Transformation as a tool to study the genetics of nodulation in Glycine max

Transformation has proven a powerful tool to elucidate the function of specific genes in animal, plant and bacterial systems. Nodulation of Glycine max is the result of a complex symbiosis between the bacterium Bradyrhizobium japonicum and the plant. The infection of root hairs by nitrogen fixing bacteria and the subsequent nodule development is regulated by numerous genes in both the plant and the bacteria. Little is known about the underlying biochemistry of this symbiosis. However, this symbiosis offers an excellent model for studying the various mechanisms which control plant cell division and development. Plant transformation will help to elucidate the role of these symbiotic plant genes. In this study methods were developed for the efficient transformation of a range of soybean nodulation mutants. The introduction of specific gene sequences has shown the ability to fundamentally alter nodule structure. Analysis of this alteration suggests changes in the nodule development are stimulated by extended cell divisions in specific areas of the nodule. This is the first time that transgenic soybean nodules have been produced or genetically altered. Abnormal maintenance and normal integration of transgenic sequences was observed in the soybean plants. Transgenic sequences appeared both autonomously maintained and integrated into the genome. These events were routinely observed in the same plant depending on the DNA sequence transferred. The non-classical transformation events were typified by autonomously replicating plasmids in the plant cell. This is the first time that such an effect has been observed in soybeans; however this observation mirrors autonomous plasmid maintenance in the animal kingdom.