Non-Nitrogen-Fixing Sinorhizobium meliloti Can Escape Sanctions in Indeterminate Alfalfa Nodules, Exhibiting Parasitic Growth.

The soil bacterium Sinorhizobium meliloti can proliferate by leveraging its nitrogen-fixing symbiosis with legumes that form indeterminate root nodules, such as Medicago sativa (alfalfa) and M. truncatula. In contrast to determinate-nodulating legumes, e.g. Glycine max (soybean) and Lotus japonicus, indeterminate-nodulating legumes impose terminal differentiation on nitrogen-fixing (N₂-fixing) rhizobia. Thus, the bacterial population is split between those that benefit the plant by N₂ fixation, but are a reproductive dead end, and those that are undifferentiated, capable of resuming free-living growth, but not fixing nitrogen. We show that, in mixed nodules colonized by nearly-isogenic strains, with one N₂-fixing and one unable to fix N₂ (Fix-), alfalfa do not preferentially penalize the Fix- strain, allowing 'cheating' at the expense of the plant and the N₂-fixer. Thus, a Fix- strain that successfully co-nodulates with a N₂-fixing strain can benefit from resources the host provides to the nodule in response to N₂ fixed by the co-nodulating strain. Co-invasion of alfalfa nodules with a N₂-fixing strain may be a successful strategy for a Fix- strain to cheat both the plant that provides fixed carbon and the N₂-fixing strain.