# New Insights into the Symbiotic Relationship between Orchids and Fungi
## 5 Conclusions and Future Perspectives  
Under natural conditions, orchid seeds cannot germinate in the absence of symbiosis with mycorrhizal fungi. It has been long believed that orchids receive not only mineral nutrients, such as N and P, but also organic C from orchid mycorrhizal fungi without rewards at the germination and the early stages in protocorm development in autotrophic orchids and the entire life cycle in myco-heterotrophic orchids. The mutualistic interaction between orchids and fungi has been focused on the flow of C without considering other potential benefits required for fungi, such as offering of vitamins and other nutrients or protection of fungal hyphae in orchid roots [71]. The recent molecular evidence in the _T. calospora-S. vomeracea_ symbiosis implies that orchids in non-photosynthetic stages may export ammonium produced from protein bodies in the embryo cells to attract mycorrhizal fungi for symbiosis. In turn, the symbiotic fungi supply N, P and C to the host orchids. However, in the case of adult green orchids, the host plants export photosynthetically fixed C to the fungal partners for exchanging mineral nutrients. Therefore, the nutrient flow in orchids could also represent mutualism as in other types of mycorrhizas. This means the tiny orchid seeds without endosperm still have to pay in order to get nutrients in return. In addition, the partially myco-heterotrophic orchids (mixotrophs) have been shown to keep receiving C from mycorrhizal fungi even after initiation of photosynthesis and their distribution may be much wider among orchids than presumed previously [72]. Several orchid and fungal importer genes have been identified; however, how fungi export the nutrients to their host orchids is still not clear. Continued study of nutrient transport system in orchid mycorrhizas would help to understand the establishment and maintenance of the interaction and assist orchid conservation in the future. Furthermore, the efforts may also contribute to better understand other types of mycorrhizas due to the easier manipulation of in vitro study of orchid mycorrhizas under axenic condition [29].  
Recent advances in NGS technologies and functional genomics methodologies provide an opportunity for a comprehensive understanding of orchid biology and offer new insights into the symbiotic relationship between orchids and orchid mycorrhizal fungi. However, the orchid-fungus symbiosis appears to be much more complex than previously thought, leading to difficulties in unravelling the mechanisms. For example, wild orchids often form symbioses with multiple fungi species within a single root [57] and the tripartite relationship among orchids, mycorrhizal fungi and the adjacent trees for nutrient exchange has been proved to be common in nature. Therefore, the future research directions in orchid mycorrhizas should consider how these three kinds of organisms interact with each other and the environment.  
Conceptualization, C.-M.Y. and W.-C.T.; literature search, C.-M.Y., K.C. and C.-K.L.; writing-original draft, C.-M.Y., K.C. and C.-K.L.; writing-review & editing, C.-M.Y., K.C., C.-K.L. and W.-C.T.; supervision, C.-M.Y. and W.-C.T.; funding acquisition, W.-C.T. This research was funded by Ministry of Science and Technology, Taiwan, grant number 107-2313-B-006-002-MY3. The authors declare no conflict of interest.