Abstract. Phytochemical characterization of the major phenolic compounds and their ultrastructural localization were carried out on onion roots (Allium cepa L.) colonized by two vesicular-arbuscular mycorrhizal (VAM) fungi: Glomus intraradix Schenck & Smith and G. versiforme (Karst.) Berch. Free and wall-bound forms of phenolic components were quantified in relation to the duration of symbiosis. Both ferulic and pcoumaric acids, as well as N-feruloyltyramine were identified as the major phenolic metabolites bound to the cell walls of VAM onion roots. Results from mycorrhized and control Plants suggest the presence of a mechanism leading to the oxidative condensation of phenols, the latter process depending on the presence or absence of symbiosis. Bioassays reveal that N-feruloyltyramine induces the branching of hyphae and reduces total fungal development. The overall results lead us to suggest that the progressive binding of phenolic compounds in VAM roots is directly involved in the control of VAM endophytic establishment and development, as it gradually reduces the plasticity and elasticity of the symbiotic matrix. Phenolic compounds bound to cell walls could also be indirectly responsible for the resistance of VAM roots to pathogenic fungi, since they result in increased resistance by the cell wall to the action of digestive enzymes.

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