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Symbiosomes: temporary moonlighting organelles

Krishnan, Hari
ARS USDA Submissions 2014 v.460 pp. 1
alpha-Proteobacteria, genes, mutants, nitrogen, nitrogen fixation, nitrogen-fixing bacteria, nitrogenase, nutrients, organelles, proteins, roots, stems, symbionts, symbiosis
Nitrogen fixation is the most important biological process on earth, second only to photosynthesis. The enzyme, nitrogenase, which catalyzes the reduction of atmospheric dinitrogen to ammonium, is encoded into the genomes of a few members of the a-, ß-and '-proteobacteria. The primary route of fixed nitrogen into the biosphere occurs via the symbiosis of nitrogen-fixing a-proteobacteria with leguminous plants. The symbiosis forms a novel organ on the roots and in a few cases on stems of the plant referred to as a nodule that contains specialized compartments affording spatial separation of the symbionts within modified plant cells. Upon infection and throughout the symbiosis the bacteria are encased within a plant derived membrane vesicle called the symbiosome (also called the peri-bacteroid membrane). Within the symbiosome, the bacteria differentiate into bacteroids, which express nitrogenase. The intervening space between the symbiosome membrane and the bacteroid is the symbiosome space. All nutrients and signals must traverse the symbiosome space, but there are few reports attempting to elucidate the functional purpose of this space. The majority of our knowledge of the symbiosome is derived from microbial mutants that affect symbiosome development and the cytolocalization of plant gene products. It is known that this space is populated by proteins contributed by both symbionts making it a unique, confluent inter-kingdom domain. Understanding the functional attributes of the space will lead to enhancing nitrogen fixation capacity and ultimately to extending the range of plant species capable of hosting symbiotic nitrogen-fixing bacteria.