Post-flowering biotic and abiotic stresses impact nitrogen use efficiency and seed filling in Arabidopsis thaliana

Nitrogen (N) is an essential nutrient that plants require for the synthesis of amino acids, proteins, and many other important metabolites. Plant metabolism and growth are consequently dependent on the amount of N that is assimilated and distributed from source leaves to developing sinks, such as fruits and seeds. The environmental stresses enhanced by climate change deeply influence seed yield and seed composition, and may disturb N use efficiency (NUE) in pants. We aimed to investigate plant responses to extreme climates with regard to NUE, N remobilization efficiency, and seed composition. By studying a collection of Arabidopsis genotypes showing a range of C:N ratios in seeds, we investigated the impact of different post-flowering growth conditions (control, heat, drought, low nitrate availability, induced senescence, and induced plant defense) on seed yield, N allocation in organs, NUE, and N remobilization efficiency. We analysed how post-flowering stresses could change seed filling and showed that post-flowering stresses change both the range of N and C concentrations and the C:N stoichiometry in seeds. Using a new trait, called delta seed composition, we measured the deviation in C:N stoichiometry of each genotype and revealed the genetic determinism of the C:N stoichiometry. Altogether, the results indicate that extreme climate impacts NUE dramatically in plants and generates different bottlenecks in N fluxes during seed filling.

Overexpression of the High-Affinity Nitrate Transporter OsNRT2.3b Driven by Different Promoters in Barley Improves Yield and Nutrient Uptake Balance

Improving nitrogen use efficiency (NUE) is very important for crops throughout the world. Rice mainly utilizes ammonium as an N source, but it also has four NRT2 genes involved in nitrate transport. The OsNRT2.3b transporter is important for maintaining cellular pH under mixed N supplies. Overexpression of this transporter driven by a ubiquitin promoter in rice greatly improved yield and NUE. This strategy for improving the NUE of crops may also be important for other cereals such as wheat and barley, which also face the challenges of nutrient uptake balance. To test this idea, we constructed transgenic barley lines overexpressing OsNRT2.3b. These transgenic barley lines overexpressing the rice transporter exhibited improved growth, yield, and NUE. We demonstrated that NRT2 family members and the partner protein HvNAR2.3 were also up-regulated by nitrate treatment (0.2 mM) in the transgenic lines. This suggests that the expression of OsNRT2.3b and other HvNRT2 family members were all up-regulated in the transgenic barley to increase the efficiency of N uptake and usage. We also compared the ubiquitin (Ubi) and a phloem-specific (RSs1) promoter-driven expression of OsNRT2.3b. The Ubi promoter failed to improve nutrient uptake balance, whereas the RSs1 promoter succeed in increasing the N, P, and Fe uptake balance. The nutrient uptake enhancement did not include Mn and Mg. Surprisingly, we found that the choice of promoter influenced the barley phenotype, not only increasing NUE and grain yield, but also improving nutrient uptake balance.

A wheat transcription factor positively sets seed vigour by regulating the grain nitrate signal

Seed vigour and early establishment are important factors determining the yield of crops. A wheat nitrate-inducible NAC transcription factor, TaNAC2, plays a critical role in promoting crop growth and nitrogen use efficiency (NUE), and now its role in seed vigour is revealed. A TaNAC2 regulated gene was identified that is a NRT2-type nitrate transporter TaNRT2.5 with a key role in seed vigour. Overexpressing TaNAC2-5A increases grain nitrate concentration and seed vigour by directly binding to the promoter of TaNRT2.5-3B and positively regulating its expression. TaNRT2.5 is expressed in developing grain, particularly the embryo and husk. In Xenopus oocyte assays TaNRT2.5 requires a partner protein TaNAR2.1 to give nitrate transport activity, and the transporter locates to the tonoplast in a tobacco leaf transient expression system. Furthermore, in the root TaNRT2.5 and TaNRT2.1 function in post-anthesis acquisition of soil nitrate. Overexpression of TaNRT2.5-3B increases seed vigour, grain nitrate concentration and yield, whereas RNA interference of TaNRT2.5 has the opposite effects. The TaNAC2-NRT2.5 module has a key role in regulating grain nitrate accumulation and seed vigour. Both genes can potentially be used to improve grain yield and NUE in wheat.

Effect of N dose, fertilisation duration and application of a nitrification inhibitor on GHG emissions from a peach orchard

Despite only occupying 5% of the worldwide arable area, fruit tree crops are of vital economic importance in many regions. Intensive cropping practices can lead to greenhouse gas (GHG) emissions. In order to reduce these emissions, numerous studies have been made on lowering N inputs or applying nitrification inhibitors (NIs) which tend to maintain or even increase yield while reducing N leaching and nitrogenous emissions to the atmosphere. However, very few studies have been conducted on potential GHG emissions from the peach crop. In this work, a three-year study was carried out in a commercial peach orchard with a split-plot design with three replicates, in which the main factor was N dose (25, 50 and 100 kg N ha^-1 year^-1, and 50 kg N ha^-1 year^-1 applied during a shorter period of time in 2015 and 2016; and only 70 kg N ha^-1 year^-1 in 2017). Subplots in the study were used to analyse the effect of the application of a NI (3,4-dimethylpyrazole phosphate; DMPP). The aim was to qualitatively compare the effect of these factors on N₂O, N₂O + N₂, CH₄ and CO₂ emissions from a peach orchard soil in order to recommend agricultural practices that minimise emissions without concurrent yield reductions. We show that N₂O and N₂O + N₂ emissions were linked to fertilisation and increased with N dose. The N₂O emissions were mitigated (up to 49%) by DMPP up to the 50 kg N ha^-1 dose (not significantly). It seems that between 70 and 100 kg N ha^-1 the application of DMPP loses effectiveness. Methane oxidation increased with N dose and decreased with DMPP application; CO₂ emissions increased with DMPP and were unaffected by N dose. The intermediate N dose (50 kg N ha^-1) applied during a shorter period of time increased yield (not significantly) and NUE without increasing GHG emissions.

Genetic Dissection of Nitrogen Use Efficiency in Tropical Maize Through Genome-Wide Association and Genomic Prediction

In sub-Saharan Africa, one of the major challenges to smallholder farmers is soil with low fertility and inability to apply nitrogen fertilizer externally due to the cost. Development of maize hybrids, which perform better in nitrogen depleted soils, is one of the promising solutions. However, breeding maize for nitrogen use efficiency (NUE) is hindered by expensive phenotypic evaluations and trait complexity under low N stress. Genome-wide association study (GWAS) and genomic prediction (GP) are promising tools to circumvent this interference. Here, we evaluated a mapping panel in diverse environments both under optimum and low N management. The objective of this study was to identify SNPs significantly associated with grain yield (GY) and other traits through GWAS and assess the potential of GP under low N and optimum conditions. Testcross progenies of 411 inbred lines were planted under optimum and low N conditions in several locations in Africa and Latin America. In all locations, low N fields were previously depleted over several seasons, and no N fertilizer was applied throughout the growing season. All inbred lines were genotyped with genotyping by sequencing. Genotypic and GxE interaction variances were significant, and heritability estimates were moderate to high for all traits under both optimum and low N conditions. Genome-wide LD decay at r ² = 0.2 and r ² = 0.34 were 0.24 and 0.19 Mbp, respectively. Chromosome-specific LD decays ranged from 0.13 to 0.34 Mbps with an average of 0.22 Mbp at r ² = 0.2. GWAS analyses revealed 38 and 45 significant SNPs under optimum and low N conditions, respectively. Out of these 83 significant SNPs, 3 SNPs on chromosomes 1, 2, and 6 were associated either with different traits or the same trait under different management conditions, suggesting pleiotropic effects of genes. A total of 136 putative candidate genes were associated with the significant SNPs, of which seven SNPs were linked with four known genes. Prediction accuracies were moderate to high for all traits under both optimum and low N conditions. These results can be used as useful resources for further applications to develop hybrids or lines with better performance under low N conditions.

Effects of Potassium Levels on Plant Growth, Accumulation and Distribution of Carbon, and Nitrate Metabolism in Apple Dwarf Rootstock Seedlings

Nitrogen (N) is one of the most required mineral elements for plant growth, and potassium (K) plays a vital role in nitrogen metabolism, both elements being widely applied as fertilizers in agricultural production. However, the exact relationship between K and nitrogen use efficiency (NUE) remains unclear. Apple dwarf rootstock seedlings (M9T337) were used to study the impacts of different K levels on plant growth, nitrogen metabolism, and carbon (C) assimilation in water culture experiments for 2 years. The results showed that both deficiency and excess K inhibited the growth and root development of M9T337 seedlings. When the K supply concentration was 0 mM and 12 mM, the biomass of each organ, root-shoot ratio, root activity and NO₃ ^- ion flow rate decreased significantly, net photosynthetic rate (P _n) and photochemical efficiency (F _v/F _m) being lower. Meanwhile, seedlings treated with 6 mM K⁺ had higher N and C metabolizing enzyme activities and higher nitrate transporter gene expression levels (NRT1.1; NRT2.1). ¹³C and ¹⁵N labeling results showed that deficiency and excess K could not only reduce ¹⁵N absorption and ¹³C assimilation accumulation of M9T337 seedlings, but also reduced the ¹⁵N distribution ratio in leaves and ¹³C distribution ratio in roots. These results suggest that appropriate K supply (6 mM) was optimal as it enhanced photoassimilate transport from leaves to roots and increased NUE by influencing photosynthesis, C and N metabolizing enzyme activities, nitrate assimilation gene activities, and nitrate transport.

Can secondary metabolites extracted from Moringa seeds suppress ammonia oxidizers to increase nitrogen use efficiency and reduce nitrate contamination in potato tubers?

Nitrification inhibition as an alleviation tool to decrease nitrogen (N) losses and increase N use efficiency (NUE) as well as reducing NO₃^- accumulation in plants is a promising technology. No study thus far has directly or indirectly to use the secondary metabolites extracted from Moringa (Moringa oleifera Lam) seeds as nitrification inhibitors. Moringa seed extract (MSE) was studied based on its content of phenolic compounds (PC) and for its antioxidant characteristic. A 2-year field experiment and 30-day incubation experiment were conducted with three treatments of control (CK), N fertilizer (300 kg N ha^-1 and 200 mg N kg^-1 soil for the field and incubation experiment, respectively), and N fertilizer with MSE (500 ppm as a TPC) to investigate the responses of ammonia-oxidizing bacteria (AOB) and archaea (AOA) to MSE and the consequences for NUE and NO₃^- accumulation in potato tubers. Total phenolics amount was 144 mg gallic acid equivalent g^-1 MSE, while flavonoid contents were 76.6 quercetin equivalent g^-1 MSE. MSE showed antioxidant activity that was comparable to the standard antioxidants TBHQ and gallic acid. MSE application with N fertilizer retarded the nitrification process, as indicated by a higher NH₄⁺-N and lower NO₃^--N content, compared with N fertilizer application alone. NH₄⁺-N content reduced to initial CK level on Day 20 under N fertilizer application alone. However, NH₄⁺-N content decreased to initial control level on Day 30 when MSE was applied. The mechanisms resulted from curbing AOB growth by phenolic compounds (TPC, TF, TAC), leading to a delay in nitrification process. AOB increased significantly when N fertilizer was applied alone; on the contrary, AOA was not sensitive to N fertilizer (with and without MSE). Increase in NUE from 37.5% to 66.3% in potato plants under MSE application with N fertilizer was also observed compared with N fertilizer application alone. The highest NO₃^- accumulation (569 mg NO₃^- kg^-1) in tubers was recorded under N fertilizer application without MSE. MSE application significantly decreased NO₃^- accumulation (92 mg NO₃^- kg^-1) in tubers which is lower than the maximum value of accepting tubers (200 mg NO₃^- kg^-1). The highest average of N uptake, fresh and dry weight, carotenoids, chlorophyll a, chlorophyll b and nitrate reductase activity was recorded when MSE was applied with N fertilizer. Accordingly, using of Moringa extracted secondary metabolites to suppress AOB growth in the soil is a significant strategy to reduce nitrification rate and N loss from soils, and therefore increase NUE as well as reducing NO₃^- accumulation in potato tubers.

Chloride as a Beneficial Macronutrient in Higher Plants: New Roles and Regulation

Chloride (Cl-) has traditionally been considered a micronutrient largely excluded by plants due to its ubiquity and abundance in nature, its antagonism with nitrate (NO3-), and its toxicity when accumulated at high concentrations. In recent years, there has been a paradigm shift in this regard since Cl- has gone from being considered a harmful ion, accidentally absorbed through NO3- transporters, to being considered a beneficial macronutrient whose transport is finely regulated by plants. As a beneficial macronutrient, Cl- determines increased fresh and dry biomass, greater leaf expansion, increased elongation of leaf and root cells, improved water relations, higher mesophyll diffusion to CO2, and better water- and nitrogen-use efficiency. While optimal growth of plants requires the synchronic supply of both Cl- and NO3- molecules, the NO3-/Cl- plant selectivity varies between species and varieties, and in the same plant it can be modified by environmental cues such as water deficit or salinity. Recently, new genes encoding transporters mediating Cl- influx (ZmNPF6.4 and ZmNPF6.6), Cl- efflux (AtSLAH3 and AtSLAH1), and Cl- compartmentalization (AtDTX33, AtDTX35, AtALMT4, and GsCLC2) have been identified and characterized. These transporters have proven to be highly relevant for nutrition, long-distance transport and compartmentalization of Cl-, as well as for cell turgor regulation and stress tolerance in plants.

Nitrogen balance of crop production in Ukraine

Lands of Ukraine are intensively used. Usage of insufficient amounts of fertilizers leads to destruction of soil organic matter, causing complete loss of soil fertility. In 2015, the total nitrogen balance in Ukraine was negative. It is -152.3 Gg or -5.8 kg N/ha/yr. In different regions, these indicators varied from -34.6-24.7 kg N/ha/yr. NUE for crop production in Ukraine in 2015 was equal to 109.1%, in some regions this indicator varied from 75.7% to 198.6%. It was a result of insufficient amounts of fertilizers applied to agricultural land. To restore the fertility of soil, it is necessary to use organic fertilizers and biological nitrogen fixation.

Exploring the Potential of Overexpressed OsCIPK2 Rice as a Nitrogen Utilization Efficient Crop and Analysis of Its Associated Rhizo-Compartmental Microbial Communities

Nitrogen (N) is one of the indispensable factors in rice growth and development. China holds a premier position in the production of rice and at the same time also faces higher N fertilizer costs along with serious damage to the environment. A better solution is much needed to address these issues, without disrupting the production of rice as an important cereal, while minimizing all the deleterious effects on the environment. Two isogenic lines Kitaake (WT) and its genetically modified line CIPK2 (RC), overexpressing the gene for Calcineurin B-like interacting protein kinase 2 (OsCIPK2) with better nitrogen use efficiency (NUE), were compared for their growth and development under low versus normal levels of N. NUE is a complex trait mainly related to a plant’s efficiency in extraction, assimilation, and recycling of N from soil. The microbial population was analyzed using high-throughput Illumina Miseq 16S rRNA sequencing and found that RC with CIPK2, specifically expressed in rice root, not only performed better without nitrogen fertilizer (LN) but also increased the diversity of bacterial communities in rice rhizosphere compartments (rhizosphere, rhizoplane, and endosphere). The relative abundance of beneficial bacteria phyla increased, which are known to promote the circulation and transformation of N in rhizosphere soil. To further explore the potential of RC regarding better performance under LN, the ion fluxes in root apical were detected by non-invasive micro-test technique (NMT). We found that RC can absorb more Ca²⁺ and NO₃⁻ under LN as compared to WT. Finally, compared to WT, RC plants exhibited better growth of root and shoot, and increased yield and N uptake under LN, whereas there was no significant difference in the growth of two rice lines under normal nitrogen (NN) treatment. We are able to get preliminary results, dealing with the OsCIPK2 overexpressed rice line, by studying the rice molecular, physiological, and chemical parameters related to NUE. The results laid the foundation for further research on N absorption and utilization in rice from the soil and the interaction with microbial communities.

A framework for analysing nitrification inhibition: A case study on 3,4-dimethylpyrazole phosphate (DMPP)

Nitrification inhibitors show great potential to reduce nitrogen losses from agricultural systems and to improve nitrogen use efficiency. The most recently developed nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) is gaining popularity due to its benefits relative to other compounds. However, the behaviour of DMPP and its effect on nitrification in soils has been characterised using inconsistent and confusing terminology. Many studies have used the term half-life to describe the persistence of DMPP but used different experimental methods to derive it leading to highly variable results. We assessed how different methodologies in experiments may have contributed to the variability in the results using a framework that describes the behaviour of DMPP and its effect on nitrification in terms of: persistence, bioactivity and longevity. We show that deriving the persistence of DMPP using ¹⁴C labelling techniques is challenging because it requires consideration of other ¹⁴C pools in the soil. We also describe the limitations of soil inorganic nitrogen measurements to characterise the bioactivity and longevity of the inhibitory effect on nitrification. We conclude by proposing experiments that can facilitate the evaluation of the benefits of DMPP across broader scales. While this study focused on DMPP, the concepts presented here are equally relevant to other nitrification inhibitors.

Importance of matching soil N transformations, crop N form preference, and climate to enhance crop yield and reducing N loss

The preferential uptake of nitrogen (N) by plant species is generally considered to be tightly associated with soil N transformation characteristics and climatic conditions. In the present study, the yield, N recovery, and N balance of wheat (an NO₃^--preferring crop) and rice (an NH₄⁺-preferring crop) planted in two sites with significantly different soil N transformations and precipitation were compared to test the connections among soil N transformations, species-specific N preferences, crop yield, N use efficiency (NUE), N loss, and climate. Nitrogen recoveries of ¹⁵N applied as either an NH₄⁺ or NO₃^- based fertilizer in crops and soil were determined, and N losses were calculated using the ¹⁵N balance. The results indicated that the match (or mismatch) of crop N preference, soil N transformation characteristics, and climate conditions could significantly affect the crop yield, recovery, and loss of applied ¹⁵N. A crop preferring NH₄⁺ (e.g., rice) planted in soil with low nitrification and low pH tended to have a higher yield and NUE, and a lower N loss compared with plants growing in soil characterized by a higher nitrification rates. In contrast, NO₃^--preferring crops (e.g. wheat) planted in soil with higher nitrification rates and lower precipitation achieved a higher yield and lower N loss than those planted in acid soil with a lower nitrification rate and higher precipitation. Replacement of ammonium N fertilizer with nitrate N fertilizer improved the NUE for NO₃^--preferring crops (wheat) planted in arid and semi-arid regions, but not in humid regions because of the mobility of NO₃^-. Thus, a good match of crop N preference, soil N transformation characteristics, and climate conditions is critical to enhance crop yield and reduce N loss. This intimate connection needs to be taken into consideration in terms of the choice of crop species and N fertilizer type to optimize N fertilization management.

Yield advantage and nitrogen fate in an additive maize-soybean relay intercropping system

Sustainable agricultural development is urgently required to satisfy future food demands while decreasing environmental costs. Intercropping can increase per-unit farmland productivity through a resource-efficient utilization. However, the fate of N in intercropping systems remains unclear. To study the yield advantages and the fate of N in additive maize-soybean relay intercropping (IMS) systems, we quantified crop yield, soil N transformation abilities, soil bacterial abundances, and the fate of 15N. This study was conducted using three planting patterns, namely, monoculture maize (Zea mays L.) (MM), monoculture soybean (Glycine max L. Merr.) (MS), and IMS, and two N application rates, specifically, no N and applied N (N1, 45 and 135 kg N ha-1 for MS and MM, correspondingly; and N for the IMS, which was the sum of the monocultures). Results showed that a higher per-unit farmland productivity and a lower land use intensity are attained in the intercropping system than in the corresponding monocultures. In addition, land equivalent ratio (LER) ranges from 1.85 to 2.20. Moreover, the fate of 15N showed that the N uptake and residual are the highest, whereas N loss in the IMS is the lowest among all planting patterns. Intercropping had an increased N use efficiency by increasing N utilization efficiency, rather than N uptake efficiency. The abundance of ammonia oxidizer and denitrifier indicated that IMS improves the structure of soil microorganisms. Furthermore, the transformation abilities of soil N denoted that intercropping strengthens ammonifying and nitrifying capacities to increase soil N residual while decreasing ammonia volatilization and N2O emission. Finally, the greenhouse warming potential and gas intensity of N2O were significantly lower in the IMS than in the corresponding monocultures. In summary, the IMS system provides an environmentally friendly approach to increasing farmland productivity.

Perspective on Wheat Yield and Quality with Reduced Nitrogen Supply

Wheat is an important cereal crop with a high demand for nitrogen (N) fertilizer to enable the grain protein accumulation that is necessary for baking and processing quality. Here, perspectives for the development of improved wheat genotypes with higher yield stability, better grain quality, and improved N use efficiency to lower environmental impacts are discussed. The development of improved wheat genotypes, for example, genotypes that lack storage proteins that do not contribute to baking quality (e.g., by genome editing), in combination with appropriate N fertilizer management to prevent N losses into the environment underpins a novel approach to improving N use efficiency. This approach may be particularly applicable to wheats grown for animal feed, which have lower quality and functionality requirements.

Nitrogen use efficiency and crop production: Patterns of regional variation in the United States, 1987-2012

National-level summaries of crop production and nutrient use efficiency, important for international comparisons, only partially elucidate agricultural dynamics within a country. Agricultural production and associated environmental impacts in large countries vary significantly because of regional differences in crops, climate, resource use and production practices. Here, we review patterns of regional crop production, nitrogen use efficiency (NUE), and major inputs of nitrogen to US crops over 1987-2012, based on the Farm Resource Regions developed by the Economic Research Service (USDA-ERS). Across the US, NUE generally decreased over time over the period studied, mainly due to increased use in mineral N fertilizer above crop N requirements. The Heartland region dominates production of major crops and thus tends to drive national patterns, showing linear response of crop production to nitrogen inputs broadly consistent with an earlier analysis of global patterns of country-scale data by Lassaletta et al. (2014). Most other regions show similar responses, but the Eastern Uplands region shows a negative response to nitrogen inputs, and the Southern Seaboard shows no significant relationship. The regional differences appear as two branches in the response of aggregate production to N inputs on a cropland area basis, but not on a total area basis, suggesting that the type of scaling used is critical under changing cropland area. Nitrogen use efficiency (NUE) is positively associated with fertilizer as a percentage of N inputs in four regions, and all regions considered together. NUE is positively associated with crop N fixation in all regions except Northern Great Plains. It is negatively associated with manure (livestock excretion); in the US, manure is still treated largely as a waste to be managed rather than a nutrient resource. This significant regional variation in patterns of crop production and NUE vs N inputs, has implications for environmental quality and food security.

Nitrate Transport, Signaling, and Use Efficiency

Nitrogen accounts for approximately 60% of the fertilizer consumed each year; thus, it represents one of the major input costs for most nonlegume crops. Nitrate is one of the two major forms of nitrogen that plants acquire from the soil. Mechanistic insights into nitrate transport and signaling have enabled new strategies for enhancing nitrogen utilization efficiency, for lowering input costs for farming, and, more importantly, for alleviating environmental impacts (e.g., eutrophication and production of the greenhouse gas N₂O). Over the past decade, significant progress has been made in understanding how nitrate is acquired from the surroundings, how it is efficiently distributed into different plant tissues in response to environmental changes, how nitrate signaling is perceived and transmitted, and how shoot and root nitrogen status is communicated. Several key components of these processes have proven to be novel tools for enhancing nitrate- and nitrogen-use efficiency. In this review, we focus on the roles of NRT1 and NRT2 in nitrate uptake and nitrate allocation among different tissues; we describe the functions of the transceptor NRT1.1, transcription factors, and small signaling peptides in nitrate signaling and tissue communication; and we compile the new strategies for improving nitrogen-use efficiency.

Nitrate Transport, Signaling, and Use Efficiency

Nitrogen accounts for approximately 60% of the fertilizer consumed each year; thus, it represents one of the major input costs for most nonlegume crops. Nitrate is one of the two major forms of nitrogen that plants acquire from the soil. Mechanistic insights into nitrate transport and signaling have enabled new strategies for enhancing nitrogen utilization efficiency, for lowering input costs for farming, and, more importantly, for alleviating environmental impacts (e.g., eutrophication and production of the greenhouse gas N₂O). Over the past decade, significant progress has been made in understanding how nitrate is acquired from the surroundings, how it is efficiently distributed into different plant tissues in response to environmental changes, how nitrate signaling is perceived and transmitted, and how shoot and root nitrogen status is communicated. Several key components of these processes have proven to be novel tools for enhancing nitrate- and nitrogen-use efficiency. In this review, we focus on the roles of NRT1 and NRT2 in nitrate uptake and nitrate allocation among different tissues; we describe the functions of the transceptor NRT1.1, transcription factors, and small signaling peptides in nitrate signaling and tissue communication; and we compile the new strategies for improving nitrogen-use efficiency.

Changes in 15NO3 - Availability and Transpiration Rate Are Associated With a Rapid Diurnal Adjustment of Anion Contents as Well as 15N and Water Fluxes Between the Roots and Shoots

Background and Aims: Understanding interactions between water and nitrate fluxes in response to nitrate availability and transpiration rate is crucial to select more efficient plants for the use of water and nitrate. Methods: Some of these interactions were investigated in intact Brassica napus plants by combining a non-destructive gravimetric device with ¹⁵NO₃ ^- labeling. The set-up allowed high-resolution measurement of the effects of a cross-combination of two concentrations of KNO₃ or KCl (0.5 and 5 mM) with two different rates of transpiration controlled by the relative humidity during a day-night cycle. Key Results: Results show that (1) high external nitrate concentrations increased root water uptake significantly whatever the transpiration rate, (2) nitrate translocation depended both on the rate of nitrate uptake and loading into xylem (3) dilution-concentration effect of nitrate in the xylem was mainly modulated by both external nitrate availability and transpiration rate, (4) dynamic changes in ¹⁵N translocation in the xylem modified shoot growth and capacitance, and (5) variations in tissue concentrations of NO₃ ^- induced by the experimental conditions were balanced by changes in concentrations of chloride and sulfate ions. These effects were even more amplified under low transpiration condition and 0.5 mM external nitrate concentration. Conclusion: Taken together, these results highlight the fine and rapid adjustment of anion contents, nitrate and water flows to changes in transpiration rate and nitrate availability during a day-night cycle. The use of this non-invasive gravimetric device is therefore a powerful tool to assess candidates genes involved in nitrogen and water use efficiency.

Phenotyping for Nitrogen Use Efficiency: Rice Genotypes Differ in N-Responsive Germination, Oxygen Consumption, Seed Urease Activities, Root Growth, Crop Duration, and Yield at Low N

The biological improvement of fertilizer nitrogen use efficiency (NUE) is hampered by the poor characterization of the phenotype and genotype for crop N response and NUE. In an attempt to identify phenotypic traits for N-response and NUE in the earliest stages of plant growth, we analyzed the N-responsive germination, respiration, urease activities, and root/shoot growth of 21 Indica genotypes of rice (Oryza sativa var. indica). We found that N delays germination from 0 to 12 h in a genotype-dependent and source-dependent manner, especially with urea and nitrate. We identified contrasting groups of fast germinating genotypes such as Aditya, Nidhi, and Swarnadhan, which were also least delayed by N and slow germinating genotypes such as Panvel 1, Triguna, and Vikramarya, which were also most delayed by N. Oxygen uptake measurements in the seeds of contrasting genotypes revealed that they were affected by N source in accordance with germination rates, especially with urea. Germinating seeds were found to have endogenous urease activity, indicating the need to explore genotypic differences in the effective urea uptake and metabolism, which remain unexplored so far. Urea was found to significantly inhibit early root growth in all genotypes but not shoot growth. Field evaluation of 15 of the above genotypes clearly showed that germination rates, crop duration, and yield are linked to NUE. Slow germinating genotypes had longer crop duration and higher yield even at lower N, indicating their higher NUE, relative to fast germinating or short duration genotypes. Moreover, longer duration genotypes suffered lesser yield losses at reduced N levels as compared to short duration genotypes, which is also a measure of their NUE. Together, these results indicate the potential of germination rates, crop duration, urea utilization and its effect on root growth in the development of novel phenotypic traits for screening genotypes and crop improvement for NUE, at least in rice.

Nitrogen nutrition in cotton and control strategies for greenhouse gas emissions: a review

Cotton (Gossypium hirustum L.) is grown globally as a major source of natural fiber. Nitrogen (N) management is cumbersome in cotton production systems; it has more impacts on yield, maturity, and lint quality of a cotton crop than other primary plant nutrient. Application and production of N fertilizers consume large amounts of energy, and excess application can cause environmental concerns, i.e., nitrate in ground water, and the production of nitrous oxide a highly potent greenhouse gas (GHG) to the atmosphere, which is a global concern. Therefore, improving nitrogen use efficiency (NUE) of cotton plant is critical in this context. Slow-release fertilizers (e.g., polymer-coated urea) have the potential to increase cotton yield and reduce environmental pollution due to more efficient use of nutrients. Limited literature is available on the mitigation of GHG emissions for cotton production. Therefore, this review focuses on the role of N fertilization, in cotton growth and GHG emission management strategies, and will assess, justify, and organize the researchable priorities. Nitrate and ammonium nitrogen are essential nutrients for successful crop production. Ammonia (NH₃) is a central intermediate in plant N metabolism. NH₃ is assimilated in cotton by the mediation of glutamine synthetase, glutamine (z-) oxoglutarate amino-transferase enzyme systems in two steps: the first step requires adenosine triphosphate (ATP) to add NH₃ to glutamate to form glutamine (Gln), and the second step transfers the NH₃ from glutamine (Gln) to α-ketoglutarate to form two glutamates. Once NH₃ has been incorporated into glutamate, it can be transferred to other carbon skeletons by various transaminases to form additional amino acids. The glutamate and glutamine formed can rapidly be used for the synthesis of low-molecular-weight organic N compounds (LMWONCs) such as amides, amino acids, ureides, amines, and peptides that are further synthesized into high-molecular-weight organic N compounds (HMWONCs) such as proteins and nucleic acids.

The role of Plant Growth Promoting Bacteria in improving nitrogen use efficiency for sustainable crop production: a focus on wheat

Due to the increase in both human population growth and environmental pressure, it is necessary to raise agricultural productivity without enhancing environmental footprint. Within this context, soil inoculation with PGPB (Plant Growth Promoting Bacteria) may be considered a promising tool of integrated management systems. In particular, PGPB may improve plant growth either directly, by facilitating resource use or modulating plant hormone levels, or indirectly by decreasing the inhibitory effects of various pathogenic agents. PGPB comprise different functional and taxonomic groups of bacteria like Pseudomonas, Bacillus, Rhizobium and others. Their ability to either mobilize mineral or organic bound nutrients from the pedosphere or to fix atmospheric N2 and make it available to the plants, is a crucial feature in their application. In literature some data are available on the use of commercial PGPB, while less efforts have been made on the study of the effect of autochthonous PGPB isolated from soils on sustainability of cropping systems; thus a literature survey on these aspects was carried out with special focus on wheat, a staple food for a large part of world population. In particular, the main topic of this review is the potential of PGPB to enhance use efficiency of agro-environmental resources focusing on the interaction PGPB-wheat for improving nitrogen use efficiency.

Effect of exogenously applied molybdenum on its absorption and nitrate metabolism in strawberry seedlings

Molybdenum (Mo)-an essential element of plants-is involved in nitrogen (N) metabolism. Plants tend to accumulate more nitrate and show lower nitrogen use efficiency (NUE) under Mo-deficient conditions. Improving NUE in fruits reduces the negative effect of large applications of chemical fertilizer, but the mechanisms underlying how Mo enhances NUE remain unclear. We cultivated strawberry seedlings sprayed with 0, 67.5, 135, 168.75, or 202.5 g Mo·ha^-1 in a non-soil culture system. The Mo concentration in every plant tissue analyzed increased gradually as Mo application level rose. Mo application affected iron, copper, and selenium adsorption in roots. Seedlings sprayed with 135 g Mo·ha^-1 had a higher [¹⁵N] shoot:root (S:R) ratio, and ¹⁵NUE, and produced higher molybdate transporter type 1 (MOT1) expression levels in the roots and leaves. Seedlings sprayed with 135 g Mo·ha^-1 also had relatively high nitrogen metabolic enzyme activities and up-regulated transcript levels of nitrate uptake genes (NRT1.1; NRT2.1) and nitrate-responsive genes. Furthermore, there was a significantly lower NO₃^- concentration in the leaves and roots, a higher NH₄⁺ concentration in leaves, and a higher glutamine/glutamate (Gln/Glu) concentration at 135 g Mo·ha^-1. Seedlings sprayed with 202.5 g Mo·ha^-1 showed the opposite trend. Taken together, these results suggest that a 135 g Mo·ha^-1 application was optimal because it enhanced NO₃^- transport from the roots to the shoots and increased NUE by mediating nitrogen metabolic enzyme activities, nitrate transport, and nitrate assimilation gene activities.

Genetic variation in traits for nitrogen use efficiency in wheat

Crop nutrient and especially nitrogen use efficiency (NUE) is both an economically and an environmentally highly desirable trait. It has been estimated that only a third of nitrogen inputs to cereal crop worldwide are recovered in grain for consumption, resulting in a huge waste of resource with major negative impacts on the environment. Most measures of NUE in wheat and other cereals are based on field assessments of crop yields at given N inputs, performance responses to added N fertilizer, or by quantifying N fertilizer recovery rates. However, NUE is a complex trait comprising two key major components, N uptake and N utilization efficiency, both also complex traits in themselves, each involving many physiological processes and biochemical pathways. A deeper understanding of the processes involved in NUE has been a target of the UK Wheat Genetic Improvement Network project (http://www.wgin.org.uk/). This has enabled the breakdown of characteristics contributing to NUE and an assessment of the variation present in those characteristics, predominantly in modern cultivars; a total of 13 years of data have been obtained to date. Significant but limited variation suggests a requirement for broader germplasm screening such as older varieties, landraces, and wild relatives.

Nitrogen assimilation system in maize is regulated by developmental and tissue-specific mechanisms

We found metabolites, enzyme activities and enzyme transcript abundances vary significantly across the maize lifecycle, but weak correlation exists between the three groups. We identified putative genes regulating nitrate assimilation. Progress in improving nitrogen (N) use efficiency (NUE) of crop plants has been hampered by the complexity of the N uptake and utilisation systems. To understand this complexity we measured the activities of seven enzymes and ten metabolites related to N metabolism in the leaf and root tissues of Gaspe Flint maize plants grown in 0.5 or 2.5 mM NO3 (-) throughout the lifecycle. The amino acids had remarkably similar profiles across the lifecycle except for transient responses, which only appeared in the leaves for aspartate or in the roots for asparagine, serine and glycine. The activities of the enzymes for N assimilation were also coordinated to a certain degree, most noticeably with a peak in root activity late in the lifecycle, but with wide variation in the activity levels over the course of development. We analysed the transcriptional data for gene sets encoding the measured enzymes and found that, unlike the enzyme activities, transcript levels of the corresponding genes did not exhibit the same coordination across the lifecycle and were only weakly correlated with the levels of various amino acids or individual enzyme activities. We identified gene sets which were correlated with the enzyme activity profiles, including seven genes located within previously known quantitative trait loci for enzyme activities and hypothesise that these genes are important for the regulation of enzyme activities. This work provides insights into the complexity of the N assimilation system throughout development and identifies candidate regulatory genes, which warrant further investigation in efforts to improve NUE in crop plants.

Variations in nitrogen use efficiency reflect the biochemical subtype while variations in water use efficiency reflect the evolutionary lineage of C4 grasses at inter-glacial CO2

C4 photosynthesis evolved multiple times in diverse lineages. Most physiological studies comparing C4 plants were not conducted at the low atmospheric CO2 prevailing during their evolution. Here, 24 C4 grasses belonging to three biochemical subtypes [nicotinamide adenine dinucleotide malic enzyme (NAD-ME), phosphoenolpyruvate carboxykinase (PCK) and nicotinamide adenine dinucleotide phosphate malic enzyme (NADP-ME)] and six major evolutionary lineages were grown under ambient (400 μL L(-1) ) and inter-glacial (280 μL L(-1) ) CO2 . We hypothesized that nitrogen-related and water-related physiological traits are associated with subtypes and lineages, respectively. Photosynthetic rate and stomatal conductance were constrained by the shared lineage, while variation in leaf mass per area (LMA), leaf N per area, plant dry mass and plant water use efficiency were influenced by the subtype. Subtype and lineage were equally important for explaining variations in photosynthetic nitrogen use efficiency (PNUE) and photosynthetic water use efficiency (PWUE). CO2 treatment impacted most parameters. Overall, higher LMA and leaf N distinguished the Chloridoideae/NAD-ME group, while NADP-ME and PCK grasses were distinguished by higher PNUE regardless of lineage. Plants were characterized by high photosynthesis and PWUE when grown at ambient CO2 and by high conductance at inter-glacial CO2 . In conclusion, the evolutionary and biochemical diversity among C4 grasses was aligned with discernible leaf physiology, but it remains unknown whether these traits represent ecophysiological adaptation.