Biological nitrogen fixation with non-legumes : An achievable target or a dogma? (2007)

Nitrogen is most often the limiting nutrient for crop production, since only a fraction of atmospheric ni- trogen is made available to the plants through biologi- cal nitrogen fixation (BNF). Extending the BNF ability to non-legumes would be a useful technology for in- creased crop yields among resource-poor farmers. The idea that genetic manipulation techniques might be used to engineer crop plants to fix nitrogen is, of course, not new. However, the more we understand about the biochemistry and physiology of BNF, the less likely it seems that this goal will be achieved by 'simply' transferring the genes for nitrogen fixation to suitable crop species. Induction of nodulation has therefore been the main target of researchers over the past few years. This review briefly describes the pro- gress made towards nodular symbiosis between rhizo- bia and other free-living atmospheric nitrogen fixers and non-legume crops. NITROGEN is an essential plant nutrient. It is the nutrient that is most commonly deficient, contributing to reduced agricultural yields throughout the world. Molecular nitrogen or dinitrogen (N2) makes up four-fifths of the atmosphere, but is metabolically unavailable directly to higher plants or animals. It is available to some microorganisms through biological nitrogen fixation (BNF) in which atmospheric nitrogen is converted to ammonia by the enzyme nitro- genase. According to statistics by FAO (2001), about 42 million tons of fertilizer N is being used annually on a global scale for the production of three major cereal crops, i.e. wheat, rice and maize (17, 9 and 16 million tons res- pectively). Crop plants are able to use about 50% of the applied fertilizer N, while 25% is lost from the soil-plant system through leaching, volatilization, denitrification and due to many other factors causing not only an annual economic loss of US$ 3 billion but also cause pollution to the environment. Some of the adverse environmental effects of excessive use of nitrogenous fertilizers are: (i) metheamoglobinemia in infants due to NO3 and NO2 in waters and food, (ii) cancer due to secondary amines, (iii) respiratory illness due to NO 3, aerosols, NO2 and HNO3, (iv) eutrophication due to N in surface water, (v) material and ecosystem damage due to HNO3 in rainwater, (vi) plant toxicity due to high levels of NO 2 and NH4 in soils, and (vii) excessive plant growth due to more available N, depletion of stratospheric ozone due to NO and N2O. If a BNF system could be assembled in the non-legume plants, it could increase the potential for nitrogen supply because fixed nitrogen would be available to the plants directly, with little or no loss. Such a system could also enhance resource conservation and environmental security, besides freeing farmers from the economic burden of purchasing fertilizer nitrogen for crop production. Thus, a significant reduction in the relative use of fertilizer N can be achieved if atmospheric N is made available to non-legumes directly through an effective associative system with some of the characteristics of legume symbiosis. Recently, several ap- proaches using techniques developed in the area of bio- technology have raised new hopes that success in this secondary objective may yet be realized. It is the authors opinion that there are now sound reasons to anticipate that at least some non-leguminous field crops may also become independent of soil nitrogen. We intend to ex- plain the reasons for this renewed optimism, against the background of knowledge accumulated in the past century that will be relevant to any ultimate su ccess in exploiting these new approaches.