Lipogenesis and Redox Balance in Nitrogen-Fixing Pea Bacteroids

Abstract

Within legume root nodules, rhizobia differentiate into bacteroids that oxidize host-derived dicarboxylic acids, which is assumed to occur via the tricarboxylic acid (TCA) cycle to generate NAD(P)H for reduction of N 2 . Metabolic flux analysis of laboratory-grown Rhizobium leguminosarum showed that the flux from [ 13 C]succinate was consistent with respiration of an obligate aerobe growing on a TCA cycle intermediate as the sole carbon source. However, the instability of fragile pea bacteroids prevented their steady-state labeling under N 2 -fixing conditions. Therefore, comparative metabolomic profiling was used to compare free-living R. leguminosarum with pea bacteroids. While the TCA cycle was shown to be essential for maximal rates of N 2 fixation, levels of pyruvate (5.5-fold reduced), acetyl coenzyme A (acetyl-CoA; 50-fold reduced), free coenzyme A (33-fold reduced), and citrate (4.5-fold reduced) were much lower in bacteroids. Instead of completely oxidizing acetyl-CoA, pea bacteroids channel it into both lipid and the lipid-like polymer poly-␤-hydroxybutyrate (PHB), the latter via a type III PHB synthase that is active only in bacteroids. Lipogenesis may be a fundamental requirement of the redox poise of electron donation to N 2 in all legume nodules. Direct reduction by NAD(P)H of the likely electron donors for nitrogenase, such as ferredoxin, is inconsistent with their redox potentials. Instead, bacteroids must balance the production of NAD(P)H from oxidation of acetyl-CoA in the TCA cycle with its storage in PHB and lipids. IMPORTANCEBiological nitrogen fixation by symbiotic bacteria (rhizobia) in legume root nodules is an energy-expensive process. Within legume root nodules, rhizobia differentiate into bacteroids that oxidize host-derived dicarboxylic acids, which is assumed to occur via the TCA cycle to generate NAD(P)H for reduction of N 2 . However, direct reduction of the likely electron donors for nitrogenase, such as ferredoxin, is inconsistent with their redox potentials. Instead, bacteroids must balance oxidation of plant-derived dicarboxylates in the TCA cycle with lipid synthesis. Pea bacteroids channel acetyl-CoA into both lipid and the lipid-like polymer poly-␤-hydroxybutyrate, the latter via a type II PHB synthase. Lipogenesis is likely to be a fundamental requirement of the redox poise of electron donation to N 2 in all legume nodules.

Metabolic Analyses of Nitrogen Fixation in the Soybean Microsymbiont Sinorhizobium fredii Using Constraint-Based Modeling

“…Pyruvate is further decarboxylated by pyruvate dehydrogenase to form acetyl-CoA. In both soybean cultivars, soybean bacteroid diverts acetyl-CoA into the TCA cycle and into the production of the lipid-like polymer PHB, both of which are pathways known for electron allocation in nitrogen-fixing bacteroids (80). Although similitudes are observed in the flux distributions, there are also differences in the use of some reactions and/or their magnitudes (Fig.…”

Auxin signalling of Arachis hypogaea activated by colonization of mutualistic fungus Phomopsis liquidambari enhances nodulation and N2 -fixation

“…In infected cells, sucrose is converted to dicarboxylic acid, primary malate. Malate is the final carbon for bacteroids, which significantly affects the development and activity of bacteroids (Terpolilli et al, ). Taken together, we conclude that P. liquidambari symbiosis‐induced auxin signalling activation enhanced rhizobial symbiosis through transcriptional activation of symbiotic pathway and promotion of nodule carbon metabolism.…”

Metabolomics of tomato xylem sap during bacterial wilt reveals Ralstonia solanacearum produces abundant putrescine, a metabolite that accelerates wilt disease

“…Interestingly, quorum sensing regulates R. solanacearum galactose catabolism such that galactose is only metabolized by R. solanacearum at high cell densities, corresponding to successful xylem colonization (Khokhani et al, 2017). 3HB is precursor of the carbon storage molecule polyhydroxybutyrate (PHB), which bacteria often produce when carbon is in excess and redox is constrained (Terpolilli et al, 2016). Microscopy and transcriptomic data suggest PHB metabolism is active when R. solanacearum grows in the xylem (Grimault et al, 1994;Brown and Allen, 2004;Jacobs et al, 2012).…”