Enhancement of the nitrogen fixation efficiency of genetically-engineered Rhizobium with high catalase activity.

The vktA catalase gene, which had been cloned from Vibrio rumoiensis S-1T having extraordinarily high catalase activity, was introduced into the root nodule bacterium, Rhizobium leguminosarum bv. phaseoli USDA 2676. The catalase activity of the vktA-transformed R. leguminosarum cells (free-living) was three orders in magnitude higher than that of the parent cells and this transformant could grow in a higher concentration of exogenous hydrogen peroxide (H2O2). The vktA-transformant was inoculated to the host plant (Phaseolus vulgaris L.) and the nodulation efficiency was evaluated. The results showed that the nitrogen-fixing activity of nodules was increased 1.7 to 2.3 times as compared to the parent. The levels of H2O2 in nodules formed by the vktA-transformant were decreased by around 73%, while those of leghemoglobins (Lba and Lbb) were increased by 1.2 (Lba) and 2.1 (Lbb) times compared with the parent. These results indicated that the increase of catalase activity in rhizobia could be useful to improve the nitrogen-fixing efficiency of nodules by the reduction of H2O2 content concomitantly with the enhancement of leghemoglobins contents.