Symbiosome

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.^[1]

Structure and formation

A symbiosome is formed as a result of a complex and coordinated interaction between the symbiont host and the endosymbiont.^[5] 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.^[5][12] This is an endocytosis-like process that forms a symbiosome rather than an endosome. In plants this process is unique.^[13]

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.^[14][12] In the plant root nodule the symbiosome membrane is also called the peribacteroid membrane.^[13]

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.^[1][13]

To enable infection the tip of the root hair

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.^[16][13] Filamentous actin (F-actin) channels the elongation of the injection threads and short F-actin fragments are dotted around the symbiosome membrane.^[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 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.^[17][18]

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.^[19][1][13] The process of differentiation is plant-driven using peptides known as nodule specific cysteine-rich peptides (NCR peptides).

NCRs are

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.^[13] 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.^[13] 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.^[15]

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.^[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.^[23] Free-living dinoflagellates are ingested into the gastrodermal cells of the host, and their symbiosome membrane is derived from the host cell.^[24] The process of symbiosome formation is often seen in the animal host to be that of phagocytosis,^[24] and it is hypothesised that the symbiosome is a phagosome that has been subject to early arrest.^[25]

Similar structures

A similar structure to the symbiosome is the

• Bacteriome
• Trophosome

References