UC Riverside

Plant-associated microbes can provide substantial benefits to crops, improving yields and reducing necessary nutrient inputs. The benefits of plant-microbial symbioses are highly variable and rely on several contextual drivers, including host symbiosis traits, makeup of available soil microbes, and environmental conditions such as soil nutrient levels and field management practices. Legume species receive fixed nitrogen from symbiosis with rhizobia, soil bacteria which can infect roots. However, many legume species still require nitrogen fertilization, suggesting breakdowns in the forces directing this symbiosis. To examine these forces, we turned to cowpeas (Vigna unguiculata)—a legume crop grown across Africa, Asia, and the Americas—and tested the effects of host genotype, microbial community, and soil conditions on plant growth benefits. In Chapter 1, we performed a full factorial soil inoculation experiment on twenty wild and domesticated cowpea genotypes to test the effects of cowpea domestication, host genotype, and soil microbial community on cowpea growth benefits in a greenhouse setting. In Chapter 2, we tested whether cowpea-associating rhizobial communities are structured spatially or by host genotype in a field that had a long history of cowpea growth and was demonstrated to have a highly beneficial soil microbial community. In Chapter 3, we planted a different subset of nineteen cowpea genotypes in the same field and an adjacent field with no history of cowpea growth to test whether seed coat inoculants could improve growth in either field. We found strong evidence that cowpea host benefits are primarily shaped by soils, which drove most of the observed variation in both nodulation and host growth. We found that cowpea host genotype has very little effect on structuring variation in host benefits from inoculation, with no significant variation in nodule rhizobial communities across host genotype. UCR Field 11 is enriched with Bradyrhizobium and dominated by a small handful of strains, with soil that offers high benefits to plants under greenhouse conditions. However, neither soil nor strains from this field were effective inoculants under field conditions, and the commercial inoculant tested also did not perform under field conditions.