Arbuscular mycorrhizas represent the most widespread, and probably most ancient, type of plant–fungus association in which the large majority of terrestrial plants must have evolved with compatibility systems towards the fungal symbionts. Cellular interactions leading to reciprocal morphofunctional integration between symbionts during mycorrhiza establishment are complex. Some plant genes and cellular events may be shared with nodulation processes, but there is evidence of molecular modifications specific to arbuscular mycorrhiza formation. Plant defence responses, which are normally weakly activated during the symbiotic state, are strongly elicited by arbuscular mycorrhizal fungi in genetically altered, resistant hosts suggesting control over defence gene expression during establishment of a successful symbiosis. Modifications are also induced in the fungal symbionts during colonization of host tissues, 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 fungi opens the possibility of identifying functional genes in order to study their regulation and role in symbiosis establishment. Key words: arbuscular mycorrhiza, reciprocal symbiosis, molecular mechanisms, plant determinants, fungal molecules.
Arbuscular Mycorrhiza in the Living Collections at the Royal Botanic Garden Edinburgh
