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Our research group studies bacterial infection. One group of bacterial soil inhabitants known as ‘rhizobia’ have the unique ability to engage in a permissive infection process on compatible host plants, leading to a special plant-derived structure called the root nodule. In this specialized organ, the bacteria multiply, invade nodule cells, and differentiate into nitrogen-fixing entities. This arrangement provides the bacteria with a carbon-rich niche, and the plant with enough fixed N to flourish in soils where it might otherwise die. This cooperative interaction occurs in widely diverse biomes, is a driving force in global nitrogen cycling, and is fundamental to the agricultural technique of crop rotation.

Beyond the ecological and economic importance of this symbiosis, it presents us with a complex developmental process, programmed into the genes of both plant and microsymbiont. What are the genes that allow the bacteria to navigate the nodule environment, and then differentiate into N-fixing entities within the nodule cells? How do plants and N-fixing bacteria recognize each other as compatible? Our research addresses these and other questions, using molecular genetic approaches.

  • In one approach we study mutants of the bacterium Sinorhizobium meliloti, which nodulates plants of the genus Medicago (including alfalfa). From a recent genetic screen, 175 independent S. meliloti mutants were isolated based on their inability to properly engage the plant and form productive nodules. We are further characterizing a subset of these mutants that seem to be defective in signaling between the host and the microbe. We are focusing on pathways that represent intersections between pathogenesis and mutualism.

  • In a second approach, we are looking at how plant-rhizobium compatibility varies in nature. By carrying out hundreds of pairwise inoculations between wild Sinorhizobium isolates and several Medicago accessions, we are characterizing many novel cases of incompatibility, and we hope to use our growing molecular genetic toolkit to elucidate the molecular nature of this ecological variation. In other words, we hope to decode compatibility rules between rhizobia and legumes.