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

TaNAR2.1 and TaNAR2.2 differ in influencing nitrogen uptake and growth of wheat (Triticum aestivum L.)

NAR2 (Nitrate assimilation related protein) is a protein chaperone involved in transporting nitrate across membranes. However, the expression pattern and function of NAR2 genes in wheat are still largely unknown. Here, we cloned two TaNAR2 genes (TaNAR2.1 and TaNAR2.2). To assess and compare the functional differences of TaNAR2.1 and TaNAR2.2, we analyzed the subcellular localization and expression pattern of the two genes in wheat under low nitrogen (LN) and high nitrogen (HN) conditions, as well as the nitrate influx and root system architecture of TaNAR2.1 and TaNAR2.2 overexpression wheat under LN and HN. Additionally, we investigated the effects of TaNAR2.1 and TaNAR2.2 overexpression on the growth phenotype, nitrogen uptake and yield of wheat throughout the growth period. There are significant differences in the expression patterns and functions of TaNAR2.1 and TaNAR2.2. TaNAR2.1 is located in the cytoplasm, nucleus and the plasma membrane, whereas TaNAR2.2 is a cytoplasm-specific protein. TaNAR2.1 appears to exhibit larger changes in expression levels and a higher capacity for nitrate influx than TaNAR2.2 under external nitrate supply. Overexpression of TaNAR2.1 significantly improves grain nitrogen use efficiency and increases grain yield, whereas overexpression of TaNAR2.2 enhances vegetative and reproductive growth of wheat roots. These findings indicate that TaNAR2.1 plays a crucial role in wheat nitrogen accumulation and yield, while TaNAR2.2 is pivotal for wheat root growth.

Nitrate transporters and mechanisms of nitrate signal transduction in Arabidopsis and rice

Nitrate (NO3 -) is a significant inorganic nitrogen source in soil, playing a crucial role in influencing crop productivity. As sessile organisms, plants have evolved complex mechanisms for nitrate uptake and response to varying soil levels. Recent advancements have enhanced our understanding of nitrate uptake and signaling pathways. This mini-review offers a comparative analysis of nitrate uptake mechanisms in Arabidopsis and rice. It also examines nitrate signal transduction, highlighting the roles of AtNRT1.1 and AtNLP7 as nitrate receptors and elucidating the OsNRT1.1B-OsSPX4-OsNLP3 cascade. Additionally, it investigates nuclear transcriptional networks that regulate nitrate-responsive genes, controlled by various transcription factors (TFs) crucial for plant development. By integrating these findings, we highlight mechanisms that may help to enhance crop nitrogen utilization.

Gene co-expression network analysis in areca floral organ and the potential role of the AcMADS17 and AcMADS23 in transgenic Arabidopsis

Areca catechu L., a monocot belonging to the palm family, is monoecious, with female and male flowers separately distributed on the same inflorescence. To discover the molecular mechanism of flower development in Areca, we sequenced different floral samples to generate tissue-specific transcriptomic profiles. We conducted a comparative analysis of the transcriptomic profiles of apical sections of the inflorescence with male flowers and the basal section of the inflorescence with female flowers. Based on the RNA sequencing dataset, we applied weighted gene co-expression network analysis (WGCNA) to identify sepal, petal, stamen, stigma and other specific modules as well as hub genes involved in specific floral organ development. The syntenic and expression patterns of AcMADS-box genes were analyzed in detail. Furthermore, we analyzed the open chromatin regions and transcription factor PI binding sites in male and female flowers by assay for transposase-accessible chromatin sequencing (ATAC-seq) assay. Heterologous expression revealed the important role of AcMADS17 and AcMADS23 in floral organ development. Our results provide a valuable genomic resource for the functional analysis of floral organ development in Areca.

miR164-MhNAC1 regulates apple root nitrogen uptake under low nitrogen stress

Nitrogen is an essential nutrient for plant growth and serves as a signaling molecule to regulate gene expression inducing physiological, growth and developmental responses. An excess or deficiency of nitrogen may have adverse effects on plants. Studying nitrogen uptake will help us understand the molecular mechanisms of utilization for targeted molecular breeding. Here, we identified and functionally validated an NAC (NAM-ATAF1/2-CUC2) transcription factor based on the transcriptomes of two apple rootstocks with different nitrogen uptake efficiency. NAC1, a target gene of miR164, directly regulates the expression of the high-affinity nitrate transporter (MhNRT2.4) and citric acid transporter (MhMATE), affecting root nitrogen uptake. To examine the role of MhNAC1 in nitrogen uptake, we produced transgenic lines that overexpressed or silenced MhNAC1. Silencing MhNAC1 promoted nitrogen uptake and citric acid secretion in roots, and enhanced plant tolerance to low nitrogen conditions, while overexpression of MhNAC1 or silencing miR164 had the opposite effect. This study not only revealed the role of the miR164-MhNAC1 module in nitrogen uptake in apple rootstocks but also confirmed that citric acid secretion in roots affected nitrogen uptake, which provides a research basis for efficient nitrogen utilization and molecular breeding in apple.

A long non-coding RNA functions as a competitive endogenous RNA to modulate TaNAC018 by acting as a decoy for tae-miR6206

Increasing evidence indicates a strong correlation between the deposition of cuticular waxes and drought tolerance. However, the precise regulatory mechanism remains elusive. Here, we conducted a comprehensive transcriptome analysis of two wheat (Triticum aestivum) near-isogenic lines, the glaucous line G-JM38 rich in cuticular waxes and the non-glaucous line NG-JM31. We identified 85,143 protein-coding mRNAs, 4,485 lncRNAs, and 1,130 miRNAs. Using the lncRNA-miRNA-mRNA network and endogenous target mimic (eTM) prediction, we discovered that lncRNA35557 acted as an eTM for the miRNA tae-miR6206, effectively preventing tae-miR6206 from cleaving the NAC transcription factor gene TaNAC018. This lncRNA-miRNA interaction led to higher transcript abundance for TaNAC018 and enhanced drought-stress tolerance. Additionally, treatment with mannitol and abscisic acid (ABA) each influenced the levels of tae-miR6206, lncRNA35557, and TaNAC018 transcript. The ectopic expression of TaNAC018 in Arabidopsis also improved tolerance toward mannitol and ABA treatment, whereas knocking down TaNAC018 transcript levels via virus-induced gene silencing in wheat rendered seedlings more sensitive to mannitol stress. Our results indicate that lncRNA35557 functions as a competing endogenous RNA to modulate TaNAC018 expression by acting as a decoy target for tae-miR6206 in glaucous wheat, suggesting that non-coding RNA has important roles in the regulatory mechanisms responsible for wheat stress tolerance.

TaLBD41 interacts with TaNAC2 to regulate nitrogen uptake and metabolism in response to nitrate availability

Nitrate is the main source of nitrogen (N) available to plants and also is a signal that triggers complex regulation of transcriptional networks to modulate a wide variety of physiological and developmental responses in plants. How plants adapt to soil nitrate fluctuations is a complex process involving a fine-tuned response to nitrate provision and N starvation, the molecular mechanisms of which remain largely uncharted. Here, we report that the wheat transcription factor TaLBD41 interacts with the nitrate-inducible transcription factor TaNAC2 and is repressed by nitrate provision. Electrophoretic mobility shift assay and dual-luciferase system show that the TaLBD41-NAC2 interaction confers homeostatic coordination of nitrate uptake, reduction, and assimilation by competitively binding to TaNRT2.1, TaNR1.2, and TaNADH-GOGAT. Knockdown of TaLBD41 expression enhances N uptake and assimilation, increases spike number, grain yield, and nitrogen harvest index under different N supply conditions. We also identified an elite haplotype of TaLBD41-2B associated with increased spike number and grain yield. Our study uncovers a novel mechanism underlying the interaction between two transcription factors in mediating wheat adaptation to nitrate availability by antagonistically regulating nitrate uptake and assimilation, providing a potential target for designing varieties with efficient N use in wheat (Triticum aestivum).

Functional analyses of the NRT2 family of nitrate transporters in Arabidopsis

Nitrogen is an essential macronutrient for plant growth and development. Nitrate is the major form of nitrogen acquired by most crops and also serves as a vital signaling molecule. Nitrate is absorbed from the soil into root cells usually by the low-affinity NRT1 NO3 - transporters and high-affinity NRT2 NO3 - transporters, with NRT2s serving to absorb NO3 - under NO3 -limiting conditions. Seven NRT2 members have been identified in Arabidopsis, and they have been shown to be involved in various biological processes. In this review, we summarize the spatiotemporal expression patterns, localization, and biotic and abiotic responses of these transporters with a focus on recent advances in the current understanding of the functions of the seven AtNRT2 genes. This review offers beneficial insight into the mechanisms by which plants adapt to changing environmental conditions and provides a theoretical basis for crop research in the near future.

Genome-wide dissection and haplotype analysis identified candidate loci for nitrogen use efficiency under drought conditions in winter wheat

Climate change causes extreme conditions like prolonged drought, which results in yield reductions due to its effects on nutrient balances such as nitrogen uptake and utilization by plants. Nitrogen (N) is a crucial nutrient element for plant growth and productivity. Understanding the mechanistic basis of nitrogen use efficiency (NUE) under drought conditions is essential to improve wheat (Triticum aestivum L.) yield. Here, we evaluated the genetic variation of NUE-related traits and photosynthesis response in a diversity panel of 200 wheat genotypes under drought and nitrogen stress conditions to uncover the inherent genetic variation and identify quantitative trait loci (QTLs) underlying these traits. The results revealed significant genetic variations among the genotypes in response to drought stress and nitrogen deprivation. Drought impacted plant performance more than N deprivation due to its effect on water and nutrient uptake. GWAS identified a total of 27 QTLs with a significant main effect on the drought-related traits, while 10 QTLs were strongly associated with the NUE traits. Haplotype analysis revealed two different haplotype blocks within the associated region on chromosomes 1B and 5A. The two haplotypes showed contrasting effects on N uptake and use efficiency traits. The in silico and transcript analyses implicated candidate gene coding for cold shock protein. This gene was the most highly expressed gene under several stress conditions, including drought stress. Upon validation, these QTLs on 1B and 5A could be used as a diagnostic marker for NUE and drought tolerance screening in wheat.

Are TaNAC Transcription Factors Involved in Promoting Wheat Yield by cis-Regulation of TaCKX Gene Family?

NAC transcription factors (TFs) are one of the largest TF families in plants, and TaNACs have been known to participate in the regulation of the transcription of many yield-regulating genes in bread wheat. The TaCKX gene family members (GFMs) have already been shown to regulate yield-related traits, including grain mass and number, leaf senescence, and root growth. The genes encode cytokinin (CK) degrading enzymes (CKXs) and are specifically expressed in different parts of developing wheat plants. The aim of the study was to identify and characterize TaNACs involved in the cis-regulation of TaCKX GFMs. After analysis of the initial transcription factor data in 1.5 Kb cis-regulatory sequences of a total of 35 homologues of TaCKX GFMs, we selected five of them, namely TaCKX1-3A, TaCKX22.1-3B, TaCKX5-3D, TaCKX9-1B, and TaCKX10, and identified five TaNAC genes: TaNACJ-1, TaNAC13a, TaNAC94, TaNACBr-1, and TaNAC6D, which are potentially involved in the cis-regulation of selected TaCKX genes, respectively. Protein feature analysis revealed that all of the selected TaNACs have a conserved NAC domain and showed a stable tertiary structure model. The expression profile of the selected TaNACs was studied in 5 day-old seedling roots, 5-6 cm inflorescences, 0, 4, 7, and 14 days-after-pollination (DAP) spikes, and the accompanying flag leaves. The expression pattern showed that all of the selected TaNACs were preferentially expressed in seedling roots, 7 and 14 DAP spikes, and flag leaves compared to 5-6 cm inflorescence and 0 and 4 DAP spikes and flag leaves in Kontesa and Ostka spring wheat cultivars (cvs.). In conclusion, the results of this study highlight the potential role of the selected TaNACs in the regulation of grain productivity, leaf senescence, root growth, and response to various stresses.

Exploring the genetic landscape of nitrogen uptake in durum wheat: genome-wide characterization and expression profiling of NPF and NRT2 gene families

Nitrate uptake by plants primarily relies on two gene families: Nitrate transporter 1/peptide transporter (NPF) and Nitrate transporter 2 (NRT2). Here, we extensively characterized the NPF and NRT2 families in the durum wheat genome, revealing 211 NPF and 20 NRT2 genes. The two families share many Cis Regulatory Elements (CREs) and Transcription Factor binding sites, highlighting a partially overlapping regulatory system and suggesting a coordinated response for nitrate transport and utilization. Analyzing RNA-seq data from 9 tissues and 20 cultivars, we explored expression profiles and co-expression relationships of both gene families. We observed a strong correlation between nucleotide variation and gene expression within the NRT2 gene family, implicating a shared selection mechanism operating on both coding and regulatory regions. Furthermore, NPF genes showed highly tissue-specific expression profiles, while NRT2s were mainly divided in two co-expression modules, one expressed in roots (NAR2/NRT3 dependent) and the other induced in anthers and/ovaries during maturation. Our evidences confirmed that the majority of these genes were retained after small-scale duplication events, suggesting a neo- or sub-functionalization of many NPFs and NRT2s. Altogether, these findings indicate that the expansion of these gene families in durum wheat could provide valuable genetic variability useful to identify NUE-related and candidate genes for future breeding programs in the context of low-impact and sustainable agriculture.

Genome-Wide Identification and Functional Analysis of Nitrate Transporter Genes (NPF, NRT2 and NRT3) in Maize

Nitrate is the primary form of nitrogen uptake in plants, mainly transported by nitrate transporters (NRTs), including NPF (NITRATE TRANSPORTER 1/PEPTIDE TRANSPORTER FAMILY), NRT2 and NRT3. In this study, we identified a total of 78 NPF, seven NRT2, and two NRT3 genes in maize. Phylogenetic analysis divided the NPF family into eight subgroups (NPF1-NPF8), consistent with the results in Arabidopsis thaliana and rice. The NRT2 family appears to have evolved more conservatively than the NPF family, as NRT2 genes contain fewer introns. The promoters of all NRTs are rich in cis-acting elements responding to biotic and abiotic stresses. The expression of NRTs varies in different tissues and developmental stages, with some NRTs only expressed in specific tissues or developmental stages. RNA-seq analysis using Xu178 revealed differential expression of NRTs in response to nitrogen starvation and nitrate resupply. Moreover, the expression patterns of six key NRTs genes (NPF6.6, NPF6.8, NRT2.1, NRT2.5 and NRT3.1A/B) varied in response to alterations in nitrogen levels across distinct maize inbred lines with different nitrogen uptake rates. This work enhances our understanding of the structure and expression of NRTs genes, and their roles in nitrate response, paving the way for improving maize nitrogen efficiency through molecular breeding.

Genome-wide systematic characterization of the NRT2 gene family and its expression profile in wheat (Triticum aestivum L.) during plant growth and in response to nitrate deficiency

BACKGROUND

Wheat (Triticum aestivum L.) is a major cereal crop that is grown worldwide, and it is highly dependent on sufficient N supply. The molecular mechanisms associated with nitrate uptake and assimilation are still poorly understood in wheat. In plants, NRT2 family proteins play a crucial role in NO₃^- acquisition and translocation under nitrate limited conditions. However, the biological functions of these genes in wheat are still unclear, especially their roles in NO₃^- uptake and assimilation.

RESULTS

In this study, a comprehensive analysis of wheat TaNRT2 genes was conducted using bioinformatics and molecular biology methods, and 49 TaNRT2 genes were identified. A phylogenetic analysis clustered the TaNRT2 genes into three clades. The genes that clustered on the same phylogenetic branch had similar gene structures and nitrate assimilation functions. The identified genes were further mapped onto the 13 wheat chromosomes, and the results showed that a large duplication event had occurred on chromosome 6. To explore the TaNRT2 gene expression profiles in wheat, we performed transcriptome sequencing after low nitrate treatment for three days. Transcriptome analysis revealed the expression levels of all TaNRT2 genes in shoots and roots, and based on the expression profiles, three highly expressed genes (TaNRT2-6A.2, TaNRT2-6A.6, and TaNRT2-6B.4) were selected for qPCR analysis in two different wheat cultivars ('Mianmai367' and 'Nanmai660') under nitrate-limited and normal conditions. All three genes were upregulated under nitrate-limited conditions and highly expressed in the high nitrogen use efficiency (NUE) wheat 'Mianmai367' under low nitrate conditions.

CONCLUSION

We systematically identified 49 NRT2 genes in wheat and analysed the transcript levels of all TaNRT2s under nitrate deficient conditions and over the whole growth period. The results suggest that these genes play important roles in nitrate absorption, distribution, and accumulation. This study provides valuable information and key candidate genes for further studies on the function of TaNRT2s in wheat.

Wheat NAC-A18 regulates grain starch and storage proteins synthesis and affects grain weight

KEY MESSAGE

Wheat NAC-A18 regulates both starch and storage protein synthesis in the grain, and a haplotype with positive effects on grain weight showed increased frequency during wheat breeding in China. Starch and seed storage protein (SSP) directly affect the processing quality of wheat grain. The synthesis of starch and SSP are also regulated at the transcriptional level. However, only a few starch and SSP regulators have been identified in wheat. In this study, we discovered a NAC transcription factor, designated as NAC-A18, which acts as a regulator of both starch and SSP synthesis. NAC-A18, is predominately expressed in wheat developing grains, encodes a transcription factor localized in the nucleus, with both activation and repression domains. Ectopic expression of wheat NAC-A18 in rice significantly decreased starch accumulation and increased SSP accumulation and grain size and weight. Dual-luciferase reporter assays indicated that NAC-A18 could reduce the expression of TaGBSSI-A1 and TaGBSSI-A2, and enhance the expression of TaLMW-D6 and TaLMW-D1. A yeast one hybrid assay demonstrated that NAC-A18 bound directly to the cis-element "ACGCAA" in the promoters of TaLMW-D6 and TaLMW-D1. Further analysis indicated that two haplotypes were formed at NAC-A18, and that NAC-A18_h1 was a favorable haplotype correlated with higher thousand grain weight. Based on limited population data, NAC-A18_h1 underwent positive selection during Chinese wheat breeding. Our study demonstrates that wheat NAC-A18 regulates starch and SSP accumulation and grain size. A molecular marker was developed for the favorable allele for breeding applications.

Transgenerational Paternal Inheritance of TaCKX GFMs Expression Patterns Indicate a Way to Select Wheat Lines with Better Parameters for Yield-Related Traits

Members of the TaCKX gene family (GFMs) encode the cytokinin oxygenase/dehydrogenase enzyme (CKX), which irreversibly degrades cytokinins in the organs of wheat plants; therefore, these genes perform a key role in the regulation of yield-related traits. The purpose of the investigation was to determine how expression patterns of these genes, together with the transcription factor-encoding gene TaNAC2-5A, and yield-related traits are inherited to apply this knowledge to speed up breeding processes. The traits were tested in 7 days after pollination (DAP) spikes and seedling roots of maternal and paternal parents and their F₂ progeny. The expression levels of most of them and the yield were inherited in F₂ from the paternal parent. Some pairs or groups of genes cooperated, and some showed opposite functions. Models of up- or down-regulation of TaCKX GFMs and TaNAC2-5A in low-yielding maternal plants crossed with higher-yielding paternal plants and their high-yielding F₂ progeny reproduced gene expression and yield of the paternal parent. The correlation coefficients between TaCKX GFMs, TaNAC2-5A, and yield-related traits in high-yielding F₂ progeny indicated which of these genes were specifically correlated with individual yield-related traits. The most common was expressed in 7 DAP spikes TaCKX2.1, which positively correlated with grain number, grain yield, spike number, and spike length, and seedling root mass. The expression levels of TaCKX1 or TaNAC2-5A in the seedling roots were negatively correlated with these traits. In contrast, the thousand grain weight (TGW) was negatively regulated by TaCKX2.2.2, TaCKX2.1, and TaCKX10 in 7 DAP spikes but positively correlated with TaCKX10 and TaNAC2-5A in seedling roots. Transmission of TaCKX GFMs and TaNAC2-5A expression patterns and yield-related traits from parents to the F₂ generation indicate their paternal imprinting. These newly shown data of nonmendelian epigenetic inheritance shed new light on crossing strategies to obtain a high-yielding F₂ generation.

Bioengineering Techniques to Improve Nitrogen Transformation and Utilization: Implications for Nitrogen Use Efficiency and Future Sustainable Crop Production

Nitrogen (N) is crucial for plant growth and development, especially in physiological and biochemical processes such as component of different proteins, enzymes, nucleic acids, and plant growth regulators. Six categories, such as transporters, nitrate absorption, signal molecules, amino acid biosynthesis, transcription factors, and miscellaneous genes, broadly encompass the genes regulating NUE in various cereal crops. Herein, we outline detailed research on bioengineering modifications of N metabolism to improve the different crop yields and biomass. We emphasize effective and precise molecular approaches and technologies, including N transporters, transgenics, omics, etc., which are opening up fascinating opportunities for a complete analysis of the molecular elements that contribute to NUE. Moreover, the detection of various types of N compounds and associated signaling pathways within plant organs have been discussed. Finally, we highlight the broader impacts of increasing NUE in crops, crucial for better agricultural yield and in the greater context of global climate change.

Integrative physiological, metabolomic and transcriptomic analysis reveals nitrogen preference and carbon and nitrogen metabolism in blackberry plants

Nitrogen (N) is an indispensable element for plant growth and development. To understand the regulation of underlying carbon (C) and N metabolism in blackberry plants, we performed integrated analyses of the physiology, metabolome and transcriptome. Blackberry plants were subjected to no N, nitrate (NO₃⁻)-N, ammonium (NH₄⁺)-N and urea treatments. Our results showed that the NH₄⁺-N treatment yielded higher values for the biomass, chlorophyll, antioxidants, N contents and antioxidant enzyme activities, as well as lower levels of free radicals and the C/N ratio compared with other treatments. Transcriptome analysis showed that different N forms significantly affected photosynthesis, flavonoid biosynthesis and the TCA cycle. Metabolome analysis indicated that the levels of lipids, carbohydrates, flavonoids and amino acids were markedly changed under different N treatments. Integrated transcriptomic and metabolomic data revealed that amino acids, including proline, arginine, L-isoleucine, L-aspartate, threonine, and L-glutamate, played important roles in maintaining normal plant growth by regulating N metabolism and amino acid metabolism. Overall, blackberry plants preferentially take up NH₄⁺-N. Under the NH₄⁺-N treatment, N assimilation was stronger, flavonoid biosynthesis was decreased, and the promoting influence of NH₄⁺-N on N metabolism was better than that of NO₃⁻-N. However, the NO₃⁻-N treatment enhanced the C/N ratio, accelerated the process of C metabolism and increased the synthesis of flavonoids, thereby accelerating the flow of N metabolism to C metabolism. These results provide deeper insight into coordinating C and N metabolism and improving N use efficiency in blackberry plants.

Millet-inspired systems metabolic engineering of NUE in crops

The use of nitrogen (N) fertilizers in agriculture has a great ability to increase crop productivity. However, their excessive use has detrimental effects on the environment. Therefore, it is necessary to develop crop varieties with improved nitrogen use efficiency (NUE) that require less N but have substantial yields. Orphan crops such as millets are cultivated in limited regions and are well adapted to lower input conditions. Therefore, they serve as a rich source of beneficial traits that can be transferred into major crops to improve their NUE. This review highlights the tremendous potential of systems biology to unravel the enzymes and pathways involved in the N metabolism of millets, which can open new possibilities to generate transgenic crops with improved NUE.

Cis-Regulation by NACs: A Promising Frontier in Wheat Crop Improvement

Crop traits are controlled by multiple genes; however, the complex spatio-temporal transcriptional behavior of genes cannot be fully understood without comprehending the role of transcription factors (TFs) and the underlying mechanisms of the binding interactions of their cis-regulatory elements. NAC belongs to one of the largest families of plant-specific TFs and has been associated with the regulation of many traits. This review provides insight into the cis-regulation of genes by wheat NACs (TaNACs) for the improvement in yield-related traits, including phytohormonal homeostasis, leaf senescence, seed traits improvement, root modulation, and biotic and abiotic stresses in wheat and other cereals. We also discussed the current potential, knowledge gaps, and prospects of TaNACs.

LjNRT2.3 plays a hierarchical role in the control of high affinity transport system for root nitrate acquisition in Lotus japonicus

Nitrate is a key mineral nutrient required for plant growth and development. Plants have evolved sophisticated mechanisms to respond to changes of nutritional availability in the surrounding environment and the optimization of root nitrate acquisition under nitrogen starvation is crucial to cope with unfavoured condition of growth. In this study we present a general description of the regulatory transcriptional and spatial profile of expression of the Lotus japonicus nitrate transporter NRT2 family. Furthermore, we report a phenotypic characterization of two independent Ljnrt2.3 knock out mutants indicating the involvement of the LjNRT2.3 gene in the root nitrate acquisition and lateral root elongation pathways occurring in response to N starvation conditions. We also report an epistatic relationship between LjNRT2.3 and LjNRT2.1 suggesting a combined mode of action of these two genes in order to optimize the Lotus response to a prolonged N starvation.

Simultaneously genetic selection of wheat yield and grain protein quality in rice-wheat and soybean-wheat cropping systems through critical nitrogen efficiency-related traits

Analyzing the contribution of nitrogen (N) uptake and its utilization in grain yield and protein quality-related traits in rice-wheat (RW) and soybean-wheat (SW) cropping systems is essential for simultaneous improvements in the two target traits. A field experiment with nine wheat genotypes was conducted in 2018-19 and 2019-20 cropping years to investigate N uptake and utilization-related traits associated with high wheat yield and good protein quality. Results showed that N uptake efficiency (NUpE) in the RW cropping system and N utilization efficiency (NUtE) in the SW cropping system explained 77.6 and 65.2% of yield variation, respectively, due to the contribution of fertile spikes and grain number per spike to grain yield varied depending on soil water and N availability in the two rotation systems. Lower grain protein content in the RW cropping system in comparison to the SW cropping system was mainly related to lower individual N accumulation at maturity, resulting from higher fertile spikes, rather than N harvest index (NHI). However, NHI in the SW cropping system accounted for greater variation in grain protein content. Both gluten index and post-anthesis N uptake were mainly affected by genotype, and low gluten index caused by high post-anthesis N uptake may be related to the simultaneous increase in kernel weight. N remobilization process associated with gluten quality was driven by increased sink N demand resulting from high grain number per unit area in the RW cropping system; confinement of low sink N demand and source capability resulted in low grain number per spike and water deficit limiting photosynthesis of flag leaf in the SW cropping system. CY-25 obtained high yield and wet gluten content at the expense of gluten index in the two wheat cropping systems, due to low plant height and high post-anthesis N uptake and kernel weight. From these results, we concluded that plant height, kernel weight, and post-anthesis N uptake were the critically agronomic and NUE-related traits for simultaneous selection of grain yield and protein quality. Our research results provided useful guidelines for improving both grain yield and protein quality by identifying desirable N-efficient genotypes in the two rotation systems.

Nitrate signaling and use efficiency in crops

Nitrate (NO₃^-) is not only an essential nutrient but also an important signaling molecule for plant growth. Low nitrogen use efficiency (NUE) of crops is causing increasingly serious environmental and ecological problems. Understanding the molecular mechanisms of NO₃^- regulation in crops is crucial for NUE improvement in agriculture. During the last several years, significant progress has been made in understanding the regulation of NO₃^- signaling in crops, and some key NO₃^- signaling factors have been shown to play important roles in NO₃^- utilization. However, no detailed reviews have yet summarized these advances. Here, we focus mainly on recent advances in crop NO₃^- signaling, including short-term signaling, long-term signaling, and the impact of environmental factors. We also review the regulation of crop NUE by crucial genes involved in NO₃^- signaling. This review provides useful information for further research on NO₃^- signaling in crops and a theoretical basis for breeding new crop varieties with high NUE, which has great significance for sustainable agriculture.

Wheat genomic study for genetic improvement of traits in China

Bread wheat (Triticum aestivum L.) is a major crop that feeds 40% of the world's population. Over the past several decades, advances in genomics have led to tremendous achievements in understanding the origin and domestication of wheat, and the genetic basis of agronomically important traits, which promote the breeding of elite varieties. In this review, we focus on progress that has been made in genomic research and genetic improvement of traits such as grain yield, end-use traits, flowering regulation, nutrient use efficiency, and biotic and abiotic stress responses, and various breeding strategies that contributed mainly by Chinese scientists. Functional genomic research in wheat is entering a new era with the availability of multiple reference wheat genome assemblies and the development of cutting-edge technologies such as precise genome editing tools, high-throughput phenotyping platforms, sequencing-based cloning strategies, high-efficiency genetic transformation systems, and speed-breeding facilities. These insights will further extend our understanding of the molecular mechanisms and regulatory networks underlying agronomic traits and facilitate the breeding process, ultimately contributing to more sustainable agriculture in China and throughout the world.

Amino acid transporter gene TaATLa1 from Triticum aestivum L. improves growth under nitrogen sufficiency and is down regulated under nitrogen deficiency

TaATLa1 was identified to respond to nitrogen deprivation through transcriptome analysis of wheat seedlings. TaATLa1 specifically transports Gln, Glu, and Asp, and affects the biomass of Arabidopsis and wheat. Nitrogen is an essential macronutrient and plays a crucial role in wheat production. Amino acids, the major form of organic nitrogen, are remobilized by amino acid transporters (AATs) in plants. AATs are commonly described as central components of essential developmental processes and yield formation via taking up and transporting amino acids in plants. However, few studies have reported the detailed biochemical properties and biological functions of these AATs in wheat. In this study, key genes encoding AATs were screened from transcriptome analysis of wheat seedlings treated with normal nitrogen (NN) and nitrogen deprivation (ND). Among them, 21 AATs were down-regulated and eight AATs were up-regulated under ND treatment. Among the homoeologs, TaATLa1.1-3A, TaATLa1.1-3B, and TaATLa1.1-3D (TaATLa1.1-3A, -3B, and -3D), belonging to amino acid transporter-like a (ATLa) subfamily, were significantly down-regulated in response to ND in wheat, and accordingly were selected for functional analyses. The results demonstrated that TaATLa1.1-3A, -3B, and -3D effectively transported glutamine (Gln), glutamate (Glu), and aspartate (Asp) in yeast. Overexpression of TaAILa1.1-3A, -3B, and -3D in Arabidopsis thaliana L. significantly increased amino acid content in leaves, storage protein content in seeds and the plant biomass under NN. Knockdown of TaATLa1.1-3A, -3B, and -3D in wheat seedlings resulted in a significant block of amino acid remobilization and growth inhibition. Taken together, TaATLa1.1-3A, -3B, and -3D contribute substantially to Arabidopsis and wheat growth. We propose that TaATLa1.1-3A, -3B, and -3D may participate in the source-sink translocation of amino acid, and they may have profound implications for wheat yield improvement.

Genetics of the Inverse Relationship between Grain Yield and Grain Protein Content in Common Wheat

Grain protein content (GPC) is one of the most important criteria to determine the quality of common wheat (Triticum aestivum). One of the major obstacles for bread wheat production is the negative correlation between GPC and grain yield (GY). Previous studies demonstrated that the deviation from this inverse relationship is highly heritable. However, little is known about the genetics controlling these deviations in common wheat. To fill this gap, we performed quantitative trait locus (QTL) analysis for GY, GPC, and four derived GY-GPC indices using an eight-way multiparent advanced generation intercross population comprising 394 lines. Interval mapping was conducted using phenotypic data from up to nine environments and genotypic data from a 20k single-nucleotide polymorphism array. The four indices were highly heritable (0.76–0.88) and showed distinct correlations to GY and GPC. Interval mapping revealed that GY, GPC, and GY-GPC indices were controlled by 6, 12, and 12 unique QTL, of which each explained only a small amount of phenotypic variance (R² ≤ 10%). Ten of the 12 index QTL were independent of loci affecting GY and GPC. QTL regions harboured several candidate genes, including Rht-1, WAPO-A1, TaTEF-7A, and NRT2.6-7A. The study confirmed the usefulness of indices to mitigate the inverse GY-GPC relationship in breeding, though the selection method should reflect their polygenic inheritance.

Genome-wide identification and in silico analysis of NPF, NRT2, CLC and SLAC1/SLAH nitrate transporters in hexaploid wheat (Triticum aestivum)

Nitrogen transport is one of the most important processes in plants mediated by specialized transmembrane proteins. Plants have two main systems for nitrogen uptake from soil and its transport within the system—a low-affinity transport system and a high-affinity transport system. Nitrate transporters are of special interest in cereal crops because large amount of money is spent on N fertilizers every year to enhance the crop productivity. Till date four gene families of nitrate transporter proteins; NPF (nitrate transporter 1/peptide transporter family), NRT2 (nitrate transporter 2 family), the CLC (chloride channel family), and the SLAC/SLAH (slow anion channel-associated homologues) have been reported in plants. In our study, in silico mining of nitrate transporter genes along with their detailed structure, phylogenetic and expression analysis was carried out. A total of 412 nitrate transporter genes were identified in hexaploid wheat genome using HMMER based homology searches in IWGSC Refseq v2.0. Out of those twenty genes were root specific, 11 leaf/shoot specific and 17 genes were grain/spike specific. The identification of nitrate transporter genes in the close proximity to the previously identified 67 marker-traits associations associated with the nitrogen use efficiency related traits in nested synthetic hexaploid wheat introgression library indicated the robustness of the reported transporter genes. The detailed crosstalk between the genome and proteome and the validation of identified putative candidate genes through expression and gene editing studies may lay down the foundation to improve nitrogen use efficiency of cereal crops.

Investigation of the direct effect of a precision Ascophyllum nodosum biostimulant on nitrogen use efficiency in wheat seedlings

Reduction in the greenhouse gas (GHG) emissions and nitrogen (N) pollution of ground water by improving nitrogen use efficiency (NUE) in crops has become an intensively investigated research topic in pursuit of a more sustainable future. Although, distinct solutions have been proposed there are only a few reports documenting the detailed interplay between observed plant growth dynamics and changes in plant N related transcriptional and biochemical changes. It was previously demonstrated that the application of a formulated biostimulant (PSI-362) derived from Ascophyllum nodosum (ANE) improves N uptake in Arabidopsis thaliana and in barley. In this study, the effect of PSI-362 on the growth dynamics of wheat seedlings was evaluated at different biostimulant and N supplementation rates. Wheat grown on N deficient MS medium was also analysed from the first hour of the treatment until the depletion of the nutrients in the medium 9 days later. During this time the biomass increase measured for PSI-362 treated plants versus untreated controls was associated with increased nitrate uptake, with surplus N assimilated by the biomass in the form of glutamate, glutamine, free amino acids, soluble proteins, and chlorophyll. Phenotypical and biochemical analysis were supported by evaluation of differential expression of genetic markers involved in nitrate perception and transport (TaNRT1.1/NPF6.3), nitrate and nitrite reduction (TaNR1 and TaNiR1) and assimilation (TaGDH2, TaGoGAT, TaGS1). Finally, a comparative analysis of the precision biostimulant PSI-362 and two generic ANEs demonstrated that the NUE effect greatly differs depending on the ANE formulation used.

Genome-Wide Characterization of High-Affinity Nitrate Transporter 2 (NRT2) Gene Family in Brassica napus

Nitrate transporter 2 (NRT2) plays an essential role in Nitrogen (N) uptake, transport, utilization, and stress resistance. In this study, the NRT2 gene family in two sequenced Brassica napus ecotypes were identified, including 31 genes in ‘Zhongshuang11’ (BnaZSNRT2s) and 19 in ‘Darmor-bzh’ (BnaDarNRT2s). The candidate genes were divided into three groups (Group I−III) based on phylogenetic analyses, supported by a conserved intron-exon structure in each group. Collinearity analysis revealed that the large expansion of BnaZSNRT2s attributed to allopolyploidization of ancestors Brassica rapa and Brassica oleracea, and small-scale duplication events in B. napus. Transcription factor (TF) binding site prediction, cis-element analysis, and microRNA prediction suggested that the expressions of BnaZSNRT2s are regulated by multiple factors, and the regulatory pattern is relatively conserved in each group and is tightly connected between groups. Expression assay showed the diverse and differentiated spatial-temporal expression profiles of BnaZSNRT2s in Group I, but conserved patterns were observed in Group II/III; and the low nitrogen (LN) stress up-regulated expression profiles were presented in Group I−III, based on RNA-seq data. RT-qPCR analyses confirmed that BnaZSNRT2.5A-1 and BnaZSNRT2.5C-1 in Group II were highly up-regulated under LN stress in B. napus roots. Our results offer valid information and candidates for further functional BnaZSNRT2s studies.

Genetic Control of Efficient Nitrogen Use for High Yield and Grain Protein Concentration in Wheat: A Review

The increasing global population and the negative effects of nitrogen (N) fertilizers on the environment challenge wheat breeding to maximize yield potential and grain protein concentration (GPC) in an economically and environmentally friendly manner. Understanding the molecular mechanisms for the response of yield components to N availability and assimilates allocation to grains provides the opportunity to increase wheat yield and GPC simultaneously. This review summarized quantitative trait loci/genes which can increase spikes and grain number by enhancing N uptake and assimilation at relative early growth stage, and 1000-grain weight and GPC by increasing post-anthesis N uptake and N allocation to grains.

Barley transcription factor HvNLP2 mediates nitrate signaling and affects nitrogen use efficiency

Plants have evolved complex mechanisms to adapt to the changing nitrogen levels in the environment. In Arabidopsis, more than a dozen nitrate signaling regulatory genes have been characterized, including the NODULE INCEPTION-LIKE PROTEIN (AtNLP) genes, which play essential roles in nitrate signaling. However, whether NLP genes in the Triticeae crops are involved in nitrate regulation and nitrogen use efficiency (NUE) remains unknown. Here, we isolated a barley (Hordeum vulgare L.) mutant, hvnlp2-1, from a TILLING (Targeting Local Lesions IN Genomes) population and constructed two RNAi lines, hvnlp2-2 and hvnlp2-3, to study the function of HvNLP2. The expression of the nitrate-responsive genes was substantially inhibited after nitrate treatment in the hvnlp2 mutants, indicating that HvNLP2 controls nitrate signaling. Nitrate content was significantly higher in the hvnlp2 mutants, which may result from the decreased assimilation of nitrogen caused by reduced nitrate reductase activity and expression of nitrate assimilatory genes. HvNLP2 is localized to the nucleus in the presence of nitrate. Further investigation showed that HvNLP2 binds to and activates the nitrate-responsive cis-elements. Moreover, hvnlp2 exhibited reduced biomass, seed yield, and NUE. Therefore, HvNLP2 controls nitrate signaling and plays an important role in NUE.

A novel NAC family transcription factor SPR suppresses seed storage protein synthesis in wheat

The synthesis of seed storage protein (SSP) is mainly regulated at the transcriptional level. However, few transcriptional regulators of SSP synthesis have been characterized in common wheat (Triticum aestivum) owing to the complex genome. As the A genome donor of common wheat, Triticum urartu could be an elite model in wheat research considering its simple genome. Here, a novel NAC family transcription factor TuSPR from T. urartu was found preferentially expressed in developing endosperm during grain-filling stages. In common wheat transgenically overexpressing TuSPR, the content of total SSPs was reduced by c. 15.97% attributed to the transcription declines of SSP genes. Both in vitro and in vivo assays showed that TuSPR bound to the cis-element 5'-CANNTG-3' distributed in SSP gene promoters and suppressed the transcription. The homolog in common wheat TaSPR shared a conserved function with TuSPR on SSP synthesis suppression. The knock-down of TaSPR in common wheat resulted in 7.07%-20.34% increases in the total SSPs. Both TuSPR and TaSPR could be superior targets in genetic engineering to manipulate SSP content in wheat, and this work undoubtedly expands our knowledge of SSP gene regulation.

Transcriptional regulators of nitrate metabolism: Key players in improving nitrogen use in crops

Green revolution has boosted crop yields by the development of varieties which rely on high fertilizer application. Since then, higher productivity has largely witnessed excessive nitrogen (N) fertilizer application resulting in many environmentally and agronomically unsustainable consequences. One possible solution to this problem is to develop varieties with efficient N use endowed with genetically superior N metabolizing machinery, thereby significantly reducing N loss in soil and facilitating gainful yield performance at lower N conditions. Nitrate (NO₃^-) is the major form of N acquired by plants in aerobic soils. Hence, its efficient acquisition, transport, assimilation into complex organic compounds, and overall homeostasis is crucial to ensure productivity under optimal and suboptimal N conditions. Transcription factors are prime regulators of these processes, and insights into their mechanism of action and the resultant effect on N metabolism are crucial to generating crops with efficient and durable nitrogen use efficiency. The present review, therefore, presents a comprehensive updated account of major N responsive transcription factor families, their cross-talk with other growth factors, and explores existing and potential areas of their biotechnological application to maximize crop yields.

The expression patterns and putative function of nitrate transporter 2.5 in plants

Nitrate transporter 2.5 (NRT2.5) was originally characterized as the transporter for nitrogen (N) limitation. In Arabidopsis, NRT2.5 is expressed mainly under extremely low NO₃^- and N starvation conditions, and this must work in conjunction with NAR2.1. NRT2.5 is expressed both in the roots and leaves in Arabidopsis, poplars, tea trees and cassava. This is also expressed in the seeds of Arabidopsis and wheat. In wheat, NRT2.5 is expressed in the embryo and shell and plays a role in the accumulation of NO₃^- in the seeds. In maize, this is also expressed in silk, cobs and tassel husk leaves. In rice, OsNRT2.5 (also known as OsNRT2.3a) may help the species to remove NO₃^- from the roots to shoots. In addition, NRT2.5 may interact with TGA3, MYC1, LBD37, LBD38, TaNAC2 and other transcription factors and participate in the transmission of NO₃^- signals. The present review summarizes the functions of NRT2.5 in different plant species, which may help plant breeders and molecular biologists to improve crop yield. Abbreviations: NRT, Nitrate transporter; NUE, nitrogen use efficiency; PTR, peptide transporter; NPF, nitrate peptide transporter family; CLC, chloride channel; LAC1/SLAH, slow anion channel-associated 1 homolog 3; LATS, low-affinity transporter systems; HATS, high-affinity transport systems; NNP, nitrate-nitrite-porter; MFS, major facilitator superfamily.

TaANR1-TaBG1 and TaWabi5-TaNRT2s/NARs Link ABA Metabolism and Nitrate Acquisition in Wheat Roots

Nitrate is the preferred form of nitrogen for most plants, acting both as a nutrient and a signaling molecule. However, the components and regulatory factors governing nitrate uptake in bread wheat (Triticum aestivum), one of the world's most important crop species, have remained unclear, largely due to the complexity of its hexaploid genome. Here, based on recently released whole-genome information for bread wheat, the high-affinity nitrate transporter2 (NRT2) and the nitrate-assimilation-related (NAR) gene family are characterized. We show that abscisic acid (ABA)- Glc ester deconjugation is stimulated in bread wheat roots by nitrate resupply following nitrate withdrawal, leading to enhanced root-tissue ABA accumulation, and that this enhancement, in turn, affects the expression of root-type NRT2/NAR genes. TaANR1 is shown to regulate nitrate-mediated ABA accumulation by directly activating TaBG1, while TaWabi5 is involved in ABA-mediated NO3 - induction of NRT2/NAR genes. Building on previous evidence establishing ABA involvement in the developmental response to high-nitrate stress, our study suggests that ABA also contributes to the optimization of nitrate uptake by regulating the expression of NRT2/NAR genes under limited nitrate supply, offering a new target for improvement of nitrate absorption in crops.