The Enhancement of Ammonium Assimilation in Rhizobium etli Prevents Nodulation of Phaseolus vulgaris. Alberto Mendoza . Centro de Investigacion sobre Fijacion de Nitrogeno, Departamento de Ecologfa Molecular, Universidad Naciona Autonoma de Mexico, Apdo. Postal 565-A, Cuernavaca, Morelos, 62271, Mexico. Alfonso Leija, Esperanza Martinez-Romero, Georgina Hernandez, and Jaime Mora. Centro de Investigacion sobre Fijacion de Nitrogeno, Departamento de Ecologfa Molecular, Universidad Nacional Autonoma de Mexico, Apdo. Postal 565-A, Cuernavaca, Morelos, 62271, Mexico. MPMI 8:584-592. Accepted 4 April 1995. Copyright 1995 The American Phytopathological Society.

The modification of the ammonium assimilation pathway or Rhizobium etli (GS-GOGAT) by adding an additional ammonium assimilation enzyme, GDH, strongly affects its symbiotic interaction with beans. The plasmid pAMla, based in the stable vector pTRlOl (M. Weinstein, R. C. Roberts, and D. R. Helsinki, J. Bacteriol. 174,7486-7489, 1992), containing the Escherichia coli gdhA gene flanked by two transcription-translation terminators was constructed. The expression of GDH in both, the wild type (CFN42/pAMla) and a ntrC mutant (CFN2012/pAMla) R. etli strains, gave a similar metabolic effect, i.e., high GDH and reduced GOGAT activities, and an increased synthesis and excretion of several amino acids. The total inhibition of bean nodulation was observed when the minimum optimal inoculum of R. etli CFN42/pAMla was used; however, an effective symbiosis occurred with the CFN2012/pAMla mutant strain. While a total inhibition of the induction of the nodA gene by bean root exudate or by naringenin was observed in the CFN42/pAMla strain, at 10 mM ammonium, the CFN2012/pAMla showed an optimal nodA gene induction. A correlation between nodA gene induction, Nod factor production, and nodulation was observed. We conclude that in R. etli, there is a down-regulation of nod gene expression and nodulation when a high internal nitrogen content is built up by the presence of a functional GDH and that NtrC is involved in such regulation. An instability of the plasmid harboring the gdhA gene was observed during symbiosis, indicating a strong selection against cells containing this plasmid.