RPE, a plant gene involved in early developmental steps of nematode feeding cells

Genes induced during early response to Meloidogyne incognita in roots of resistant and susceptible alfalfa cultivars

A cDNA library made to RNA from roots of Meloidogyne incognita (root-knot nematode) susceptible alfalfa cv. Lahontan seedlings 72 h after root-knot nematode inoculation was differentially screened with cDNA made from uninoculated control and M. incognita infested (72 h) root RNA. Of the six cDNAs isolated, the deduced amino acid sequences of four showed significant homology to sequences present in the databank, while two were pioneer sequences. The four cDNAs with matches to known sequences include those for glycine-rich protein, the gluconeogenic pathway enzyme phosphoenolpyruvate carboxykinase, an isoflavone reductase-like protein, and metallothionein. We have followed the expression of these genes during the course of nematode infection in both the susceptible and resistant host and also in different plant organs. Based on these analyses, the genes induced early in nematode infection are related either to metabolic pathways or to stress/defense.

Antibody-based resistance to plant pathogens

Plant diseases are a major threat to the world food supply, as up to 15% of production is lost to pathogens. In the past, disease control and the generation of resistant plant lines protected against viral, bacterial or fungal pathogens, was achieved using conventional breeding based on crossings, mutant screenings and backcrossing. Many approaches in this field have failed or the resistance obtained has been rapidly broken by the pathogens. Recent advances in molecular biotechnology have made it possible to obtain and to modify genes that are useful for generating disease resistant crops. Several strategies, including expression of pathogen-derived sequences or anti-pathogenic agents, have been developed to engineer improved pathogen resistance in transgenic plants. Antibody-based resistance is a novel strategy for generating transgenic plants resistant to pathogens. Decades ago it was shown that polyclonal and monoclonal antibodies can neutralize viruses, bacteria and selected fungi. This approach has been improved recently by the development of recombinant antibodies (rAbs). Crop resistance can be engineered by the expression of pathogen-specific antibodies, antibody fragments or antibody fusion proteins. The advantages of this approach are that rAbs can be engineered against almost any target molecule, and it has been demonstrated that expression of functional pathogen-specific rAbs in plants confers effective pathogen protection. The efficacy of antibody-based resistance was first shown for plant viruses and its application to other plant pathogens is becoming more established. However, successful use of antibodies to generate plant pathogen resistance relies on appropriate target selection, careful antibody design, efficient antibody expression, stability and targeting to appropriate cellular compartments.

Arabidopsis thaliana genes expressed in the early compatible interaction with root-knot nematodes

In the compatible interaction between Arabidopsis thaliana and the endoparasitic nematode Meloidogyne incognita, galls are formed on the roots of the host plant. Differential display was used to identify alterations of gene expression in young A. thaliana root galls caused by M. incognita. Six genes were confirmed as plant genes by DNA gel blot hybridizations. Significant homology was found with a trypsin inhibitor, peroxidase, mitochondrial uncoupling protein, endomembrane protein, 20S proteasome alpha-subunit, and diaminopimelate decarboxylase. The cellular and temporal expression of each of the six genes was analyzed by mRNA in situ hybridizations.

Identification of an Arabidopsis thaliana mutant accumulating threonine resulting from mutation in a new dihydrodipicolinate synthase gene

A novel Arabidopsis DHDPS gene named DHDPS2 was found through identification of a mutant by promoter trapping. The mutation promotes a reduction of growth resulting from combination of a defect in lysine biosynthesis and accumulation of a toxic level of threonine or derived products. The mutant also modifies the amino acid composition issuing from the pyruvate and aspartate pathways, affecting mainly the root compartment. These data are in accordance with the expression of DHDPS2 in the root apex as visualized by expression of the GUS reporter gene. This suggests that a large proportion of the amino acids derived from pyruvate and aspartate are synthesized in this organ.

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.

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.

Identification, cloning, and properties of cytosolic D-ribulose-5-phosphate 3-epimerase from higher plants

Plant cells contain a complete oxidative pentose phosphate pathway in the chloroplasts, but an incomplete pathway was proposed to be present in the cytosol, with cytosolic (cyt) isoforms of ribulose-5-phosphate 3-epimerase (RPEase) and other non-oxidative branch enzymes being undetectable. Here we present for the first time the identification, cloning, and properties of a cyt-RPEase in rice (Oryza sativa) and presence of its homologues in other plant species. Recombinant cyt-RPEase is a homodimer of 24.3-kDa subunits such as in the case of the animal and yeast enzymes, whereas the chloroplast (chl) RPEase is a hexamer. Cytosolic and chloroplastic RPEases cannot be separated by anion exchange chromatography. Since plant cyt-RPEase is more closely related in its primary structure to homologous enzymes in animal and yeast cells than to the chloroplast RPEase, the plant nuclear genes coding for cytosolic and chloroplast RPEases were most likely derived from eubacteria and cyanobacteria, respectively. Accumulation of cyt-RPEase-mRNA and protein is high in root cells, lacking chl-RPEase, and lower in green tissue. These and other observations support the view that green and non-green plant cells possess a complete oxidative pentose phosphate pathway in the cytosol.