A critical evaluation of the prospects for nitrogen fixation with non-legumes

Some twenty years ago many speculations were made about possibilities to improve the use of the well-known nitrogen-fixing plant symbioses and to extend the use of atmospheric nitrogen to the most important agricultural plants, like cereals. Since then our understanding of the molecular biology of nitrogenase and the symbiotic interactions during root nodule formation and activity enable a reconsideration of these original speculations. Besides the possibilities for better practical use of the existing systems some more far-reaching speculations will be discussed: the introduction ofnif genes into cells of higher plants, the extension of the host range ofRhizobium orFrankia and the possibilities to transform rhizosphere bacteria, likeAzospirillum, into more efficient endosymbionts. In all cases it will be evident that the more we know, the more we realize what we still have to understand.