Effects of root proanthocyanidins on mycorrhizal colonization and nitrogen uptake in poplar

Proanthocyanidins (PAs), or condensed tannins, are widespread plant secondary metabolites common in trees. Proanthocyanidins play roles in plant defense and soil nutrient cycling, and have applications in human medicine and diet. Although PA function in plant shoots is well studied, there is less information on the role of PAs in roots. Proanthocyanidins can act as anti-fungal compounds, suggesting PAs in roots could negatively affect beneficial mycorrhizal fungi, and thus nutrient uptake. Poplars (Populus spp.) are known to produce a wide range of phenolic compounds, and for this work a transformable (P. tremula L. x P. tremuloides Michx.) hybrid was utilized. Transgenic lines with high and low tissue PA concentrations were used to test the hypothesis that high root PA levels would impede mycorrhizal colonization, and consequently, nitrogen uptake. Plants were grown in a sandwich tissue culture system allowing co-culture of the mycorrhizal fungi and roots. Plants from each line were inoculated with either the ectomycorrhizal (EcM) fungus Laccaria bicolor (Maire) P.D. Orton or the arbuscular mycorrhizal (AM) fungus Rhizophagus irregularis (Błaszk., Wubet, Renker & Buscot) C. Walker & A. Schüßler, or were kept as a non-inoculated control. Uptake of ammonium and nitrate by plant roots was measured by 15N-labeling. Successful EcM colonization on poplar roots was confirmed in all the plant lines, while no AM structures were observed. The low-PAs/phenolics line was less colonized by EcM fungi. When inoculated with EcM fungi, plants from all lines tended to have lower root PA concentrations. No significant differences in nitrogen uptake among plant lines were observed, but ammonium uptake was greater than nitrate uptake. Results suggest that PA content is reduced during colonization and that phenylpropanoids may play essential roles in establishing ectomycorrhizal symbioses.