The expression of the Rhizobium tropici guaB gene is required in the early stages of bean nodulation

COLLAVINO, M.M., GRASSO, D.H., RICILLO, P.M. AND AGUILAR O.M.

Bejing, China

Congreso; XIV Congreso Internacional de Fijación de Nitrógeno; 2004

The expression of the Rhizobium tropici guaB gene is required in the early stages of bean nodulation.  Mónica Collavino1,  Daniel H. Grasso 2, Pablo M. Riccillo1,  and O. Mario Aguilar1.  1IBBM-Facultad de Ciencias Exactas, UNLP. Calle  47 y 115,  La Plata, Argentina; 2 Inst. de Suelo, INTA Castelar, Argentina.   The guaB encodes for the enzyme inosin-monophasphate deydrogenase. In this study, the guaB gene of R. tropici under the control of the symbiotic promoters, nodA, bacA and nifH, were used to assess the timing of guaB requirement during nodulation. A promoterless gus gene was cloned downstream from guaB in order to monitor guaB transcription. The guaB mutant strain R. tropici CIAT899-10.T carrying the three fusions were tested for symbiotic phenotype by assessing nodulation, nitrogenase activity, and histological examination. guaB under the control of the nod promoter restores the symbiotic wild type phenotype. In contrast, the nodules formed by the mutant strain PnifH-guaB fusion and by the mutant, were found to be identical. guaB under the control of the bacA promoter supports partial complementation, providing limited ability for the mutant to invade only certain zones of the nodule. In addition, the expression patterns of the symbiotics promotors during the different stages of nodulation were in agreement with that previously reported. Conclusions: the rhizobial guaB is required from early steps of nodulation, and that the timing of this expression is important for nodule development. On the other side, mutations in the guaB of Sinorhizobium meliloti 1021 do not affect symbiosis with alfalfa. Supported by ANPCyT PICT No.7072/99