Enhanced Nodulation and Nitrogen Fixation by a Revertant of a Nodulation-Defective Bradyrhizobium japonicum Tryptophan Auxotroph

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Abstract
In greenhouse studies, the symbiotic properties of a prototrophic revertant (TA11 NOD + ) of a nodulation defective tryptophan auxotroph of Bradyrhizobium japonicum were compared with those of the normally nodulating wild-type strain, B. japonicum I-110 ARS. Strain I-110 ARS was the parent of auxotrophic mutant TA11. Plants inoculated with TA11 NOD + contained significantly more nitrogen per plant than did plants inoculated with wild-type bacteria (275.9 ± 35 versus 184 ± 18 mg). Also, plants that received the revertant were larger, averaging 8.4 ± 0.9 g (dry weight) versus 6.4 ± 0.6 g for those that received the wild-type bacterial strain. Additionally, plants that received the NOD + strain had 56% more nodules and 41% more nodule mass than did control plants. With both inocula, average nodule size and amount of nitrogen fixed per gram of nodule were about the same. These data indicated that the improvement in nitrogen fixation observed with the TA11 NOD + resulted from an increase in the overall nodule number. The physiological basis for this increase in nodulation is not known, but enhanced tryptophan catabolism does not appear to be involved.