Phenolic Compounds and Resistance to Fungal Pathogens Induced in Primary Roots of Douglas-Fir Seedlings by the Ectomycorrhizal FungusLaccaria laccata

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Abstract
A mycorrhizal strain of L. laccata, which can protect primary roots of seedlings of Douglas-fir (P. menziesii) from lethal root rot by Fusarium oxysporum, induced an accumulation of osmiophilic materials in cortical cells of primary roots. Histochemical tests demonstrated that the osmiophilic materials were primarily phenolic. One month old primary roots, incubated with L. laccata, its cell-free metabolites, or killed inoculum, were challenged with F. oxysporum or Pythium irregulare. Rate of advance of hyphae toward roots, extent of surface colonization and cortical infection by the pathogens, and amount of phenolic material in the cortex were estimated by optical techniques. Cortical infection by each pathogen was significantly less in seedlings affected by L. laccata or its metabolites than in controls. Hyphal frequency was inversely proportional to the concentration of osmiophilic materials. Rates of growth of the pathogens toward the root and intensity of rhizoplane colonization were not influenced by L. laccata. Antibiosis was unimportant in root protection although L. laccata was capable of inhibiting F. oxysporum in vitro. Phenolics induced by L. laccata in the primary root may be the basis for root protection.