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.
History 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. 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. At the point of entry into a symbiont host cell, part of the cell's membrane envelops the endosymbiont and breaks off into the cytoplasm as a discrete unit, an organelle-like vacuole called the symbiosome. 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. To enable infection the tip of the root hair curls over the rhizobia and by an inward growth produces an infection thread to carry the endosymbionts into the cortical cells. At the same time the cortical cells divide to produce the tough root nodules that will house and protect the bacteria. The bacterial production of extracellular polymeric substance (EPS) is seen to be necessary for enabling infection. The rhizobia infect the plant in large numbers, only seen to be actively dividing at the tip of the injection thread, where they are released into the cells inside symbiosomes. The symbiosome is formed as a result of an endocytosis-like process that produces an endosome. Typically endosomes target to lysosomes, but the symbiosome re-targets the host-cell proteins. The changes in the plant needed to form the infection thread, the increased division of the cortical cells, the formation of the root nodule, and symbiosome, are brought about by dynamic changes in the actin cytoskeleton. Filamentous actin (F-actin) channels the elongation of the injection threads and short F-actin fragments are dotted around the symbiosome membrane. The bacteria are released as injection drops into the host root nodule cells where the plasma membrane encloses them in the organelle-like structure of the symbiosome. In most plants a symbiosome encloses a single endosymbiont bacterium but some types may contain more than one. A negative feedback loop called the autoregulation of nodulation works to balance the need for nitrogen and thus the formation of nodules.
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