UREASE-NULL AND HYDROGENASE-NULL PHENOTYPES OF A PHYLLOPLANE BACTERIUM REVEAL ALTERED NICKEL METABOLISM IN 2 SOYBEAN MUTANTS

Mutation at either of two genetic loci (Eu2 or Eu3) in soybean (Glycin e max [L.] Merr.) results in a pleiotropic elimination of the activity of both major urease isozymes. Surprisingly, the phenotype of a phyll oplane bacterium, Methylobacterium mesophilicum, living on the leaves of eu2/eu2 or eu3-e1/eu3-e1 mutants is also affected by these plant mu tations. The bacteria isolated from leaves of these soybean mutants ha ve transient urease- and hydrogenase-deficient phenotypes that can be corrected by the addition of nickel to free-living cultures. The same bacterium growing on wild-type soybeans or on urease mutants eu1-sun/e u1-sun or eu4/eu4, each deficient in only one urease isozyme, are urea se-positive. These results suggest that the bacterium living on the eu 2/eu2 or eu3-e1/eu3-e1 mutant is unable to produce an active urease or hydrogenase because it is effectively starved for nickel. We infer th at mutations at Eu2 or Eu3 result in defects in nickel metabolism but not in Ni2+ uptake or transport, because eu2/eu2 and eu3-e1/eu3-e1 mut ants exhibit normal uptake of (NiCl2)-Ni-63. Moreover, wild-type plant s grafted on mutant rootstocks produce seeds with fully active urease, indicating unimpeded transport of nickel through mutant roots and ste ms.