Orchids and their mycorrhizal fungi: an insufficiently explored relationship

Orchids are associated with diverse fungal taxa, including nonmycorrhizal endophytic fungi as well as mycorrhizal fungi. The orchid mycorrhizal (OM) symbiosis is an excellent model for investigating the biological interactions between plants and fungi due to their high dependency on these symbionts...

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Bibliographic Details
Published in:Mycorrhiza 2020, Vol.30 (1), p.5-22
Main Authors: Favre-Godal, Quentin, Gourguillon, Lorène, Lordel-Madeleine, Sonia, Gindro, Katia, Choisy, Patrick
Format: Article
Language:English
Subjects:
Agriculture
Analytical chemistry
Arbuscular mycorrhizas
Bioactive compounds
Biomedical and Life Sciences
Chemical Sciences
Colonization
Crosstalk
Ecological function
Ecology
Endophytes
Flavonoids
Forestry
Fungi
Genomics
Life Sciences
Metabolites
Metabolomics
Microbiology
Molecular interactions
Myc protein
Mycorrhizae
Orchidaceae
Phytohormones
Plant Sciences
Proteomics
Review
Secondary metabolites
Symbionts
Symbiosis
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Summary:Orchids are associated with diverse fungal taxa, including nonmycorrhizal endophytic fungi as well as mycorrhizal fungi. The orchid mycorrhizal (OM) symbiosis is an excellent model for investigating the biological interactions between plants and fungi due to their high dependency on these symbionts for growth and survival. To capture the complexity of OM interactions, significant genomic, numerous transcriptomic, and proteomic studies have been performed, unraveling partly the role of each partner. On the other hand, several papers studied the bioactive metabolites from each partner but rarely interpreted their significance in this symbiotic relationship. In this review, we focus from a biochemical viewpoint on the OM dynamics and its molecular interactions. The ecological functions of OM in plant development and stress resistance are described first, summarizing recent literature. Secondly, because only few studies have specifically looked on OM molecular interactions, the signaling pathways and compounds allowing the establishment/maintenance of mycorrhizal association involved in arbuscular mycorrhiza (AM) are discussed in parallel with OM. Based on mechanistic similarities between OM and AM, and recent findings on orchids’ endophytes, a putative model representing the different molecular strategies that OM fungi might employ to establish this association is proposed. It is hypothesized here that (i) orchids would excrete plant molecule signals such as strigolactones and flavonoids but also other secondary metabolites; (ii) in response, OM fungi would secrete mycorrhizal factors (Myc factors) or similar compounds to activate the common symbiosis genes (CSGs); (iii) overcome the defense mechanism by evasion of the pathogen-associated molecular patterns (PAMPs)-triggered immunity and by secretion of effectors such as small inhibitor proteins; and (iv) finally, secrete phytohormones to help the colonization or disrupt the crosstalk of plant defense phytohormones. To challenge this putative model, targeted and untargeted metabolomics studies with special attention to each partner’s contribution are finally encouraged and some technical approaches are proposed.
ISSN:0940-6360
1432-1890
DOI:10.1007/s00572-020-00934-2