The long-term objective of the research in my laboratory is to understand the mechanisms of symbiotic interactions in non-legume crops and to manipulate these responses for sustainable agriculture. The research is currently focused on:
Determining the mechanism of rhizobial-cereal interactions and its link to mycorrhizal symbiotic pathway. Because cereal growth is limited by nitrogen, the high yields of these crops are currently dependent on fertilizers that are a major on-farm expense and cause significant environmental damages. In view of the increasing demand for higher production of these crops, the development of alternative strategies to enhance crop production while reducing fertilizer use is of high priority. One such strategy is to use rhizobia that form intimate endophytic association with these crops and increase their growth. Rhizobia are soil bacteria that form a very efficient nitrogen-fixing association with legumes. Recent studies have shown that rhizobia hijacked the genetic machinery of legumes involved in symbiosis with mycorrhizal fungi. Similar to legumes, rice forms symbiosis with mycorrhizal fungi and contain the homologues of legume genes necessary for interactions with rhizobia. Preliminary results from the investigators' laboratories indicate that a rhizobial strain utilizes the mycorrhizal symbiotic pathway to colonize rice tissues. The goal of this project is to determine the molecular and cellular mechanisms of both rhizobia and rice that result in this beneficial association as well as to evaluate the potential of rhizobial nitrogen fixation with rice. This rhizobial-cereal interaction provides a unique opportunity to study the initial relationships and mutual benefits that probably allowed legume nodulation to appear but also, in the long term, to use this system to engineer nitrogen fixation in cereals by following the path of legume evolution.
Bacterial sulfur metabolism and its role in symbiotic and pathogenic interactions. Rhizobial symbionts of legumes require large amounts of sulfur for nitrogenase, glutathione, and many other proteins and cofactors essential for bacterial physiology. While sulfur deficiency often limits nitrogen fixation, very little is known about the sources of sulfur that rhizobia utilize during symbiosis. Despite what is often surmised, sulfur in soils and in planta is predominantly in the organic form as sulfonates and s-esters. While the annotated genomes of all rhizobial strains contain homologues of organic sulfur utilization genes that have been characterized in other bacteria, they do not contain homolog to well characterized sulfur regulatory proteins found in many bacteria studied thus far. The goal of this project is to determine the genetic and regulatory mechanisms of sulfur metabolism in rhizobia and the role of these mechanisms in symbiotic nitrogen fixation. Additionally, we are exploring the sulfur metabolism and its links to bacterial pathogenesis in plants and animals.