NodW Is Essential for Full Expression of the Common Nodulation Genes in Bradyrhizobium japonicum. Juan Sanjuan. Center for Legume Research and Department of Microbiology, University of Tennessee, M409 Walters Life Science Building, Knoxville 37996 U.S.A. Philipp Grob(2), Michael Gottfert(2), Hauke Hennecke(2), and Gary Stacey(1). (1)Center for Legume Research and Department of Microbiology, University of Tennessee, M409 Walters Life Science Building, Knoxville 37996 U.S.A., and (2)Mikrobiologisches Institut, Eidgenossische Technische Hochschule, ETH-Zentrum, Schmelzbergstrasse 7, CH-8092 Zurich, Switzerland. MPMI 7:364-369. Accepted 28 February 1994. Copyright 1994 The American Phytopathological Society.

The Bradyrhizobium japonicum nodW gene is required for nodulation of siratro, mung bean, and cowpea roots, but much less so for soybean nodulation. We found that a nodW mutant was unable to induce expression of the common nodDl and nodYABCSUIJ gene operons in the presence of either of the isoflavones genistein and daid-zein. Under these conditions, the nodW mutant did not produce detectable amounts of the lipooligosaccharide nodulation factors. However, the mutant produced a detectable amount of the wild-type nodulation signal in the presence of soybean seed extracts. Furthermore, the host-specific nodulation phenotype of this mutant could be complemented with a plasmid carrying the nodYABCS genes that were expressed under the control of the tac promoter, indicating that the nodulation defect in the nodW mutant is due to a lack of expression of the common nod genes. A B. japonicum nodDl-nodW double mutant was virtually Nod- on all plant hosts tested, including soybean. Likewise, nod gene expression and nod factor production by this mutant were severely impaired. Our data indicate that in B. japonicum, expression of the common nodulation genes in response to plant isoflavones and nodulation of legume hosts are mediated by both nodDl and nodW