The production of the phytotoxin rhizobitoxine by Bradyrhizobium japonicum in soybean root nodules confers a pathogenic character to the normally mutually beneficial symbiosis by causing a transient foliar chlorosis, thus blurring the distinction between mutualistic and parasitic symbiosis. Toxin synthesis is dependent upon the genotype of both the host plant and bacteria. By selecting for mutants that no longer synthesize rhizobitoxine in culture, several mutants with altered symbiotic phenotypes have been identified. One group of mutants does not make toxin in culture or in planta. These include mutations in structural genes in rhizobitoxine biosynthesis. Interestingly, one of these genes requires a translation frameshift for expression. Symbiotically, the most interesting group of mutants are those that fail to synthesize rhizobitoxine in cluture, but do produce rhizobitoxine in planta at levels greater than wild type. Consequently, these mutants produce more severe chlorosis from which the plant does not recover. In addition, this group of mutants has altered mutualistic symbiosis. The parental strain is restricted in its ability to nodulate soybeans that carry the Rj4 gene. These mutants are able to nodulate Rj4 soybeans at rates similar to unrestricted strains. This group of mutants demonstrates a relationship between rhizobitoxine biosynthesis and host range functions of Bradyrhizobium. The goals of this proposal are to examine the biochemical and genetic mechanisms controlling these parasitic and mutualistic symbiotic functions.
