Chapter 11 - Plant Symbioses with Frankia and Cyanobacteria

Abstract

In addition to rhizobia, two other groups of prokaryotes enter N2-fixing symbioses with plants. This chapter discusses plant symbioses with Frankia and Cyanobacteria. Actinorhizal nodules are formed in symbioses between the members of the actinomycetous eubacterial genus Frankia and the roots of more than 200 dicotyledonous plant species belonging to 24 genera from eight plant families. In planta, the shape, septation, and subcellular localization of Frankia vesicles are determined by the host plant, which therefore, can direct bacterial differentiation. Frankia can also form multilocular sporangia. The members of the Fabales are colonized intracellularly, whereas the members of the other plant groups are colonized intercellularly. Cyanobacterial symbioses covers the general distribution of cyanobacterial symbioses, the plant organs, the cyanobionts, evolutionary aspects, the colonization units— -hormogonia, signals, the colonization process, and adaptations and exchange of metabolites. A recent study implies that the organ colonized in Gunnera (stem glands) only develops under combined N deprivation. The largest variation in cyanobionts is found in lichens, ranging in morphology from unicellular to more complex filamentous forms. The vital role of hormogonia in the colonization process has also been demonstrated in reconstitution experiments. The importance of hormogonia in symbiosis is also manifested in their chemotactic behavior. The colonization of plants by cyanobacteria has been intensively studied in the Gunnera symbiosis. The host's control of cyanbiont proliferation is supported by the observation that the relative proportion of cyanobacterial biomass compared to plant biomass in symbiotic organs always remains relatively small.