Molecular and functional diversity of culturable bacterial strains associated with Tuber borchii fruit bodies from different Italian sites

Tuber borchii is an ectomycorrhizal fungus establishing important beneficial symbiosis with the roots of several tree species from different botanical families. Tuber borchii fruit bodies are associated with diverse bacterial communities, characterized by their plant growth promoting (PGP) abilities, such as the production of compounds capable of contrasting the growth of common soil-inhabiting phytopathogens. In this work we studied the molecular and functional diversity of bacterial strains, referable to the Pseudomonas fluorescens, selectively isolated from Tuber borchii samples, collected in three distinct Tuscany geographic areas with high truffle vocation. To this aim, 120 fluorescent colonies were randomly selected and isolated on King’s B agar, molecular characterized by Amplified Ribosomal DNA Restriction Analysis and then identified by amplicon sequencing of the 16S rRNA gene and two housekeeping genes, namely gyrase subunit B (gyrB) and RNA polymerase sigma factor (rpoD). Most of them were affiliated to the Pseudomonas fluorescens group and some isolates to Paenibacillus taiwanensis. Such isolates were functionally characterized assessing their ability to produce siderophores, using the universal Chrome Azurol Sulphonate (CAS) assay. The results obtained showed that siderophores producing isolates were 81,4% of the total, as revealed by a visible halo on CAS agar. Actually, 42,4% produced a discoloration halo from 0.3 to 0.8 cm and 27,9% from 0.9 to 1.4%. Finally, 11% accumulated the higher amounts, producing discoloration halos larger than 1,5 cm. In conclusion, our results allow us to identify functional bacterial strains to be used as specific bacterial inocula for both biocontrol, plant growth promotion and improvement of fungal performance and mycorrhizal establishment.