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PLANTS INTERACTING WITH OTHER ORGANISMS

Nitrogen Fixation Mutants of Medicago truncatula Fail to Support Plant and Bacterial Symbiotic Gene Expression^1,[W],[OA]

Department of Biological Sciences, Stanford University, Stanford, California 94305

The Rhizobium-legume symbiosis culminates in the exchange of nutrients in the root nodule. Bacteria within the nodule reduce molecular nitrogen for plant use and plants provide bacteria with carbon-containing compounds. Following the initial signaling events that lead to plant infection, little is known about the plant requirements for establishment and maintenance of the symbiosis. We screened 44,000 M2 plants from fast neutron-irradiated Medicago truncatula seeds and isolated eight independent mutant lines that are defective in nitrogen fixation. The eight mutants are monogenic and represent seven complementation groups. To monitor bacterial status in mutant nodules, we assayed Sinorhizobium meliloti symbiosis gene promoters (nodF, exoY, bacA, and nifH) in the defective in nitrogen fixation mutants. Additionally, we used an Affymetrix oligonucleotide microarray to monitor gene expression changes in wild-type and three mutant plants during the nodulation process. These analyses suggest the mutants can be separated into three classes: one class that supports little to no nitrogen fixation and minimal bacterial expression of nifH; another class that supports no nitrogen fixation and minimal bacterial expression of nodF, bacA, and nifH; and a final class that supports low levels of both nitrogen fixation and bacterial nifH expression.

² Present address: Department of Plant Biology, University of Minnesota, 250 Biological Sciences Center, St. Paul, MN 55108.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Sharon R. Long (srl{at}stanford.edu).

^[W] The online version of this article contains Web-only data.

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Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.105.072132.

^* Corresponding author; e-mail srl{at}stanford.edu; fax 650–725–8309.

Received October 4, 2005; returned for revision November 22, 2005; accepted November 23, 2005.

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