When Does the Self-Regulatory Response Elicited in Soybean Root after Inoculation Occur?

The inoculation of soybean (Glycine max L.) roots with Bradyrhizobium japonicum produces a regulatory response that inhibits nodulation in the younger regions of the roots. By exposing the soybean roots to live homologous bacteria for only a short period of time, the question of whether or not early interactions of rhizobia with root cells, prior to infection, elicit this regulatory response has been explored. B. japonicum cells mixed with infective bacteriophages were applied to the roots and then 6 or 24 hours later roots were again inoculated with phage-resistant rhizobia. Mixing of the rhizobia and bacteriophages caused bacterial lysis in 6 to 8 hours and allowed the bacteria to act as live symbionts on the root for only a few hours. However, the interaction of live homologous bacteria with the soybean roots for a few hours did not cause inhibition of nodulation in the younger regions of the roots. Results of these experiments indicate that the self-regulatory response in soybean is not rapidly produced by the early, pre-infection, interactions between rhizobia and the root cells.