ABSTRACT
Rhizobium and Brady rhizobium bacteria are unique among microorganisms in their ability to induce the formation of nitrogen-fixing nodules on leguminous plants. Nodule formation involves a specific recognition between the prokaryotic and eukaryotic partners, invasion of plant cells by bacteria, and many changes in the structure and biochemistry of both organisms as the nodule develops. The development of nitrogen-fixing nodules has been characterized as occurring in a series of stages, each of which may be influenced by one or more genes in each symbiotic partner. Many specialized vectors and techniques have been developed for cloning, mobilization, and mutagenesis of genes in the Rhizobiaceae, as discussed elsewhere. In some heterologous strain/plant combinations, there is no interaction, even at the earliest stages of nodulation: root hairs are not markedly curled, and no nodule-like structures are formed. Host-specificity nod genes are those that influence the specificity of strains for specific host plants.
