Transceptor-mediated nitrogen sensing in legumes

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Legumes engage in a root symbiosis called nodulation with rhizobial bacteria. In this interaction rhizobia are taken up into root nodule cells where they fix N. Nodulation and N-fixation are suppressed by high levels of soil N. In Arabidopsis, recent work has shown that soil nitrate availability is sensed by the nitrate transporter NRT1.1. This plasma membrane protein can transport nitrate in both high (micromolar) and low (mM) affinity ranges. In Arabidopsis NRT1.1 is phosphorylated at Thr101 in response to changes in external nitrate supply, switching between high and low concentration ranges. NRT1.1 was recently shown to transport auxin and nrt1.1 mutants showed altered root architectural response to nitrate availability. Medicago truncatula has three NRT1.1 homologues, two of them are expressed in roots and have a conserved Thr101, and one of these is induced in N treated nodules. Non-legumes and the primitive legume Chamaecrista fasciculata have only one copy of NRT1.1. Our goals include the electrophysiological characterization of the M. truncatula, and C. fasciculata NRT1.1 homologues. We will test their ability to transport nitrate (high and low affinity), auxin and other substrates and determine their phosphorylation status under different N regimes. Medicago NRT1.1 mutants will be obtained to test their role in N suppression of nodulation. The ultimate aims of the study are to 1) identify and characterize components of the legume N-sensing apparatus 2) understand the links between N perception and nodulation 3) gain insights into the evolution of N-sensing in legumes and 3) to develop a pea that can fix N at high soil nitrate levels. The last aim has important applications in sustainable agriculture, for instance as green manures in crop rotation to lower N fertilizer inputs for wheat but also for legume crops like pea and soybean where early N applications that are used to establish the crop can also inhibit nodulation.

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