Symbiosome

[1] 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.

This unit provides an inter-kingdom, micro-environment for the production of nitrogen for the plant,[3][6] and the receipt of malate for energy for the bacteroid.

[7] 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.

A few years later in 1989, Lauren Roth with Gary Stacey [8] as well as Robert B Mellor[9] applied this concept to the nitrogen-fixing unit seen in the plant root nodule,[1] previously called an infection vacuole.

[10] 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.

[11] In the animal models, the symbiosome has a more complex arrangement of membranes, such that it has proved difficult to isolate, purify and study.

At the same time the cortical cells divide to produce the tough root nodules that will house and protect the bacteria.

[13] 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.

[16] 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 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.

In addition the lipopolysaccharide produced by the bacteria is modified by an unusual fatty acid that also gives protection against environmental stresses.

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.

[1][22] The endosymbiont dinoflagellates are used for their ability to photosynthesise and provide energy, giving the host cnidarians such as corals, and anemones, plant properties.

[25] A similar structure to the symbiosome is the parasitophorous vacuole formed within host cells infected by apicomplexan parasites.