Colonization strategy of the endophytic plant growth promoting strains of Pseudomonas fluorescens and Klebsiella oxytoca on the seeds, seedlings and roots of the epiphytic orchid, Dendrobium nobile Lindl.

AIMS: Orchids form strong mycorrhizal associations, but their interactions with bacteria are poorly understood. We aimed to investigate the distribution of plant growth promoting rhizobacteria at different stages of orchid development and to study if there is any selective specificity in choosing PGPR partners.

METHODS AND RESULTS: Colonization patterns of gfp-tagged Pseudomonas fluorescens and Klebsiella oxytoca were studied on roots, seeds, and seedlings of Dendrobium nobile. Endophytic rhizobacteria rapidly colonized velamen and core parenchyma entering through exodermis and the passage cells, whereas at the early stages, they stayed restricted to the surface and the outer layers of the protocorms and rhizoids. The highest amounts of auxin (indole-3-acetic acid) were produced by K. oxytoca and P. fluorescens in the nitrogen-limiting and NO3 -containing media, respectively. Bacterization of D. nobile seeds resulted in promotion of their in vitro germination. The plant showed no selective specificity to the tested strains. K. oxytoca demonstrated more intense colonization activity and more efficient growth promoting impact under tryptophan supplementation, while P. fluorescens revealed its growth promoting capacity without tryptophan.

CONCLUSIONS: Both strategies are regarded as complementary, improving adaptive potentials of the orchid when different microbial populations colonize the plant.

SIGNIFICANCE AND IMPACT OF STUDY: This study enlarges our knowledge on orchid-microbial interactions, and provides new features on application of the non-orchid PGPR in orchid seed germination and conservation. This article is protected by copyright. All rights reserved.