As one of the largest families of flowering plants, Orchidaceae is well-known for its high diversity and complex life cycles. Interestingly, such exquisite plants originate from minute seeds, going through challenges to germinate and establish in nature. Alternatively, orchid utilization as an economically important plant gradually decreases its natural population, therefore, driving the need for conservation. As with any conservation attempts, broad knowledge is required, including the species’ interaction with other organisms. All orchids establish mycorrhizal symbiosis with certain lineages of fungi to germinate naturally. Since the whole in situ study is considerably complex, in vitro symbiotic germination study is a promising alternative. It serves as a tool for extensive studies at morphophysiological and molecular levels. In addition, it provides insights before reintroduction into its natural habitat. Here we reviewed how mycorrhiza contributes to orchid lifecycles, methods to conduct in vitro study, and how it can be utilized for conservation needs.

Epiphytic orchids are commonly found in exposed environments, which plausibly lead to different root fungal community structures from terrestrial orchids. Until recently, no studies have been conducted to show the fungal community structure during the growth of a photosynthetic and epiphytic orchid in its natural growing state. In this study, the Vanda falcata (commonly known as Neofinetia falcata ), one of Japan’s ornamental orchids, was used to characterize the fungal community structure at different developmental stages. Amplicon sequencing analysis showed that all development stages contain a similar fungal community: Ascomycota dominates half of the community while one-third of the community is Basidiomycota. Rhizoctonia- like fungi, a term to group the most common basidiomycetous fungi that form orchid mycorrhiza, exist even in a smaller portion (around a quarter) compared to other Basidiomycota members. While ascomycetous fungi exhibit pathogenicity, two Ceratobasidium strains isolated from young and adult plants could initiate seed germination in vitro . It was also found that the colonization of mycorrhizal fungi was concentrated in the lower part of the root where it directly attaches to the phorophyte bark, while ascomycetous fungi were distributed in the velamen but never colonized cortical cells. Additionally, lower root parts attached to the bark have denser exodermal passage cells, and these cells were colonized only by mycorrhizal fungi that further infiltrated the cortical area. Therefore, we propose that physical regulation of fungal entry to partition the ascomycetes and mycorrhizal fungi results in the balanced mycorrhizal symbiosis in this orchid.

Orchids and arbuscular mycorrhiza (AM) plants evolved independently and have different structures and fungal partners, but they both facilitate nutrient uptake. Orchid mycorrhiza (OM) supports orchid seed germination, but unlike AM, its role in disease resistance of mature plants is largely unknown. Here, we examined whether OM induces systemic disease resistance against a necrotrophic pathogen in a similar fashion to AM. We investigated the priming effect of mycorrhizal fungi inoculation on resistance of a terrestrial orchid, Bletilla striata , to soft rot caused by Dickeya fangzhongdai . We found that root colonization by a compatible OM fungus primed B. striata seedlings and induced systemic resistance against the infection. Transcriptome analysis showed that priming was mediated by the downregulation of jasmonate and ethylene pathways and that these pathways are upregulated once infection occurs. Comparison with the reported transcriptome of AM fungus–colonized rice leaves revealed similar mechanisms in B. striata and in rice. These findings highlight a novel aspect of commonality between OM and AM plants in terms of induced systemic resistance. Highlight: Colonization by a compatible mycorrhizal fungus primes induced systemic resistance against a necrotrophic pathogen in a terrestrial orchid, Bletilla striata , by regulating jasmonate and ethylene pathways, similar to arbuscular mycorrhizal plants.