Identification of Rhizobium Leguminosarum Genes and Signal Compounds Involved in the Induction of Early Nodulin Gene Expression

The process of root nodule formation on legumes, induced by Rhizobium, can be looked upon as a sequence of several distinct steps. These steps have been defined by cytological studies on developing wild-type root nodules, and by analyses of nodules formed by either plant or bacterial mutants (Vincent 1980). Nowadays attachment of bacteria, root hair deformation and curling, induction of cortical cell division, infection thread formation, nodule development, bacterial release from infection threads, bacteroid development and effective nitrogen fixation are recognized as successive steps in root nodule formation (Vincent 1980). The multistep nature of root nodule formation has led to the hypothesis that at several stages in the Rhizobium-plant interaction signal molecules from either symbiontic partner are involved in inducing a process in the other partner Identification of the different bacterial and plant signals and analysis of the mode of action of each seperate compound would then significantly enlarge our knowledge about the establishment of symbiosis.