The broad host range of arbuscular mycorrhizal (AM) fungi differs from the specificity encountered in biotrophic pathogens. We summarize here the basic plant strategies of defence plant responses to colonization with pathogenic micro-organisms and evaluate their possible involvement in AM performance. Detailed evidence is presented that, during early colonization of plant roots by symbiotic Glomus, defence-related root responses are induced and then subsequently suppressed. In AM interactions, the broad host-range infection capacity and the induction of defence-related genes suggests that the compatible interaction is dominant. These results are discussed in relation to the possible conclusion that AM fungi might lack avirulence homologues or alternatively, that the factor causing suppression of the host defence response might be dominant.