Early Nodulation Signals of the Wild Type and Symbiotic Mutants of Soybean (Glycine max). Tara D. Sutherland. Plant Molecular Genetics (OHLD), Institute of Agriculture, University of Tennessee, Knoxville 37901-1071 U.S.A. Brant J. Bassam, Lesley J. Schuller, and Peter M. Gresshoff. Plant Molecular Genetics (OHLD), Institute of Agriculture, University of Tennessee, Knoxville 37901-1071 U.S.A. MPMI 3:122-128. Accepted 7 November 1989. Copyright 1990 The American Phytopathological Society.

Factors affecting Bradyrhizobium nod gene expression derived from extracts and root exudates of soybean cultivar Bragg were compared using a bioassay that monitors expression of the plant-inducible nodYABC operon from Bradyrhizobium japonicum strain USDA110. Wild-type soybean plants (inoculated and uninoculated with B. japonicum) were compared with an auto-regulatory (“supernodulating”) and two nonnodulating mutants. Analyses of extracts from seeds, cotyledons, roots, stems, and leaves showed levels of nod gene induction characteristic for each tissue type; however, no significant variation was observed between each plant genotype tested. Extracts from root tips of inoculated and uninoculated plants showed similar abilities to induce the nodYABC operon in a time course experiment. Root exudates from wild-type and mutant plants also had similar nod gene inducing ability. The validity of the nod gene induction bioassay was confirmed using HPLC analysis, which showed that the peak size of the inducer daidzein in fractionated root extracts correlated with nod gene inducibility. The results indicate that the autoregulatory and nonnodulating plant mutants do not have significantly different levels of bacterial nod gene signaling molecules compared to wild-type plants. It appears unlikely that alterations in levels of plant signals are responsible for the mutant symbiotic plant phenotypes examined.