Back to top


Figure 4.49 Transport and metabolism in an infected nodule cell. Sucrose from the shoot is converted to malate in the plant and imported across the symbiosome membrane and into bacteroids, where it fuels nitrogen fixation. The product of the nitrogen fixation is then exported back to the plant, where it is assimilated into asparagine (Asn) for export to the shoot (blue arrows). In many legumes, such as soybean, the export products are ureides instead of Asn. The plant must provide metals and ions to the bacteroid, although only some of the transport systems on the symbiosome and bacteroid membranes are defined. Many rhizobia lack the ability to make homocitrate or become symbiotic auxotrophs for supply of branched-chain amino acids and become dependent on the plant. (Reproduced with permission by Annual Reviews from M. Udvardi and P.S. Poole, Annu Rev Plant Biol 64: 781-805, 2013)