Dendrobium nobile Lindl. seed germination in co-cultures with diverse associated bacteria

The conservation of orchids is challenging due to their strong biotic relations and tiny seeds, requiring mycorrhiza for germination. This is aggravated when tropical plants are maintained in artificial conditions of greenhouses. We aimed to select the plant growth promoting rhizobacteria (PGPR) for orchid seed germination, to study plant–microbial interactions, and to determine whether there is any specificity between two species of Dendrobium plants in choosing bacterial partners. By the isolation of rhizoplane and endophytic rhizobacteria from Dendrobium moschatum roots, the known PGPR (Azospirillum, Enterobacter, Streptomyces) and less popular (Roseomonas, Agrococcus) strains were tested for the production of biologically active auxin. The bacterization of another orchid, D. noblie, with several newly selected strains and previously isolated ones (Mycobacterium sp., Bacillus pumilus) revealed that the orchids did not express evident specificity in relations with favorable bacteria, but refused to establish associations with Streptomyces and Azospirillum. Endophytic Agrococcus and Sphingomonas strains showed significant promotion of orchid germination. Mycobacterium and B. pumilus were also stable in their positive influence on the acceleration of D. noblie seed development. The active colonization of the seed surface and the inner tissues by associated bacteria was observed under electron microscopy. The analysis of orchid–bacteria relations was made. Altogether, the data shows that selection provides a good strategy for choosing the active strains for orchid seeds’ bacterization, since not all known PGPR are useful and successful in building associative frameworks with orchid seeds. The stable activity of the strains guarantees their long-term and effective application in orchid in vitro biotechnology.