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Lipogenesis and Redox Balance in Nitrogen-Fixing Pea Bacteroids
Jason J Terpolilli1, Shyam K Masakapalli2, Ramakrishnan Karunakaran3
1Centre for Rhizobium Studies, Murdoch University, Perth, Australia Department of Molecular Microbiology, John Innes Centre, Norwich, United Kingdom.
Journal of Bacteriology
|August 10, 2016
Summary
Rhizobia in legume nodules use the tricarboxylic acid (TCA) cycle for energy but store acetyl-CoA in lipids and poly-β-hydroxybutyrate (PHB) instead of complete oxidation. This lipid synthesis is crucial for nitrogen (N2) fixation efficiency.
Area of Science:
- Microbiology
- Biochemistry
- Plant Science
Background:
- Biological nitrogen fixation by symbiotic rhizobia in legume root nodules is vital but energy-intensive.
- Rhizobia differentiate into bacteroids within nodules, oxidizing host-derived dicarboxylic acids via the tricarboxylic acid (TCA) cycle.
Purpose of the Study:
- To investigate the metabolic fate of acetyl-CoA in pea bacteroids during symbiotic nitrogen fixation.
- To understand the role of lipid synthesis and poly-β-hydroxybutyrate (PHB) production in bacteroid metabolism and N2 fixation.
Main Methods:
- Comparative metabolomic profiling of free-living Rhizobium leguminosarum and pea bacteroids.
- Analysis of metabolic flux from [(13)C]succinate in laboratory-grown R. leguminosarum.
Main Results:
- The TCA cycle is essential for maximal N2 fixation rates.
- Pea bacteroids exhibit significantly lower levels of pyruvate, acetyl-CoA, free CoA, and citrate compared to free-living bacteria.
- Bacteroids channel acetyl-CoA into lipid synthesis and poly-β-hydroxybutyrate (PHB) production via a type III PHB synthase.
Conclusions:
- Lipogenesis and PHB synthesis are fundamental for balancing NAD(P)H production and redox poise required for N2 fixation.
- Bacteroid metabolism deviates from complete acetyl-CoA oxidation, prioritizing energy storage and redox balance for symbiosis.
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