Moloney A. H., Guy R. D., Layzell D. B.
Department of Biology, Queen's University, Kingston, Ontario, Canada K7L 3N6.
Plant Physiol. 1994 Feb;104(2):541-550. doi: 10.1104/pp.104.2.541.
In N2-fixing legumes, the proportion of total electron flow through nitrogenase (total nitrogenase activity, TNA) that is used for N2 fixation is called the electron allocation coefficient (EAC). Previous studies have proposed that EAC is regulated by the competitive inhibition of H2 on N2 fixation and that the degree of H2 inhibition can be affected by a nodule's permeability to gas diffusion. To test this hypothesis, EAC was measured in soybean (Glycine max L. Merr.) nodules exposed to various partial pressures of H2 and N2, with or without changes in TNA or nodule permeability to gas diffusion, and the results were compared with the predictions of a mathematical model that combined equations for gas diffusion and competitive inhibition of N2 fixation (A. Moloney and D.B. Layzell [1993] Plant Physiol 103: 421-428). The empirical data clearly showed that decreases in EAC were associated with increases in external pH2, decreases in external pN2, and decreases in nodule permeability to O2 diffusion. The model predicted similar trends in EAC, and the small deviations that occurred between measured and predicted values could be readily accounted for by altering one or more of the following model assumptions: K1(H2) of nitrogenase (range from 2-4% H2), Km(N2) of nitrogenase (range from 4-5% N2), the allocation of less than 100% of whole-nodule respiration to tissues within the diffusion barrier, and the presence of a diffusion pathway that is open pore versus closed pore. The differences in the open-pore and closed-pore versions of the model suggest that it may be possible to use EAC measurements as a tool for the study of legume nodule diffusion barrier structure and function. The ability of the model to predict EAC provided strong support for the hypothesis that H2 inhibition of N2 fixation plays a major role in the in vivo control of EAC and that the presence of a variable barrier to gas diffusion affects the H2 and N2 concentration in the infected cell and, therefore, the degree of H2 inhibition.
在固氮豆科植物中，通过固氮酶的总电子流比例（总固氮酶活性，TNA）用于固氮的部分被称为电子分配系数（EAC）。先前的研究提出，EAC受H₂对固氮的竞争性抑制调节，且H₂抑制程度会受到根瘤对气体扩散的通透性影响。为验证该假设，在暴露于不同H₂和N₂分压的大豆（Glycine max L. Merr.）根瘤中测量EAC，同时观察TNA或根瘤对气体扩散的通透性有无变化，并将结果与结合了气体扩散方程和固氮竞争性抑制方程的数学模型预测值进行比较（A. Moloney和D.B. Layzell [1993] Plant Physiol 103: 421 - 428）。实验数据清楚地表明，EAC的降低与外部pH₂升高、外部pN₂降低以及根瘤对O₂扩散的通透性降低有关。模型预测了EAC的类似趋势，测量值与预测值之间出现的小偏差可以通过改变以下一个或多个模型假设轻松解释：固氮酶的K1(H₂)（范围为2 - 4% H₂）、固氮酶的Km(N₂)（范围为4 - 5% N₂）、扩散屏障内组织的全根瘤呼吸分配比例小于100%以及存在开放孔道与封闭孔道的扩散途径。模型开放孔道和封闭孔道版本的差异表明，有可能将EAC测量用作研究豆科植物根瘤扩散屏障结构和功能的工具。模型预测EAC的能力为以下假设提供了有力支持：H₂对固氮的抑制在体内对EAC的控制中起主要作用，并且可变气体扩散屏障的存在会影响受感染细胞中的H₂和N₂浓度，进而影响H₂抑制程度。
