Pierre Abad

Plant-parasitic nematodes Meloidogyne spp induce an elaborate permanent feeding site characterized by the redifferentiation of root cells into multinucleate and hypertrophied giant cells. We have isolated by a promoter trap strategy an Arabidopsis thaliana formin gene, AtFH6, which is upregulated during giant cell formation. Formins are actin-nucleating proteins that stimulate de novo polymerization of actin filaments. We show here that three type-I formins were upregulated in giant cells and that the AtFH6 protein was anchored to the plasma membrane and uniformly distributed. Suppression of the budding defect of the Saccharomyces cerevisiae bni1Delta bnr1Delta mutant showed that AtFH6 regulates polarized growth by controlling the assembly of actin cables. Our results suggest that AtFH6 might be involved in the isotropic growth of hypertrophied feeding cells via the reorganization of the actin cytoskeleton. The actin cables would serve as tracks for vesicle trafficking needed for extensive plasma membrane and cell wall biogenesis. Therefore, determining how plant parasitic nematodes modify root cells into giant cells represents an attractive system to identify genes that regulate cell growth and morphogenesis.

SUMMARY: Taxonomy: Eukaryota; Metazoa; Nematoda; Chromadorea; order Tylenchida; Tylenchoidea; Heteroderidae; genus Meloidogyne.  Physical properties: Microscopic‐non‐segmented worms. Meloidogyne species can reproduce by apomixis, facultative meiotic parthenogenesis or obligate mitotic parthenogenesis. Obligate biotrophic parasites inducing the re‐differentiation of plant cells into specialized feeding cells.  Hosts: Meloidogyne spp. can infest more than 3000 plant species including vegetables, fruit trees, cereals and ornamental flowers.  Symptoms: Root swellings called galls. Alteration of the root vascular system.  Disease control: Cultural control, chemical control, resistant cultivars.  Agronomic importance: Major threat to agriculture in temperate and tropical regions.

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.

The phytoparasitic nematodeBursaphelenchus xylophilusis the causal agent of pine wilt disease. However, it is difficult to differentiate isolates ofB. xylophilusfrom the closely related speciesB. mucronatus, which is not pathogenic to pine trees under field conditions. Here, we describe the development of a species‐specific DNA probe using a satellite DNA sequence fromB. xylophilus.This unusually abundant satellite DNA constitutes up to 30% of the nematode genome. It is represented as a tandemly repeatedMspI site‐containing sequence with a monomeric unit of 160 bp. We show that thisMspI satDNA probe is effective directly on a single squashedB. xylophilusnematode spotted onto a filter or by using polymerase chain reaction (PCR) amplification. Thus, in the pine wood‐nematode species complex, identification of the plant pathogenic speciesB. xylophilusmay be accomplished both rapidly and reliably.

An AluI satellite DNA family has been cloned from the entomopathogenic nematode Heterorhabditis indicus. This repeated sequence appears to be an unusually abundant satellite DNA, since it constitutes about 45% of the H. indicus genome. The consensus sequence is 174 nucleotides long and has an A + T content of 56%, with the presence of direct and inverted repeat clusters. DNA sequence data reveal that monomers are quite homogeneous. Such homogeneity suggests that some mechanism is acting to maintain the homogeneity of this satellite DNA, despite its abundance, or that this repeated sequence could have appeared recently in the genome of H. indicus. Hybridization analysis of genomic DNAs from different Heterorhabditis species shows that this satellite DNA sequence is specific to the H. indicus genome. Considering the species specificity and the high copy number of this AluI satellite DNA sequence, it could provide a rapid and powerful tool for identifying H. indicus strains.Key words: AluI repeated DNA, tandem repeats, species-specific sequence, nucleotide sequence analysis.