Developmental expression of secretory beta-1,4-endoglucanases in the subventral esophageal glands of Heterodera glycines

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.

Molecular characterisation and expression of two venom allergen-like protein genes in Heterodera glycines

Secretory proteins encoded by genes expressed in the oesophageal gland cells of plant-parasitic nematodes have key roles in nematode parasitism of plants. Two venom allergen-like protein cDNAs (designated hg-vap-1 and hg-vap-2)were isolated from Heterodera glycines gland cell cDNA libraries. Both cDNAs hybridised to genomic DNA of H. glycines in Southern blots. The hg-vap-1 cDNA contained an open reading frame encoding 215 amino acids with the first 25 amino acids being a putative secretion signal. The hg-vap-2 cDNA contained an open reading frame encoding 212 amino acids with the first 19 amino acids being a putative secretion signal. Genes of hg-vap-1 and hg-vap-2 contained four introns, which ranged in size from 44 to 574 bp, and five exons ranging in size from 43 to 279 bp. In situ hybridisation analyses showed that mRNAs of both vap genes accumulated specifically in the subventral gland cells of H. glycines during parasitism. The gland cell-specific expression and presence of predicted secretion signal peptides in both VAPs suggest that these proteins are secreted from the nematode and may play a role in the infection of host plants by this parasite.

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.

Susceptibility to the sugar beet cyst nematode is modulated by ethylene signal transduction in Arabidopsis thaliana

Previously, we identified Arabidopsis thaliana mutant rhd1-4 as hypersusceptible to the sugar beet cyst nematode Heterodera schachtii. We assessed rhd1-4 as well as two other rhd1 alleles and found that each exhibited, in addition to H. schachtii hypersusceptibility, decreased root length, increased root hair length and density, and deformation of the root epidermal cells compared with wild-type A. thaliana ecotype Columbia (Col-0). Treatment of rhd1-4 and Col-0 with the ethylene inhibitors 2-aminoethoxyvinylglycine and silver nitrate and the ethylene precursor 1-aminocyclopropane-1-carboxylic acid suggests that the rhd1-4 hypersusceptibility and root morphology phenotypes are the result of an increased ethylene response. Assessment of known ethylene mutants further support the finding that ethylene plays a role in mediating A. thaliana susceptibility to H. schachtii because mutants that overproduce ethylene (eto1-1, eto2, and eto3) are hypersusceptible to H. schachtii and mutants that are ethylene-insensitive (etr1-1, ein2-1, ein3-1, eir1-1, and axr2) are less susceptible to H. schachtii. Because the ethylene mutants tested show altered susceptibility and altered root hair density and length, a discrimination between the effects of altered ethylene signal transduction and root hair density on susceptibility was accomplished by analyzing the ttg and gl2 mutants, which produce ectopic root hairs that result in greatly increased root hair densities while maintaining normal ethylene signal transduction. The observed normal susceptibilities to H. schachtii of ttg and g12 indicate that increased root hair density, per se, does not cause hypersusceptibility. Furthermore, the results of nematode attraction assays suggest that the hypersusceptibility of rhd1-4 and the ethylene-overproducing mutant eto3 may be the result of increased attraction of H. schachtii-infective juveniles to root exudates of these plants. Our findings indicate that rhd1 is altered in its ethylene response and that ethylene signal transduction positively influences plant susceptibility to cyst nematodes.

Signal peptide-selection of cDNA cloned directly from the esophageal gland cells of the soybean cyst nematode Heterodera glycines

Secretions from the esophageal gland cells of plant-parasitic nematodes play critical roles in the nematode-parasitic cycle. A novel method to isolate cDNA encoding putative nematode secretory proteins was developed that utilizes mRNA for reverse transcription-polymerase chain reaction derived from microaspiration of the esophageal gland cell contents of parasitic stages of the soybean cyst nematode Heterodera glycines. The resulting H. glycines gland cell cDNA was cloned into the pRK18 vector, and plasmid DNA was transformed into a mutated yeast host for specific selection of cDNA inserts that encode proteins with functional signal peptides. Of the 223 cDNA clones recovered from selection in yeast, 97% of the clones encoded a predicted signal peptide. Fourteen unique cDNA clones hybridized to genomic DNA of H. glycines on Southern blots and, among them, nine cDNA clones encoded putative extracellular proteins, as predicted by PSORT II computer analysis. Four cDNA clones hybridized to transcripts within the dorsal esophageal gland cell of parasitic stages of H. glycines, and in situ hybridization within H. glycines was not detected for eight cDNA clones. The protocol provides a direct means to isolate potential plant-parasitic nematode esophageal gland secretory protein genes.

Functional screening yields a new beta-1,4-endoglucanase gene from Heterodera glycines that may be the product of recent gene duplication

Clones with secreted cellulolytic activity were identified when a cDNA library constructed from poly A(+) RNA of preparasitic second-stage juveniles of Heterodera glycines, the soybean cyst nematode, was expressed in the Escherichia coli SOLR strain and overlaid with a carboxymethylcellulose (CMC) substrate. Twenty CMC-degrading clones were analyzed, and all were either identical or strongly similar to a beta-1,4-endoglucanase gene (HG-eng-2), previously isolated from H. glycines. A subgroup of identical "HG-eng-2-like" clones had considerable differences in the 5' untranslated region compared with HG-eng-2 and were designated HG-eng-3. One H. glycines genomic clone contained HG-eng-2 and HG-eng-3 full-length genes, separated by a distance of approximately 8 kb, and a second genomic clone contained two copies of HG-eng-2, separated by approximately 6.5 kb, suggesting the presence of endoglucanase gene clusters in H. glycines. The HG-eng-2 and HG-eng-3 genes were in opposite transcriptional orientation, with considerable nucleotide differences in their 5' flanking regions. The highly conserved nucleotide sequence in the introns and exons and their close proximity within the genome suggest that HG-eng-2 and HG-eng-3 are the products of recent gene duplication and inversion.

Nematode Parasitism Genes

The ability of nematodes to live on plant hosts involves multiple parasitism genes. The most pronounced morphological adaptations of nematodes for plant parasitism include a hollow, protrusible stylet (feeding spear) connected to three enlarged esophageal gland cells that express products that are secreted into plant tissues through the stylet. Reverse genetic and expressed sequence tag (EST) approaches are being used to discover the parasitism genes expressed in nematode esophageal gland cells. Some genes cloned from root-knot (Meloidogyne spp.) and cyst (Heterodera and Globodera spp.) nematodes have homologues reported in genomic analyses of Caenorhabditis elegans and animal-parasitic nematodes. To date, however, the candidate parasitism genes endogenous to the esophageal glands of plant nematodes (such as the ß-1,4-endoglucanases) have their greatest similarity to microbial genes, prompting speculation that genes for plant parasitism by nematodes may have been acquired by horizontal gene transfer.

An efficient cDNA-AFLP-based strategy for the identification of putative pathogenicity factors from the potato cyst nematode Globodera rostochiensis

A new strategy has been designed to identify putative pathogenicity factors from the dorsal or subventral esophageal glands of the potato cyst nematode Globodera rostochiensis. Three independent criteria were used for selection. First, genes of interest should predominantly be expressed in infective second-stage juveniles, and not, or to a far lesser extent, in younger developmental stages. For this, gene expression profiles from five different developmental stages were generated with cDNA-AFLP (amplified fragment length polymorphism). Secondly, the mRNA corresponding to such a putative pathogenicity factor should predominantly be present in the esophageal glands of pre-parasitic juveniles. This was checked by in situ hybridization. As a third criterion, these proteinaceous factors should be preceded by a signal peptide for secretion. Expression profiles of more than 4,000 genes were generated and three up-regulated, dorsal gland-specific proteins preceded by signal peptide for secretion were identified. No dorsal gland genes have been cloned before from plant-parasitic nematodes. The partial sequence of these three factors, A4, A18, and A41, showed no significant homology to any known gene. Their presence in the dorsal glands of infective juveniles suggests that these proteins could be involved in feeding cell initiation, and not in migration in the plant root or in protection against plant defense responses. Finally, the applicability of this new strategy in other plant-microbe interactions is discussed.

Changes in mRNA abundance within Heterodera schachtii-infected roots of Arabidopsis thaliana

Gene expression changes in plant roots infected by plant-parasitic cyst nematodes are involved in the formation of nematode feeding sites. We analyzed mRNA abundance changes within roots of Arabidopsis thaliana during the early compatible interaction with Heterodera schachtii, the sugarbeet cyst nematode. Approximately 1,600 root sections, each containing a single parasitic nematode and its feeding site, and 1,600 adjacent, nematode-free root sections were excised from aseptic A. thaliana cultures 3 to 4 days after inoculation with H. schachtii. These tissue samples were termed infected and uninfected, respectively. Preparasitic nematodes were added to the uninfected tissue sample to maintain the nematode to plant tissue proportion. mRNA extracted from these two tissue samples was subjected to differential display analysis. Thirty-six cDNA clones corresponding to mRNA species with different abundance between both tissue samples were isolated. Of these clones, 24 were of A. thaliana origin and 12 were from H. schachtii. Differential display data predicted that the A. thaliana cDNA clones corresponded to 13 transcripts that were more abundant in the infected root sections and 11 transcripts that were more abundant in the uninfected root sections. H. schachtii cDNA clones were predicted to correspond to four transcripts that were more abundant in parasitic nematodes and to eight transcripts that were more abundant in preparasitic nematodes. In situ hybridization experiments confirmed the mRNA abundance changes in A. thaliana roots predicted by the differential display analyses for two A. thaliana clones.