CHARACTERIZATION AND LOCALIZATION OF PLANT PHENOLICS LIKELY INVOLVED IN THE PATHOGEN RESISTANCE EXPRESSED BY ENDOMYCORRHIZAL ROOTS

Phytochemical characterization of the major phenolic compounds and the ir ultrastructural localization were carried out on onion roots (Alliu m cepa L.) colonized by two vesicular-arbuscular mycorrhizal (VAM) fun gi: 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 p-coumaric aci ds, as well as N-ferulovltyramine were identified as the major phenoli c metabolites bound to the cell walls of VAM onion roots. Results from mycorrhized and control plants suggest the presence of a mechanism le ading to the oxidative condensation of phenols, the latter process dep ending on the presence or absence of symbiosis. Bioassays reveal that N-feruloyltyramine induces the branching of hyphae and reduces total f ungal development. The overall results lead us to suggest that the pro gressive binding of phenolic compounds in VAM roots is directly involv ed in the control of VAM endophytic establishment and development, as it gradually reduces the plasticity and elasticity of the symbiotic ma trix. 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.