Fungal specificity bottlenecks during orchid germination and development

Abstract

Fungus-subsidized growth through the seedling stage is the most critical feature of the life history for the thousands of mycorrhizal plant species that propagate by means of 'dust seeds.' We investigated the extent of specificity towards fungi shown by orchids in the genera Cephalanthera and Epipactis at three stages of their life cycle: (i) initiation of germination, (ii) during seedling development, and (iii) in the mature photosynthetic plant. It is known that in the mature phase, plants of these genera can be mycorrhizal with a number of fungi that are simultaneously ectomycorrhizal with the roots of neighbouring forest trees. The extent to which earlier developmental stages use the same or a distinctive suite of fungi was unclear. To address this question, a total of 1500 packets containing orchid seeds were buried for up to 3 years in diverse European forest sites which either supported or lacked populations of helleborine orchids. After harvest, the fungi associated with the three developmental stages, and with tree roots, were identified via cultivation-independent molecular methods. While our results show that most fungal symbionts are ectomycorrhizal, differences were observed between orchids in the representation of fungi at the three life stages. In Cephalanthera damasonium and C. longifolia, the fungi detected in seedlings were only a subset of the wider range seen in germinating seeds and mature plants. In Epipactis atrorubens, the fungi detected were similar at all three life stages, but different fungal lineages produced a difference in seedling germination performance. Our results demonstrate that there can be a narrow checkpoint for mycorrhizal range during seedling growth relative to the more promiscuous germination and mature stages of these plants' life cycle.

Enigmatic Sebacinales

“…They found multiple mycorrhizal strains of one species close to S. vermifera s.l., and suggested longer-term growth tests to obtain sufficient information on successive interactive functions. A comparative study of germination, seedling development, and mature plants in Cephalanthera and Epipactis species together with neighboring ectomycorrhizal trees and their simultaneously specific ECM mycobionts, including Sebacinales, in various forests of England, France, Germany, and Finland, has been carried out by Bidartondo and Read (2008) and Tesitelova et al (2012). One general result of these two studies was that fungal specificity is a bottleneck during germination and development in Cephalanthera but not in Epipactis, which did not show change of its symbionts during development.…”

Interactions with mycorrhizal fungi in two closely related hybridizing orchid species

“…The level of specificity between fungus and orchid is an important factor determining chances of successful seedling establishment (BIDARTONDO and READ 2008). Early molecular studies have revealed that terrestrial orchids interact with a wide range of mycorrhizal fungi.…”

Analysis of fungal diversity in Orchis tridentata Scopoli

“…Although mycorrhizae are considered essential for orchid biology, little is known about the diversity of mycorrhizal fungi associated with orchids in nature, especially as far as photosynthetic terrestrial species are concerned [12,13].…”