Regulation of nitrogen fixation in infected cells of leguminous root nodules in relation to O 2 supply

Abstract

Respiration and nitrogen fixation in legume root nodules is considered to be limited by the rate at which O2 from the atmosphere can enter nodules. A thin diffusion barrier in the inner cortex, restricts access to the central tissue where there is a high demand for and low concentration of O2. Observed variations in rates of nodule activities in response to imposed stresses, are often attributed to variations in the diffusion resistance of the barrier. In the present work, alternative or supplementary metabolic mechanisms are considered. Aspects of nodule structure and of metabolism underlying nodule activities are reviewed in terms of components of the symbiotic system, the nature of steady states and in relation to homeostasis of low concentration of O2 within the bacteroid-filled host cells. It is suggested that variations in O2-demand of both mitochondria and bacteroids, serve to preserve nitrogenase activity by poising O2 concentration within ‘safe’ limits. Further, data from isolated soybean bacteroids suggest that nitrogenase is converted to a less active but more robust form, in the presence of O2 in excess of about 70 nM, thus protecting nitrogenase from irreversible inactivation by excess O2. This regulation is rapidly-reversible when O2 concentration falls below about 0.1 µM. Respiration by large numbers of host mitochondria in the periphery of infected nodule cells, adjacent to gas-filled intercellular spaces, is considered to play an important part in maintaining a steep gradient of O2 concentration in this zone. Also, it is possible that variations in nodule O2 demand may be involved in the apparent variations in resistance of the diffusion barrier. It is concluded that there are many biochemical components which should be considered, along with possible changes to the diffusion barrier, when the effects of imposed stresses on nodule activities are being analysed.