Mycorrhizal diversity and community composition in co-occurring Cypripedium species

Orchids commonly rely on mycorrhizal fungi to obtain the necessary resources for seed germination and growth. Whereas most photosynthetic orchids typically associate with so-called rhizoctonia fungi to complete their life cycle, there is increasing evidence that other fungi may be involved as well and that the mycorrhizal communities associated with orchids may be more diverse. Coexisting orchid species also tend to associate with different fungi to reduce competition for similar resources and to increase long-term population viability. However, few studies have related the mycorrhizal communities in the rhizosphere to communities found in the roots of closely related coexisting orchid species. In this study, we used high-throughput sequencing to investigate the diversity and community composition of orchid mycorrhizal fungi in the roots and the rhizosphere of four Cypripedium species growing in forests in Northeast China. The results showed that the investigated Cypripedium species associated with a wide variety of fungi including members of Tulasnellaceae, Psathyrellaceae, and Herpotrichiellaceae, whereas members of Russulaceae, Cortinariaceae, Thelephoraceae, and Herpotrichiellaceae showed high abundance in rhizosphere soils. The diversity of fungi detected in the rhizosphere soil was much higher than that in the roots. The observed variation in fungal communities in Cypripedium roots was not related to forest site or orchid species. On the other hand, variation in mycorrhizal communities of rhizosphere soil was significantly related to sampling site. These results indicate that orchid mycorrhizal communities in the rhizosphere display considerable variation among sites and that orchids use only a subset of the locally available fungi. Future studies focusing on the fine-scale spatial distribution of orchid mycorrhizal fungi and more detailed assessments of local environmental conditions will provide novel insights into the mechanisms explaining variation of fungal communities in both orchid roots and the rhizosphere.