Non-specific symbiotic germination of Cynorkis purpurea (Thouars) Kraezl., a habitat-specific terrestrial orchid from the Central Highlands of Madagascar

Abstract

Orchids, particularly terrestrial taxa, rely mostly on basidiomycete fungi in the Cantharellales and Sebacinales that trigger the process of seed germination and/or initiate the full development of the seedling. During the course of development, orchids may associate with the same fungus, or they may enlist other types of fungi for their developmental needs leading to resilience in a natural setting. This study examined in vitro seed germination and seedling developmental behavior of Cynorkis purpurea, a terrestrial orchid from the Central Highlands of Madagascar. This species is mostly restricted to gallery forests in the Itremo Massif, in moist substrate between rocks bordering streams. The main objective was to understand the influence of diverse mycorrhizal fungi on seed germination and further development of C. purpurea. The study aims to compare symbiotic versus asymbiotic germination and seedling development with seeds and fungi collected from a 13-km(2) area in the Itremo region. Seeds collected from the wild were sown with diverse orchid mycorrhizal fungi (OMF) spanning 12 operational taxonomic units (OTUs) in three genera (Tulasnella, Ceratobasidium, and Sebacina) acquired from different habitats. Treatments were assessed in terms of the percentage of germinated seeds and fully developed seedlings against those in asymbiotic control media treatments. Overall, OMF significantly improved seedling development within the 12-week experiment period. Sebacina as a genus was the most effective at promoting seedling development of C. purpurea, as well as having the ability to enter into successful symbiotic relationships with orchids of different life forms; this new knowledge may be especially useful for orchid conservation practiced in tropical areas like Madagascar. A Sebacina isolate from an epiphytic seedling of Polystachya concreta was the most effective at inducing rapid seedling development and was among the five that outperformed fungi isolated from roots of C. purpurea. C. purpurea was found to be a mycorrhizal generalist, despite its specific habitat preference, highlighting the complex interaction between the plant, fungi, and the environment. The potential impact on conservation strategies of understanding the requirements for orchid seed germination and development by identifying and using OMF from diverse sources is discussed in detail.

Root endophytic fungi promote in vitro seed germination in Pleurothallis coriacardia (Orchidaceae)

“…However, our results suggest that either Phytamax TM is not well suited for P. coriacardia germination in vitro, or the germination of this species is particularly slow. We believe that the nutrient richness of this medium may be inhibitory to germination of P. coriacardia seeds, as previously demonstrated with other orchid species (Rafter et al 2016). We are cautious to suggest a highly specialized relationship with P. coriacardia, despite our results, because we only isolated fungi from adult plants, and as it has been reported, mature plants are not implicated in the symbiotic germination of orchids seeds (Ovando et al 2005).…”

Effect of different mycobionts on symbiotic germination and seedling growth of Dendrobium officinale, an important medicinal orchid

“…4a, b). In orchids, mycorrhizal symbionts may switch in different developmental stages (McCormick et al 2004;Rafter et al 2016). Previous studies have demonstrated that compatible fungi for promoting seed germination may not be able to support subsequent seedling development (Bidartondo and Read 2008;Huynh et al 2009;Rasmussen et al 2015).…”

Co-Cultures of Mycorrhizal Fungi Do Not Increase Germination and Seedling Development in the Epiphytic Orchid Dendrobium nobile

“…Germination success was categorized into three stages and scored as described by (Arditti, 1967) with slight adjustments (Table 1). To avoid the possibility that embryo swelling (stage 1) was the result of water imbibition and not the result of a true association with the fungus (Rafter et al, 2016), only seeds that reached stage 2 or more advanced stages were considered to have truly germinated. Seed germination success was assessed by calculating the number of seeds reaching stages 2 and 3 divided by the number of viable seeds (i.e., protocorm formation) and the number of seeds developing into a seedling divided by the number of viable seeds (i.e., seedling development).…”