Frankia and Actinorhizal Plants: Symbiotic Nitrogen Fixation

Abstract

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 underly- ing the evolution of an intracellular plant-bacterial symbiosis. In contrast with the unique stem-like structure of legume nodules, actinorhizal nodules are com- posed of modified lateral roots with infected cells in the expanded cortex. In contrast with rhizobia, Frankia strains can protect the oxygen-sensitive nitroge- nase enzyme complex, and thus nitrogen fixation, from oxygen. Therefore, oxy- gen 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.