The Diverse Assemblage of Fungal Endophytes from Orchids in Madagascar Linked to Abiotic Factors and Seasonality

Abstract

The inselbergs of the Central Highlands of Madagascar are one of many ‘micro-hotspots’ of biodiversity on the island, particularly for Orchidaceae. In this region are several genera that have a large number of endemic species that are in serious decline or edging towards extinction. Studies relating to diversity of orchids and their fungal partners (both mycorrhizal and non-mycorrhizal root associates) deserve more attention, as climate change and human induced decline in resilience of species in the wild is at an all-time high. Identification of mycorrhizal fungi (MF) via conventional seed baited-protocorms has limitations for large scale studies and its application for time-bound conservation projects. The paper describes the value of understanding fungal diversity in the roots of orchids at different stages of maturity. The first part of the study was a preliminary investigation mainly to identify culturable Rhizoctonia endophytes, and the second part looked at all life forms of available taxa together with associated soil characteristics. We isolated and identified 19 putative MF from 18 of the 50 taxa spread over an area of 250 sq. km, covering three life forms, growth phases of the orchid taxa, and habitat types. In the rest of the taxa, we were unable to detect any putative MF, but had varying numbers of non-mycorrhizal endophytes. We also found that diversity of putative MF was higher in plants from soils with the lowest P levels recorded. Putative mycorrhizal OTUs were predominantly from the Tulasnella lineage, followed by Ceratobasidium and Serendipita. Within a small subset of samples, a difference in colonised endophytes depending on the collection season was observed. In vitro germination studies using 10 OTUs of mycorrhizal fungi in 14 orchid species showed mostly generalist associations. When orchid seed and fungal sources were studied irrespective of habitat, life form, and distance from each other (orchid seed and fungal source), compatibility for symbiotic seed germination was observed in most cases. Issues with the identification of compatible MF and symbiotic system of seed germination are discussed.

Fungal Diversity of Selected Habitat Specific Cynorkis Species (Orchidaceae) in the Central Highlands of Madagascar

“…Therefore, for the purpose of understanding specific requirements for reintroduction, targeted environmental analysis is required. Previous studies indicate that low P content may be linked to higher mycorrhizal diversity in Cephalanthera rubra [40] and several orchids from Madagascar [11]. Likewise, Van der Ent et al [41] found that the rhizosphere soil concentrations of P and K was low in soil in the study site where Paphiopedilum rothschildianum was growing.…”

“…About 10% of all angiosperm species are orchids which are associated with roughly 25,000 Basidiomycota and some Ascomycota as fungal partners [36,37]. These associations are guided by soil characteristics, seasonality, and life forms of the orchids [9][10][11].…”

“…These biotic factors therefore transcend all parts of the plant throughout every phase of its life. The orchid, and the organisms they rely on, are also intimately tied to the abiotic factors present in their surroundings such as moisture [5], soil pH [6], sunlight [7], and nutrients [8], all of which may vary considerably along the terrain in each habitat [9][10][11]. As a result, orchids are widespread but limited in where they grow, and they are acutely sensitive to subtle environmental changes once they become established from seed.…”

Preventing Extinction of the Critically Endangered Dactylorhiza incarnata subsp. ochroleuca in Britain Using Symbiotic Seedlings for Reintroduction

“…As this site is hosting a diverse range of orchids seed-germination-compatible Tulasnella OTU from southern marsh orchid (from a chalky wet habitat) was tested among many other isolates of putative mycorrhizal fungi for yellow early marsh. The lineage of Tulasnellaceae, a common mycorrhizal fungus in orchids shows generalist behavior as reported in many Dactylorhiza and other terrestrial orchids [ 28 , 29 , 30 , 31 , 32 ].…”

“…The current study used fungi available from plants at different growth phases and found that Tulsanella OTU from mature phase D. praetermissa was a germination-compatible fungus. This has been proved as a successful approach in identifying putative mycorrhizal fungi in orchids from Madagascar [ 17 , 24 , 31 , 32 , 39 ]. Putative mycorrhizal fungus isolate from seedlings was more efficient at supporting seed germination compared to those isolated from adult roots in Dendrobium exile [ 38 , 40 ] suggested that the fungi used in conservation must be limited to mycorrhizal fungi acquired from the same or nearby populations.…”