Previous studies have shown that only a small fraction of the infections initiated in legume roots by nitrogen-fixing rhizobia successfully develop into nodules. For unknown reasons, most initiations terminate prior to completion of the process at all stages of development. The plant hormone ethylene has been demonstrated to inhibit nodule formation. Conversely, studies in Dr. Peter's laboratory have shown that the number of nodules formed on alfalfa roots by Rhizobium meliloti is significantly increased by exposure to 1-a-amino-ethoxyvinylglycine (AVG), a potent inhibitor of ethylene biosynthesis. Together, these results suggest that ethylene biosynthesis may have an important role in regulating the efficiency of infection and nodule formation. This possibility is consistent with the fact that many strains of Bradyrhizobium japonicum synthesize a structural and functional analog of AVG. This compund, called rhizobitoxine, is also a potent inhibitor of ethylene biosynthesis in plants. The PI will examine the role of rhizobitoxine and ethylene biosynthesis in regulation of nodule development. Mutants of B. japonicum that have lost the ability to synthesize rhizobitoxine will be isolated and inoculated onto host plants to analyze the effects of rhizobitoxine on ethylene biosynthesis and nodule development. The PI will determine whether rhizobitoxine and AVG enhance nodule formation by increasing the total number of infections or by reducing infection abortion. Genes required for rhizobitoxine synthesis will be isolated and characterized. Although all plants require nitrogen for growth, only a few plants (legumes mostly) can take atmospheric nitrogen and convert it to useable nitrogen compounds. The present research will examine the mechanism by which legumes initiate the nitrogen fixation process, allowing those plants to grow without the economic and environmental cost associated with nitrogen fertilization.
