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The major projects in our lab are focused on understanding the cellular and molecular mechanisms used to regulate the establishment of symbiotic partnerships between plant roots and soil microbes. A model for many of our studies is the development of nitrogen-fixing root nodules resulting from the specific interaction between legume roots and Rhizobium bacteria. Root nodules are unique symbiotic organs that allow host plants to colonize nitrogen-poor environments and provide critical nitrogen inputs into the agricultural crops that supply dietary protein for much of the world's population. We are currently investigating two aspects of this important plant-microbe symbiosis: the hormonal regulation of nodule morphogenesis, and the controlled infection of host tissues by the endosymbiotic bacteria. Nodule morphogenesis is induced by lipo-oligosaccharide signal molecules secreted by Rhizobium, and the effects of these specific signals can be mimicked by manipulating endogenous levels of cytokinins and auxins. Recently we discovered that a similar hormonal mechanism also regulates the morphogenesis of a very different plant-microbe symbiosis resulting from the interaction of ectomycorrhizal fungi with pine roots, thus providing a common hormonal mechanism underlying extremely diverse plant-microbe symbioses. Most of our work on nodule infection is focused on the unusual family of plant extracellular matrix proteins called proline-rich proteins, or PRPs. PRPs form a covalently crosslinked structural network in plant cell walls, and the PRP gene family is tightly regulated during both root and nodule development. Clones encoding several members of the PRP family were isolated and characterized, and are being used to study the function and regulation of the PRP family. Most of our experimental strategies utilize a combination of biochemistry and cell biology, and increasingly the use of transgenic host plants. We found that Rhizobium rapidly attenuates the expression of normal root PRPs and induces the expression of nodule-specific PRPs and we are currently working to characterize the novel signal molecules that mediate this regulation of host cell wall architecture. Antibodies were raised against synthetic PRP domains and confocal microscopy is being used to investigate the localized deposition of PRP family members during the process of nodule infection. Since PRPs are structural cell wall proteins, we are also interested in defining the three-dimensional structure of these unusual proteins, and are starting to use a combination of molecular modeling and physical biochemistry to solve the structure of several synthetic PRP model peptides. Our ultimate goal is to develop an understanding of the mechanisms regulating symbiotic infection that will prove necessary for developing novel strategies to improve and extend the role of Rhizobium symbioses in agriculture. |
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