Frankia and Actinorhizal Plants: Symbiotic Nitrogen Fixation

Two types of symbioses are known where nitrogen-fixing soil bacteria induce the formation of special organs, i.e. nodules, on the roots of their dicotyledonous host plants; legume-rhizobia symbioses and actinorhizal symbioses. The later are the symbioses between actinobacteria of the genus Frankia and a group of mostly woody plant species from eight families and three different orders (Fagales, Rosales, Cucurbitales). While so far, research has mostly focused on legume-rhizobia symbioses, actinorhizal symbioses with their wider phylogenetic range are more likely to hold the key to understanding the common principles underlying the evolution of an intracellular plant-bacterial symbiosis. In contrast with the unique stem-like structure of legume nodules, actinorhizal nodules are composed of modified lateral roots with infected cells in the expanded cortex. In contrast with rhizobia, Frankia strains can protect the oxygen-sensitive nitrogenase enzyme complex, and thus nitrogen fixation, from oxygen. Therefore, oxygen protection systems established in actinorhizal nodules from different host plants involve contributions of both symbiotic partners. In this chapter, structural and developmental features of actinorhizal symbioses are described.