Establishing symbiotic nitrogen fixation in cereals and other non-legume crops: The Greener Nitrogen Revolution

Abstract

Haber's invention of the synthesis of ammonia from its elements is one of the cornerstones of modern civilization. For nearly a century, agriculture has come to rely on synthetic nitrogen fertilizers produced from ammonia. This largescale production is now supporting nearly half of the world's population through increased food production. But whilst the use of synthetic nitrogen fertilizers brought enormous benefits, including those of the Green Revolution, the world needs to disengage from our ever-increasing reliance on nitrogen fertilizers produced from fossil fuels. Their pollution of the atmosphere and water systems has become a major global environmental and economic concern. Naturally, legume crops such as peas and beans can fix nitrogen symbiotically by interacting with soil nitrogen-fixing rhizobia, bacteria that become established intracellularly within root nodules. Ever since this was first demonstrated in 1888, consistent attempts have been made to extend the symbiotic interaction of legumes with nitrogen-fixing bacteria to non-legume crops, particularly cereals. In 1988, a fresh impetus arose from the discovery of Gluconacetobacter diazotrophicus (Gd), a non-nodulating, non-rhizobial, nitrogen-fixing bacterium isolated from the intercellular juice of sugarcane. Subsequently, strains of Gd inoculated under specific conditions were shown to intracellularly colonize the roots and shoots of the cereals: wheat, maize (corn) and rice, as well as crops as diverse as potato, tea, oilseed rape, grass and tomato. An extensive field trials programme using a seed inoculum technology based on Gd (NFix ® ) indicates that NFix ® is able to significantly improve yields of wheat, maize, oilseed rape and grasses, in both the presence and absence of synthetic nitrogen fertilizers. Evidence suggests that these benefits are accruing through a possible combination of intracellular symbiotic nitrogen fixation, enhanced rates of photosynthesis and the presence of additional plant growth factors. Here, we discuss the research events that have led to this important development and present results demonstrating the efficacy of NFix ® technology in non-legume crops, in particular cereals.

Non-nodular Endophytic Bacterial Symbiosis and the Nitrogen Fixation of Gluconacetobacter diazotrophicus

“…However, there are a number of key attributes that distinguish this bacterium from others and point to the reasons why it is able to achieve the types and levels of impact demonstrated in Figures 1 and 2, when colonizing crop plants. Among these attributes G. diazotrophicus has the ability to cope with high sucrose concentrations, low oxygen and pH levels and the ability to intracellularly colonize and fix nitrogen in a wide range of crop plants [12,23,24].…”

“…These techniques include chlorophyll levels and leaf percentage nitrogen [17], nitrogenase activity measured through an acetylene reduction assay (ARA) [136,137], nif gene mutant studies [138], labeled nitrogen 15 N2 studies [137][138], enhanced photosynthetic rates [16] and plant growth and yield benefits [12,19,139,140].…”

“…In Brazil, a primary driver for growing sugarcane has been ethanol production for use as a sustainable substitute fuel for petrochemicals. For many years, and in deed for decades, in sugarcane offer some prospect for the development of non-legume crop symbiotic nitrogen fixation, not only to increase crop yields but also to potentially reduce nitrogen fertilizer use, and this prospect is now beginning to be realized [12].…”

“…However, more significant yield enhancement has been demonstrated in recent independent field trial research utilizing proprietary NFix® technology (Patent Number: WO2016/016629) based on G. diazotrophicus of around 1 tonne per hectare in both maize and wheat (Figures 1 and 2) at any level of nitrogen fertilizer [12,21].…”

Metatranscriptomics and nitrogen fixation from the rhizoplane of maize plantlets inoculated with a group of PGPRs

“…Nitrogen fixation in cereals has been long pursued since cereals such as rice, maize and wheat became widely consumed worldwide (23), considering they use the largest amounts of chemical nitrogen fertilizers which represent a substantial cost for farmers, not to mention the high toll these fertilizers take on the environment. Maize originated and was domesticated in Mexico where many native races and varieties exist (24).…”