To identify plant genes involved in giant cell formation induced by the root-knot nematode Meloidogyne incognita, a promoter-trapping strategy was developed in Arabidopsis thaliana. We identified a gene, NEC, early upregulated in the giant cells. During root development, this gene is expressed in meristems including initiation sites of lateral roots. This gene encodes a calcium-binding protein with an EF-hand motif. In order to investigate its function, analysis of homozygous nec plants and plants over-expressing antisense gene were carried out. Secondly, we investigated whether nitrogen-fixing nodules and giant cell formation induce distinct or overlapping regulatory pathways in Medicago truncatula. We studied expression patterns of nodule-expressed genes after infection with M. incognita. Analysis of promoter-GUS fusions indicates an accumulation of the cell cycle gene CCS52a in giant cells, while the early nodulin gene ENOD40 expression was detected only in cells surrounding the giant cells. Interestingly, on transgenic plants over-expressing ENOD40, more galls were formed. In addition, results obtained on macroarray analysis will be discussed. Taken together, these results suggest that certain events might be common between giant cell formation and nodule development