Leguminous plants provide an important protein source for about one-third of the world's population. Nitrogen-fixing nodules on the roots of these plants produce relative large amounts of carbon monoxide (CO). Although CO plays important and well-known roles in the chemistry of the troposphere and in cellular biology, the roles of CO in ecological interactions between legumes and their microbial symbionts are poorly known. Preliminary results suggest that use of CO by many legume symbionts (including strains of agriculturally important nitrogen-fixing bacteria) may promote their survival and viability in soils. Symbiont survival and viability in turn can affect plant performance and ultimately impact the role of legumes in a variety of ecosystems. To test this concept, we will use pairs of legume symbionts. One strain in each pair will be manipulated genetically to eliminate the capacity for CO utilization. We will monitor relative and absolute survival of each member of these pairs singly and in mixed populations with and without added CO. We also propose that bacterial CO oxidation controls in part net CO emission by nodules and overall plant performance. We will test this notion using pairs of legumes inoculated with CO-oxidizing symbionts or mutants that do not oxidize CO. Finally, we will use cultivation methods and our recently developed molecular tools to determine relationships between the distribution of CO-oxidizing rhizobia in soil and root and nodule CO production. We will focus on bulk soil, the rhizosphere and the rhizoplane in greenhouse and field studies. In order to expand the impact of our work, we will incorporate appropriate aspects of our research into modules that will be used in science education programs at a K-8 school in South Bristol, Maine.
