Symbiotic Relationships in Actinorhizae

The importance of nitrogen-fixing non-legumes for the nitrogen economy of certain ecosystems has been known for a long time (see Silvester, 1977), and some of these plants probably played an essential part in soil reconstruction during and after the formation of glaciers in the Pleistocene age when species of Dryas, Hippophae, Elaeagnus, Alnus and Shepherdia covered large areas of northern Europe and Canada (Lawrence et al., 1967, Silvester, 1974). Although the nodules of non-legumes were first described in 1829 and nodule formation by an endophyte was demonstrated in 1866 (see Goodchild, 1977), these plants were considered of little agricultural interest and received virtually no attention for nearly a century. It was only in the 1950’s, when their potentiality for forestry was recognized, that studies really began on the symbiotic nitrogen-fixing associations formed by non-legumes. However, fundamental knowledge concerning this type of symbiosis is still very fragmentary, as compared to that known about the symbiotic associations between Rhizobia and legumes, and there is a particular lack of information concerning the microbial partner, the infection processes and the physiological relationships existing within the symbiosis.