Non-Nitrogen-Fixing <i>Sinorhizobium meliloti</i> Can Escape Sanctions in Indeterminate Alfalfa Nodules, Exhibiting Parasitic Growth

The soil bacterium <i>Sinorhizobium meliloti</i> can proliferate by leveraging its nitrogen-fixing symbiosis with legumes that form indeterminate root nodules, such as <i>Medicago sativa</i> (alfalfa) and <i>M</i>. <i>truncatula</i>. In contrast to determinate-nodulating legumes, such as <i>Glycine max</i> (soybean) and <i>Lotus japonicus</i>, 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 conodulates with an N₂-fixing strain can benefit from resources the host provides to the nodule in response to N₂ fixed by the conodulating strain. Coinvasion of alfalfa nodules with an 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. [Formula: see text] Copyright © 2025 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.