Symbiosome

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. as well as Robert B Mellor applied this concept to the nitrogen-fixing unit seen in the plant root nodule, This has since engendered a great deal of research, one result of this has been the provision of a more detailed description of the symbiosome (peribacteroid) membrane, as well as comparisons with similar structures in Vesicular Arbuscular Mycorrhizal symbioses in plants. In the animal models, the symbiosome has a more complex arrangement of membranes, such that it has proved difficult to isolate, purify and study. ==Structure and formation==

A symbiosome is formed as a result of a complex and coordinated interaction between the symbiont host and the endosymbiont. This is an endocytosis-like process that forms a symbiosome rather than an endosome. In plants this process is unique. The symbiosome membrane is separated from the endosymbiont membrane by a space known as the symbiosome space, which allows for the exchange of solutes between the symbionts. In the plant root nodule the symbiosome membrane is also called the peribacteroid membrane. ==In the plant==

In the legume-rhizobia symbioses the symbiosome is the nitrogen-fixing unit in the plant, formed by an interaction of plant and bacterial signals, and their cooperation. The legumes are protein-rich, and have a high demand for nitrogen that is usually available from nitrates in the soil. When these are scarce the plant secretes flavonoids that attract free-living diazotrophic (nitrogen-fixing) rhizobia to their root hairs. In turn the bacteria release Nod factors that stimulate the infection process in the plant. Differentiation The outer host-cell derived symbiosome membrane encloses a space called the symbisome space or the peribacteroid space that surrounds the endosymbiont. In order for the symbiosome to be established as a nitrogen-fixing unit the enclosed bacterium has to be terminally differentiated into a morphologically changed bacteroid. The bacterium in the soil is free-living and motile. In the symbiosome it has to change its gene expression to adapt to a non-motile, non-reproductive form as the bacteroid. This change is noted by an increase in the size of the bacterium and its elongation. The bacterial membrane is also made permeable. In order to survive the NCR activities, the bacteria need to produce a protein called BacA. In addition the lipopolysaccharide produced by the bacteria is modified by an unusual fatty acid that also gives protection against environmental stresses. These defensive measures help the differentiation process and ensures their survival as bacteroids. Some strains of rhizobia produce a peptidase that degrades the NCRs. Nitrogen-fixing unit The established bacteroid is able to fix nitrogen into a chemically usable form of ammonium for the plant. This is an energy-demanding process fuelled by the plant's carbohydrates. Transport vesicles form in the symbiosome membrane allowing the passage of ammonium into the symbiosome space from the bacteroid, and the passage of plant nutrients to the bacteroid. The rhizobia infect the plant in large numbers where they are released into the cells inside symbiosomes. They are protected by the tough structure of the root nodule. ==In the animal==

The most well studied symbiosis involving an animal host is that between the cnidaria and the dinoflagellates, most commonly the single-celled zooxanthellae. The symbiosis of the Chlorella–Hydra first described the symbiosome. The coral Zoanthus robustus has been used as a model organism to study the symbiosis with its microsymbiont algal species of Symbiodinium, with a focus on the symbiosome and its membranes. Methods for isolating the symbiosome membranes have been looked for – the symbiont in the animal host has a multilayered membrane complex which has proved resistant to disruption making their isolation difficult. The endosymbiont dinoflagellates are used for their ability to photosynthesise and provide energy, giving the host cnidarians such as corals, and anemones, plant properties. Free-living dinoflagellates are ingested into the gastrodermal cells of the host, and their symbiosome membrane is derived from the host cell. The process of symbiosome formation is often seen in the animal host to be that of phagocytosis, ==Similar structures==

A similar structure to the symbiosome is the parasitophorous vacuole formed within host cells infected by apicomplexan parasites. The vacuole is derived from the host cell plasma membrane. It is made safe from the host's endolysomal system by modifying-proteins released by the parasite. The parasitophorous vacuole membrane is greatly remodelled by the parasite. ==See also==