Abstract
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 calospora in 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.
Highlights
Plant-bacteria interactions have been studied for several decades. Hiltner (1904) was the first to observe that microorganisms were more abundant in the soil surrounding the roots than in the bulk soil far from the root
We investigated the influence of endophytic bacteria on the growth of the orchid mycorrhizal fungus Tulasnella calospora (Boud.) Juel, in order to understand possible interactions between bacterial and fungal orchid endophytes
Bacterial endophytes were isolated from all organs only for S. vomeracea
Summary
Plant-bacteria interactions have been studied for several decades. Hiltner (1904) was the first to observe that microorganisms were more abundant in the soil surrounding the roots (the rhizosphere) than in the bulk soil far from the root. Subsequent investigations have clearly demonstrated that several plant-associated bacteria positively influence plant health and growth (Hardoim et al 2008; Liu et al 2017). Plants can select beneficial bacteria, including those living within their tissues as endophytes (Marasco et al 2012; Rashid et al 2012; Agnolucci et al 2019). Beneficial plant-associated bacteria, including endophytic ones, are known as Plant Growth-Promoting Bacteria (PGPB). PGPB can affect plant growth through direct as well as indirect mechanisms (Gamalero and Glick 2011; Glick 2015). For example, the ability of PGPB to reduce the deleterious effects of plant pathogens by producing antibiotics, lytic enzymes and siderophores, or by strengthening the plant defense responses by triggering induced systemic resistance (Thomashow et al 1990; Arora et al 2001; Whipps 2001)
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