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. 2013 Mar 4;14(3):5198–5213. doi: 10.3390/ijms14035198

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© 2013 by the authors; licensee MDPI, Basel, Switzerland.

This article is an open-access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/3.0/).

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Exchange of metabolites between the host plant and Sm2011 during N₂ fixation in M. truncatula and M. sativa. Sucrose is delivered to the nodules via the phloem where it is cleaved to malate which is ultimately provided to the bacteroid through the peribacteroid membrane. Within the bacteroid, malate is oxidized by TCA cycle to provide reductants for nitrogenase and for the respiratory chain that fuels nitrogenase with ATP. The presence of leghemoglobin in the host cell controls the concentration of O₂ available to the respiratory chain in peribacteroid membrane. The bacteroid returns NH₄⁺ to the host plant which is assimilated into asparagine and other amino acids that are exported from the nodule via the xylem. AAs, amino acids; ASN, asparagine; Lb, leghemoglobin; N₂ase, nitrogenase; PBM, peribacteroid membrane; PH, phloem; RC, respiratory chain; XY, xylem.