Abstract
Sinorhizobium meliloti is a nitrogen-fixing soil bacterium that forms a facultative mutualism with its host alfalfa. S. meliloti strains exhibit diverse colony morphologies on solid culture media that span a continuum from dry to mucoid. Studies on how this trait affects symbiosis rely on mutations in exopolysaccharide II production genes; however, whether natural variation in morphology confers a selective advantage on rhizobia in their natural habitats remains unknown. Here, we used 90 mucoid and 27 dry natural isolates to test whether colony morphology is associated with rhizobial fitness in the soil and in the host, or with the nitrogen-fixation benefits they provide to the plant. Colony morphologies were widely distributed across a phylogenetic tree, suggesting that morphological shifts occurred repeatedly and no single genetic variant was strongly associated with this trait. We inoculated soil mesocosms and plants with a mixture of dry and mucoid bacteria. We observed that dry rhizobia have higher fitness, particularly at 22°C and 32°C, but not under cold or drought conditions, independent of the initial rhizobial load. Conversely, mucoid bacteria have a modestly higher relative fitness in host nodules, and single inoculations showed that they tend to confer greater benefits to all three alfalfa varieties. Over time, we also observed rare, repeated conversion from mucoid to dry morphologies in soil and host plants. Taken together, our study reveals that morphological variation within a species in a single agricultural field is important for bacterial fitness in the soil and can have cascading effects on the rhizobia's plant benefits.