Suppression of the Symbiotic Supernodulation Symptoms of Soybean

Supernodulation mutants of soybean (Glycine max L. Merr.) produce very high numbers of nodules and increased nodule mass compared to the parent cultivar Bragg in the absence or presence of nitrate. All (12 were tested) mutants also display a nitrate tolerant symbiotic (nts) as well as a supernodulation phenotype suggesting that nitrate inhibition of nodulation and endogenous autoregulation are at least in part jointly controlled by plant genes. Genetic analysis suggests that single recessive mendelian alleles at a single locus are involved. Supernodulation of nts mutants was suppressed by a variety of means. These include (a) suppression by low inoculum, (b) suppression by grafting, (c) suppression by another gene (epistatic), and (d) suppression by wild type vascular sap or methanol extract refeeding. These methods show that (1) nitrate tolerance of nodulation can be expressed even if supernodulation phenotype is not expressed, (ii) non-nodulation mutants epistatically suppress supernodulation, (iii) shoots of wild-type or non-nodulation mutant soybeans, or Glycine soja Sieb. and Zucc., suppress supernodulation and (iv) methanol extracts from inoculated wild type plants (but not uninoculated wild type or mutant nts382 plants) suppress supernodulation by 60 to 80 %. We are presently using these tools and the relevant material to investigate further the genetic basis of autoregulation of nodulation in legumes.