Symbiotic Hydrogenase Activity in Bradyrhizobium sp. (Vigna) Increases Nitrogen Content in Vigna unguiculata Plants

Bradyrhizobium sp. (Lupinus) and Bradyrhizobium sp. (Vigna) mutants in which hydrogenase (hup) activity was affected were constructed and analyzed. Vigna unguiculata plants inoculated with the Bradyrhizobium sp. (Vigna) hup mutant showed reduced nitrogenase activity and also a significant decrease in nitrogen content, suggesting a relevant contribution of hydrogenase activity to plant yield. Bacteria belonging to the group collectively known as rhizo-bia form nitrogen-fixing nodules in symbiosis with leguminous plants. In these symbioses, a large amount of hydrogen is released from nodules as a by-product of the nitrogenase re-action. Certain rhizobial strains are able to oxidize this hydro-gen through the expression of uptake hydrogenases. Hydrogen recycling has been shown to reduce energy losses associated with diazotrophy (19). Consequently, incorporation of the hy-drogen oxidation capability into rhizobial strains has been pro-posed as a way to improve symbiotic nitrogen fixation (11). However, enhancement of legume host productivity associate