Horizontal gene transfer in nematodes: a catalyst for plant parasitism?

The origin of plant parasitism within the phylum Nematoda is intriguing. The ability to parasitize plants has originated independently at least three times during nematode evolution and, as more molecular data has emerged, it has become clear that multiple instances of horizontal gene transfer (HGT) from bacteria and fungi have played a crucial role in the nematode's adaptation to this new lifestyle. The first reported HGT cases in plant-parasitic nematodes were genes encoding plant cell wall-degrading enzymes. Other putative examples of HGT were subsequently described, including genes that may be involved in the modulation of the plant's defense system, the establishment of a nematode feeding site, and the synthesis or processing of nutrients. Although, in many cases, it is difficult to pinpoint the donor organism, candidate donors are usually soil dwelling and are either plant-pathogenic or plant-associated microorganisms, hence occupying the same ecological niche as the nematodes. The exact mechanisms of transfer are unknown, although close contacts with donor microorganisms, such as symbiotic or trophic interactions, are a possibility. The widespread occurrence of horizontally transferred genes in evolutionarily independent plant-parasitic nematode lineages suggests that HGT may be a prerequisite for successful plant parasitism in nematodes.     

An endosymbiotic bacterium in a plant-parasitic nematode: Member of a new Wolbachia supergroup

Wolbachia is an endosymbiotic bacterium widely present in arthropods and animal-parasitic nematodes. Despite previous efforts, it has never been identified in plant-parasitic nematodes. Random sequencing of genes expressed by the burrowing nematode Radopholus similis resulted in several sequences with similarity to Wolbachia genes. The presence of a Wolbachia-like endosymbiont in this plant-parasitic nematode was investigated using both morphological and molecular approaches. Transmission electron microscopy, fluorescent immunolocalisation and staining with DAPI confirmed the presence of the endosymbiont within the reproductive tract of female adults.16S rDNA, ftsZ and groEL gene sequences showed that the endosymbiont of R. similis is distantly related to the known Wolbachia supergroups. Finally, based on our initial success in finding sequences of this endosymbiont by screening an expressed sequence tag (EST) dataset, all nematode ESTs were mined for Wolbachia-like sequences. Although the retained sequences belonged to six different nematode species, R. similis was the only plant-parasitic nematode with traces of Wolbachia. Based on our phylogenetic study and the current literature we designate the endosymbiont of R. similis to a new supergroup (supergroup I) rather than considering it as a new species. Although its role remains unknown, the endosymbiont was found in all individuals tested, pointing towards an essential function of the bacteria.     

Plant parasitic nematode proteins and the host parasite interaction.

This review focuses on the proteins and secretions of sedentary plant parasitic nematodes potentially important for plant-nematode interactions. These nematodes are well equipped for parasitism of plants. Having acquired the ability to manipulate fundamental aspects of plant biology, they are able to hijack host-cell development to make their feeding site. They feed exclusively from feeding sites as they complete their life cycle, satisfying their nutritional demands for development and reproduction. Biochemical and genomic approaches have been used successfully to identify a number of nematode parasitism genes. So far, 65 204 expressed sequence tags (ESTs) have been generated for six Meloidogyne species and sequencing projects, currently in progress, will underpin genomic comparisons of Meloidogyne spp. with sequences of other pathogens and generate genechip microarrays to undertake profiling studies of up- and down-regulated genes during the infection process. RNA interference provides a molecular genetic tool to study gene function in parasitism. These methods should provide new data to help our understanding of how parasitic nematodes infect their hosts, leading to the identification of novel pathogenicity genes.     

Four transthyretin-like genes of the migratory plant-parasitic nematode Radopholus similis: members of an extensive nematode-specific family

Screening 1154 ESTs from the plant-parasitic nematode Radopholus similis resulted in seven tags coding for proteins holding a transthyretin-like domain (PF01060). The seven ESTs corresponded to four different genes which were cloned from a cDNA library (accession numbers AM691117, AM691118, AM691119, AM691120). Transthyretin-like genes belong to a large family, different from the transthyretin and the transthyretin-related genes with whom they share some sequence similarity at the protein level. This similarity has caused an inconsistent use of different names and abbreviations in the past. To avoid further confusion, we introduce a standardized nomenclature for this gene family, and chose to name this barely characterized gene family ttl (as for transthyretin-like). Further examination of the identified genes, named Rs-ttl-1 to -4, showed that they are expressed in both juveniles and adults, but not in young embryos. Whole mount in situ hybridization revealed a distinct spatial expression pattern for two of the genes: Rs-ttl-1 is expressed in the tissues surrounding the vulva, whereas Rs-ttl-2 is expressed in the ventral nerve cord. The deduced protein sequences contain a putative signal peptide for secretion, pointing to an extracellular function of the mature proteins. Database screens showed that the ttl family is restricted to nematodes. Moreover, a HMMER search revealed that ESTs derived from ttl genes are more abundant in parasitic nematode libraries, with a bias towards the parasitic stages. Despite their abundance in nematodes, including the extensively studied model organism Caenorhabditis elegans, the function of TTL proteins remains obscure. Our data suggest a role in the nervous system. Even without insight into their biological function, the nematode-specific nature of this gene family makes it a promising target for nematicides or RNAi mediated control strategies against parasitic nematodes.     

植物寄生线虫效应蛋白功能分析方法的研究进展

植物寄生线虫在侵染寄主过程中分泌许多与寄生相关的蛋白,这一类蛋白称为效应蛋白,这些效应蛋白在植物细胞内发挥各种作用,从而有利于线虫侵染、寄生和生长发育。研究这些效应蛋白的功能对于掌握线虫侵染植物的分子机理非常重要,也是寻找新的植物线虫病害防治方法的理论基础。对目前应用于研究植物寄生线虫效应蛋白功能的主要方法进行了概述。