Plant growth promoting potential of bacterial endophytes from three terrestrial mediterranean orchid species

Orchids are highly dependent on symbiotic microorganisms during their entire life cycle. Whereas an important role in orchid seed germination and early plant development is well established for mycorrhizal fungi, the influence of endophytic bacteria on orchid growth has been less investigated. Here, we report the isolation of endophytic bacteria from different organs of three terrestrial Mediterranean orchid species (Spiranthes spiralis, Serapias vomeracea and Neottia ovata), the investigation of their potential Plant Growth-Promoting (PGP) traits and their interaction with the orchid mycorrhizal (OM) fungus Tulasnella calosporain vitro. Little overlap was found among endophytic bacteria isolated from the different organs of the three orchid species. Taxonomic identification, based on the 16S rRNA gene, of fifty dereplicated bacterial isolates revealed that they belong to the genera Pseudomonas, Pantoea, Rahnella, Staphylococcus, Sphingomonas, Microbacterium, Streptomyces, Fictibacillus and Bacillus. Most bacterial isolates exhibited some potential PGP traits, such as nutrient solubilization, ACC deaminase activities and/or IAA biosynthesis. Although some Pseudomonas reduced growth of the OM fungus Tulasnella calospora, most isolates did not affect fungal growth. These results increase our understanding of the diversity and potential PGP functions of bacterial endophytes in terrestrial orchids, and suggest a role as beneficial partners in the orchid microbiota.