DERIVATIVES OF RHIZOBIUM-MELILOTI STRAINS CARRYING A PLASMID OF RHIZOBIUM-LEGUMINOSARUM SPECIFYING HYDROGEN UPTAKE AND PEA-SPECIFIC SYMBIOTIC FUNCTIONS

pIJ1008, a R. leguminosarum plasmid which determines H2 uptake ability and symbiotic functions in pea, was transferable to 3 of 7 natural isolates of R. melliloti tested. In these 3 strains, pIJ1008 was maintained stably with the respective sym megaplasmid indigenous to each R. meliloti strain. These strains carrying both plasmids nodulated alfalfa but not pea. By reisolation and examination of the strains from alfalfa nodule tissue, it was shown that pIJ1008 continued to be maintained but that pea nodulation ability was suppressed. In 1 strain of R. meliloti which carries a 200 kilobase cryptic plasmid (in addition to a megaplasmid), the transfer and selection for pIJ1008 resulted in the loss of the cryptic plasmid. In 3 separate plant growth experiments, alfalfa nodules induced by each of the R. meliloti strains carrying both sym plasmids were assayed for H2 uptake activity. The average activity was 40-, 3.5- and 2-fold higher than with the respective pIJ1008-free strains. This higher activity was not accompanied by an increase in plant biomass or N content of shoots.