Development and Partial Characterization of Nearly Isogenic Pea Lines (Pisum sativum L.) that Alter Uptake Hydrogenase Activity in Symbiotic Rhizobium

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Abstract
Some Rhizobium bacteria have H2-uptake (Hup) systems that oxidize H2 evolved from nitrogenase in leguminous root nodules. Pea (Pisum sativum L.) cultivars ''JI1205'' and ''Alaska'' produce high Hup (Hup++) and moderate Hup (Hup+) phenotypes, respectively, in Rhizobium leguminosarum 128C53. The physiological significance and biochemical basis of this host plant genetic effect are unknown. The purpose of this investigation was to advance basic Hup studies by developing nearly isogenic lines of peas that alter Hup phenotypes in R. leguminosarum strains containing hup genes. Eight pairs of nearly isogenic pea lines that produce Hup++ and Hup+ phenotypes in R. leguminosarum 128C53 were identified in 173 F2-derived F6 families produced from crosses between JI1205 and Alaska. Tests with the pea isolines and three strains of hup-containing R. leguminosarum showed that the isolines altered Hup activity significantly (P .ltoreq. 0.05) in 19 of 24 symbolic combinations. Analyses of Hup phenotypes in F6 families, the F1 population, and two backcrosses suggested involvement of a single genetic locus. Three of the eight pairs of isolines were identified as being suitable for physiological studies, because the two lines in each pair showed similar growth, N assimilation, and flowering traits under nonsymbiotic conditions. Tests of those lines under N2-dependent conditions with isogenic Hup+ and negligible Hup (Hup-) mutants of R. leguminosarum 128C53 showed that, in symbioses with Hup+ rhizobia, two out of three Hup++ pea lines decreased N2 fixation relative to Hup+ peas. In one of those cases, however, the Hup++ plant line also decreased fixation by Hup- rhizobia. When results were averaged across all rhizobia tested, Hup+ pea isolines had 8.2% higher dry weight (P .ltoreq. 0.05) and fixed 12.6% more N2 (P .ltoreq. 0.05) than Hup++ isolines. Pea lines described here may help identify host plant factors that unfluence rhizobial Hup activity and should assist in clarifying how Hup systems influence other physiological processes.