Rhizobium trifolii Mutant Interactions During the Establishment of Nodulation in White Clover

Abstract

A more refined bacteroid isolation procedure was developed and used to investigate the establishment of nodulation of white clovers by Rhizobium tri/olii strain T1. Combinations of a variety of bacterial strains were mixed together for inoculation of clovers. The results showed that in nodules containing two different strains of Rhizobium only one type of inoculum strain could be reisolated from any one plant cell. Furthermore, it was found that nodules which resulted from inoculation with cell mixtures containing the non-invasive Agrobacterium tume/aciens strain B6S3 and a nodulating Rhizobium strain at various cell ratios did not contain strain B6S3. However, a number of invasive but non-nodulating mutants of Rhizobium could be readily reisolated from nodules resulting from inoculation with cell mixtures containing both the mutant and its parent strain. Moreover, some combinations of symbiotic mutants showed a cooperative interaction giving rise to nitrogen-fixing nodules. From an investigation of the genetic characteristics of the bacterial cells isolated from nodules, it was concluded that this interaction between different symbiotically defective mutants of Rhizobium is a form of cross-feeding and not genetic complementation. Furthermore, there are four possible steps in the nodulation process where the intra-nodule interactions can occur between different strains of Rhizobium.