Abstract
In Rhizobium-legume symbiosis, the plant host controls and optimizes the nodulation process by autoregulation. Tn5 mutants of Rhizobium leguminosarum bv. phaseoli TAL 182 which are impaired at various stages of symbiotic development, were used to examine autoregulation in the common bean (Phaseolus vulgaris L.). Class I mutants were nonnodulating, class II mutants induced small, distinct swellings on the roots, and a class III mutant formed pink, bacterium-containing, but ineffective nodules. A purine mutant (Ade-) was nonnodulating, while a pyrimidine mutant (Ura-) formed small swellings on the roots. Amino acid mutants (Leu-, Phe-, and Cys-) formed mostly empty white nodules. Each of the mutants was used as a primary inoculant on one side of a split-root system to assess its ability to suppress secondary nodulation by the wild type on the other side. All mutants with defects in nodulation ability, regardless of the particular stage of blockage, failed to induce a suppression response from the host. Only the nodulation-competent, bacterium-containing, but ineffective class III mutant induced a suppression response similar to that induced by the wild type. Suppression was correlated with the ability of the microsymbiont to proliferate inside the nodules but not with the ability to initiate nodule formation or the ability to fix nitrogen. Thus, the presence of bacteria inside the nodules may be required for the induction of nodulation suppression in the common bean.