Molecular analysis of migratory plant-parasitic nematodes with a focus on plant cell wall modifying enzymes

Plant-parasitic nematodes cause a considerable amount of crop damage worldwide. These nematodes all have a stylet to damage plant cells mechanically, but through which they also secrete proteins important for their infection process. Most studies on genes encoding these so-called parasitism proteins were conducted on sedentary plant-parasitic nematodes which manipulate the plant to induce a specialized nematode feeding site. In this doctoral thesis parasitism genes of the less specialized migratory endoparasitic nematodes were investigated. The focus was on plant cell wall modifying enzymes that are being secreted by the nematode to enable migration through the rigid plant cell wall. The main component of the plant cell wall is cellulose, the most abundant sugar polymer on earth. In nematodes, cellulases or endo-1,4-β-glucanases which can degrade cellulose were mainly identified in sedentary species. A large part of this thesis is about the characterization of endoglucanases in migratory endoparasitic nematodes, namely in the species Radopholus similis, Ditylenchus africanus and Pratylenchus coffeae. Additionally, a detailed model for the evolution of the gene structure of endoglucanases within nematodes was proposed. The second most abundant sugar polymer in a plant cell wall is xylan. An endoxylanase from R. similis putatively able to degrade xylan was characterized. When this gene was downregulated in nematodes by RNAi, the infection on the plant was reduced by 60%, showing that the xylanase is important for the nematodes to infect their host plant. Another technique to identify parasitism genes is expressed sequence tag analysis, by randomly sequencing pieces of genes that are being expressed by the nematode. This approach was applied to D. africanus and 4847 ESTs were analyzed. Scientific research sometimes leads to unexpected discoveries. During an EST analysis of R. similis several ESTs were found with significant similarity to sequences from a Wolbachia endosymbiotic bacterium. This finding was further investigated and the presence of an intracellular Wolbachia bacterium in the nematode was demonstrated. Although this bacterium is often found in filarial nematodes, it is the first time it is discovered in a plant-parasitic nematode. Future research will be needed to determine what role this endosymbiont plays within the nematode