| Popis: |
The nitrogenase enzyme requires large amounts of ATP which, within legume symbioses, is provided by oxidative phosphorylation. However, nitrogenase is irreversibly inhibited by O2, so for normal nodule function, bacteroids must be maintained in a low O2 environment, but supplied with a high O2 flux to satisfy their respiratory needs. Our current understanding of how legume nodules regulate O2 concentration and supply rate can be summarized as follows: (a) O2 enters nodules by passive diffusion. A barrier to gas diffusion, thought to be present in the inner cortex, maintains the O2 concentration in the central, infected region of the nodule at a level which is below the detection limit of O2 microelectrodes (Witty et al 1987, Tjepkema, Yocum 1974). Numerous studies have shown that nodules have the ability to regulate their permeability to O2 diffusion and thereby control O2 concentration in the infected cells (Layzell, Hunt 1990). (b) To maintain a high O2 flux at the low O2 concentration in the central infected zone, legume nodules have interconnecting gas-filled spaces between the cells and also possess leghemoglobin (Lb), an O2-binding protein. Lb is thought to facilitate the diffusion of O2 from the surface of the infected cells to the bacteroids. Mathematical models of leghemoglobin function (Sheehy, Bergersen 1986, Hunt et al 1988) have predicted that the concentration of this protein is in great excess of that needed to facilitate the estimated O2 flux. |
