Rhizobium leguminosarum is a soil bacterium that is able to form symbiotic relationship with legumes and to fix nitrogen, even under various severe conditions and arid climate. This symbiosis between the bacteria and legumes is mutually beneficial. The main benefit for the plant is the fact that these bacteria can increase plant growth promotions (PGP). Several direct and indirect ways of PGP can be identified in relation to this process.
As mentioned above, one of the direct ways in which Rhizobium leguminosarum is used to promote growth in legumes is through the biological fixation of nitrogen. The ability to fix atmospheric nitrogen with the use of specific microorganisms is extremely important today, as it helps to mitigate the limitations of chemical fertilization. Another important direct way the symbiosis benefits plants is Phosphate solubilization. Several species of rhizobia, such as R. leguminosarum, R. meliloti, and M. mediterraneum, are able to solubilize phosphorus, as they produce low molecular weight organic acids that can act on inorganic phosphorus. Other direct ways include Siderophore formation and Phytohormone production, which are also processes highly important for plant growth promotion.
In turn, indirect ways in which Rhizobium leguminosarum symbiosis with legumes enhances PGP are biological control of plant disease and antagonistic effects of rhizobia to pathogens and pest. These also include induction of plant defense by rhizobia against pests and diseases, resistance of rhizobia to abiotic stress factors, soil salinity, the ability to mitigate water deficiency and drought, high temperature and heat stress, and acid soils and soil acidification. All the benefits mentioned above can demonstrate that the symbiosis also benefits the microorganism as it is protected against extreme conditions, such as high temperatures and avid climatic conditions.