Interaction between Frankia and actinorhizal plants.

Abstract

A significant part of the nitrogen in living organisms is derived from atmospheric dinitrogen which gets incorporated into organic compounds by biological or chemical nitrogen fixation. Biological nitrogen fixation is an energy-consuming process performed by the enzyme nitrogenase, which is irreversibly denatured by oxygen. Nitrogenase is formed only by prokaryotes, which in some cases fix nitrogen in symbiosis with higher plants. Two groups of bacteria can enter symbioses with higher plants which lead to the formation of special organs, the root nodules, where the bacteria fix nitrogen while being hosted inside plant cells. The product of nitrogen fixation, ammonium, is exported to the plant, while the plant in turn provides its symbiont with energy sources. Azorhizobium, Bradyrhizobium, and Rhizobium enter symbioses with leguminous plants (with one exception, Parasponia from the Ulmaceae family; Trinick, 1979), and actinomycetes of the genus Frankia induce nodules on the roots of a diverse group of dicotyledonous plants, mostly woody shrubs, which are collectively referred to as actinorhizal plants (Benson and Silvester, 1993). While rhizobia are unicellular, Frankia usually grows in hyphal form, but can also form sporangia and, under certain conditions, nitrogen-fixing vesicles (Benson and Silvester, 1993). Vesicles are formed at the ends of hyphae or on short side branches either in the free-living state under nitrogen starvation and aerobic conditions, or in symbiosis. Within the vesicles, nitrogenase is protected from oxygen (Parsons et al., 1987) by the vesicle envelope, which consists of multiple layers of hopanoids, bacterial steroid lipids (Berry et al., 1993). In contrast to Frankia, rhizobia can use N2 as nitrogen source only in symbiosis (with one exception, Azorhizobium caulinodans ORS571; reviewed by de Bruijn, 1989).