Institut de Systématique, Évolution, Biodiversité (UMR 7205 - CNRS, MNHN, UPMC, EPHE), Muséum national d'Histoire naturelle, Sorbonne Universités, 57 rue Cuvier, 75005 Paris, France.
Department of Plant Taxonomy and Nature Conservation, University of Gdańsk, Wita Stwosza 59, 80-308, Gdańsk, Poland.
Ann Bot. 2022 Feb 11;129(3):259-270. doi: 10.1093/aob/mcab134.
As in most land plants, the roots of orchids (Orchidaceae) associate with soil fungi. Recent studies have highlighted the diversity of the fungal partners involved, mostly within Basidiomycotas. The association with a polyphyletic group of fungi collectively called rhizoctonias (Ceratobasidiaceae, Tulasnellaceae and Serendipitaceae) is the most frequent. Yet, several orchid species target other fungal taxa that differ from rhizoctonias by their phylogenetic position and/or ecological traits related to their nutrition out of the orchid roots (e.g. soil saprobic or ectomycorrhizal fungi). We offer an evolutionary framework for these symbiotic associations.
Our view is based on the 'Waiting Room Hypothesis', an evolutionary scenario stating that mycorrhizal fungi of land flora were recruited from ancestors that initially colonized roots as endophytes. Endophytes biotrophically colonize tissues in a diffuse way, contrasting with mycorrhizae by the absence of morphological differentiation and of contribution to the plant's nutrition. The association with rhizoctonias is probably the ancestral symbiosis that persists in most extant orchids, while during orchid evolution numerous secondary transitions occurred to other fungal taxa. We suggest that both the rhizoctonia partners and the secondarily acquired ones are from fungal taxa that have broad endophytic ability, as exemplified in non-orchid roots. We review evidence that endophytism in non-orchid plants is the current ecology of many rhizoctonias, which suggests that their ancestors may have been endophytic in orchid ancestors. This also applies to the non-rhizoctonia fungi that were secondarily recruited by several orchid lineages as mycorrhizal partners. Indeed, from our review of the published literature, they are often detected, probably as endophytes, in extant rhizoctonia-associated orchids.
The orchid family offers one of the best documented examples of the 'Waiting Room Hypothesis': their mycorrhizal symbioses support the idea that extant mycorrhizal fungi have been recruited among endophytic fungi that colonized orchid ancestors.
与大多数陆生植物一样，兰花（Orchidaceae）的根系与土壤真菌共生。最近的研究强调了参与共生的真菌多样性，这些真菌主要属于担子菌门。与一种被称为根结菌（Ceratobasidiaceae、Tulasnellaceae 和 Serendipitaceae）的多系真菌的共生关系最为普遍。然而，一些兰花物种与其他真菌类群形成共生关系，这些真菌与根结菌在系统发育位置和/或与兰花根系外营养相关的生态特征上存在差异（例如土壤腐生或外生菌根真菌）。我们为这些共生关系提供了一个进化框架。
我们的观点基于“候诊室假说”，这是一种进化情景，表明陆地植物的菌根真菌是从最初作为内生菌定植在根部的祖先中招募而来的。内生菌以弥散方式生物定植组织，与菌根不同的是，内生菌没有形态分化，也不为植物的营养做出贡献。与根结菌的共生关系可能是现存大多数兰花中保留的原始共生关系，而在兰花进化过程中，发生了许多向其他真菌类群的次级转变。我们认为，根结菌的共生伙伴和次级获得的共生伙伴都来自具有广泛内生能力的真菌类群，正如非兰花植物根系中所例证的那样。我们回顾了内生菌在非兰花植物中的证据，这些内生菌是许多根结菌的当前生态学，这表明它们的祖先可能是兰花祖先的内生菌。这也适用于被几个兰花谱系作为菌根共生伙伴次级招募的非根结菌真菌。事实上，从我们对已发表文献的综述中可以看出，它们经常被检测到，可能作为内生菌存在于现存与根结菌相关的兰花中。
兰花科为“候诊室假说”提供了一个最好的例证之一：它们的菌根共生关系支持了现存菌根真菌是从定植在兰花祖先中的内生真菌中招募而来的观点。
