides but alternative strategies are required as effective chemicals are withdrawn from use. Here, we review the different approaches that are being developed to provide resistance to a range of nematode species. Natural, R gene-based resistance is currently exploited in traditional breeding programmes and research is ongoing to characterize the molecular basis for the observed resistant phenotypes. A number of transgenic approaches hold promise, the best described being the expression of proteinase inhibitors to disrupt nematode digestion. The application of plant-delivered RNA interference (RNAi) to silence essential nematode genes has recently emerged as a potentially valuable resistance strategy.

Papain-like cysteine proteases: key players at molecular battlefields employed by both plants and their invaders

Papain-like cysteine proteases (PLCPs) play crucial roles in plant-pathogen/pest interactions. During these parasitic interactions, PLCPs act on non-self substrates, provoking the selection of counteracting inhibitors and other means to evade proteolysis. We review examples of PLCPs acting on molecular battlefields in the extracellular space, plant cytoplasm and herbivore gut. Examples are maize Mir1 (Maize inbred resistance 1), tomato Rcr3 (Required for Cladosporium resistance-3), Pseudomonas AvrRpt2 and AurPphB, insect DvCAL1 (Diabrotica virgifera cathepsin L-like protease-1) and nematode MiCpl1 (Meloidogyne incognita cathepsin L-like protease 1). The data suggest that PLCPs cleave specific proteins and that their translocation, activation and inhibition of PLCPs are tightly regulated.

Detection of putative secreted proteins in the plant-parasitic nematode Heterodera schachtii

The beet cyst nematode Heterodera schachtii is an important pathogen worldwide, but its molecular characterization has been limited to studying individual genes of interest. We undertook a high-throughput genomic approach and drastically increased the number of available sequences for this parasite. A total of 2,662 expressed sequence tags were grouped into 1,212 clusters representing a nonredundant catalog of H. schachtii genes. Implementing a bioinformatic workflow, we identified 50 sequences coding for candidate secreted proteins. All of these contain a putative signal peptide required for entry into the secretory pathway and lack any transmembrane domain. Included are previously postulated cell-wall-degrading enzymes and other parasitism-related genes. Moreover, we provide the first report of an arabinogalactan endo-1,4-beta-galactosidase enzyme (EC 3.2.1.89) in animals. As sequence data increase at a rapid rate, developing high-throughput genomic screening is a necessity. The in silico approach described here is an effective way to identify putative secreted proteins and prioritize candidates for further studies.

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.

Assessment of Nematode Resistance in Wheat Transgenic Plants Expressing Potato Proteinase Inhibitor (PIN2) Gene

Serine proteinase inhibitors (IP's) are proteins found naturally in a wide range of plants with a significant role in the natural defense system of plants against herbivores. The question addressed in the present study involves assessing the ability of the serine proteinase inhibitor in combating nematode infestation. The present study involves engineering a plant serine proteinase inhibitor (pin2) gene into T. durum PDW215 by Agrobacterium-mediated transformation to combat cereal cyst nematode (Heterodera avenae) infestation. Putative T(0) transformants were screened and positive segregating lines analysed further for the study of the stable integration, expression and segregation of the genes. PCR, Southern analysis along with bar gene expression studies corroborate the stable integration pattern of the respective genes. The transformation efficiency is 3%, while the frequency of escapes was 35.71%. chi(2) analysis reveals the stable integration and segregation of the genes in both the T(1) and T(2) progeny lines. The PIN2 systemic expression confers satisfactory nematode resistance. The correlation analysis suggests that at p < 0.05 level of significance the relative proteinase inhibitor (PI) values show a direct positive correlation vis-à-vis plant height, plant seed weight and also the seed number.

RNA interference and plant parasitic nematodes

RNA interference (RNAi) has recently been demonstrated in plant parasitic nematodes. It is a potentially powerful investigative tool for the genome-wide identification of gene function that should help improve our understanding of plant parasitic nematodes. RNAi should help identify gene and, hence, protein targets for nematode control strategies. Prospects for novel resistance depend on the plant generating an effective form of double-stranded RNA in the absence of an endogenous target gene without detriment to itself. These RNA molecules must then become available to the nematode and be capable of ingestion via its feeding tube. If these requirements can be met, crop resistance could be achieved by a plant delivering a dsRNA that targets a nematode gene and induces a lethal or highly damaging RNAi effect on the parasite.

Expressed sequence tags from Diabrotica virgifera virgifera midgut identify a coleopteran cadherin and a diversity of cathepsins

The Western corn rootworm is the major pest of corn in the USA and has recently become the target for insect-resistant transgenic crops. Transgenic crops have switched the focus for identifying insecticide targets from the insect nervous system to the midgut. Here we describe a collection of 691 sequences from the Western corn rootworm midgut, 27% of which predict proteins with no matches in current databases. Of the remaining sequences, most predict proteins with either catalytic (62%) or binding (19%) functions, as expected for proteins expressed in the insect midgut. The utility of this approach for the identification of targets for novel toxins is demonstrated by analysis of the first coleopteran cadherin gene, a putative Bt receptor, and a large class of cysteine-proteases, the cathepsins.

Isolation and characterization of a cDNA encoding a serine proteinase from the root-knot nematode Meloidogyne incognita

This report describes the first serine proteinase gene isolated from the sedentary nematode Meloidogyne incognita. Using degenerate primers, a 1372bp cDNA encoding a chymotrypsin-like serine proteinase (Mi-ser1) was amplified from total RNA of adult females by RT-PCR and 5' and 3' rapid amplification of cDNA ends. The deduced amino acid sequence of Mi-ser1 encoded a putative signal peptide and a prodomain of 22 and 33 amino acids, respectively, and a mature proteinase of 341 amino acids with a predicted molecular mass of 37,680Da. Sequence identity with the top serine proteinases matches from the databases ranged from 23 to 27%, including sequences from insects, mammals, and other nematodes. Southern blot analysis suggested that Mi-ser1 is encoded by a single or few gene copies. The pattern of developmental expression analyzed by Northern blot and RT-PCR indicated that Mi-ser1 was transcribed mainly in females. The domain architecture composed of a single chymotrypsin-like catalytic domain and the detection of a putative signal peptide suggested a digestive role for Mi-ser1.

Expression of Oryzacystatin I and II in Alfalfa Increases Resistance to the Root-Lesion Nematode

ABSTRACT Digestive cysteine proteinases have been isolated from plant-parasitic nematodes as well as coleopteran and hemipteran insects. Phytocystatins, inhibitors of cysteine proteinases, are found in a number of plants where they may play a role in defense against pathogens and pests. The cDNAs of the phytocystatins from rice, oryzacystatin I (OC-I) and oryzacystatin II (OC-II), were expressed in alfalfa (Medicago sativa) plants under the control of the potato protease inhibitor II (PinII) promoter and the plants were evaluated for resistance to the root-lesion nematode (Pratylenchus penetrans). A PinII-beta-glucuronidase (GUS) gene was introduced into alfalfa to determine the pattern of gene expression from this promoter. Constitutive GUS expression was observed in leaf and root vascular tissue, and in some plants, expression was observed in leaf mesophyll cells. Mechanical wounding of leaves increased GUS expression approximately twofold over 24 h. Inoculation with root-lesion nematodes resulted in localized GUS expression. Populations of root-lesion nematodes in alfalfa roots from one line containing the PinII::OC-I transgene and one line containing the PinII::OC-II transgene were reduced 29 and 32%, respectively, compared with a transgenic control line. These results suggest that oryzacystatins have the potential to confer increased resistance to the root-lesion nematode in alfalfa.

Engineering plants for nematode resistance

Biotechnology offers sustainable solutions to the problem of plant parasitic nematode control. There are several possible approaches for developing transgenic plants with improved nematode resistance; these include anti-invasion and migration strategies, feeding-cell attenuation, and antinematode feeding and development strategies. The essential elements of an effective control strategy are (a) genes that encode an antinematode effector protein, peptide or interfering RNA and (b) promoters that direct a specific pattern of expression for that effector. This review summarizes information on effectors that act directly against the nematode as well as those aimed at disrupting the nematode feeding site. We discuss patterns of promoter activity that could deliver expression of these effectors in a restricted and directed manner. Societal opposition to the technology of GM-nematode control is also discussed.

Identification of putative parasitism genes expressed in the esophageal gland cells of the soybean cyst nematode Heterodera glycines

Cloning parasitism genes encoding secretory proteins expressed in the esophageal gland cells is the key to understanding the molecular basis of nematode parasitism of plants. Suppression subtractive hybridization (SSH) with the microaspirated contents from Heterodera glycines esophageal gland cells and intestinal region was used to isolate genes expressed preferentially in the gland cells of parasitic stages. Twenty-three unique cDNA sequences from a SSH cDNA library were identified and hybridized to the genomic DNA of H. glycines in Southern blots. Full-length cDNAs of 21 clones were obtained by screening a gland-cell long-distance polymerase chain reaction cDNA library. Deduced proteins of 10 clones were preceded by a signal peptide for secretion, and PSORT II computer analysis predicted eight proteins as extracellular, one as nuclear, and one as plasmalemma localized. In situ hybridization showed that four of the predicted extracellular clones were expressed specifically in the dorsal gland cell, one in the subventral gland cells, and three in the intestine in H. glycines. The predicted nuclear clone and the plasmalemma-localized clone were expressed in the subventral gland cells and the dorsal gland cell, respectively. SSH is an efficient method for cloning putative parasitism genes encoding esophageal gland cell secretory proteins that may have a role in H. glycines parasitism of soybean.

Phage display selection of hairpin loop soyacystatin variants that mediate high affinity inhibition of a cysteine proteinase

Two hairpin-loop domains in cystatin family proteinase inhibitors form an interface surface region that slots into the active site cleft of papain-like cysteine proteinases, and determine binding affinity. The slot region surface architecture of the soybean cysteine proteinase inhibitor (soyacystatin N, scN) was engineered using techniques of in vitro molecular evolution to define residues that facilitate interaction with the proteinase cleft and modulate inhibitor affinity and function. Combinatorial phage display libraries of scN variants that contain mutations in the essential motifs of the first (QVVAG) and second (EW) hairpin-loop regions were constructed. Approximately 1010-1011 phages expressing recombinant scN proteins were subjected to biopanning selection based on binding affinity to immobilized papain. The QVVAG motif in the first hairpin loop was invariant in all functional scN proteins. All selected variants (30) had W79 in the second hairpin-loop motif, but there was diversity for hydrophobic and basic amino acids in residue 78. Kinetic analysis of isolated scN variants identified a novel scN isoform scN(LW) with higher papain affinity than the wild-type molecule. The variant contained an E78L substitution and had a twofold lower Ki (2.1 pM) than parental scN, due to its increased association rate constant (2.6 +/- 0.09 x 107 M-1sec-1). These results define residues in the first and second hairpin-loop regions which are essential for optimal interaction between phytocystatins and papain, a prototypical cysteine proteinase. Furthermore, the isolated variants are a biochemical platform for further integration of mutations to optimize cystatin affinity for specific biological targets.

A plant defensive cystatin (soyacystatin) targets cathepsin L-like digestive cysteine proteinases (DvCALs) in the larval midgut of western corn rootworm (Diabrotica virgifera virgifera)

Feeding bioassay results established that the soybean cysteine proteinase inhibitor N (soyacystatin N, scN) substantially inhibits growth and development of western corn rootworm (WCR), by attenuating digestive proteolysis [Zhao, Y. et al. (1996) Plant Physiol. 111, 1299-1306]. Recombinant scN was more inhibitory than the potent and broad specificity cysteine proteinase inhibitor E-64. WCR digestive proteolytic activity was separated by mildly denaturing SDS-PAGE into two fractions and in-gel assays confirmed that the proteinase activities of each were largely scN-sensitive. Since binding affinity to the target proteinase [Koiwa, H. et al. (1998) Plant J. 14, 371-380] governs the effectiveness of scN as a proteinase inhibitor and an insecticide, five peptides (28-33 kDa) were isolated from WCR gut extracts by scN affinity chromatographic separation. Analysis of the N-terminal sequence of these peptides revealed similarity to a cathepsin L-like cysteine proteinase (DvCAL1, Diabrotica virgifera virgifera cathepsin L) encoded by a WCR cDNA. Our results indicate that cathepsin L orthologs are pivotal digestive proteinases of WCR larvae, and are targets of plant defensive cystatins (phytocystatins), like scN.

Parasitic nematodes, proteinases and transgenic plants

Parasite proteinases have important functions in host-parasite interactions. Consequently, they have been investigated as targets for the control of both plant and animal parasites. Plant parasitic nematodes cause estimated annual losses to world agriculture of US$100 billion and, currently, their control often relies on highly toxic nematicides, with associated environmental risks. The potential of disrupting digestive proteinases for plant parasitic nematode control, via expression of proteinase inhibitors in transgenic plants, is summarized here by Catherine Lilley, Pauline Devlin, Peter Urwin and Howard Atkinson. They then consider whether the approach of expressing antinematode proteins in plants can be adapted for control of certain animal parasitic nematodes.

Proteinases and associated genes of parasitic helminths

Many parasites have deployed proteinases to accomplish some of the tasks imposed by a parasitic life style, including tissue penetration, digestion of host tissue for nutrition and evasion of host immune responses. Information on proteinases from trematodes, cestodes and nematode parasites is reviewed, concentrating on those worms of major medical and economical importance. Their biochemical characterization is discussed, along with their putative biological roles and, where available, their associated genes. For example, proteinases expressed by the various stages of the schistosome life-cycle, in particular the well-characterized cercarial elastase which is involved in the penetration of the host skin and the variety of proteinases, such as cathepsin B (Sm31), cathepsin L1, cathepsin L2, cathepsin D, cathepsin C and legumain (Sm32), which are believed to be involved in the catabolism of host haemoglobin. The various endo- and exoproteinases of Fasciola hepatica, the causative agent of liver fluke disease, are reviewed, and recent reports of how these enzymes have been successfully employed in cocktail vaccines are discussed. The various proteinases of cestodes and of the diverse superfamilies of parasitic nematodes are detailed, with special attention being given to those parasites for which most is known, including species of Taenia, Echinococcus, Spirometra, Necator, Acylostoma and Haemonchus. By far the largest number of papers in the literature and entries to the sequence data bases dealing with proteinases of parasitic helminths report on enzymes belonging to the papain superfamily of cysteine proteinases. Accordingly, the final section of the review is devoted to a phylogenetic analysis of this superfamily using over 150 published sequences. This analysis shows that the papain superfamily can be divided into two major branches. Branch A contains the cathepin Bs, the cathepsin Cs and a novel family termed cathepsin Xs, while Branch B contains the cruzipains, cathepsin Ls, papain-like and aleurain/cathepsin H-like proteinases. The relationships of the helminth proteinases, and similar proteinases from protozoan parasites and other organisms, within these groups are discussed.

Characterization of cDNAs encoding serine proteinases from the soybean cyst nematode Heterodera glycines

Three cDNAs encoding serine proteinases (HGSPI-III) were isolated from a cDNA library constructed from feeding females of Heterodera glycines. The library was screened with three separate serine proteinase gene fragments amplified from cDNA of H. glycines using consensus oligonucleotide primers. Each predicted protein contains a secretion signal sequence, a propeptide and a mature protein of 226-296 amino acids. One of the predicted enzymes, HGSP-II has 41% identity to a chymotrypsin-like enzyme from the mollusc, Haliotis rufescens, and analysis of key residues involved in substrate binding also suggests a chymotrypsin-like specificity. HGSP-I and HGSP-III show greatest homology to kallikreins but sequence analysis does not allow prediction of their substrate preferences. Southern blot analysis suggests that HGSP-II and HGSP-III are encoded by single-copy genes in contrast to HGSP-I which may have two or more homologues. The regions encoding the mature proteinases were cloned into an expression vector and recombinant protein produced in Escherichia coli. Both HGSP-I and HGSP-II were shown, after refolding, to cleave the synthetic peptide N-CBZ-Phe-Arg-7-amido-4-methylcoumarin, and this activity could be inhibited by the cowpea trypsin inhibitor, CpTI. HGSP-III showed no activity against the synthetic substrates tested. The information gained from these studies indicates that serine proteinases are an important group of enzymes in H. glycines and further characterization will aid the development of a proteinase inhibitor-based approach for transgenic plant resistance to plant parasitic nematodes.

Continual green-fluorescent protein monitoring of cauliflower mosaic virus 35S promoter activity in nematode-induced feeding cells in Arabidopsis thaliana

The responsiveness of the cauliflower mosaic virus 35S promoter in feeding sites developed by both sexes of Heterodera schachtii and female Meloidogyne incognita has been studied. The objective was to establish the value of green-fluorescent protein (GFP) as a nondestructive reporter gene system for characterizing promoter activity at nematode feeding sites in vivo. Growth units were devised that allowed individual feeding sites in roots of Arabidopsis thaliana to be observed by both bright-field and epifluorescent illumination. Changes in GFP expression were visually observed under experimental conditions that resulted in chloroplast formation in syncytia but not other root cells. Changes in GFP levels altered the extent of quenching, by this protein, of red light emitted by chlorophyll within the chloroplasts under violet excitation. Image analysis provided a semiquantitative basis for simultaneous measurement of changes in GFP fluorescence and the unquenched emission by chlorophyll. GFP levels were constant in cells surrounding the syncytium induced by H. schachtii, but they fell progressive from 10 to 35 days postinfection within this structure. Significant reduction in GFP levels was not limited to the early part of the time course but also occurred between 27 and 35 days postinfection. GFP was detected by immunoblotting in females of M. incognita but not in H. schachtii parasitizing similar GFP-expressing roots.