Keeping the nitrogen-fixation dream alive

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Abstract
The conversion of inert N2 gas to a metabolically tractable form, such as ammonia, is called nitrogen fixation. In biology, nitrogen fixation is a highly oxygen-sensitive process restricted to a select group of diverse microorganisms, often collectively referred to as diazotrophs, or “nitrogen eaters.” The sparse availability of fixed nitrogen, also known as fertilizer, has historically limited worldwide food production (1⇓⇓–4). Since about 1920, the situation has been significantly ameliorated by application of industrially produced fertilizer. Indeed, the Haber–Bosch process for industrial fertilizer production has been touted as the technological advance that has had the most impact on the modern world, driving the green revolution of the last century and fueling unprecedented population growth (1). However, the practice of applying industrially produced fertilizer to augment agricultural yield has also proven to incur severe economic, agronomic, and environmental penalties. Among these “penalites” are consumption of nonrenewable fossil fuels, prodigious production of greenhouse gases, spoiling of watersheds as a consequence of fertilizer run-off, costs associated with fertilizer distribution and application and, of course, socio-political issues associated with unbridled population growth (1⇓⇓–4). Emergence of the recombinant DNA era in the mid-1970s led to the ambitious goal of endowing higher plants, in particular cereal plants, with the capacity to produce their own nitrogenous fertilizers by transfer of the microbial genetic determinants to plants (5). This effort was anticipated to result in sustainable agricultural practices that could reduce the demand for production of industrial fertilizers and avoid the costs and adverse effects associated with their transportation and application. Some 40 y later, this dream remains unrealized and can be reasonably considered as biotechnology’s most significant failure. Now, with a complete understanding of the fundamental genetic determinants necessary to sustain nitrogen fixation in microbes, and at …

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