Cyanobacteria-eukaryotic plant symbioses.

N2-fixing heterocystous cyanobacteria develop in symbiotic association with a small number of eukaryotic plant species belonging to the algae, fungi, liverworts, ferns, gymnosperms and angiosperm. When the free-living cyanobacteria develop in symbiosis, they become modified morphologically, physiologically and biochemically. The symbiosis are relatively specific, and among the changes which occur in the endophytic cyanobacteria are increases in the size of the vegetative cells, changes in the ultrastructure of the vegetative cells, a tendency for a reduction in the filamentous habit, an increased heterocyst frequency when another photosynthetic partner is present, reduced activities of glutamine synthetase and glutamate synthase, and NH4+ release by; the endophytic cyanobacteria. These and other aspects are considered, emphasizing, in particular, work carried out in the authors' laboratory.