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.

中文翻译：

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兰花及其菌根真菌：探索不足的关系。

兰花与各种真菌类群有关，包括非菌根内生真菌和菌根真菌。兰花菌根（OM）共生是研究植物与真菌之间生物相互作用的出色模型，因为它们高度依赖于这些共生体的生长和存活。为了捕获OM相互作用的复杂性，已经进行了重要的基因组研究，大量的转录组学和蛋白质组学研究，部分阐明了每个伴侣的作用。另一方面，几篇论文研究了每个伴侣的生物活性代谢物，但很少解释它们在这种共生关系中的重要性。在这篇综述中，我们从生化角度着眼于OM动力学及其分子相互作用。首先概述了OM在植物发育和抗逆性方面的生态功能，并总结了最近的文献。其次，由于只有很少的研究专门针对OM分子相互作用，因此与OM并行讨论了允许建立/维持与丛枝菌根（AM）有关的菌根缔合的信号传导途径和化合物。基于OM和AM之间的机理相似性，以及对兰花内生菌的最新发现，提出了一种代表模型，该模型代表了OM真菌可能采用的不同分子策略来建立这种联系。在此假设：（i）兰花会分泌植物分子信号，例如内酯内酯和类黄酮以及其他次生代谢产物；（ii）作为回应，OM真菌会分泌菌根因子（Myc因子）或类似的化合物来激活常见的共生基因（CSG）。（iii）通过逃避病原体相关分子模式（PAMPs）触发的免疫力和效应子（例如小抑制剂蛋白）的分泌来克服防御机制；（iv）最后，分泌植物激素以帮助定植或破坏植物防御植物激素的串扰。为了挑战这种推定的模型，最后鼓励特别关注每个伙伴贡献的靶向和非靶向代谢组学研究，并提出一些技术方法。（iii）通过逃避病原体相关分子模式（PAMPs）触发的免疫力和效应子（例如小抑制剂蛋白）的分泌来克服防御机制；（iv）最后，分泌植物激素以帮助定植或破坏植物防御植物激素的串扰。为了挑战这种推定的模型，最后鼓励特别关注每个伙伴贡献的靶向和非靶向代谢组学研究，并提出一些技术方法。（iii）通过逃避病原体相关分子模式（PAMPs）触发的免疫力和效应子（例如小抑制剂蛋白）的分泌来克服防御机制；（iv）最后，分泌植物激素以帮助定植或破坏植物防御植物激素的串扰。为了挑战这种推定的模型，最后鼓励特别关注每个伙伴贡献的靶向和非靶向代谢组学研究，并提出一些技术方法。