Induced defence responses limit Hartig net formation in ectomycorrhizal birch roots

<jats:p>Roots of clonal birches (<jats:italic>Betula pendula</jats:italic>) were inoculated with the ectomycorrhizal fungi <jats:italic>Paxillus involutus</jats:italic> (isolates P0 and Mi) and <jats:italic>Hebeloma cylindrosporum</jats:italic> (strains D1 and D105). These fungi showed different rates of mycorrhiza formation <jats:italic>in vitro</jats:italic>. Mature mycorrhizas were obtained after only 2–4 d with <jats:italic>H. cylindrosporum</jats:italic>, whereas 6–8 d were necessary with <jats:italic>P. involutus</jats:italic> isolate P0, and <jats:italic>P. involutus</jats:italic> isolate Mi was not able to form mature mycorrhiza during the 10 d of the experiment. Temporal changes in PAL activity and the expression of genes encoding intracellular pathogenesis‐related proteins were followed after inoculating birch roots with these fungi. Transient increase of PAL activity, and transient induction of expression of the wound‐inducible <jats:italic>Bet v1‐SC1</jats:italic> gene, were observed in roots challenged with both <jats:italic>H. cylindrosporum</jats:italic> strains and the <jats:italic>P. involutus</jats:italic> isolate P0. These changes were found to coincide with hyphal penetration between root cells during Hartig net formation, and were never observed in roots inoculated with the poorly aggressive <jats:italic>P. involutus</jats:italic> isolate Mi. Examination of mycorrhizal root sections under u.v. light indicated the presence of phenolic compounds in the host cell walls at the vicinity of the Hartig net. These results strongly suggest that hyphal penetration between the root cells triggers a transient defence response which, in turn, could limit Hartig net formation to the outer layer of the root cortex.</jats:p>