Rhizobia independently adapt to soil and legume host environments, but soil conditions influence the abundance of high-quality partners.

UNLABELLED: Rhizobia live as free-living microorganisms in the soil and in association with legume hosts. Both environments exert selective pressures on rhizobia, influencing the reproductive success of individual strains (e.g., fitness). The soil, a heterogeneous and fluctuating environment, is often overlooked, and little is known about whether selection in the soil influences the outcomes of the rhizobium-legume mutualism. We exposed a mixture of 68 Sinorhizobium meliloti strains to soil-mediated selection using eight different treatments (temperature, osmotic, and texture perturbations) and host-mediated selection with two Medicago species as hosts. We found that cold (4°C) and warm (32°C) temperatures, as well as salt addition, had the strongest effects on diversity, population composition, or population size. Strain relative fitness was strongly positively correlated among soil treatments, except cold, warm, and salinity, suggesting strains undergo similar selection in the soil. Genome-wide association analysis revealed a complex genetic architecture for soil fitness, characterized by numerous loci of small effect that did not show significant associations. In contrast, when comparing rhizobial fitness between soil and host environments, we found minimal strain fitness correlations, suggesting an independent genetic basis and habitat-specific adaptations. Finally, by examining the relationship between rhizobial fitness in soil and their benefits to the host plant, we found that soil selection influenced the relative abundance of high- and low-quality strains; however, whether these effects were positive or negative for the plant was host-dependent. Our results suggest that rhizobial evolution in soil and host are largely independent, but soil selection can alter mutualism benefits.

IMPORTANCE: Rhizobium-legume mutualism is crucial for introducing nitrogen into agricultural and natural ecosystems, and rhizobial persistence in the soil is an important component of agroecosystems. However, we know little about how populations of rhizobia persist and adapt to this environment, especially in the context of the soil's spatial and temporal variations (temperature, moisture, and soil texture). We found that rhizobia are similarly selected across abiotic soil conditions, but their reproductive success in the soil is independent from their reproductive success in the host. Intriguingly, we found that certain soil conditions increase (or decrease) the relative abundance of more beneficial strains. Understanding how rhizobia adapt to diverse environments is crucial for developing effective bioinoculants that persist in the soil while remaining highly competitive for host colonization and beneficial to the plant.