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Bradyrhizobium japonicum Mutants with Enhanced Sensitivity to Genistein Resulting in Altered nod Gene Regulation

December 2001 , Volume 14 , Number  12
Pages  1,404 - 1,410

H. Ip , 1 F. D'Aoust , 2 A. A. Begum , 2 H. Zhang , 3 D. L. Smith , 3 B. T. Driscoll , 2 and T. C. Charles 1

1Department of Biology, University of Waterloo, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada; 2Department of Natural Resource Sciences, McGill University, 21,111 Lakeshore Road, Ste-Anne-de-Bellevue, Quebec H9X 3V9, Canada; 3Department of Plant Science, McGill University, 21,111 Lakeshore Road, Ste-Anne-de-Bellevue, QC H9X 3V9, Canada

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Accepted 27 August 2001.

Bradyrhizobium japonicum mutants with altered nod gene induction characteristics were isolated by screening mutants for genistein-independent nod gene expression. Plasmid pZB32, carrying a nodYlacZ transcriptional gene fusion, was introduced into B. japonicum cells that had been subjected to UV mutagenesis. Ten independent transformants producing a blue color on plates containing 5-bromo-4chloro-3indolyl-β-d-galactopyranoside but lacking genistein, indicative of constitutive expression of the nodYlacZ reporter gene, were isolated. β-Galactosidase activity assays revealed that while all of the 10 strains were sensitive to low concentrations of genistein, none exhibited truly constitutive nodYlacZ expression in liquid culture. Soybean plants inoculated with three of the mutants were chlorotic and stunted, with shoot dry weights close to those of the uninoculated plants, indicating the absence of nitrogen fixation. Differences in the kinetics of nodYlacZ expression and lipochitin oligosaccharide Nod signal production suggested that the strains carried different mutations. Some of these strains may be useful in mitigating the low root zone temperature-associated delay in soybean nodulation at the northern extent of soybean cultivation.

© 2001 The American Phytopathological Society