CELLULAR AND MOLECULAR APPROACHES IN THE CHARACTERIZATION OF SYMBIOTIC EVENTS IN FUNCTIONAL ARBUSCULAR MYCORRHIZAL ASSOCIATIONS

Arbuscular mycorrhizas represent the most widespread, and probably mos t ancient, type of plant-fungus association in which the large majorit y of terrestrial plants must have evolved with compatibility systems t owards the fungal symbionts. Cellular interactions Lading to reciproca l morphofunctional integration between symbionts during mycorrhiza est ablishment are complex. Some plant genes and cellular events may be sh ared with nodulation processes, but there is evidence of molecular mod ifications specific to arbuscular mycorrhiza formation. Plant defence responses, which are normally weakly activated during the symbiotic st ate, are strongly elicited by arbuscular mycorrhizal fungi in genetica lly altered, resistant hosts suggesting control over defence gene expr ession during establishment of a successful symbiosis. Modifications a re also induced in the fungal symbionts during colonization of host ti ssues, with changes in wall metabolism and protein expression. Nothing is known of the genetic make-up of arbuscular mycorrhizal fungi which are recalcitrant to pure culture. Recent cloning of DNA from these fu ngi opens the possibility of identifying functional genes in order to study their regulation and role in symbiosis establishment.