DIAZOTROPHIC ENDOPHYTES - PROGRESS AND PROSPECTS FOR NITROGEN-FIXATION IN MONOCOTS

The development of nitrogen fixation in maize can be considered the '' holy grail'' of nitrogen fixation research. As nitrogen fertilization is one of the highest costs of corn production, the development of a s ymbiosis between diazotrophic bacteria and corn would be of enormous e conomic value. Such a discovery would also improve human health as it would decrease the amount of nitrate in ground water as well as in cor n cultured for human consumption. Several proposals have been made tow ard this end. These include: a) the transfer of root nodulation genes from a legume to maize; b) the expression of the bacterial nif regulon in maize organelles; and c) the development of corn lines with the ab ility to accept fixed nitrogen from diazotrophs in the rhizosphere. Al l of these proposals have enormous technical problems to overcome such that the development of nitrogen-fixing corn in the near term has bee n considered unlikely. An alternative and less-technically challenging approach may be a thorough study of non-pathogenic bacterial endophyt es that already inhabit the corn plant. The discovery of a nitrogen-fi xing bacterial-sugar cane association by Dobereiner and coworkers in B razil illustrates the enormous potential of endophytic bacteria to enh ance grass biomass in the absence of nitrogen fertilizer. Dobereiner a nd coworkers have discovered diazotrophic strains of Acetobacter diazo trophicus and Herbaspirillum seropedicae in lines of sugar cane that w ere bred in the absence of nitrogen fertilizer. The Brazilian group ha s also demonstrated that sugar cane plants infected with these diazotr ophs are capable of deriving all of their nitrogen needs from N-2. Rec ently, the presence of non-pathogenic endophytic bacteria in corn has been shown. Based on this evidence and using the sugar cane paradigm a s an example, investigators are working toward the discovery and analy sis of diazotrophic endophytes in corn which includes the search for c orn germplasm that would readily benefit from an association with thes e bacteria. Several diazotrophic endophytes have been identified in gr ass species that are members of the alpha-, beta-, and gamma-subclasse s of the proteobacteria. Our understanding of the ability of these bac teria to enhance the growth of grasses through nitrogen fixation is on ly beginning to be explored but this approach is thought to be far les s technically challenging than are other proposals to develop 'nitroge n fixation in maize.