Detection of uptake hydrogenase activity in Rhizobium meliloti in symbiosis with Medicago sativa

Uptake hydrogenase (Hup) activity in N2-fixing microsymbionts recovers some of the energy wasted during nitrogenase enzyme activity, but is relatively limited in Rhizobium meliloti. Uptake hydrogenase activity of R. meliloti strains was evaluated with procedures involving radioactive hydrogen utilizing nodules on inoculated alfalfa, Medicago sativa L cv Spredor 2. Nodules were harvested, weighed, and incubated in the presence of H-3(2) (50-mu-Ci) gas, and Hup activity of R. meliloti strains was measured. All strains formed nodules, and except for strains SS-4 and Cp113, which were considered Hup-, all exhibited some degree of Hup activity and formed effective root nodules as determined visually by the presence of leghemoglobin and by plant growth relative to the growth of noninoculated plants. Strain 1024 (USDA collection) showed the highest Hup activity (43 nmol H-2 mg-1 fresh nodule weight h-1). Strains 55-5, BL-1, RMZ-2, and SL-5 had moderate levels of Hup activity, whereas the remaining strains produced very little Hup activity. There was very little decline in activity after 5 days in strains 1024, 55-5, BL-1, RMZ-2, and SL-5, which was attributed to incorporation of H-3(2) into nodular cells. Inoculating superior alfalfas with these Hup+ Rhizobium could enhance N2-fixation efficiency and increase productivity.