Orchid Root Associated Bacteria: Linchpins or Accessories?

Abstract

Besides the plant-fungus symbiosis in arbuscular mycorrhizal (AM) and ectomycorrhizal (EM) plants, many endorhizal and rhizosphere bacteria (Root Associated Bacteria, or RAB) also enhance plant fitness, diversity, and coexistence among plants via bi- or tripartite interactions with plant hosts and mycorrhizal fungi. Assuming that bacterial associations are just as important for the obligate mycorrhizal plant family Orchidaceae, surprisingly little is known about the RAB associated with orchids. Herein, we first present the current, underwhelming state of RAB research including their interactions with fungi and the influence of holobionts on plant fitness. We then delineate the need for novel investigations specifically in orchid RAB ecology, and sketch out questions and hypotheses which, when addressed, will advance plant-microbial ecology. We specifically discuss the potential effects of beneficial RAB on orchids as: (1) Plant Growth Promoting Rhizobacteria (PGPR), (2) Mycorrhization Helper Bacteria (MHB), and (3) constituents of an orchid holobiont. We further posit that a hologenomic view should be considered as a framework for addressing co-evolution of the plant host, their obligate Orchid Mycorrhizal Fungi (OMF), and orchid RAB. We conclude by discussing implications of the suggested research for conservation of orchids, their microbial partners, and their collective habitats.

Auxin biosynthesis by Microbacterium testaceum Y411 associated with orchid aerial roots and their efficacy in micropropagation

“…The orchid root-associated microbial community is determined by its species or root type, where the variation may be attributed to the composition of root exudates ( Kaur and Sharma, 2021 ). Plant exudates also provide tryptophan, the primary precursor in microbial IAA production ( Kravchenko et al., 2004 ; Raut et al., 2017 ).…”

The effect of global warming on the Australian endemic orchid Cryptostylis leptochila and its pollinator

“…leptochila . Orchids have numerous symbiotic relationships with endophytic fungi [ 114 , 115 ] and bacteria [ 116 ]. The interaction with a mycorrhizal fungus, which provides essential nutrients for seedling development is vital for orchids [ 117 ] and the seedling recruitment success is strongly associated with the availability of a suitable fungal strain.…”

Traditional, Therapeutic Uses and Phytochemistry of Terrestrial European Orchids and Implications for Conservation

“…However, apart for the latter which specifies that local native orchids are propagated with the aim to help dwindling populations, other companies do not indicate the provenience of seed material. This sets potential problems of genetic pollution, increasing hybridization with local orchid species ([ 277 ] and references therein), and also the introduction of allochthonous microorganisms used in the potting mix, in particular bacteria and fungi, which could compete with the endophytes found in native populations, i.e., [ 278 , 279 , 280 ]. Given that recent studies seem to suggest that orchids can act as reservoir of orchid mycorrhizal fungi [ 281 ], conservation efforts would not only be pivotal for orchid populations persistence but also for the hosted microorganisms, which deserve more scientific attention as potential sources of active signalling molecules of ecological, agronomical, and industrial interest [ 263 ].…”