A variety of intimate, symbiotic relationships exist between the roots of plants and the microorganisms in the soil. For instance, mycorrhizal fungi colonize the surface of plant roots, effectively extending them further through the soil to collect nutrients otherwise out of reach. These mycorrhizal branching structures, known as hyphae, emanating from plant roots also improve soil aggregation and hence improve water infiltration and aeration. In return, Mycorrhiza can demand up to 20-40% of photosynthetically derived carbon from their plant hosts. In the world of rhizospheric bacteria, Rhizobia are well known for their key role in fixing atmospheric nitrogen for plant uptake. Yet there are, additionally, more than two dozen known genera of rhizobacteria that help plants grow, either directly by releasing growth stimulants (phytohormones) and enhancing mineral uptake, or indirectly by fighting off plant pathogens.
McNear, David Jr., 2013, The Rhizosphere Roots, Soil and Everything In Between, Nature Education Knowledge 4( 3):1, http://www.nature.com/scitable/knowledge/library/the-rhizosphere-roots-soil-and-67500617
 Rhizobia are nitrogen-fixing bacteria living in nodules formed in the roots of leguminous plants.