Adenylate Gradients and Ar:O2 Effects on Legume Nodules: I. Mathematical Models

Mathematical models were developed to test the likelihood that large cytosolic adenylate concentration gradients existacross the bacteria-infected cells of legume nodules. Previous studies hypothesized that this may be the case to account forthe unusually low adenylate energy charge (AEC; 0.65) measured in the plant fraction of metabolically active nodules (M.M.Kuzma, H. Winter, P. Storer, I. Oresnik, C.A. Atkins, D.B. Layzell [1999] Plant Physiol 119: 399–407). Simulations coupledleghemoglobin-facilitated O2 diffusion into the infected cell, through bacteroid nitrogenase activity, with the ATP demandfor transport and ammonia assimilation in the plant fraction of ureide- and amide-producing nodules. Although largecytosolic adenylate gradients were predicted to exist in both nodule types, amide nodules were predicted to have steeperAEC gradients (0.82–0.52) than ureide nodules (0.82–0.61). The differences were attributed to an additional ATP demand forAsn synthesis in the amide nodule. Simulations for nodules transferred to an Ar:O2 atmosphere predicted a major reductionin the magnitude of adenylate gradients and an increase in the AEC of the plant fraction. Results were consistent with anumber of experimental studies and were used to propose an experimental test of the models.