# New Insights into the Symbiotic Relationship between Orchids and Fungi
## 3 Nutrient Exchange between Orchids and Mycorrhizal Fungi  
Mycorrhizal symbioses are important for the growth and survival of many plant species and are responsible for up to 80% of nitrogen (N) and phosphorus (P) uptake by plants [23]. In general, a mutualistic relationship for nutrient exchanges is established between plants and mycorrhizal fungi in that plants provide photosynthetically fixed C to the symbiotic fungi and they benefit from fungi with regard to absorption of mineral nutrients, such as N and P [24; 25; 26]. However, recent evidence suggests that not only carbohydrates but also lipids can be transferred from host plants to fungi as C resources [27; 28]. The mechanisms of how ectomycorrhizal and arbuscular mycorrhizal fungi take up and transfer mineral nutrients to their host plants for exchanging C resources are extensively studied; however, much less attention has been paid to ericoid and orchid mycorrhizal fungi [3; 23; 29]. Several detailed reviews on the mutualistic interactions between plants and fungi that form ectomycorrhizal or arbuscular mycorrhizal symbiosis have been published [3; 23; 24; 25; 26; 30]. Here, only the current understanding of mechanisms of nutrient exchange in orchid mycorrhizals is summarized. Compared to the other types of mycorrhizals, the orchid-fungus symbiosis is considered to be unique since the nutrient movement is not bi-directional in the early developmental stages and even in adults of some orchid species [29]. As the orchid seeds are very tiny with very little storage, colonization by fungi is required for their germination and/or early protocorm development [15]. Therefore, the mycorrhizal fungi were thought to provide both mineral nutrients and C to the non-photosynthetic orchid seedlings and to receive no reward [29]. However, similar to all other mycorrhizal symbioses, the mycorrhizal fungi receive C as a reward for exchanging mineral nutrients after orchids develop green leaves for photosynthesis [31]. Although the majority of orchids are autotrophic with photosynthetic ability, more than 100 species are myco-heterotrophic which are completely achorophyllous and nutritionally dependent on their symbiotic fungi throughout the whole life cycle [15; 32]. In addition, mixotrophy, also called partial myco-heterotrophy, which consists of both nutritional strategies of autotrophy and myco-heterotrophy, has been reported [33; 34]. Mixotrophic orchids, meeting their C demand via mycorrhizal fungi even when they are photosynthetic at the adult stage, may be the evolutionary intermediates between achorophyllous and autotrophic orchids [15; 31]. This partial myco-heterotrophy may enable orchids to adapt to the shaded environment of forests and the conditions when the photosynthetic abilities are reduced [35; 36]. The well-known non-mutualistic interaction between non-photosynthetic orchids and their symbiotic fungi was challenged by a recent study using mycorrhizals formed between _Serapias vomeracea_ (orchid) and _Tulasnella calospora_ (fungus) to investigate expression of orchid and fungal N transport and assimilation genes [29; 37]. It was demonstrated that the fungal low-affinity ammonium transporter gene, _TcAMT2_, is significantly upregulated in symbiosis and the transcripts corresponding to _TcAMT2_ are detected in the orchid protocorm cells containing fungal pelotons by laser microdissection analysis. In addition, _TcGS1_ and _TcGOGAT_, the fungal genes encoding glutamine synthetase and glutamate synthase in the N assimilation pathway, are also up-regulated in mycorrhizal protocorms. A commentary of this study by [PERSON] and [PERSON] (2017) concluded that the fungus, _T. calospora_, may receive ammonium from its orchid host, _S. vomeracea_, during seed germination and protocorm development. Therefore, a true mutualism may be also present in the initially mycheterotrophic stage of autotrophic orchids or myco-heterotrophic orchids [29].