Detection of uptake hydrogenase activity inRhizobium melilotiin symbiosis withMedicago sativa

Abstract

Abstract 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 3H2 (50 μCi) 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 H2 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 pr...