SUMMARY: Transfer of plasmid pIJ1008, a recombinant of two indigenous plasmids into strain 300 produced strain 3960, which reduced significantly more N in pea root nodules than did strain 300 itself. Strain 3960 was superior to the field isolate 128C53, from which the symbiotic determinants of pIJ1008 were derived, and transfer of plasmid pIJ1008 into two other genetic backgrounds also improved symbiotic performance compared to the introduction of other nodulation plasmids.

As plasmid pIJ1008 carries genetic determinants for an uptake hydrogenase activity (Hup) as well as nodulation capability (Nod) and other determinants for symbiotic nitrogen fixation (Fix), the increased effectiveness of strains carrying pIJ1008 may result from their capacity to conserve energy by recovering H evolved by nitrogenase.

Intact plant studies with N showed that the superior N fixation capability associated with pIJ1008 was enhanced by 2 m-NO , a common concentration of soil N. It was also shown that, as plants grew older, the hydrogenase determined by pIJ1008 was not able to recycle all the hydrogen evolved by pea root nodules.


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