A Model of the Regulation of Nitrogenase Electron Allocation in Legume Nodules (I. The Diffusion Barrier and H2 Inhibition of N2 Fixation)

Abstract

A mathematical model is presented to explain the regulation of nitrogenase electron allocation to N, fixation (FAC) in legume nodules. l h e model is based on two assumptions: (a) that Hz inhibits Nz fixation in a competitive manner; and (b) that O,, H,, and N, move into and out of nodules by diffusion and their movement is impeded by a diffusion barrier, the permeability of which is controlled to maintain a very low infeded cell O, concentration. When the model was used to simulate nodules displaying a range of values for total nitrogenase activity (TNA), maximum FAC values were predicted to be between 0.69 and 0.71, and a negative correlation was predicted to exist between FAC and TNA. These predictions were in good agreement with empirically derived values reported in the literature and support the suggestion that Hz inhibition of Nz fixation is a major determinant in the regulation of nitrogenase FAC in legume nodules. Two versions of the model were constructed. A closed-pore model assumed that the diffusion barrier consisted of a solid shell of water of variable thickness in the nodule cortex. An open-pore model assumed that a small number of gas-filled intercellular spaces connected the nodule central zone with the root atmosphere and these pores were opened or closed by water to provide variations in the nodule's permeability to gas diffusion. Because of differences in the diffusivity of gases in the gaseous and aqueous phases, the model predicted that, at a given infected cell 0, concentration, an openpore diffusion barrier would result in less Hz accumulation in the infected cells than a closed-pore diffusion barrier. Therefore, the model may be used to test specific hypotheses about the physical structure of the barrier to gas diffusion in legume nodules.Nitrogenase, the bacterial enzyme responsible for Nz fixation, simultaneously reduces Nz to NH3 and reduces protons to Hz gas. The EAC of nitrogenase is a measure of the proportion of total electron flow through the enzyme that is used to reduce Nz. EAC is thus a measure of the efficiency of NZ fixation (Edie, 1983;Edie and Phillips, 1983). HZ evolution seems to be associated with the binding of NZ to nitrogenase, and at least 25% of the total electron flow through nitrogen- ase during NZ fixation is expended in the reduction of protons (Simpson and Bums, 1984; Thomeley and Lowe, 1985). Therefore, the maximum theoretical EAC for NZ fixation is 0.75. Most investigators of legume symbioses have reported EAC values under ambient conditions at less than the theoretical maximum, usually between 0.60 and 0.72 (Edie, 1983;Mahon and Nelson, 1986; Walsh and Layzell, 1986;Hunt et al., 1987;Walsh et al., 1987;Vessey et al., 1988aVessey et al., , 1988b. The reasons for this apparent inefficiency of NZ fixation and the mechanisms responsible for the regulation of EAC in vivo are not known.In vitro studies have shown that the EAC of isolated nitrogenase can be increased by manipulating factors that increase the rate of electron flux through the MoFe protein componen...

A Re-Evaluation of the Role of the Infected Cell in the Control of O2 Diffusion in Legume Nodules

“…AI1 these mechanisms are based on the fact that solubility and diffusion of 0 2 in an aqueous liquid phase are much lower than in a gas phase. Mathematical models of gas diffusion in nodules (Sheehy et al, 1985(Sheehy et al, , 1987Sheehy and Bergersen, 1986;Hunt et al, 1988;Denison, 1992;Moloney and Layzell, 1993) are consistent with the presence of a variable bamer in the cortex, which functions to maintain an extremely low O2 concentration throughout the entire central infected zone of the nodule.…”

Can genotypes of soybean (Glycine max) selected for nitrate tolerance provide good “models” for studying the mechanism of nitrate inhibition of nitrogenase activity?

“…This difference was consistent with the results of previous studies (Sung et al 1991, Diaz del Castillo et al 1992) which have reported a small decline in EAC in nodules in which TNA has been inhibited. Moloney and Layzell (1993) have provided evidence that the EAC shift is a consequence of a greater banier to O2, H2 and N2 diffusion that results in more competitive inhibition of N2 fixation by Hj, gas within the infected cells.…”