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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. [2] The majority of such symbioses are made between legumes and diazotrophic Rhizobia bacteria.
Common Symbiotic Pathway - a simplified presentation based on McLean, Bravo and Harrison 2017 A LysM Domain. The common symbiosis signaling pathway (CSSP) is a signaling cascade in plants that allows them to interact with symbiotic microbes.
From this microcolony, the bacteria enter the developing nodule through the infection thread, which grows through the root hair into the basal part of the epidermis cell, and onwards into the root cortex; they are then surrounded by a plant-derived symbiosome membrane and differentiate into bacteroids that fix nitrogen.
The host plant provides the bacteria with amino acids so they do not need to assimilate ammonia. [5] The amino acids are then shuttled back to the plant with newly fixed nitrogen. Nitrogenase is an enzyme involved in nitrogen fixation and requires anaerobic conditions. Membranes within root nodules are able to provide these conditions.
Certain plants establish a symbiotic relationship with bacteria, enabling them to produce nodules that facilitate the conversion of atmospheric nitrogen to ammonia. In this connection, cytokinins have been found to play a role in the development of root fixing nodules. [ 6 ]
The partner choice hypothesis proposes that the plant uses prenodulation signals from the rhizobia to decide whether to allow nodulation, and chooses only noncheating rhizobia. There is evidence for sanctions in soybean plants, which reduce rhizobium reproduction (perhaps by limiting oxygen supply) in nodules that fix less nitrogen. [26]
The theory states that in plant endosymbioses, the micro-symbiont and the macro-symbiont generally share their lytic vacuolar compartments. This stems from Mellor's earlier 1988 symbiosome (or "symbiosome is a lysosome") theory, which states that the organelle that microsymbionts inhabit partially takes over the lysosomal functions in these plant cells.
There is evidence that plants and endophytes engage in communication with each other that can aid symbiosis. For example, plant chemical signals have been shown to activate gene expression in endophytes. One example of this plant-endosymbiont interaction occurs between dicotyledonous plants in the Convolvulaceae and clavicipitaceous fungi.