# New Insights into the Symbiotic Relationship between Orchids and Fungi
## 3 Nutrient Exchange between Orchids and Mycorrhizal Fungi
The finding that orchid seeds and protocorms may also pay for exchanging nutrients from their fungal partners could explain how a compatible mycorrhizal fungus recognizes and subsequently colonizes the seeds or protocorms of the host orchid in the soil. Although the orchid seeds have little storage, the mature embryo cells contain protein bodies, a protein storage organelle, which could potentially produce ammonium through a proper catalysis process after seed imbibition [38; 39]. The ammonium produced from protein bodies may be used to attract fungi for colonization [29; 37]. Growth of _T. calospora_ on the media with different N sources indicated the fungus prefers utilizing ammonium and organic N including glutamic acid and glutamine. Poor growth on the nitrate-containing medium is consistent with the molecular evidence that the genome of _T. calospora_ does not have the genes encoding nitrate uptake and assimilation. In addition, the research conducted by [PERSON] et al. (2017) also suggests that _S. romeracea_ may receive organic N from _T. calospora_ because the fungal arginase and urease genes involved in amino acid breakdown are not significantly induced or lowly expressed, but the fungal _Argininosuccinate lyase_ gene for arginine biosynthesis and the orchid amino acid transporter genes are significantly upregulated in symbiotic protocorms [29; 37]. This finding is consistent with the previous study that _Ceratobosidium cornigerum_ transfers amino acids to its orchid hosts [40]. However, [PERSON] et al. (2014) found that orchids obtain ammonium from mycorrhizal fungi by using isotope tracing experiments [41]. According to the above research, a novel model could be proposed that non-photosynthetic and photosynthetic orchids export ammonium and sugars, respectively, to their fungal partners for exchanging the required nutrients (Figure 1). Furthermore, orchids may be able to receive both ammonium and amino acids as N sources from mycorrhizal fungi. The acquisition and movement of nutrients between orchids and their fungal partners could be more complex than the above single-directional or mutualistic mechanisms. In nature, many fully myo-heterotrophic orchids or orchids that have a myo-heterotrophic juvenile phase have been found to grow exclusively with specific autotrophic trees [42]. Therefore, it has been speculated that these orchids may obtain C from their symbiotic fungi which can also form ectomycorrhizas with those trees [43]. [PERSON] (1970) firstly found that the rhizomes of three _Corallorhiza_ orchids are often very close to tree roots, and the hyphae connecting the tree roots and the _Corallorhiza_ orchids are structures typical of many ectomycorrhizal basidiomycetes [44]. The fungi associated with both _Corallorhiza_ orchids and their adjacent trees were indeed confirmed as the ectomycorrhizal fungi [45; 46]. By autoradiographical and tissue oxidation analyses, [PERSON] et al. (2000) demonstrated that  
Figure 1: The possible model of nutrient exchange in orchid mycorrhizas. In natural conditions, orchid seeds form symbiosis with orchid mycorrhizal fungi to receive nutrients for germination and further protocorm development. The hyphae of orchid mycorrhizal fungi can enter orchid cells and form coiled complexes called pelotons. At nonphotosynthetic stages, orchid cells export ammonium (NH4+) to their fungal partners and receive nitrogen (N), phosphorus (P) and carbon (C) for germination. In addition, the senescent pelotons can also release N, P and C to orchid cells when hyphal coils are digested. After orchids develop green leaves, mycorrhizal fungi can obtain photosynthetically fixed C as a reward. However, photosynthetic orchids may not further receive N from fungal cells and only P is exchanged to orchid cells [29]. \({}^{14}\)C can move from the autotrophic shrubs (_Betula pendula_ or _Salix repens_) to the mycheterotrophic orchid (_C. trifida_) through the mycorrhizal fungus (_Ceratobasidium cornigerum_) [42; 43].