What Do Orchid- Associated Fungi Do For a Living?

I. Introduction

There are many different species of orchids in the Orchidaceae family. The family Orchidacae include many different and diverse fungi which all orchids thought to form mycorrhizae. The mycorrhizae are very important during seed germination in orchids and supply nutrients for them to properly grow. ^[1] Outside the interactions with orchids, fungi participate in different roles depending on their environment ^[1].

The types of orchids that will be focused on are fully mycoheterotrophic orchids and fully autotrophic orchids. Both mixotrophic and fully mycoheterotropic orchids seem to have associations with more diverse fungal lineages ^[2]. There are also orchids that are considered partially mycoheterotrophic and mixotrophic. It is still unclear whether fungi have specificity towards certain orchid species. This needs to be further addressed by doing research on what fungi do outside their environments. The role of fungal associates in mature orchids is poorly understood and little is known about what role fungal diversity plays in affecting an orchid's distribution, population size, and genetic diversity ^[3].

II. Non-photosynthetic orchids outside tropics

Non-photosynthetic, also known as fully mycoheterotrophic orchids, often associate with ectomycorrhizal fungi ^[4] . Non-photosynthetic orchid species live in temperate regions ^[4].In temperate regions orchid species interact with near by trees through mycorrhizal networks where they can receive photosythates. ^[5]

Associated species

Corallorhiza spp. is a genus of orchids that associates with ectomycorrhizal fungi, and studies have shown that nearby tree roots lay in very close proximity to orchid rhizomes ^[6]. Fungi that form ectomycorrhiza, associate with nearby trees and trade materials such as nitrogen and carbon from trees to the fungus ^[6].This indirectly gives carbon to these myco-heterotropic orchids ^[6].

Armillaria mellea- A fungus that associates with the orchid Gastrodia cunninghammii ^[7].This fungus species is a parasite on the roots of adjacent forest trees ^[7].More specifically, the fungal cytoplasm is penetrated/released into the basal rhizome which then provides nourishment, which is important to this type of orchid species which lack chlorophyll ^[7].

III. Non-photosynthetic orchid species in tropics

Plants and fungi often have different mechanisms for gaining nutrients to survive. One method to gain nutrients is through the decaying or organic matter from wood-decomposer or litter decaying fungi ^[8]. Some of the orchids that associate with these types of fungi belong to the sub-family Epidendroideae ^[9].Some fungi associated with orchids are saprotrophic, and gain carbon and nitrogen from wood to help the fungus grow and survive ^[8]. Although the saprotrophic fungi do not gain as much nitrogen as ectomycorrhiza fungi, like in the terrestrial non-photosynthetic orchids ^[8]. Rather the wood-decaying fungi are enriched with an abundant amount of carbon from the decaying of wood: an organic source ^[8]. Saprotrophic fungi are associated with non-rhizoctonia fungi in non-photosynthetic orchids ^[8]. However, there are very few non-photosynthetic orchids that form a symbiotic relationship with the saprotrophic fungi.

Associated Species and Plants

One species of fungus is the Gastrodia confusa from a wet-temperate bamboo forest which associated with several species of wood-decomposer Mycena fungi ^[8].

Secondly, the world's largest non-photosynthetic plant, Erythrorchis ochobiensis, is reported to form mycorrhizas with a wide range of wood-rotting fungi ^[10].

Why Do the Fungi Favor the Tropics?

In non-photosynthetic, fully mycoheterotropic orchids, saprotrophic fungi are associated in tropical and sub-tropical forests ^[11]. In these locations,there is often a litter-rich forest floor, and the fungi decay organic matter on the forest floor as well as stumps and logs ^[9].

It is hypothesized that the temperature and climate in these conditions favor the saprotrophic fungi, and result in a need for specificity between orchids and this type of fungus ^[11] . Saptrophic fungi in the topics has the resource of water which allows the fungus to secrete the enzymes it need to break down the lignin in the wood ^[11]. The enzymes released from the fungus allows the fungus to gain access to enriched carbon and nitrogen sources in the wood ^[8].

Lack of Specificity

Saprotrophic fungi often do not demonstrate specificity in non-photosynthetic plants in comparison to photosynthetic orchids ^{[11] [12]}. It has been hypothesized that the saptrophic fungi may not express specificity because of the variable environment in the tropical region ^[11]. However, further investigation is need to understand how the taxonomy of the saptrophic fungi work in their region, as well as further mechanism of how the fungi function.

II. Photosynthetic/Green orchids

Green orchids develop photosynthetic leaves and become fully autotrophic, and they usually associate with saprotrophic fungi belonging to the genus Rhizoctonia. ^[13]It includes basidiomycete members in the Ceratobasidiaceae, Tulasnellaceae and Sebacinales. ^[13]

Rhizoctonia Solani- A fungus that is pathogenic to many different hosts, and can form a symbiosis with orchid seeds ^[14].

Tulasnella- A fungus that forms an mycorrhizal association with several Australian terrestrial orchids ^[15] . Some isolates in this genus use ammonium as a inorganic nitrogen form, but not nitrate. ^[16] It has been found that they have amino acid permeases and peptide transporters, allowing for organic nitrogen uptake ^[16]. Still what these fungi do is still unknown.

Sebacina- A fungus that appears to be saprotrophic, but some species are ectomycorrhizal. Sebacina can grow on organic nitrogen and ammonium ^[16].

Additional Information

Different theories on the diversity of orchid and fungi associations typically like to divide the groups of orchids based on ecology, for example, if they are terrestrial or epphytic orchids, and the orchid's photosynthetic ability ^[3].

Resources

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