Symbiosome

A symbiosome is a specialised compartment in a host cell that houses an endosymbiont in a symbiotic relationship. The term was first used in 1983 to describe the vacuole structure in the symbiosis between the animal host the Hydra, and the endosymbiont Chlorella. Symbiosomes are also seen in other cnidaria-dinoflagellate symbioses, including those found in coral-algal symbioses. In 1989 the concept was applied to the similar structure found in the nitrogen-fixing root nodules of certain plants. The symbiosome in the root nodules has been much more successfully researched due in part to the complexity of isolating the symbiosome membrane in animal hosts. The symbiosome in a root nodule cell in a plant is an organelle-like structure that has formed in a symbiotic relationship with nitrogen-fixing bacteria. The plant symbiosome is unique to those plants that produce root nodules. The majority of such symbioses are made between legumes and diazotrophic Rhizobia bacteria. The rhizobia-legume symbioses are the most studied due to the importance in agriculture. Each symbiosome in a root nodule cell encloses a single rhizobium that differentiates into a bacteroid. However, in some cases a symbiosome may house several bacteroids. The symbiosome membrane, or peribacteroid membrane, surrounds the bacteroid membrane, separated by a symbiosome space. This unit provides an inter-kingdom, micro-environment for the production of nitrogen for the plant, and the receipt of malate for energy for the bacteroid. The concept of the symbiosome was first described in 1983, by Neckelmann and Muscatine, as seen in the symbiotic relationship between Chlorella ( a class of green algae, and Hydra a cnidarian animal host. Until then it had been described as a vacuole. A few years later in 1989, Lauren Roth with Gary Stacey as well as Robert B Mellor applied this concept to the nitrogen-fixing unit seen in the plant root nodule, previously called an infection vacuole.