Boston University News Service – Before synthetic nitrogen fertilizers existed, plants and bacteria worked together to return nutrients to the soil. A type of bacteria living in plant roots, called nitrogen-fixing bacteria or Rhizobia, enriches the soil with nitrogen from the atmosphere, making it available to the host plants. But not all plants can host Rhizobia, because the plants’ immune systems repel the bacteria. Scientists have long believed that only legumes, or plants like soybean, pea, and alfalfa, could chemically communicate, and therefore accept, the nitrogen-fixing bacteria.