Early defence reactions in Norway spruce seedlings inoculated with the mycorrhizal fungus Pisolithus tinctorius (Persoon) Coker &; Couch and the pathogen Heterobasidion annosum (Fr.) Bref.

The activities of the enzymes responsible for cell-wall strengthening and salicylic acid (SA) content in Norway spruce seedlings were investigated after inoculation with the ectomycorrhizal fungus Pisolithus tinctorius or the pathogen Heterobasidion annosum, and after treatment with elicitors from both of these fungi. Inoculation with both fungi increased guaiacol peroxidase (POD) activity in the roots of the pathogen-inoculated seedlings during the earliest phases of colonisation, and induced the activities of several POD isoforms. Two of these were only seen in pathogen-inoculated seedlings and corresponded with increased POD activity against ferulic acid. Colonisation with H. annosum triggered an increase in phenylalanine ammonia lyase (PAL) activity in the roots of the spruce seedlings, which was followed by an accumulation of free SA. One month after inoculation levels of free SA were increased also in the shoots of H. annosum-inoculated seedlings. In contrast increase in free SA content in the roots of P. tinctorius-inoculated seedlings was only transient. Similarly to inoculation, treatment with elicitors of H. annosum increased the PAL and POD activity, as well as SA content in the roots of spruce seedlings. A positive correlation between PAL activity and SA content in the H. annosum-inoculated seedlings and accumulation of SA precursors in the phenylpropanoid pathway indicate that the plant defence mechanisms, during which SA is synthesised through the PAL pathway, are exploited by H. annosum for facilitation of colonisation.