Why are most rhizobia beneficial to their plant hosts, rather than parasitic?
Multiple strains per plant and root-to-root (not seed-borne) transmission should favor rhizobia that invest in their own reproduction, rather than symbiotic N2 fixation, as analogous factors may favor pathogen virulence. But legumes can select for greater mutualism, controlling nodule O2 supply and reducing reproduction of rhizobia that fix less N2.
