SUPPRESSION OF DEFENSE RESPONSES IN MYCORRHIZAL ALFALFA AND TOBACCO ROOTS

The broad host range of arbuscular mycorrhizal (AM) fungi differs from the specificity encountered in biotrophic pathogens. We summarize her e the basic plant strategies of defence plant responses to colonizatio n with pathogenic micro-organisms and evaluate their possible involvem ent in AM performance. Detailed evidence is presented that, during ear ly colonization of plant roots by symbiotic Glomus, defence-related ro ot responses are induced and then subsequently suppressed. In AM inter actions, the broad host-range infection capacity and the induction of defence-related genes suggests that the compatible interaction is domi nant. These results are discussed in relation to the possible conclusi on that AM fungi might lack avirulence homologues or alternatively, th at the factor causing suppression of the host defence response might b e dominant.