E.M. Lodwig1, K. Findlay2, A.J. Downie2, P.S. Poole1
1 School of AMS, University of Reading, Whiteknights, PO Box 228, Reading, RG6 6AJ, UK
2John Innes Centre, Norwich Research Park, Colney, Norwich, NR4 7UH, UK
The role of the carbon storage compounds poly-P-hydroxybutyrate and glycogen in bacteroids is not fully understood. Both compounds are synthesized in bacteroids however the overall pool sizes vary considerably. Furthermore, mutation of the biosynthetic pathways has been shown to affect the efficiency of symbiotic nitrogen fixation in beans (Cevallos et al. 1996; Marroqui et al. 2001). Therefore the pathways appear to be involved in the regulation of bacteroid metabolism. We have constructed single mutants in phaC (PHB synthase) and glgA (glycogen synthase), and a double mutant, to assess their impact on the R. leguminosarum bv. viciae symbiosis. A mutant in phaC in R. leguminosarum strain A34 (mutant RU1328) which is unable to accumulate PHB in free-living cells exhibits a 50% reduction in nitrogenase activity and a 40% reduction in the dry weight of plants. However, the efficiency of the RU1328 symbiosis is not consistent as in a second experiment there were no differences in nitrogenase activity of RU1328 or dry weight of peas. A mutant in glgA (RU1448) which is unable to accumulate glycogen in free-living cells exhibits no significant difference in nitrogenase activity or dry weight when inoculated onto peas. Both mutants in phaC and glgA alter the carbon balance in infected plant cells therefore carbon metabolism of the plant may have a strong influence on the effects of the mutations. The mutant in both genes (RU1478) nodulates pea but forms bacteroids that senesce prematurely and appear to give a Fix" phenotype (nitrogenase activity is not yet available). Together these data suggest that at least one pathway is required for nitrogen fixation but that carbon metabolism via either pathway is relatively plastic.
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