Two types of root nodule symbioses between higher plants and nitrogen-fixing soil bacteria are known, the legume symbiosis with Gram-negative rhizobia and the actinorhizal symbiosis between a diverse group of ca. 280 species from eight different families of three orders, Fagales, Cucurbitales and Rosales. All plants able to enter a root nodule symbiosis go back to a common ancestor, i.e., they belong to a single clade. Phylogenomic studies have revealed that non-symbiotic members of this clade have lost symbiosis-related genes. Thus, the common ancestor had evolved a symbiosis which was subsequently lost in the majority of lineages. The microsymbiont of the common ancestor must have been a Frankia strain; eventually, microsymbiont exchange has taken place in legumes. It has long been an aim to tranfer the capability to enter a root nodule symbiosis to non-symbiotic crop plants to render them independent of nitrogen fertilizer. In a project financed by the Tryggers foundation, we want to do family- or order-specific comparisons of the genes induced in nodules compared to roots, and the presence/absence of nodulation-related genes identified in legume symbioses, in different groups of actinorhizal plants.