Metabolic constraints on nitrogen fixation by rhizobia in legume nodules

Preprint

Rhizobia induce nodule formation on legume roots and differentiate into bacteroids, which use plant-derived dicarboxylates as energy and electron sources for reduction of atmospheric N2 into ammonia for secretion to plants. Using heterogeneous genome-scale datasets, we reconstructed a model of bacteroid metabolism to investigate the effects of varying dicarboxylate and oxygen supply on carbon and nitrogen allocation. Modelling and ¹³C metabolic flux analysis in bacteroids indicate that microaerobiosis restricts the decarboxylating arm of the TCA cycle and limits ammonia assimilation into glutamate. Catabolism of dicarboxylates induces a higher oxygen demand but also a higher NADH/NAD⁺ ratio compared to sugars. Carbon polymer synthesis and alanine secretion by bacteroids facilitate redox balance in microaerobic nodules with alanine secretion increasing as oxygen tension decreases. Our results provide a framework for understanding fundamental constraints on rhizobial metabolism during symbiotic nitrogen fixation.

Authors: Schulte, CCM; Borah, K; Wheatley, RM; Terpolilli, JJ; Saalbach, G

• Publication status: Published
• Peer review status: Not peer reviewed
• Preprint server: bioRxiv
• Publication date: 2021-02-16
• DOI: 10.1101/2021.02.16.431433
• EISSN: 2692-8205
• Language: English