User:Kpina27/Orchid mycorrhiza

Introduction
The Orchidaceae is one of the most diverse groups and currently the largest plant family with over 25,000 species. An important aspect of Orchids ability to survive and adapt to their environment has to do with their symbiotic relationship that they have with mycorrhizae. Mycorrhizae are fungus that grow in symbiotic association with the roots (in/on) of plants that allow for the plant to obtain nutrients and increase their susceptibility to survive and reproduce. Specifically, they play an important role in Orchids increasing their ability to grow and develop in different environments. This is because Orchids lack the ability to get sufficient nutrient acquisition from the soil directly due to them lacking endospores and having limiting storage reserves (#5). The lack of endospores and limited storage space inhibits their ability to successfully germinate and acquire the proper nutrients therefore, they use these various types of mycorrhizae to aide in helping them acquire what they need to survive. Orchid mycorrhizae play an essential role in in supplying the flowering plants nutrients such as nitrogen and phosphorus which the Orchids then exchange for organic carbon resources (#1). Previous research has identified that all terrestrial Orchids which the species are currently known rely on mycorrhizal fungi to help them obtain these nutrients which further implicates their significant role shared in the symbiotic relationship between the two (#7). It is understood that mycorrhizae do indeed have a significant role in Orchids ability to survive and reproduce but what needs to be further researched is how those nutrients are transferred between fungi to host.

Article body
At infection of an orchid by a mycorrhizal fungus both partners are altered considerably to allow for nutrient transfer and symbiosis. Nutrient transfer mechanisms and the symbiotic mycorrhizal peloton organs start to appear only shortly after infection around 20–36 hours after initial contact. There is significant genetic upregulation and downregulation of many different genes to facilitate the creation of the symbiotic organ, and the pathways with which nutrients travel. As the fungus enters the parenchyma cells of the orchid the plasma membrane invaginates to facilitate fungal infection and growth. This newly invaginated plasma membrane surrounds the growing pelotons and creates a huge surface area from which nutrients can be exchanged. The pelotons of orchid mycorrhiza are intensely coiled dense fungal hyphae that are often more extensive in comparison to endomycorrhizal structures of arbuscular mycorrhiza. The surrounding plant membrane essentially becomes rough endoplasmic reticulum with high amounts of ribosomes and a plethora of transporter proteins, and aquaporins. Additionally there is evidence from electron microscopy that indicates the occurrence of exocytosis from the plant membrane. This highly convoluted and transporter rich membrane expertly performs the duties of nutrient exchange between the plant and fungus and allows for molecular manipulation by ribosomes and excreted enzymes within the interfacial apoplast. Pelotons are not permanent structures and are readily degraded and digested within 30 to 40 hours of their formation in orchid mycorrhiza. This happens in all endomycorrhizal associations but orchid plants readily digest fungal pelotons sooner after formation and more often than is seen in arbuscular mycorrhizal interactions. It is proposed that the occurrence of this more extreme digestive pattern may have something to do with necrotorphic nutrient transfer which is the absorption of nutrients form dead cells. The key nutrients involved in the majority of the transfer between fungi and orchid plants are carbon, nitrogen and phosphorus.

Orchid mycorrhizal interactions are unique in the flow of nutrients. Typically in arbuscular mycorrhizal interactions the plants will unidirectionally supply the fungi with carbon in exchange for phosphorus or nitrogen or both depending on the environment, but orchid mycorrhizal nutrient transfer is less specific (but no less regulated) and there is often bidirectional flow of carbon between the fungus and plant, as well as flow of nitrogen and phosphorus from the fungus to plant. In around 400 species of plants there is no flow of carbon from plant and all of the nutrients of the plant are supplied by the fungus. However, the net carbon gain by the plant in these interactions is positive in majority of the observed interactions.