Characterizing a Novel Sinorhizobium activity that Increases Legume Nodulation

Washington State University, Pullman WA

Investigators

Abstract

Symbiosis literally means "living together" and by living together organisms can often do things that they cannot accomplish on their own. The nitrogen-fixing symbiosis between bacteria called rhizobia and legume plants benefits both partners. For the legume, the symbiosis gives them access to biologically active nitrogen, an essential nutrient that the rhizobia can "fix" using inert nitrogen gas. The bacteria get a place to live in root nodules, organs formed on the legume after bacterial infection. Nodule formation requires the rhizobia to communicate with the host plant. The plant wants successful infection by bacteria that can fix nitrogen but must still defend itself against pathogens. Because rhizobia want to infect where they can, legumes also limit nodule formation using selectively applied defenses. One plant defense, called autoregulation of nodulation (AON), blocks the formation of too many nodules. A newly identified rhizobial gene, glx, increases nodulation in pea and in alfalfa, possibly by interfering with AON. This project investigates the physiological and biochemical mechanisms by which glx stimulates nodulation. The research will contribute a better understanding of one of the most accessible stories about a mutualistic symbiosis, a highly beneficial relationship between humble, non-pathogenic rhizobia and their green and often delicious legume hosts. Since poor nodulation can limit nitrogen fixation and plant growth, being able to increase nodulation could be helpful in agriculture, where it might reduce the need to apply ammonia fertilizer and would decrease the associated environmental problems. Broader impacts include the training of graduate students and working with undergraduate and high school students to develop their understanding of hands-on science. In addition, outreach to grower communities in the region, especially with regard to the major pulse and forage legumes produced in the state of Washington, will focus on how new scientific developments might influence agriculture. Control of nodulation by legumes is not understood very well. In order to establish a symbiotic interaction, signaling and signal transduction are crucial. The glx gene, which significantly increases nodulation when introduced into rhizobia that nodulate pea and barrel medic, shows that glx can alter development in two different symbioses and hints that it may act by affecting fundamental circuitry. In a pathogenic context, glx would be called a virulence determinant because it stimulates infection. The systemic plant defense called autoregulation of nodulation (AON) limits the formation of too many nodules after an initial infection. glx appears to be able to bypass or overcome AON. AON is mediated by peptide signals that are generated in a newly infected root then transferred to the shoot, where a receptor then generates a factor, called the shoot-derived inhibitor, that is transported to the roots and inhibits further nodulation. The glx gene is annotated as a glyoxalase/dioxygenase, suggesting it is involved in oxidation of small molecules. The putative small molecules may interact with the AON signal transduction pathway or with similar circuits the legume uses to limit nodulation in response to abundant nitrogen or low pH. Determining how the glx gene acts to increase nodulation through its activity or metabolites that it generates will provide a bacterial probe into the controlled response that the plant uses to optimize its interaction with rhizobia. The knowledge gained will increase fundamental understanding of plant-microbe communications and may have practical consequences in agriculture.