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 symbiosis between diazotrophic bacteria and corn would be of enormous economic value. Such a discovery would also improve human health as it would decrease the amount of nitrate in ground water as well as in corn cultured for human consumption. Several proposals have been made toward 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 ability to accept fixed nitrogen from diazotrophs in the rhizosphere. All of these proposals have enormous technical problems to overcome such that the development of nitrogen-fixing corn in the near term has been considered unlikely. An alternative and less-technically challenging approach may be a thorough study of non-pathogenic bacterial endophytes that already inhabit the corn plant. The discovery of a nitrogen-fixing bacterial-sugar cane association by Dobereiner and coworkers in Brazil illustrates the enormous potential of endophytic bacteria to enhance grass biomass in the absence of nitrogen fertilizer. Dobereiner and coworkers have discovered diazotrophic strains of Acetobacter diazotrophicus and Herbaspirillum seropedicae in lines of sugar cane that were bred in the absence of nitrogen fertilizer. The Brazilian group has also demonstrated that sugar cane plants infected with these diazotrophs are capable of deriving all of their nitrogen needs from N-2. Recently, the presence of non-pathogenic endophytic bacteria in corn has been shown. Based on this evidence and using the sugar cane paradigm as an example, investigators are working toward the discovery and analysis of diazotrophic endophytes in corn which includes the search for corn germplasm that would readily benefit from an association with these bacteria. Several diazotrophic endophytes have been identified in grass species that are members of the alpha-, beta-, and gamma-subclasses of the proteobacteria. Our understanding of the ability of these bacteria to enhance the growth of grasses through nitrogen fixation is only beginning to be explored but this approach is thought to be far less technically challenging than are other proposals to develop 'nitrogen fixation in maize.