Colonization of roots of rice (Oryza sativa) by symbiotic Nostoc strains

•   The lack of nitrogen in agriculture, and negative environmental effects of fertilizers, have stimulated interest in creating artificial associations between N₂ -fixing cyanobacteria and rice (Oryza sativa). •   For the first time, numerous (57) Nostoc isolates from natural symbioses were screened for their ability to associate with rice. Successful colonizers were tested for N₂ -fixation by acetylene reduction_, and for their ability to adsorb to roots by chlorophyll a measurements. Paranodules were induced by 2,4-dichlorophenoxyacetic acid. And genetic fingerprints of the cyanobacteria were obtained for identification. Ultrastructural investigations were made by light and scanning electron microscopy. •   Twenty-one symbiotic Nostoc isolates associated with rice roots, colonizing surfaces and intercellular spaces. Adsorption was high and appeared biphasic. The rates of N₂ fixation by associated cyanobacteria were higher compared with those in free-living cyanobacteria. Paranodules were formed and colonized, but root growth was adversely affected. •   Under laboratory conditions, artificial associations were created between one-third of the screened symbiotic cyanobacteria and rice. The agricultural potential for the association appears high since the cyanobacteria adsorb tightly and fix more N₂ than when free-living.