From the Cover: Enlisting wild grass genes to combat nitrification in wheat farming: A nature-based solution

Active nitrifiers and rapid nitrification are major contributing factors to nitrogen losses in global wheat production. Suppressing nitrifier activity is an effective strategy to limit N losses from agriculture. Production and release of nitrification inhibitors from plant roots is termed “biological nitrification inhibition” (BNI). Here, we report the discovery of a chromosome region that controls BNI production in “wheat grass” Leymus racemosus (Lam.) Tzvelev, located on the short arm of the “Lr#3Ns^b” (Lr#n), which can be transferred to wheat as T3BL.3Ns^bS (denoted Lr#n-SA), where 3BS arm of chromosome 3B of wheat was replaced by 3Ns^bS of L. racemosus. We successfully introduced T3BL.3Ns^bS into the wheat cultivar “Chinese Spring” (CS-Lr#n-SA, referred to as “BNI-CS”), which resulted in the doubling of its BNI capacity. T3BL.3Ns^bS from BNI-CS was then transferred to several elite high-yielding hexaploid wheat cultivars, leading to near doubling of BNI production in “BNI-MUNAL” and “BNI-ROELFS.” Laboratory incubation studies with root-zone soil from field-grown BNI-MUNAL confirmed BNI trait expression, evident from suppression of soil nitrifier activity, reduced nitrification potential, and N₂O emissions. Changes in N metabolism included reductions in both leaf nitrate, nitrate reductase activity, and enhanced glutamine synthetase activity, indicating a shift toward ammonium nutrition. Nitrogen uptake from soil organic matter mineralization improved under low N conditions. Biomass production, grain yields, and N uptake were significantly higher in BNI-MUNAL across N treatments. Grain protein levels and breadmaking attributes were not negatively impacted. Wide use of BNI functions in wheat breeding may combat nitrification in high N input–intensive farming but also can improve adaptation to low N input marginal areas.

Nitrification and denitrification are critical soil biological processes, which, left unchecked, can accelerate generation of harmful reactive nitrogen (N) forms (NO₃ ⁻, N₂O, and NOx) that trigger a “nitrogen cascade,” damaging ecosystems, water systems, and soil fertility (1 –8). Excessive nitrifier activity and a rapid generation of soil nitrates plague modern cereal production systems. This has led to shifting crop N nutrition toward an “all nitrate form,” which is largely responsible for N losses and a decline in agronomic nitrogen-use efficiency (NUE) (6, 7, 9 –11).Wheat, one of the three founding crops for food security (12), consumes nearly a fifth of factory-produced N fertilizers, and it has an average NUE of 33%, which has remained unchanged for the last two decades (13 –15). Regulating soil nitrifier activity to slow the rate of soil nitrate formation should provide more balanced N forms (NH₄ ⁺ and NO₃ ⁻) for plant uptake (rather than nearly “all NO₃ ⁻” at present), reduce N losses, and facilitate the assimilation of dual N forms. This optimizes the utilization of biochemical machinery for N assimilation, improving stability and possibly enhancing yield potential (16). In addition, the assimilation of NH₄ ⁺ is energetically more efficient (requiring 40% less metabolic energy) than NO₃ ⁻ assimilation (16). Often, a stimulatory growth response is observed in wheat, when 15 to 30% of NO₃ ⁻ is replaced with NH₄ ⁺ in nutrient solutions (17, 18).Synthetic nitrification inhibitors (SNIs) have been shown to suppress N₂O emissions, reduce N losses, and improve agronomic NUE in several cereal crops including wheat (6, 19 –21). However, the lack of cost effectiveness, inconsistency in field performance, inability to function in tropical environments, and the concerns related to the entering of SNIs into food chains have limited their adoption in production agriculture (6, 7, 19, 20).Biological nitrification inhibition (BNI) is a plant function whereby nitrification inhibitors (BNIs) are produced from root systems to suppress soil nitrifier activity (22 –26). Earlier, we reported that the BNI capacity in the root systems of cultivated wheat lack adequate strength to effectively suppress soil nitrifier activity in the rhizosphere (24, 25). Leymus racemosus (hereafter referred to as “wild grass”), a perennial Triticeae evolutionarily related to wheat, produces extensive root systems ( SI Appendix, Fig. S1) and was discovered to have a high BNI capacity several times higher than cultivated wheat. It was also effective in suppressing soil nitrifier activity and in reducing soi -nitrate formation ( SI Appendix, Fig. S2) (25). Subsequently, the chromosome Lr#n = 3Ns^b was found to be controlling a major part of BNI capacity in wild grass, and it is the focus of our current research (25, 27, 28). Earlier, we reported that Lr#I and Lr#J had a minor impact on BNI capacity, but they are not the focus of this research (25).We transferred the Lr#n chromosome (Lr#n-SA = T3BL.3Ns^bS) controlling BNI capacity (hereafter referred to as BNI trait) into the cultivated wheat, Chinese Spring (CS). The results of the transfer of this BNI trait into several elite wheat types with a grain-yield (GY) potential >10 t ha⁻¹, resulting in substantial improvements of BNI capacity in root systems, are reported in this paper.