Nitrate in 2020: Thirty Years from Transport to Signaling Networks

Unlocking the potentials of nitrate transporters at improving plant nitrogen use efficiency

Nitrate ( NO 3 - ) transporters have been identified as the primary targets involved in plant nitrogen (N) uptake, transport, assimilation, and remobilization, all of which are key determinants of nitrogen use efficiency (NUE). However, less attention has been directed toward the influence of plant nutrients and environmental cues on the expression and activities of NO 3 - transporters. To better understand how these transporters function in improving plant NUE, this review critically examined the roles of NO 3 - transporters in N uptake, transport, and distribution processes. It also described their influence on crop productivity and NUE, especially when co-expressed with other transcription factors, and discussed these transporters' functional roles in helping plants cope with adverse environmental conditions. We equally established the possible impacts of NO 3 - transporters on the uptake and utilization efficiency of other plant nutrients while suggesting possible strategic approaches to improving NUE in plants. Understanding the specificity of these determinants is crucial to achieving better N utilization efficiency in crops within a given environment.

Insights on Phytohormonal Crosstalk in Plant Response to Nitrogen Stress: A Focus on Plant Root Growth and Development

Nitrogen (N) is a vital mineral component that can restrict the growth and development of plants if supplied inappropriately. In order to benefit their growth and development, plants have complex physiological and structural responses to changes in their nitrogen supply. As higher plants have multiple organs with varying functions and nutritional requirements, they coordinate their responses at the whole-plant level based on local and long-distance signaling pathways. It has been suggested that phytohormones are signaling substances in such pathways. The nitrogen signaling pathway is closely associated with phytohormones such as auxin (AUX), abscisic acid (ABA), cytokinins (CKs), ethylene (ETH), brassinosteroid (BR), strigolactones (SLs), jasmonic acid (JA), and salicylic acid (SA). Recent research has shed light on how nitrogen and phytohormones interact to modulate physiology and morphology. This review provides a summary of the research on how phytohormone signaling affects root system architecture (RSA) in response to nitrogen availability. Overall, this review contributes to identifying recent developments in the interaction between phytohormones and N, as well as serving as a foundation for further study.

The intermediate in a nitrate-responsive ω-amidase pathway in plants may signal ammonium assimilation status

A metabolite of ammonium assimilation was previously theorized to be involved in the coordination of the overall nitrate response in plants. Here we show that 2-hydroxy-5-oxoproline, made by transamination of glutamine, the first product of ammonium assimilation, may be involved in signaling a plant's ammonium assimilation status. In leaves, 2-hydroxy-5-oxoproline met four foundational requirements to be such a signal. First, when it was applied to foliage, enzyme activities of nitrate reduction and ammonium assimilation increased; the activities of key tricarboxylic acid cycle-associated enzymes that help to supply carbon skeletons for amino acid synthesis also increased. Second, its leaf pools increased as nitrate availability increased. Third, the pool size of its precursor, Gln, reflected ammonium assimilation rather than photorespiration. Fourth, it was widely conserved among monocots, dicots, legumes, and nonlegumes and in plants with C3 or C4 metabolism. Made directly from the first product of ammonium assimilation, 2-hydroxy-5-oxoproline acted as a nitrate uptake stimulant. When 2-hydroxy-5-oxoproline was provided to roots, the plant's nitrate uptake rate approximately doubled. Plants exogenously provided with 2-hydroxy-5-oxoproline to either roots or leaves accumulated greater biomass. A model was constructed that included the proposed roles of 2-hydroxy-5-oxoproline as a signal molecule of ammonium assimilation status in leaves, as a stimulator of nitrate uptake by roots and nitrate downloading from the xylem. In summary, a glutamine metabolite made in the ω-amidase pathway stimulated nitrate uptake by roots and was likely to be a signal of ammonium assimilation status in leaves. A chemical synthesis method for 2-hydroxy-5-oxoproline was also developed.

Genome-wide identification and analyses of cotton high-affinity nitrate transporter 2 family genes and their responses to stress

Nitrate transporters (NRTs) are crucial for the uptake, use, and storage of nitrogen by plants. In this study, 42 members of the GhNRT2 (Nitrate Transporter 2 family) were found in the four different cotton species. The conserved domains, phylogenetic relationships, physicochemical properties, subcellular localization, conserved motifs, gene structure, cis-acting elements, and promoter region expression patterns of these 42 members were analyzed. The findings confirmed that members of the NRT2 family behaved typically, and subcellular localization tests confirmed that they were hydrophobic proteins that were mostly located on the cytoplasmic membrane. The NRT2 family of genes with A.thaliana and rice underwent phylogenetic analysis, and the results revealed that GhNRT2 could be divided into three groups. The same taxa also shared similar gene structure and motif distribution. The composition of cis-acting elements suggests that most of the expression of GhNRT2 may be related to plant hormones, abiotic stress, and photoreactions. The GhNRT2 gene was highly expressed, mainly in roots. Drought, salt, and extreme temperature stress showed that GhNRT2 gene expression was significantly up-regulated or down-regulated, indicating that it may be involved in the stress response of cotton. In general, the genes of the NRT2 family of cotton were comprehensively analyzed, and their potential nitrogen uptake and utilization functions in cotton were preliminarily predicted. Additionally, we provide an experimental basis for the adverse stress conditions in which they may function.

The cassava (Manihot-esculenta Crantz)'s nitrate transporter NPF4.5, expressed in seedling roots, involved in nitrate flux and osmotic stress

AtNPF4.5/AIT2, which was predicted to be a low-affinity transporter capable for nitrate uptake, was screened by ABA receptor complex in Arabidopsis ten years ago. However, the molecular and biochemical characterizations of AtNPF4.5 in plants remained largely unclear. In this study, the function of a plasma-membrane-localized and root-specifically-expressed gene MeNPF4.5 (Manihot-esculenta NITRATE TRANSPORTER 1 PTR FAMILY4.5), an ortholog of the Arabidopsis thaliana NPF4.5, was investigated in cassava roots as a nitrate efflux transporter on low nitrate medium and an influx transporter following exposure to high concentration of external nitrates. Moreover, RNA interference (RNAi) of MeNPF4.5 reduced the nitrate efflux capacity but the overexpressing cassava seedlings increased the ability of efflux from the elongation to the mature zone of root under low nitrate treatments. Besides, MeNPF4.5-RNAi expression reduced the nitrate influx capacity but enhanced nitrate absorption in parts of overexpressing plants from the meristem, elongation to mature zone of roots under high nitrate conditions. Furthermore, MeNPF4.5-RNAi seedlings survived owing to roots that could grow normally, but the MeNPF4.5-over-expressors showed adverse growth under 7% PEG6000 stress, suggesting that MeNPF4.5 negatively regulated the osmotic stress and was involved in nitrate flux through cassava seedlings.

The Arabidopsis NLP7-HB52/54-VAR2 pathway modulates energy utilization in diverse light and nitrogen conditions

In plants, nitrate is the dominant nitrogen (N) source and a critical nutrient signal regulating various physiological and developmental processes.^1,2,3,4 Nitrate-responsive gene regulatory networks are widely believed to control growth, development, and life cycle in addition to N acquisition and utilization,^1,2,3,4 and NIN-LIKE PROTEIN (NLP) transcriptional activators have been identified as the master regulators governing the networks.^5,6,7 However, it remains to be elucidated how nitrate signaling regulates respective physiological and developmental processes. Here, we have identified a new nitrate-activated transcriptional cascade involved in chloroplast development and the maintenance of chloroplast function in Arabidopsis. This cascade consisting of NLP7 and two homeodomain-leucine zipper (HD-Zip) class I transcription factors, HOMEOBOX PROTEIN52 (HB52) and HB54,^8,9 was responsible for nitrate- and light-dependent expression of VAR2 encoding the FtsH2 subunit of the chloroplast FtsH protease involved in the quality control of photodamaged thylakoid membrane proteins.^10,11 Consistently, the nitrate-activated NLP7-HB52/54-VAR2 pathway underpinned photosynthetic light energy utilization, especially in high light environments. Furthermore, genetically enhancing the NLP7-HB52/54-VAR2 pathway resulted in improved light energy utilization under high light and low N conditions, a superior agronomic trait. These findings shed light on a new role of nitrate signaling and a novel mechanism for integrating information on N nutrient and light environments, providing a hint for enhancing the light energy utilization of plants in low N environments.

New Insights into MdSPS4-Mediated Sucrose Accumulation under Different Nitrogen Levels Revealed by Physiological and Transcriptomic Analysis

Nitrogen nutrition participates in many physiological processes and understanding the physiological and molecular mechanisms of apple responses to nitrogen is very significant for improving apple quality. This study excavated crucial genes that regulates sugar metabolism in response to nitrogen in apples through physiology and transcriptome analysis, so as to lay a theoretical foundation for improving fruit quality. In this paper, the content of sugar and organic acid in apple fruit at different developmental periods under different nitrogen levels (0, 150, 300, and 600 kg·hm^-2) were determined. Then, the transcriptomic analysis was performed in 120 days after bloom (DAB) and 150 DAB. The results showed that the fructose and glucose content were the highest at 120 DAB under 600 kg·hm^-2 nitrogen level. Meanwhile, different nitrogen treatments decreased malate content in 30 and 60 DAB. RNA-seq analysis revealed a total of 4537 UniGenes were identified as differentially expressed genes (DEGs) under nitrogen treatments. Among these DEGs, 2362 (52.06%) were up-regulated and 2175 (47.94%) were down-regulated. The gene co-expression clusters revealed that most DEGs were significantly annotated in the photosynthesis, glycolysis/gluconeogenesis, pyruvate metabolism, carbon metabolism, carbon fixation in photosynthetic organisms and plant hormone signal transduction pathways. The key transcription factor genes (ERF, NAC, WRKY, and C2H2 genes) were differentially expressed in apple fruit. Sugar and acid metabolism-related genes (e.g., HXK1, SPS4, SS2, PPC16-2, and MDH2 genes) exhibited significantly up-regulated expression at 120 DAB, whereas they were down-regulated at 150 DAB. Furthermore, the MdSPS4 gene overexpression positively promoted sucrose accumulation in apple callus and fruit. In conclusion, the combinational analysis of transcriptome and the functional validation of the MdSPS4 gene provides new insights into apple responses to different nitrogen levels.

Traceability of nitrate polluted hotspots in plain river networks of the Yangtze River delta by nitrogen and oxygen isotopes coupling bayesian model

The adverse effects of increased nitrate (NO₃^-) pollution especially from the non-point source on the hydrosphere and anthroposphere are becoming more prominent. The non-point-derived NO₃^- in the rivers supplying the upstream threatens the aquatic ecosystem of Taihu Lake. Here, dual-stable isotopes (δ¹⁵N and δ¹⁸O) of NO₃^- were applied to the Bayesian model (SIAR) for quantitative source identification of reactive nitrogen (Nr) in a mixed agricultural and urban region along the complex river network of the Yangze River delta. The results showed that the NO₃^- concentrations in the rivers ranged from 1.09 to 4.44 mg L^-1 and decreased from the highly urbanized areas to the lakeside rural areas. The specific isotopic characteristics of four sources (atmospheric deposition, AD; chemical fertilizer, CF; manure and sewage, MS; and soil leachate, SL) by the SIAR isotope model indicated that the MS source made the greatest contribution (46.56%) to the total NO₃^- load, followed by SL (27.86%), CF (23.77%), and AD (1.81%). The highly urbanized areas and the hybrid areas, which contained a mix of industrialized, populated, and agricultural areas, were identified as hotspot areas with heavy Nr pollution, responsible for spatial patterns of δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-. These hotspot areas were characterized by a less well-developed sewage pipeline system with high Nr emissions from cash crops. The changes in wastewater treatment level, the agricultural production structure, and meteorological changes were the main factors of spatial variation of Nr concentration and source in the upstream Taihu Lake Basin. The variation in Nr concentration across Taihu Lake would respond to these anthropogenic-driven Nr loads. These findings suggest that MS was the predominant source had the strongest effect on the overall riverine NO₃^- source which was the primary problem that needed to be solved.

Transcriptional repressor RST1 controls salt tolerance and grain yield in rice by regulating gene expression of asparagine synthetase

Salt stress impairs nutrient metabolism in plant cells, leading to growth and yield penalties. However, the mechanism by which plants alter their nutrient metabolism processes in response to salt stress remains elusive. In this study, we identified and characterized the rice (Oryza sativa) rice salt tolerant 1 (rst1) mutant, which displayed improved salt tolerance and grain yield. Map-based cloning revealed that the gene RST1 encoded an auxin response factor (OsARF18). Molecular analyses showed that RST1 directly repressed the expression of the gene encoding asparagine synthetase 1 (OsAS1). Loss of RST1 function increased the expression of OsAS1 and improved nitrogen (N) utilization by promoting asparagine production and avoiding excess ammonium (NH₄⁺) accumulation. RST1 was undergoing directional selection during domestication. The superior haplotype RST1^{Hap III} decreased its transcriptional repression activity and contributed to salt tolerance and grain weight. Together, our findings unravel a synergistic regulator of growth and salt tolerance associated with N metabolism and provide a new strategy for the development of tolerant cultivars.

Does Potassium (K+) Contribute to High-Nitrate (NO3-) Weakening of a Plant's Defense System against Necrotrophic Fungi?

In this opinion article, we have analyzed the relevancy of a hypothesis which is based on the idea that in Arabidopsis thaliana jasmonic acid, a (JA)-mediated defense system against necrotrophic fungi is weakened when NO₃^- supply is high. Such a hypothesis is based on the fact that when NO₃^- supply is high, it induces an increase in the amount of bioactive ABA which induces the sequestration of the phosphatase ABI2 (PP2C) into the PYR/PYL/RCAR receptor. Consequently, the Ca sensors CBL1/9-CIPK23 are not dephosphorylated by ABI2, thus remaining able to phosphorylate targets such as AtNPF6.3 and AtKAT1, which are NO₃^- and K⁺ transporters, respectively. Therefore, the impact of phosphorylation on the regulation of these two transporters, could (1) reduce NO₃^- influx as in its phosphorylated state AtNPF6.3 shifts to low capacity state and (2) increase K⁺ influx, as in its phosphorylated state KAT1 becomes more active. It is also well known that in roots, K⁺ loading in the xylem and its transport to the shoot is activated in the presence of NO₃^-. As such, the enrichment of plant tissues in K⁺ can impair a jasmonic acid (JA) regulatory pathway and the induction of the corresponding biomarkers. The latter are known to be up-regulated under K⁺ deficiency and inhibited when K⁺ is resupplied. We therefore suggest that increased K⁺ uptake and tissue content induced by high NO₃^- supply modifies the JA regulatory pathway, resulting in a weakened JA-mediated plant's defense system against necrotrophic fungi.

Nitrate availability controls translocation of the transcription factor NAC075 for cell-type-specific reprogramming of root growth

Plant root architecture flexibly adapts to changing nitrate (NO₃^-) availability in the soil; however, the underlying molecular mechanism of this adaptive development remains under-studied. To explore the regulation of NO₃^--mediated root growth, we screened for low-nitrate-resistant mutant (lonr) and identified mutants that were defective in the NAC transcription factor NAC075 (lonr1) as being less sensitive to low NO₃^- in terms of primary root growth. We show that NAC075 is a mobile transcription factor relocating from the root stele tissues to the endodermis based on NO₃^- availability. Under low-NO₃^- availability, the kinase CBL-interacting protein kinase 1 (CIPK1) is activated, and it phosphorylates NAC075, restricting its movement from the stele, which leads to the transcriptional regulation of downstream target WRKY53, consequently leading to adapted root architecture. Our work thus identifies an adaptive mechanism involving translocation of transcription factor based on nutrient availability and leading to cell-specific reprogramming of plant root growth.

Genome-wide association study of agronomic traits related to nitrogen use efficiency in wheat

GWAS identified 347 QTLs associated with eight traits related to nitrogen use efficiency in a 389-count wheat panel. Four novel candidate transcription factor genes were verified using qRT-PCR. Nitrogen is an essential nutrient for plants that determines crop yield. Improving nitrogen use efficiency (NUE) should considerably increase wheat yield and reduce the use of nitrogen fertilisers. However, knowledge on the genetic basis of NUE during wheat maturity is limited. In this study, a diversity panel incorporating 389 wheat accessions was phenotyped for eight NUE-related agronomic traits across five different environments. A total of 347 quantitative trait loci (QTLs) for low nitrogen tolerance indices (ratio of agronomic characters under low and high nitrogen conditions) were identified through a genome-wide association study utilising 397,384 single nucleotide polymorphisms (SNPs) within the MLM (Q + K) model, including 11 stable QTLs. Furthermore, 69 candidate genes were predicted for low nitrogen tolerance indices of best linear unbiased predictions values of the eight studied agronomic traits, and four novel candidate transcription factors (TraesCS5A02G237500 for qFsnR5A.2, TraesCS5B02G384500 and TraesCS5B02G384600 for qSLR5B.1, and TraesCS3B02G068800 for qTKWR3B.1) showed differing expression patterns in contrasting low-nitrogen-tolerant wheat genotypes. Moreover, the number of favourable marker alleles calculated using NUE that were significantly related to SNP in accessions decreased over the decades, indicating a decline in the NUE of the 389 wheat varieties. These findings denote promising NUE markers that could be useful in breeding high-NUE wheat varieties, and the candidate genes could further detail the NUE-related regulation network in wheat.

Genome-wide association studies reveal genetic basis of ionomic variation in cassava

As one of the most important food crops, cassava (Manihot esculenta) is the main dietary source of micronutrients for about 1 billion people. However, the ionomic variation in cassava and the underlying genetic mechanisms remain unclear so far. Herein, genome-wide association studies were performed to reveal the specific single nucleotide polymorphisms (SNPs) that affect the ionomic variation in cassava. We identified 164 SNPs with P-values lower than the threshold located in 88 loci associated with divergent ionomic variations. Among them, 13 SNPs are related to both calcium (Ca) and magnesium (Mg), and many loci for different ionomic traits seem to be clustered on specific chromosome regions. Moreover, we identified the peak SNPs in the promoter regions of Sc10g003170 (encoding methionyl-tRNA synthetase [MetRS]) and Sc18g015190 (encoding the transcriptional regulatory protein AlgP) for nitrogen (N) and phosphorus (P) accumulation, respectively. Notably, these two SNPs (chr10_32807962 and chr18_31343738) were directly correlated with the transcript levels of Sc10g003170 (MetRS) and Sc18g015190 (AlgP), which positively modulated N accumulation and P concentration in cassava, respectively. Taken together, this study provides important insight into the genetic basis of cassava natural ionomic variation, which will promote genetic breeding to improve nutrient use and accumulation of elements in cassava.

Effect of l-amino acid-based biostimulants on nitrogen use efficiency (NUE) in lettuce plants

BACKGROUND

Biostimulants are increasingly integrated into production systems with the goal of modifying physiological processes in plants to optimize productivity. Specifically, l-α-amino acid-based biostimulants enhance plant productivity through improved photosynthesis and increased assimilation of essential nutrients such as nitrogen (N). This element is a major component of fertilizers, which usually are applied in excess. Thus, the inefficient use of N fertilizers has generated a serious environmental pollution issue. The use of biostimulants has the potential to address problems related to N fertilization. Therefore, the objective of this study is to analyze whether two biostimulants based on l-α-amino acid (Terra Sorb® radicular and Terramin® Pro) designed by Bioiberica, S.A.U company can compensate deficient N fertilization and test its effect on lettuce plants. Growth, photosynthetic, N accumulation, and N use efficiency (NUE) parameters were analyzed on lettuce leaves.

RESULTS

Results showed that regardless of N fertilization, the use of both biostimulants, especially Terramin® Pro, increased biomass production. Moreover, both biostimulants enhanced photosynthetic, NO3 - and total N accumulations as well as NUE parameters.

CONCLUSION

Therefore, Terra Sorb® radicular and Terramin® Pro constitute a useful tool for crops development in N-limiting areas, and in intensive agricultural areas without N deficiency allowing the reduction of N inputs without impairing crop yields and reducing environmental impact. © 2022 The Authors. Journal of The Science of Food and Agriculture published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Nitrogen-inducible GLK1 modulates phosphate starvation response via the PHR1-dependent pathway

Phosphorus (P) is a limiting nutrient for plant growth and productivity. Thus, a deep understanding of the molecular mechanisms of plants' response to phosphate starvation is significant when breeding crops with higher phosphorus-use efficiency. Here, we found that GARP-type transcription factor GLK1 acted as a positive regulator for phosphate-starvation response (PSR) via the PHR1-dependent pathway in Arabidopsis thaliana. GLK1 increased the transcription activity of PHR1 through the direct physical interaction and regulated the multiple responses to inorganic orthophosphate (Pi) starvation. Nitrogen (N) is a key factor in the regulation of PSR. We also found that the N status controlled the function of the GLK1-PHR1 signaling module under Pi-deficient (LP) conditions by regulating the accumulation of GLK1 and PHR1. Ultimately, we showed that the presence of GLK1 effectively promoted the protein accumulation of PHR1 at low N concentrations, and this action was helpful to maintain the activation of PSR. According to these findings, we establish the working model for GLK1 in PSR and propose that GLK1 mediates the interaction between N and P by influencing the effect of N on PHR1 in Arabidopsis thaliana.

Physiological Responses of Cigar Tobacco Crop to Nitrogen Deficiency and Genome-Wide Characterization of the NtNPF Family Genes

Tobacco prefers nitrate as a nitrogen (N) source. However, little is known about the molecular components responsible for nitrate uptake and the physiological responses of cigar tobacco to N deficiency. In this study, a total of 117 nitrate transporter 1 (NRT1) and peptide transporter (PTR) family (NPF) genes were comprehensively identified and systematically characterized in the whole tobacco genome. The NtNPF members showed significant genetic diversity within and across subfamilies but showed conservation between subfamilies. The NtNPF genes are dispersed unevenly across the chromosomes. The phylogenetic analysis revealed that eight subfamilies of NtNPF genes are tightly grouped with their orthologues in Arabidopsis. The promoter regions of the NtNPF genes had extensive cis-regulatory elements. Twelve core NtNPF genes, which were strongly induced by N limitation, were identified based on the RNA-seq data. Furthermore, N deprivation severely impaired plant growth of two cigar tobaccos, and CX26 may be more sensitive to N deficiency than CX14. Moreover, 12 hub genes respond differently to N deficiency between the two cultivars, indicating the vital roles in regulating N uptake and transport in cigar tobacco. The findings here contribute towards a better knowledge of the NtNPF genes and lay the foundation for further functional analysis of cigar tobacco.

Comprehensive screening of low nitrogen tolerant maize based on multiple traits at the seedling stage

Background

Plants tolerant to low nitrogen are a quantitative trait affected by many factors, and the different parameters were used for stress-tolerant plant screening in different investigations. But there is no agreement on the use of these indicators. Therefore, a method that can integrate different parameters to evaluate stress tolerance is urgently needed.

Methods

Six maize genotypes were subject to low nitrogen stress for twenty days. Then seventeen traits of the six maize genotypes related to nitrogen were investigated. Nitrogen tolerance coefficient (NTC) was calculated as low nitrogen traits to high nitrogen traits. Then principal component analysis was conducted based on the NTC. Based on fuzzy mathematics theory, a D value (decimal comprehensive evaluation value) was introduced to evaluate maize tolerant to low nitrogen.

Results

Three maize (SY998, GEMS42-I and GEMS42-II) with the higher D value have better growth and higher nitrogen accumulation under low nitrogen conditions. In contrast, Ji846 with the lowest D value has the lowest nitrogen accumulation and biomass in response to nitrogen limitation. These results indicated that the D value could help to screen low nitrogen tolerant maize, given that the D value was positively correlated with low nitrogen tolerance in maize seedlings.

Conclusions

The present study introduced the D value to evaluate stress tolerance. The higher the D value, the greater tolerance of maize to low nitrogen stress. This method may reduce the complexity of the investigated traits and enhance the accuracy of stress-tolerant evaluation. In addition, this method not only can screen potentially tolerant germplasm for low-nitrogen tolerance quickly, but also can comprise the correlated traits as many as possible to avoid the one-sidedness of a single parameter.

Histone methyltransferases SDG33 and SDG34 regulate organ-specific nitrogen responses in tomato

Histone posttranslational modifications shape the chromatin landscape of the plant genome and affect gene expression in response to developmental and environmental cues. To date, the role of histone modifications in regulating plant responses to environmental nutrient availability, especially in agriculturally important species, remains largely unknown. We describe the functions of two histone lysine methyltransferases, SET Domain Group 33 (SDG33) and SDG34, in mediating nitrogen (N) responses of shoots and roots in tomato. By comparing the transcriptomes of CRISPR edited tomato lines sdg33 and sdg34 with wild-type plants under N-supplied and N-starved conditions, we uncovered that SDG33 and SDG34 regulate overlapping yet distinct downstream gene targets. In response to N level changes, both SDG33 and SDG34 mediate gene regulation in an organ-specific manner: in roots, SDG33 and SDG34 regulate a gene network including Nitrate Transporter 1.1 (NRT1.1) and Small Auxin Up-regulated RNA (SAUR) genes. In agreement with this, mutations in sdg33 or sdg34 abolish the root growth response triggered by an N-supply; In shoots, SDG33 and SDG34 affect the expression of photosynthesis genes and photosynthetic parameters in response to N. Our analysis thus revealed that SDG33 and SDG34 regulate N-responsive gene expression and physiological changes in an organ-specific manner, thus presenting previously unknown candidate genes as targets for selection and engineering to improve N uptake and usage in crop plants.

Inhibition of shoot-expressed NRT1.1 improves reutilization of apoplastic iron under iron-deficient conditions

Iron deficiency is a major constraint for plant growth in calcareous soils. The interplay between NO₃ ^- and Fe nutrition affects plant performance under Fe-deficient conditions. However, how NO₃ ^- negatively regulates Fe nutrition at the molecular level in plants remains elusive. Here, we showed that the key nitrate transporter NRT1.1 in Arabidopsis plants, especially in the shoots, was markedly downregulated at post-translational levels by Fe deficiency. However, loss of NRT1.1 function alleviated Fe deficiency chlorosis, suggesting that downregulation of NRT1.1 by Fe deficiency favors plant tolerance to Fe deficiency. Further analysis showed that although disruption of NRT1.1 did not alter Fe levels in both the shoots and roots, it improved the reutilization of apoplastic Fe in shoots but not in roots. In addition, disruption of NRT1.1 prevented Fe deficiency-induced apoplastic alkalization in shoots by inhibiting apoplastic H⁺ depletion via NO₃ ^- uptake. In vitro analysis showed that reduced pH facilitates release of cell wall-bound Fe. Thus, foliar spray with an acidic buffer promoted the reutilization of Fe in the leaf apoplast to enhance plant tolerance to Fe deficiency, while the opposite was true for the foliar spray with a neutral buffer. Thus, downregulation of the shoot-part function of NRT1.1 prevents apoplastic alkalization to ensure the reutilization of apoplastic Fe under Fe-deficient conditions. Our findings may provide a basis for elucidating the link between N and Fe nutrition in plants and insight to scrutinize the relevance of shoot-expressed NRT1.1 to the plant response to stress.

Nitrate–Nitrite–Nitric Oxide Pathway: A Mechanism of Hypoxia and Anoxia Tolerance in Plants

Oxygen (O₂) is the most crucial substrate for numerous biochemical processes in plants. Its deprivation is a critical factor that affects plant growth and may lead to death if it lasts for a long time. However, various biotic and abiotic factors cause O₂ deprivation, leading to hypoxia and anoxia in plant tissues. To survive under hypoxia and/or anoxia, plants deploy various mechanisms such as fermentation paths, reactive oxygen species (ROS), reactive nitrogen species (RNS), antioxidant enzymes, aerenchyma, and adventitious root formation, while nitrate (NO₃⁻), nitrite (NO₂⁻), and nitric oxide (NO) have shown numerous beneficial roles through modulating these mechanisms. Therefore, in this review, we highlight the role of reductive pathways of NO formation which lessen the deleterious effects of oxidative damages and increase the adaptation capacity of plants during hypoxia and anoxia. Meanwhile, the overproduction of NO through reductive pathways during hypoxia and anoxia leads to cellular dysfunction and cell death. Thus, its scavenging or inhibition is equally important for plant survival. As plants are also reported to produce a potent greenhouse gas nitrous oxide (N₂O) when supplied with NO₃⁻ and NO₂⁻, resembling bacterial denitrification, its role during hypoxia and anoxia tolerance is discussed here. We point out that NO reduction to N₂O along with the phytoglobin-NO cycle could be the most important NO-scavenging mechanism that would reduce nitro-oxidative stress, thus enhancing plants’ survival during O₂-limited conditions. Hence, understanding the molecular mechanisms involved in reducing NO toxicity would not only provide insight into its role in plant physiology, but also address the uncertainties seen in the global N₂O budget.

Interaction of BES1 and LBD37 transcription factors modulates brassinosteroid-regulated root forging response under low nitrogen in arabidopsis

Brassinosteriod (BR) plays important roles in regulation of plant growth, development and environmental responses. BR signaling regulates multiple biological processes through controlling the activity of BES1/BZR1 regulators. Apart from the roles in the promotion of plant growth, BR is also involved in regulation of the root foraging response under low nitrogen, however how BR signaling regulate this process remains unclear. Here we show that BES1 and LBD37 antagonistically regulate root foraging response under low nitrogen conditions. Both the transcriptional level and dephosphorylated level of BES1, is significant induced by low nitrogen, predominantly in root. Phenotypic analysis showed that BES1 gain-of-function mutant or BES1 overexpression transgenic plants exhibits progressive outgrowth of lateral root in response to low nitrogen and BES1 negatively regulates repressors of nitrate signaling pathway and positively regulates several key genes required for NO₃ ^- uptake and signaling. In contrast, BES1 knock-down mutant BES1-RNAi exhibited a dramatical reduction of lateral root elongation in response to low N. Furthermore, we identified a BES1 interacting protein, LBD37, which is a negative repressor of N availability signals. Our results showed that BES1 can inhibit LBD37 transcriptional repression on N-responsive genes. Our results thus demonstrated that BES1-LBD37 module acts critical nodes to integrate BR signaling and nitrogen signaling to modulate the root forging response at LN condition.

Enhancement of nitrogen use efficiency through agronomic and molecular based approaches in cotton

Cotton is a major fiber crop grown worldwide. Nitrogen (N) is an essential nutrient for cotton production and supports efficient crop production. It is a crucial nutrient that is required more than any other. Nitrogen management is a daunting task for plants; thus, various strategies, individually and collectively, have been adopted to improve its efficacy. The negative environmental impacts of excessive N application on cotton production have become harmful to consumers and growers. The 4R's of nutrient stewardship (right product, right rate, right time, and right place) is a newly developed agronomic practice that provides a solid foundation for achieving nitrogen use efficiency (NUE) in cotton production. Cropping systems are equally crucial for increasing production, profitability, environmental growth protection, and sustainability. This concept incorporates the right fertilizer source at the right rate, time, and place. In addition to agronomic practices, molecular approaches are equally important for improving cotton NUE. This could be achieved by increasing the efficacy of metabolic pathways at the cellular, organ, and structural levels and NUE-regulating enzymes and genes. This is a potential method to improve the role of N transporters in plants, resulting in better utilization and remobilization of N in cotton plants. Therefore, we suggest effective methods for accelerating NUE in cotton. This review aims to provide a detailed overview of agronomic and molecular approaches for improving NUE in cotton production, which benefits both the environment and growers.

NIN-like protein 7 transcription factor is a plant nitrate sensor

Nitrate is an essential nutrient and signaling molecule for plant growth. Plants sense intracellular nitrate to adjust their metabolic and growth responses. Here we identify the primary nitrate sensor in plants. We found that mutation of all seven Arabidopsis NIN-like protein (NLP) transcription factors abolished plants' primary nitrate responses and developmental programs. Analyses of NIN-NLP7 chimeras and nitrate binding revealed that NLP7 is derepressed upon nitrate perception via its amino terminus. A genetically encoded fluorescent split biosensor, mCitrine-NLP7, enabled visualization of single-cell nitrate dynamics in planta. The nitrate sensor domain of NLP7 resembles the bacterial nitrate sensor NreA. Substitutions of conserved residues in the ligand-binding pocket impaired the ability of nitrate-triggered NLP7 to control transcription, transport, metabolism, development, and biomass. We propose that NLP7 represents a nitrate sensor in land plants.

Systematic Annotation Reveals CEP Function in Tomato Root Development and Abiotic Stress Response

Tomato (Solanum lycopersicum) is one of the most important vegetable crops worldwide; however, environmental stressors severely restrict tomato growth and yield. Therefore, it is of great interest to discover novel regulators to improve tomato growth and environmental stress adaptions. Here, we applied a comprehensive bioinformatics approach to identify putative tomato C-TERMINALLY ENCODED PEPTIDE (CEP) genes and to explore their potential physiological function in tomato root development and abiotic stress responses. A total of 17 tomato CEP genes were identified and grouped into two subgroups based on the similarity of CEP motifs. The public RNA-Seq data revealed that tomato CEP genes displayed a diverse expression pattern in tomato tissues. Additionally, CEP genes expression was differentially regulated by nitrate or ammonium status in roots and shoots, respectively. The differences in expression levels of CEP genes induced by nitrogen indicate a potential involvement of CEPs in tomato nitrogen acquisition. The synthetic CEP peptides promoted tomato primary root growth, which requires nitric oxide (NO) and calcium signaling. Furthermore, we also revealed that CEP peptides improved tomato root resistance to salinity. Overall, our work will contribute to provide novel genetic breeding strategies for tomato cultivation under adverse environments.

A Koshihikari X Oryza rufipogon Introgression Line with a High Capacity to Take up Nitrogen to Maintain Growth and Panicle Development under Low Nitrogen Conditions

Nitrogen (N) is an important macronutrient for plant growth and development. Currently, N fertilizers are required for the efficient production of modern crops such as rice due to their limited capacity to take up N when present at low concentrations. Wild rice represents a useful genetic resource for improving crop responses to low nutrient stress. Here, we describe the isolation and characterization of an introgression line, KRIL37, that carries a small region of the Oryza rufipogon genome in the Oryza sativa L. cv Koshihikari (KH) background. This line was found to grow better under low N conditions and have similar or lower C/N ratios in aerial portions compared to those in the parental KH cultivar, suggesting that KRIL37 has a higher capacity to take up and assimilate N when present at low concentrations. KRIL37 performance in the field was also better than that of KH cultivated without N and fertilizer (-F). Transcriptome analyses of 3-week-old seedlings based on RNA-sequencing revealed that KH induced a wider suite of genes than the tolerant line KRIL37 in response to low N conditions. Some ammonium transporters and N assimilation genes were found to be induced under low N in KRIL37, but not in KH. Our findings suggest that the superior growth performance of KRIL37 under limited N conditions could be due to the expression of wild alleles influencing N uptake and assimilation. Our study demonstrates the potential to use wild rice genomes to improve modern crops for low nutrient tolerance.

OsAMT1;1 and OsAMT1;2 Coordinate Root Morphological and Physiological Responses to Ammonium for Efficient Nitrogen Foraging in Rice

Optimal plant growth and development rely on morphological and physiological adaptions of the root system to forage heterogeneously distributed nitrogen (N) in soils. Rice grows mainly in the paddy soil where ammonium (NH4+) is present as the major N source. Although root NH4+ foraging behaviors are expected to be agronomically relevant, the underlying mechanism remains largely unknown. Here, we showed that NH4+ supply transiently enhanced the high-affinity NH4+ uptake and stimulated lateral root (LR) branching and elongation. These synergistic physiological and morphological responses were closely related to NH4+-induced expression of NH4+ transporters OsAMT1;1 and OsAMT1;2 in roots. The two independent double mutants (dko) defective in OsAMT1;1 and OsAMT1;2 failed to induce NH4+ uptake and stimulate LR formation, suggesting that OsAMT1s conferred the substrate-dependent root NH4+ foraging. In dko plants, NH4+ was unable to activate the expression of OsPIN2, and the OsPIN2 mutant (lra1) exhibited a strong reduction in NH4+-triggered LR branching, suggesting that the auxin pathway was likely involved in OsAMT1s-dependent LR branching. Importantly, OsAMT1s-dependent root NH4+ foraging behaviors facilitated rice growth and N acquisition under fluctuating NH4+ supply. These results revealed an essential role of OsAMT1s in synergizing root morphological and physiological processes, allowing for efficient root NH4+ foraging to optimize N capture under fluctuating N availabilities.

Regulatory functions of cellular energy sensor SnRK1 for nitrate signalling through NLP7 repression

The coordinated metabolism of carbon and nitrogen is essential for optimal plant growth and development. Nitrate is an important molecular signal for plant adaptation to a changing environment, but how nitrate regulates plant growth under carbon deficiency conditions remains unclear. Here we show that the evolutionarily conserved energy sensor SnRK1 negatively regulates the nitrate signalling pathway. Nitrate promoted plant growth and downstream gene expression, but such effects were repressed when plants were grown under carbon deficiency conditions. Mutation of KIN10, the α-catalytic subunit of SnRK1, partially suppressed the inhibitory effects of carbon deficiency on nitrate-mediated plant growth. KIN10 phosphorylated NLP7, the master regulator of the nitrate signalling pathway, to promote its cytoplasmic localization and degradation. Furthermore, nitrate depletion induced KIN10 accumulation, whereas nitrate treatment promoted KIN10 degradation. Such KIN10-mediated NLP7 regulation allows carbon and nitrate availability to control optimal nitrate signalling and ensures the coordination of carbon and nitrogen metabolism in plants.

The transcription factor GNC optimizes nitrogen use efficiency and growth by up-regulating the expression of nitrate uptake and assimilation genes in poplar

Plants have evolved complex mechanisms to cope with the fluctuating environmental availability of nitrogen. However, potential genes modulating plant responses to nitrate are yet to be characterized. Here, a poplar GATA transcription factor gene PdGNC (GATA nitrate-inducible carbon-metabolism-involved) was found to be strongly induced by low nitrate. Overexpressing PdGNC in poplar clone 717-1B4 (P. tremula × alba) significantly improved nitrate uptake, remobilization, and assimilation with higher nitrogen use efficiency (NUE) and faster growth, particularly under low nitrate conditions. Conversely, CRISPR/Cas9-mediated poplar mutant gnc exhibited decreased nitrate uptake, relocation, and assimilation, combined with lower NUE and slower growth. Assays with yeast one-hybrid, electrophoretic mobility shift, and a dual-luciferase reporter showed that PdGNC directly activated the promoters of nitrogen pathway genes PdNRT2.4b, PdNR, PdNiR, and PdGS2, leading to a significant increase in nitrate utilization in poplar. As expected, the enhanced NUE promoted growth under low nitrate availability. Taken together, our data show that PdGNC plays an important role in the regulation of NUE and growth in poplar by improving nitrate acquisition, remobilization, and assimilation, and provide a promising strategy for molecular breeding to improve productivity under nitrogen limitation in trees.

Root nitrate uptake in sugarcane (Saccharum spp.) is modulated by transcriptional and presumably posttranscriptional regulation of the NRT2.1/NRT3.1 transport system

KEY MESSAGE

Nitrate uptake in sugarcane roots is regulated at the transcriptional and posttranscriptional levels based on the physiological status of the plant and is likely a determinant mechanism for discrimination against nitrate. Sugarcane (Saccharum spp.) is one of the most suitable energy crops for biofuel feedstock, but the reduced recovery of nitrogen (N) fertilizer by sugarcane roots increases the crop carbon footprint. The low nitrogen use efficiency (NUE) of sugarcane has been associated with the significantly low nitrate uptake, which limits the utilization of the large amount of nitrate available in agricultural soils. To understand the regulation of nitrate uptake in sugarcane roots, we identified the major canonical nitrate transporter genes (NRTs-NITRATE TRANSPORTERS) and then determined their expression profiles in roots under contrasting N conditions. Correlation of gene expression with ¹⁵N-nitrate uptake revealed that under N deprivation or inorganic N (ammonium or nitrate) supply in N-sufficient roots, the regulation of ScNRT2.1 and ScNRT3.1 expression is the predominant mechanism for the modulation of the activity of the nitrate high-affinity transport system. Conversely, in N-deficient roots, the induction of ScNRT2.1 and ScNRT3.1 transcription is not correlated with the marked repression of nitrate uptake in response to nitrate resupply or high N provision, which suggested the existence of a posttranscriptional regulatory mechanism. Our findings suggested that high-affinity nitrate uptake is regulated at the transcriptional and presumably at the posttranscriptional levels based on the physiological N status and that the regulation of NRT2.1 and NRT3.1 activity is likely a determinant mechanism for the discrimination against nitrate uptake observed in sugarcane roots, which contributes to the low NUE in this crop species.

The Original Form of C4-Photosynthetic Phosphoenolpyruvate Carboxylase Is Retained in Pooids but Lost in Rice

Poaceae is the most prominent monocot family that contains the primary cereal crops wheat, rice, and maize. These cereal species exhibit physiological diversity, such as different photosynthetic systems and environmental stress tolerance. Phosphoenolpyruvate carboxylase (PEPC) in Poaceae is encoded by a small multigene family and plays a central role in C4-photosynthesis and dicarboxylic acid metabolism. Here, to better understand the molecular basis of the cereal species diversity, we analyzed the PEPC gene family in wheat together with other grass species. We could designate seven plant-type and one bacterial-type grass PEPC groups, ppc1a, ppc1b, ppc2a, ppc2b, ppc3, ppc4, ppcC4, and ppc-b, respectively, among which ppc1b is an uncharacterized type of PEPC. Evolutionary inference revealed that these PEPCs were derived from five types of ancient PEPCs (ppc1, ppc2, ppc3, ppc4, and ppc-b) in three chromosomal blocks of the ancestral Poaceae genome. C4-photosynthetic PEPC (ppcC4 ) had evolved from ppc1b, which seemed to be arisen by a chromosomal duplication event. We observed that ppc1b was lost in many Oryza species but preserved in Pooideae after natural selection. In silico analysis of cereal RNA-Seq data highlighted the preferential expression of ppc1b in upper ground organs, selective up-regulation of ppc1b under osmotic stress conditions, and nitrogen response of ppc1b. Characterization of wheat ppc1b showed high levels of gene expression in young leaves, transcriptional responses under nitrogen and abiotic stress, and the presence of a Dof1 binding site, similar to ppcC4 in maize. Our results indicate the evolving status of Poaceae PEPCs and suggest the functional association of ppc1-derivatives with adaptation to environmental changes.

The interplay of auxin and brassinosteroid signaling tunes root growth under low and different nitrogen forms

The coordinated signaling activity of auxin and brassinosteroids (BRs) is critical for optimal plant growth and development. Nutrient-derived signals regulate root growth by modulating the levels and spatial distribution of growth hormones to optimize nutrient uptake and assimilation. However, the effect of the interaction of these two hormones and their signaling on root plasticity during low and differential availability of nitrogen (N) forms (NH4+/NO3-) remains elusive. We demonstrate that root elongation under low N (LN) is an outcome of the interdependent activity of auxin and BR signaling pathways in Arabidopsis (Arabidopsis thaliana). LN promotes root elongation by increasing BR-induced auxin transport activity in the roots. Increased nuclear auxin signaling and its transport efficiency have a distinct impact on root elongation under LN conditions. High auxin levels reversibly inhibit BR signaling via BRI1 KINASE INHIBITOR1. Using the tissue-specific approach, we show that BR signaling from root vasculature (stele) tissues is sufficient to promote cell elongation and, hence, root growth under LN condition. Further, we show that N form-defined root growth attenuation or enhancement depends on the fine balance of BR and auxin signaling activity. NH4+ as a sole N source represses BR signaling and response, which in turn inhibits auxin response and transport, whereas NO3- promotes root elongation in a BR signaling-dependent manner. In this study, we demonstrate the interplay of auxin and BR-derived signals, which are critical for root growth in a heterogeneous N environment and appear essential for root N foraging response and adaptation.

Cassava (Manihot esculenta) Slow Anion Channel (MeSLAH4) Gene Overexpression Enhances Nitrogen Assimilation, Growth, and Yield in Rice

Nitrogen is one of the most important nutrient elements required for plant growth and development, which is also immensely related to the efficient use of nitrogen by crop plants. Therefore, plants evolved sophisticated mechanisms and anion channels to extract inorganic nitrogen (nitrate) from the soil or nutrient solutions, assimilate, and recycle the organic nitrogen. Hence, developing crop plants with a greater capability of using nitrogen efficiently is the fundamental research objective for attaining better agricultural productivity and environmental sustainability. In this context, an in-depth investigation has been conducted into the cassava slow type anion channels (SLAHs) gene family, including genome-wide expression analysis, phylogenetic relationships with other related organisms, chromosome localization, and functional analysis. A potential and nitrogen-responsive gene of cassava (MeSLAH4) was identified and selected for overexpression (OE) analysis in rice, which increased the grain yield and root growth related performance. The morpho-physiological response of OE lines was better under low nitrogen (0.01 mm NH₄NO₃) conditions compared to the wild type (WT) and OE lines under normal nitrogen (0.5 mm NH₄NO₃) conditions. The relative expression of the MeSLAH4 gene was higher (about 80-fold) in the OE line than in the wild type. The accumulation and flux assay showed higher accumulation of NO 3 - and more expansion of root cells and grain dimension of OE lines compared to the wild type plants. The results of this experiment demonstrated that the MeSLAH4 gene may play a vital role in enhancing the efficient use of nitrogen in rice, which could be utilized for high-yielding crop production.

An Integrated Regulatory Network of mRNAs, microRNAs, and lncRNAs Involved in Nitrogen Metabolism of Moso Bamboo

Nitrogen is a key macronutrient essential for plant growth and development, and its availability has a strong influence on biological processes. Nitrogen fertilizer has been widely applied in bamboo forests in recent decades; however, the mechanism of nitrogen metabolism in bamboo is not fully elucidated. Here, we characterized the morphological, physiological, and transcriptome changes of moso bamboo in response to different schemes for nitrogen addition to illuminate the regulation mechanism of nitrogen metabolism. The appropriate addition of nitrogen improved the chlorophyll content and Pn (net photosynthetic rate) of leaves, the nitrogen and ammonium contents of the seedling roots, the biomass of the whole seedling, the number of lateral roots, and the activity of enzymes involved in nitrogen metabolism in the roots. Based on the whole transcriptome data of the roots, a total of 8,632 differentially expressed mRNAs (DEGs) were identified under different nitrogen additions, such as 52 nitrate transporter genes, 6 nitrate reductase genes, 2 nitrite reductase genes, 2 glutamine synthase genes, 2 glutamate synthase genes (GOGAT), 3 glutamate dehydrogenase genes, and 431 TFs belonging to 23 families. Meanwhile, 123 differentially expressed miRNAs (DEMs) and 396 differentially expressed lncRNAs (DELs) were characterized as nitrogen responsive, respectively. Furthermore, 94 DEM-DEG pairs and 23 DEL-DEG pairs involved in nitrogen metabolism were identified. Finally, a predicted regulatory network of nitrogen metabolism was initially constructed, which included 17 nitrogen metabolic pathway genes, 15 TFs, 4 miRNAs, and 10 lncRNAs by conjoint analysis of DEGs, DEMs, and DELs and their regulatory relationships, which was supported by RNA-seq data and qPCR results. The lncRNA-miRNA-mRNA network provides new insights into the regulation mechanism of nitrogen metabolism in bamboo, which facilitates further genetic improvement for bamboo to adapt to the fluctuating nitrogen environment.

Root system size and root hair length are key phenes for nitrate acquisition and biomass production across natural variation in Arabidopsis

The role of root phenes in nitrogen (N) acquisition and biomass production was evaluated in 10 contrasting natural accessions of Arabidopsis thaliana L. Seedlings were grown on vertical agar plates with two different nitrate supplies. The low N treatment increased the root to shoot biomass ratio and promoted the proliferation of lateral roots and root hairs. The cost of a larger root system did not impact shoot biomass. Greater biomass production could be achieved through increased root length or through specific root hair characteristics. A greater number of root hairs may provide a low-resistance pathway under elevated N conditions, while root hair length may enhance root zone exploration under low N conditions. The variability of N uptake and the expression levels of genes encoding nitrate transporters were measured. A positive correlation was found between root system size and high-affinity nitrate uptake, emphasizing the benefits of an exploratory root organ in N acquisition. The expression levels of NRT1.2/NPF4.6, NRT2.2, and NRT1.5/NPF7.3 negatively correlated with some root morphological traits. Such basic knowledge in Arabidopsis demonstrates the importance of root phenes to improve N acquisition and paves the way to design eudicot ideotypes.

Nitrate-dependent regulation of miR444-OsMADS27 signalling cascade controls root development in rice

Nitrate is an important nutrient and a key signalling molecule for plant development. A number of transcription factors involved in the response to nitrate and their regulatory mechanisms have been identified. However, little is known about the transcription factors involved in nitrate sensing and their regulatory mechanisms among crop plants. In this study, we identified functions of a nitrate-responsive miR444:MADS-box transcription factor OsMADS27 module and its downstream targets mediating rice root growth and stress responses. Transgenic rice plants expressing miR444 target mimic improved rice root growth. Although miR444 has the potential to target multiple genes, we identified OsMADS27 as the major miR444 target that regulates the expression of nitrate transporters, as well as several key genes including expansins, and those associated with auxin signalling, to promote root growth. In agreement with this, overexpression of miRNA-resistant OsMADS27 improved root development and tolerance to abiotic stresses, while its silencing suppressed root growth. OsMADS27 mediated robust stress tolerance in plants through its ability to bind to the promoters of specific stress regulators, as observed in ChIP-seq analysis. Our results provide evidence of a nitrate-dependent miR444-OsMADS27 signalling cascade involved in the regulation of rice root growth, as well as its surprising role in stress responses.

Sulfur signaling and starvation response in Arabidopsis

As sessile organisms, plants have developed sophisticated mechanism to sense and utilize nutrients from the environment, and modulate their growth and development according to the nutrient availability. Research in the past two decades revealed that nutrient assimilation is not occurring spontaneously, but nutrient signaling networks are complexly regulated and integrate sensing and signaling, gene expression, and metabolism to ensure homeostasis and coordination with plant energy conversion and other processes. Here, we review the importance of the macronutrient sulfur (S) and compare the knowledge of S signaling with other important macronutrients, such as nitrogen (N) and phosphorus (P). We focus on key advances in understanding sulfur sensing and signaling, uptake and assimilation, and we provide new analysis of published literature, to identify core genes regulated by the key transcriptional factor in S starvation response, SLIM1/EIL3, and compare the impact on other nutrient deficiency and stresses on S-related genes.

Improving Crop Nitrogen Use Efficiency Toward Sustainable Green Revolution

The Green Revolution of the 1960s improved crop yields in part through the widespread cultivation of semidwarf plant varieties, which resist lodging but require a high-nitrogen (N) fertilizer input. Because environmentally degrading synthetic fertilizer use underlies current worldwide cereal yields, future agricultural sustainability demands enhanced N use efficiency (NUE). Here, we summarize the current understanding of how plants sense, uptake, and respond to N availability in the model plants that can be used to improve sustainable productivity in agriculture. Recent progress in unlocking the genetic basis of NUE within the broader context of plant systems biology has provided insights into the coordination of plant growth and nutrient assimilation and inspired the implementation of a new breeding strategy to cut fertilizer use in high-yield cereal crops. We conclude that identifying fresh targets for N sensing and response in crops would simultaneously enable improved grain productivity and NUE to launch a new Green Revolution and promote future food security.

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.

CsATG101 Delays Growth and Accelerates Senescence Response to Low Nitrogen Stress in Arabidopsis thaliana

For tea plants, nitrogen (N) is a foundational element and large quantities of N are required during periods of roundly vigorous growth. However, the fluctuation of N in the tea garden could not always meet the dynamic demand of the tea plants. Autophagy, an intracellular degradation process for materials recycling in eukaryotes, plays an important role in nutrient remobilization upon stressful conditions and leaf senescence. Studies have proven that numerous autophagy-related genes (ATGs) are involved in N utilization efficiency in Arabidopsis thaliana and other species. Here, we identified an ATG gene, CsATG101, and characterized the potential functions in response to N in A. thaliana. The expression patterns of CsATG101 in four categories of aging gradient leaves among 24 tea cultivars indicated that autophagy mainly occurred in mature leaves at a relatively high level. Further, the in planta heterologous expression of CsATG101 in A. thaliana was employed to investigate the response of CsATG101 to low N stress. The results illustrated a delayed transition from vegetative to reproductive growth under normal N conditions, while premature senescence under N deficient conditions in transgenic plants vs. the wild type. The expression profiles of 12 AtATGs confirmed the autophagy process, especially in mature leaves of transgenic plants. Also, the relatively high expression levels for AtAAP1, AtLHT1, AtGLN1;1, and AtNIA1 in mature leaves illustrated that the mature leaves act as the source leaves in transgenic plants. Altogether, the findings demonstrated that CsATG101 is a candidate gene for improving annual fresh tea leaves yield under both deficient and sufficient N conditions via the autophagy process.

Glutamate: A multifunctional amino acid in plants

Glutamate (Glu) is a versatile metabolite and a signaling molecule in plants. Glu biosynthesis is associated with the primary nitrogen assimilation pathway. The conversion between Glu and 2-oxoglutarate connects Glu metabolism to the tricarboxylic acid cycle, carbon metabolism, and energy production. Glu is the predominant amino donor for transamination reactions in the cell. In addition to protein synthesis, Glu is a building block for tetrapyrroles, glutathione, and folate. Glu is the precursor of γ-aminobutyric acid that plays an important role in balancing carbon/nitrogen metabolism and various cellular processes. Glu can conjugate to the major auxin indole 3-acetic acid (IAA), and IAA-Glu is destined for oxidative degradation. Glu also conjugates with isochorismate for the production of salicylic acid. Accumulating evidence indicates that Glu functions as a signaling molecule to regulate plant growth, development, and defense responses. The ligand-gated Glu receptor-like proteins (GLRs) mediate some of these responses. However, many of the Glu signaling events are GLR-independent. The receptor perceiving extracellular Glu as a danger signal is still unknown. In addition to GLRs, Glu may act on receptor-like kinases or receptor-like proteins to trigger immune responses. Glu metabolism and Glu signaling may entwine to regulate growth, development, and defense responses in plants.

Physio-molecular traits of contrasting bread wheat genotypes associated with 15N influx exhibiting homeolog expression bias in nitrate transporter genes under different external nitrate concentrations

The high affinity nitrate transport system is a potential target for improving nitrogen use efficiency of bread wheat growing either under optimal or limiting nitrate concentration. Nitrate uptake is one of the most important traits to take into account to improve nitrogen use efficiency in wheat (Triticum aestivum L.). In this study, we aimed to gain an insight into the regulation of NO₃^- -uptake and translocation systems in two contrasting wheat genotypes [K9107(K9) vs. Choti Lerma (CL)]. Different conditions, such as NO₃^--uptake rates, soil-types, N-free solid external media, and external NO₃^- levels at the seedling stage, were considered. We also studied the contribution of homeolog expression of five genes encoding two nitrate transporters in the root tissue, along with their overall transcript expression levels relative to specific external nitrate availability. We observed that K9107 had a higher ¹⁵N influx than Choti Lerma under both limiting as well as optimum external N conditions in vermiculite-perlite (i.e., N-free solid) medium, with the improved translocation efficiency in Choti Lerma. However, in different soil types, different levels of ¹⁵N-enrichment in both the genotypes were found. Our results also demonstrated that the partitioning of dry matter in root and shoot was different under these growing conditions. Moreover, K9107 showed significantly higher relative expression of TaNRT2.1 at the lowest and TaNPF6.1 and TaNPF6.2 at the highest external nitrate concentrations. We also observed genotype-specific and nitrate starvation-dependent homeolog expression bias in all five nitrate transporter genes. Our data suggest that K9107 had a higher NO₃^- influx capacity, involving different nitrate transporters, than Choti Lerma at the seedling stage.

Identification and characterization of apple MdNLP7 transcription factor in the nitrate response

Nitrogen is an essential nutrient for plant growth and development. Low utilization of nitrogen fertilizer during agricultural production causes a series of environmental problems, such as water eutrophication, soil acidity, and air pollution. Investigating the patterns and mechanisms of crop NO₃^- absorption and utilization therefore key to fully improving crop nitrogen utilization rates and promoting sustainable agricultural development. Apple is one of the most important horticultural crops in the world. Its nitrogen demand by apple during the growth period is very high, but few studies have been performed on apple genes, that regulate the NO₃^- response. Here, we found that the apple transcription factor MdNLP7 promoted nitrogen absorption and assimilation by activating the expression of MdNIA2 and MdNRT1.1. MdNLP7 also regulated H₂O₂ content by increasing catalase activity, which may also influence nitrate utilization. Our findings provide insight into the mechanisms by which MdNLP7 controls nitrate utilization in apple.

Small signaling peptides mediate plant adaptions to abiotic environmental stress

Peptide-receptor complexes activate distinct downstream regulatory networks to mediate plant adaptions to abiotic environmental stress. Plants are constantly exposed to various adverse environmental factors; thus they must adjust their growth accordingly. Plants recruit small secretory peptides to adapt to these detrimental environments. These small peptides, which are perceived by their corresponding receptors and/or co-receptors, act as local- or long-distance mobile signaling molecules to establish cell-to-cell regulatory networks, resulting in optimal cellular and physiological outputs. In this review, we highlight recent advances on the regulatory role of small peptides in plant abiotic responses and nutrients signaling.

MdMYB10 affects nitrogen uptake and reallocation by regulating the nitrate transporter MdNRT2.4-1 in the red flesh apple

Nitrate is the major nitrogen sources for higher plants. In addition to serving not only as a nutrient, it is also a signaling molecule that regulates plant growth and development. Although membrane-bound nitrate transporter/peptide transporters (NRT/PTR) have been extensively studied and shown to regulate nitrate uptake and movement, little is known about how these factors are regulated by the external nitrogen environment. Red flesh apple, the coloration of which is determined by the transcription factor MdMYB10, had higher nitrate uptake efficiency than non-red flesh apple. Nitrate assimilation and utilization were increased in red flesh apple cultivar, and comparative transcriptome analysis showed that the expression of genes encoding the NRT2s was increased in red flesh apple. In vitro and in vivo experiments showed that MdMYB10 directly bound to the MdNRT2.4-1 promoter to transcriptionally activate its expression, resulting in enhanced nitrate uptake. MdMYB10 also controlled nitrate reallocation from old leaves to new leaves through MdNRT2.4-1. Overall, our findings provide novel insights into the mechanism by which MdMYB10 controls nitrate uptake and reallocation in apple, which facilitates adaptation to low nitrogen environment.

Genome-wide identification of nitrate transporter 2 (NRT2) gene family and functional analysis of MeNRT2.2 in cassava (Manihot esculenta Crantz)

Nitrate transporter 2 (NRT2) proteins play an important role in nitrate uptake and utilization in plants. The NRT2 family has been identified and functionally characterized in many plants. However, no systematic identification of NRT2 family members has been reported in cassava (Manihot esculenta Crantz). In this study, six MeNRT2 genes were identified from cassava genome and named as MeNRT2.1-2.6 according to their chromosomal locations. Phylogenetic tree showed that NRT2 proteins were divided into four main subgroups, which was further supported by their gene structure and conserved motifs. All six MeNRT2 genes are randomly distributed on 4 chromosomes (LG8, LG11, LG13, and LG17), two tandem duplicated genes (MeNRT2.3/MeNRT2.4) and a pair of segmental duplicated gene (MeNRT2.1/MeNRT2.2) was detected. Subsequently, expression profiles of MeNRT2 genes in eight different tissues and in response to nitrate deficient treatment were analyzed. The results showed that the MeNRT2 genes had differential expression patterns. All of MeNRT2 genes induced by nitrate deficiency, of them the MeNRT2.2 had the highest expression level after treatment. Arabidopis transformed with MeNRT2.2 gene showed higher fresh weight than wild type plants in response to N starvation, suggesting that MeNRT2.2 play important role in adapting to low nitrogen. Taken together, our results provide the reference for further analyses of the molecular functions of the MeNRT2 gene family, but also some candidate genes for developing nitrogen efficient crops.