Trophic relationships: effect of bacteria, fungi and nematodes interactions on P nutrition from phytate in the rhizosphere of Pinus pinaster seedlings

International audienceAims and Background: Phytate accounts for large P fractions accumulating in soil. However, plants are poorly able to access this source of P because of their low phytate mineralizing capacity. In contrast to plants, some rhizosphere bacteria are able to mineralize phytate. This activity could benefit first to bacterial grow th and plant P availability could be enhanced only through bacterial grazers liberating nutrients locked up in bacterial biomass. We hypothesize that phytase- producing bacteria could mobilize plant unavailable phytate P into new nutrient pools which become available to plants w hen bacteria are grazed by nematodes. Methods : We used a simplified system with phytate and combinations made of Pinus pinaster seedlings, an ectomycorrhizal fungus (Hebeloma cylindrosporum), populations of bacteria ( Bacillus subtilis) displaying phytase activity and one of their nematode grazer (Rhabditis sp). Results: The presence of bacteria, alone or in combination with the ectomycorrhizal fungus, was not sufficient to improve plant P status. In contrast, the nematode graz ing activity led to a dramatic increase of plant P accumulation up to 200% compared to control ones. Conclusion: Microbial grazing by nematodes represents therefore an important mechanism to improve P availability to plants, by accelerating phytate mineralization and/or releasing locked microbial P