Over the past millions of years, plants and bacteria have developed a large number of interactions. From those, symbiosis is one of the most interesting. An overview is given of the possible origin of photosynthetic eukaryotic organisms through various endosymbiotic relationships of Archaeal cells with primitive spirochaetes, aerobic bacteria and cyanobacteria. Subsequent extention of the potentiality of eukaryotes has resulted in a wide variety of endo- and exo-symbiotic relations. From those, the nitrogen-fixing symbiosis is the best known relationship. Nif genes are widely distributed within the phylogenetic tree of Bacteria and Archaea. Only a few micro-organims have developed symbiotic relationships with a large variety of eukaryotes. Remarkable similarities occur in the processes of recognition of the partners and the regulation of the nitrogen metabolism of the symbiont. Most information is now available on the Rhizobiumlegume symbiosis. Plants excrete flavonoids and the bacteria produce oligosaccharides (nod-factors). Both signal molecules are highly specific and work at the gene expression level of the partners. In the nitrogen-fixing stage, the growth of the microsymbiont is retarded, the ammonia-assimilating system is inhibited and ammonia is excreted in the cytoplasm of the host. The uniformity of symbiotic events may indicate that construction of new types of nitrogen-fixing symbioses through recombinant DNA techniques is not excluded. Rapid success with this approach, however, remains far from realistic.