Orchid mycorrhizal fungus surveys are widespread in orchid species, but little is known about orchid-associated bacteria (OAB), although tripartite associations between plants, fungus and bacteria have been reported to play an important role in enhancing fitness of plant that utilize arbuscular and ectomycorrhizal associations. In this study, we asked questions to determine whether congeneric orchids exhibit species-specific recruitment of bacterial communities and whether it is linked to their respective population size, phenological stages or habitat soil. In this multi-year, range-wide study, we examined large and small populations of two rare orchids, Platanthera cooperi (S. Watson) R. M. Bateman from California Floristic Province and Platanthera praeclara Sheviak and Bowles from North American tallgrass prairie. We collected roots from seedlings, vegetative and reproductive individuals of each species. Simultaneously, we collected habitat soil. The bacterial communities in root and soil were assessed by amplifying the V4 and V5 hypervariable regions of the 16S ribosomal DNA.
Results/Conclusions
We detected a differential recruitment of bacterial communities within roots of two orchid species (P < 0.05), where P. cooperi recruited higher abundances of Burkholderiaceae and Pseudomonadaceae while P. praeclara showed dominance of Bacillales [incertae sedis] and Halomonadaceae. Across two species, the recruitment differences were also linked to their population size (P < 0.05) and phenological stages (P < 0.05). Large populations of P. cooperi and P. praeclara showed dominance of Burkholderiaceae and Pseudomonadaceae, respectively when compared to small populations. Among the three phenological stages, reproductive individuals of P. cooperi and P. praeclara showed also dominance of Burkholderiaceae and Pseudomonadaceae, respectively when compared to seedling or vegetative stages. We found no influence of soil bacterial communities on the recruitment differences between two species. In conclusion, the species-specific bacterial recruitment observed in our study, and its subsequent variation among host population size and phenological stages, suggest the important role of bacterial communities in the niche space of orchid species.