Abstract
Cyanobacteria are oxygenic photosynthetic bacteria, many of which have ability to fix nitrogen. Filamentous cyanobacteria of the genus Nostoc produce differentiated cells such as heterocysts (specialized cells for nitrogen fixation) and hormogonia (transient motile filements which function as infective units), and many of them are known to establish symbiotic association with various eukaryotic hosts, including plants, algae and fangi.
Molecular-phylogenetic analyses of free-living and symbiotic Nostoc species suggest that symbiotic association may have evoloved polyphyletically within the clade. Co-culturing experiments using a bryophyte host indicate that their ability of hormogonia formation may not correspond to plant-infection efficiency.
Continued molecular-biological and physiological study of Nostoc species will provide a clue to understand mechanisms and evolution of the cyanobacterium-plant symbioses.
