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NifA-Dependent Expression of Glutamate Dehydrogenase in Rhizobium etli Modifies Nitrogen Partitioning During Symbiosis

Constitutive expression of foreign glutamate dehydrogenase in Rhizobium etli inhibits bean plant nodulation (A. Mendoza, A. Leija, E. Martínez-Romero, G. Hernández, and J. Mora. Mol. Plant-Microbe Interact. 8:584-592, 1995). Here we report that this inhibition is overcome when controlling gdhA expression by NifA, thus delaying the GDH activity onset after nodule establishment. Expression of gdhA modifies the nitrogen partitioning inside the bacteroid, where newly synthesized ammonia is preferentially incorporated into the amino acid pool instead of being exported to the infected cells. As a consequence, the fixed nitrogen transport to the leaves, measured as the ureides content in xylem sap, is significantly reduced. Nitrogenase activity, although not nifHDK expression, is significantly reduced in bacteroids expressing gdhA, probably due to the utilization of energy and reducing power for nitrogen assimilation. Here we show that ammonia assimilation inside R. etli bacteroids is active, albeit at low levels, and when enhanced is deleterious to the symbiotic performance. This leads us to believe that further reduction of the basal nitrogen metabolism in the bacteroid might stimulate the nitrogenase activity and increase the nitrogen supply to the plant.