Effect of Selected Truffle-Associated Bacteria and Fungi on the Mycorrhization of Quercus ilex Seedlings with Tuber melanosporum

The success of truffle cultivation is especially dependent on the quality of truffle-mycorrhized seedlings, which are typically produced in nurseries under aseptic conditions to avoid root colonization by undesired ectomycorrhizal fungi. However, such practices may also eliminate beneficial microorganisms that could support truffle symbiosis and improve seedling quality. In this study, twelve endophytic bacterial and fungal strains, isolated from the <i>Tuber melanosporum</i> environment (gleba tissue, mycorrhizae and truffle <i>brûlé</i>), were tested for their effect on <i>T. melanosporum</i> mycorrhization levels in inoculated <i>Quercus ilex</i> seedlings under nursery conditions. Co-inoculation with a strain of <i>Agrobacterium tumefaciens</i> significantly enhanced root colonization by <i>T. melanosporum</i>, supporting its potential role as mycorrhizal helper bacterium. In contrast, a strain of <i>Trichoderma harzianum</i> negatively affected mycorrhization. The remaining strains did not show significant effects on seedling mycorrhization or seedling growth. Our findings support the hypothesis that specific bacterial strains associated with truffles can act as mycorrhizal helper bacteria, highlighting the potential for co-inoculation strategies to enhance quality of truffle-inoculated seedlings in nurseries. However, further research is needed to gain a deeper understanding of the interactions within the mycorrhizosphere that could contribute to improving nursery seedling quality.