A Rapid Regulatory Response Governing Nodulation in Soybean

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Abstract
The number of nodules which develop on the primary root of soybean seedlings after inoculation with Rhizobium japonicum is substantially diminished in the region of the root developmentally 10-15 h younger than the region maximally susceptible to nodulation at the time of inoculation. This rapid inhibition of nodulation was investigated by inoculating soybean seedlings with rhizobia at 2 different times, 15 h apart. Living R. japonicum cells, but not heterologous rhizobia or UV-killed cells of the homologous bacterium, were capable of eliciting the rapid inhibitory response. Nodulation responses to varying inoculum concentrations showed that bacterial dosages could be superoptimal, resulting in reduced nodulation and reduced inhibition of nodulation. When suspensions of R. japonicum were dripped uniformly onto the root surfaces, the degree of inhibition of nodulation in developmentally younger regions of the root was correlated with the number of nodules formed in the older and initially most susceptible region of the root. Nodulation in the developmentally younger region of the root, however, was affected very little if the 1st inoculum was restricted to contact with root cells in the region initially most susceptible to nodulation. The rapid regulatory response may be an important factor contributing to the clustering of nodules in the crown region of soybean roots in field-grown plants and the sparse nodulation commonly observed in younger regions of the root.