The Legume–Rhizobia Symbiosis

Bargaz, AdnaneSwedish University of Agricultural Sciences,Sveriges lantbruksuniversitet,Biosystem och teknologi,Department of Biosystems and Technology (författare)

The symbiotic nitrogen fixation (SNF) with legumes is the primary source of biologically fixed nitrogen for agricultural system. It is performed by a group of bacteria commonly called rhizobia. It is characterized by a host preference, and the differences among symbioses between rhizobial strains and legume genotypes are related to infection, nodule development and effectiveness in N2fixation. The interaction between a rhizobia and the legume is mediated by a lipochitin oligosaccharide secreted by the rhizobia, and called “Nod factor”. It is recognized by transmembrane receptors on the root-hair cells of the legume. It can regulate the nodule organogenesis by inducing changes in the cytokinin balance of the root, during nodule initiation. N2fixation in legume nodules is catalyzed by the nitrogenase enzyme depending upon the photosynthate supply, the O2concentration, and the fixed-N export. Among environmental factors that influence the SNF, the temperature is essential for nodule formation; the salinity and drought decrease the nodule permeability to O2and the photosynthate supply to the nodule, the phosphorus deficiency inhibits the nodule development and the total N2fixation. Rhizobia strains differ in their efficiency in N2fixation with host legume. There is evidence of genotypic variability for SNF at different levels of available P which show a possibility of selecting cultivars able to support biological N2fixation under low P soils.

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