The term symbiosis (from the Greek: syn "with"; and biosis "living") commonly describes close and often long-term interactions between different biological species. The term was first used in 1879 by the German mycologist, Heinrich Anton de Bary, who defined it as: "the living together of unlike organisms".

The definition of symbiosis is in flux and the term has been applied to a wide range of biological interactions. The symbiotic relationship may be categorized as being mutualistic, parasitic, or commensal in nature. Others define it more narrowly, as only those relationships from which both organisms benefit, in which case it would be synonymous with mutualism.

Symbiotic relationships included those associations in which one organisms lives on another (ectosymbiosis, such as mistletoe), or where one partner lives inside another (endosymbiosis, such as lactobacilli and other bacteria in humans or zooxanthelles in corals). Symbiotic relationships may be either obligate, i.e., necessary to the survival of at least one of the organisms involved, or facultative, where the relationship is beneficial but not essential to survival of the organisms.

Orchid mycorrhiza are a symbiotic relationship between the roots of plants of the family Orchidaceae and a variety of fungi. All orchids are mycoheterotrophic at some point in their life cycle. Orchid mycorrhiza are critically important during orchid germination, as orchid seed has virtually no energy reserve and obtains its carbon from the fungal symbiont. Many adult orchids retain their fungal symbionts, although the benefits to the adult photosynthetic orchid and the fungus remain largely unexplored.

The fungi that form orchid mycorrhiza are typically basidiomycetes. These fungi come from a range of taxa including Ceratobasidium (Rhizoctonia), Sebacina, Tulasnella and Russula species. Some orchids associate with saprotrophic, or pathogenic fungi, while other orchids associate with ectomycorrhizal fungal species. These latter associations are often called tripartate associations as they involve the orchid, the ectomycorrhizal fungus and the ectomycorrhizal host plant.

For More Information: Orchids

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