Roles of a Rhizobium-induced Plant Peroxidase in Infection and Nodulation

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9507535 VandenBosch Nitrogen-fixing root nodules, which form on legume roots in response to infection with bacteria in the family Rhizobiaceae, are a key source of nitrogen available to plants in both natural and agricultural environments. Because of its ecological and economic importance, this symbiosis has stimulated many studies into the interactions between host plant and symbiont. Although key determinative events of the legume/Rhizobium symbiosis are likely to precede bacterial infection, no plant genes have been identified previously that are expressed strongly prior to infection and nodule morphogenesis. We have used subtractive hybridization to enrich for plant genes induced during this early, pre-infection phase of the Medicago truncatula /Rhizobium meliloti symbiosis. One gene so identified encodes a putative plant peroxidase, named MtRip1 for Rhizobium-induced peroxidase. The timing, localization and manner of induction of Mtrip1 indicate that it may be of key importance to symbiotic establishment. Although the role of the Rip1 protein during symbiotic establishment is uncertain, it is hypothesized that the enzyme may be required for nodule morphogenesis, and/or it may be used during arrest of rhizobial infection, and/or it may be a sink for hydrogen peroxide. The experimental plan of this project is designed to investigate the function(s) of the Rhizobium-induced peroxidase using several complementary approaches. The outcome of this work will allow us to address several long-standing questions in nodulation research, including how the plant regulates infection by its symbiont Rhizobium, and how Rhizobium avoids triggering a generalized defense response as occurs in response to plant pathogens. More fundamentally, the research will advance our understanding of the functional consequences of changes in cell walls during development.