Orchids and their mycorrhizal fungi: an insufficiently explored relationship

Orchids are associated with diverse fungal taxa, including non-mycorrhizal endophytic fungi and mycorrhizas. Orchid mycorrhiza (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 proteomics studies have been performed, unravelling partly the role of each partner. In this review, the orchid and mycorrhizal fungus relationship will be described summarizing the recent published literature on OM with special attention to the nutrient exchange model, the correlation on fitness and distribution of orchid populations, and finally the chemical communication and defense mechanisms. Based on the recent finding on orchids endophytes, OM relationship, arbuscular mycorrhiza (AM) and OM similarities [1], [2], a putative model representing the different strategies that OM fungi might employ to establish this symbiosis is proposed. It is hypothesized here that (i) orchids would excrete signaling molecules such as strigolactones and flavonoids to facilitate the establishment of the symbiosis. In response, (ii) OM fungi would secrete mycorrhizal (Myc) factors to activate the common symbiosis genes, (iii) evade the pathogen associated molecular patterns triggered immunity and secrete effectors to overcome the defense mechanism (iv) and finally secrete phytohormones to help the colonization or disrupt the crosstalk of plant defense phytohormones. To challenge this supposed model, metabolomics studies with special attention to each partner contribution are encouraged and some technical approaches are proposed.

• References

• 1 Miura C, Yamaguchi K, Miyahara R, Yamamoto T, Fuji M, Yagame T. et al. The mycoheterotrophic symbiosis between Orchids and mycorrhizal fungi possesses major components shared with mutualistic plant-mycorrhizal symbioses MPMI. 2018; 31: 1032-1047
• 2 Kohler A, Kuo A, Nagy LG, Morin E, Barry KW, Buscot F. et al. Convergent losses of decay mechanisms and rapid turnover of symbiosis genes in mycorrhizal mutualists Nat Genet. 2015; 47: 410-415