# New Insights into the Symbiotic Relationship between Orchids and Fungi
## 2 Orchid Mycorrhiza and Its Role in Orchid Seed Germination  
The early stages of orchid reproduction and development, especially the germination of orchid seeds, are highly influenced by the relationship with their appropriate mycorrhizal fungi as well as their surrounding environments. Like dust particles, orchid seeds are incredibly tiny, ranging in weigh from 0.3 to 14 ug (micrograms), and each seed capsule may contain 1300 to 4 million seeds that are adapted for wind dispersal [12,13]. These characteristics are correlated to their reduced size of embryo and the absence of endosperm which causes difficulty in germination. Therefore, various attempts have been made to initiate the germination of orchid seeds. Currently, the symbiotic germination using mycorrhizal fungi is still considered for inducing a higher germination rate even though other methods by modulating the factors for germination including nutrients, photoperiod, temperature and plant hormone need to be combined [14]. The most important contribution of mycorrhizal fungi to orchids is supplying nutrients for growth and development. Orchid seeds under natural conditions are unable to germinate without a supply of C, mineral nutrients as well as vitamins from their symbiotic partners. Additionally, mycorrhizal fungi also contribute to supplying and retaining water for orchid species during germination. This is also a highly essential role of mycorrhizal fungi because orchid seeds commonly have a hydrophobic testa to avoid imbibition in unfavorable environments [12,14,15,16].  
Under the co-existence with mycorrhizal fungi, orchid seeds can germinate and develop into unique seedling structures called protocorms that consist of parenchyma cells [17]. Then, further development of the protocorm into plant is also supported by their mycorrhizal fungi. In protocorms of orchids, the entered fungal hyphae form coiled complexes called pelotons. Orchids can obtain nutrients by digesting them [18]. It is likely that the site of hyphal entry can be the determinant factor for the success in symbiotic associations [17]. Although many fungal hyphae prefer suspensors for their entry, it has been shown that only the fungal hyphae which enter through rhizoids and not suspensors can establish successful mycorrhizal association in _Dactyloriza majalis_[19]. Therefore, the presence or absence of suspensors during orchid development and the preferred entry sites of mycorrhizal fungi might be the determinants of the specificity between orchids and their fungi.  
Many environmental factors including abiotic factors, such as light, soil structure, moisture, and biotic factors, such as microorganisms, are required to be considered for the successful germination of orchid seeds and further development. These factors affect not only orchid seeds but also the success in symbiotic association between orchids and mycorrhizal fungi [12,16,20]. For example, it has been shown that organ development of symbiotic orchid seedlings and development of photosynthetic structures are affected by light intensity, whereas development of mycotrophic tissues is suppressed by light [20]. Furthermore, non-symbiotic microorganisms can act as competitors of the symbiotic partners and also influence microenvironments in soil [21,22]. Therefore, future studies on mechanismsof orchid seed germination should also pay attention on the effects of abiotic and biotic factors other than mycorrhizal fungi.