In low transpiring conditions, nitrate and water fluxes for growth of B. napus plantlets correlate with changes in BnNrt2.1 and BnNrt1.1 transporter expression

Plant nitrogen availability and crosstalk with phytohormones signallings and their biotechnology breeding application in crops

Nitrogen (N), one of the most important nutrients, limits plant growth and crop yields in sustainable agriculture system, in which phytohormones are known to play essential roles in N availability. Hence, it is not surprising that massive studies about the crosstalk between N and phytohormones have been constantly emerging. In this review, with the intellectual landscape of N and phytohormones crosstalk provided by the bibliometric analysis, we trace the research story of best-known crosstalk between N and various phytohormones over the last 20 years. Then, we discuss how N regulates various phytohormones biosynthesis and transport in plants. In reverse, we also summarize how phytohormones signallings modulate root system architecture (RSA) in response to N availability. Besides, we expand to outline how phytohormones signallings regulate uptake, transport, and assimilation of N in plants. Further, we conclude advanced biotechnology strategies, explain their application, and provide potential phytohormones-regulated N use efficiency (NUE) targets in crops. Collectively, this review provides not only a better understanding on the recent progress of crosstalk between N and phytohormones, but also targeted strategies for improvement of NUE to increase crop yields in future biotechnology breeding of crops.

NITROGEN RESPONSE DEFICIENCY 1-mediated CHL1 induction contributes to optimized growth performance during altered nitrate availability in Arabidopsis

Plants cannot fix nitrogen directly; they must absorb it from the soil through their roots, or in rare cases, form associations with nitrogen-fixing bacteria. The efficiency of nitrogen use in most domesticated crops is low, and more than half of the available nitrogen in the soil can leach into the environment. Understanding the nitrogen signaling pathways is essential for maximizing the efficiency of nitrogen use in crops. In the present study, we characterized the Myeloblastosis (Myb)-like gene NITROGEN RESPONSE DEFICIENCY 1 (NID1). We observed that the growth performance of nid1 knockout (KO) mutant Arabidopsis plants was better than that of wild-type Col-0 plants under very low-nitrate conditions, leading to improved growth performance in the nid1 KO plants. The results of chromatin immunoprecipitation and electrophoretic mobility shift assays indicated that NID1 binds to the promoter of the NITRATE TRANSPORTER (NRT)1.1 gene. Furthermore, nid1 KO plants exhibited similar growth performance to the nid1 KO/chl1-5 (nrt1.1 KO) double mutant and chl1-5 (nrt1.1 KO) plants in response to low-nitrate conditions. We suggest that NID1 plays a crucial role as a transcription factor in optimizing plant growth by modulating the transcript abundance of the nitrate transceptor CHL1, leading to enhanced ABA accumulation in low-nitrate conditions.

Combined Allosteric Responses Explain the Bifurcation in Non-Linear Dynamics of 15N Root Fluxes Under Nutritional Steady-State Conditions for Nitrate

With regard to thermodynamics out of equilibrium, seedlings are open systems that dissipate energy towards their environment. Accordingly, under nutritional steady-state conditions, changes in external concentrations of one single ion provokes instability and reorganization in the metabolic and structure/architecture of the seedling that is more favorable to the fluxes of energy and matter. This reorganization is called a bifurcation and is described in mathematics as a non-linear dynamic system. In this study, we investigate the non-linear dynamics of ¹⁵N fluxes among cellular compartments of B. napus seedlings in response to a wide range of external NO 3 - 15 concentrations (from 0.05 to 20 mM): this allows to determine whether any stationary states and bifurcations could be found. The biphasic behavior of the root NO 3 - 15 uptake rate (v_in ) was explained by the combined cooperative properties between the v_app (N uptake, storage and assimilation rate) and v_out (N translocation rate) ¹⁵N fluxes that revealed a unique and stable stationary state around 0.28 mM nitrate. The disappearance of this stationary state around 0.5 mM external nitrate concentrations provokes a dramatic bifurcation in ¹⁵N flux pattern. This bifurcation in the v_in and v_out ¹⁵N fluxes fits better with the increase of BnNPF6.3/NRT1.1 expression than BnNRT2.1 nitrate transporter genes, confirming the allosteric property of the BnNPF6/NRT1.1 transporter, as reported in the literature between low and high nitrate concentrations. Moreover, several statistically significant power-law equations were found between variations in the shoots tryptophan concentrations (i.e., IAA precursor) with changes in the v_app and v_out ¹⁵N fluxes as well as a synthetic parameter of plant N status estimated from the root/shoot ratio of total free amino acids concentrations. These relationships designate IAA as one of the major biological parameters related to metabolic and structural-morphological reorganization coupled with the N and water fluxes induced by nitrate. The results seriously challenge the scientific grounds of the concept of high- and low-affinity of nitrate transporters and are therefore discussed in terms of the ecological significance and physiological implications on the basis of recent agronomic, physiological and molecular data of the literature.

Comparative genome and transcriptome analysis unravels key factors of nitrogen use efficiency in Brassica napus L

Considerable genetic variation in agronomic nitrogen (N) use efficiency (NUE) has been reported among genotypes of Brassica napus. However, the physiological and molecular mechanisms underpinning these differences remain poorly understood. In this study, physiological and genetic factors impacting NUE were identified in field trials and hydroponic experiments using two B. napus genotypes with contrasting NUE. The results showed that the N-efficient genotype (D4-15) had greater N uptake and utilization efficiencies, more root tips, larger root surface and root volume, and higher N assimilation and photosynthesis capacity than the N-inefficient genotype (D2-1). Genomic analysis revealed that D4-15 had a greater genome diversity related to NUE than D2-1. By combining genomic and transcriptomic analysis, genes involved in photosynthesis and C/N metabolism were implicated in conferring NUE. Co-expression network analysis of genes that differed between the two genotypes suggested gene clusters impacting NUE. A nitrate transporter gene BnaA06g04560D (NRT2.1) and two vacuole nitrate transporter CLC genes (BnaA02g11800D and BnaA02g28670D) were up-regulated by N starvation in D4-15 but not in D2-1. The study revealed that high N uptake and utilization efficiencies, maintained photosynthesis and coordinated C/N metabolism confer high NUE in B. napus, and identified candidate genes that could facilitate breeding for enhanced NUE in B. napus.

Inhibition of Aminotransferases by Aminoethoxyvinylglycine Triggers a Nitrogen Limitation Condition and Deregulation of Histidine Homeostasis That Impact Root and Shoot Development and Nitrate Uptake

Background and Aims: Although AVG (aminoethoxyvinylglycine) is intensely used to decipher signaling in ethylene/indol-3-acetic acid (IAA) interactions on root morphogenesis, AVG is not a specific inhibitor of aminocyclopropane-1-carboxylate synthase (ACS) and tryptophan aminotransferase (TAA) and tryptophan aminotransferase related (TAR) activities since it is able to inhibit several aminotransferases involved in N metabolism. Indeed, 1 mM glutamate (Glu) supply to the roots in plants treated with 10 μM AVG partially restores the root growth. Here, we highlight the changes induced by AVG and AVG + Glu treatments on the N metabolism impairment and root morphogenetic program. Methods: Root nitrate uptake induced by AVG and AVG + Glu treatments was measured by a differential labeling with 15NO3 - and 15Nglutamate. In parallel a profiling of amino acids (AA) was performed to decipher the impairment of AA metabolism. Key Results: 10 μM AVG treatment increases K15NO3 uptake and 15N translocation during root growth inhibition whereas 10 μM AVG + 1 mM 15Nglutamate treatment inhibits K15NO3 uptake and increases 15Nglutamate uptake during partial root growth restoration. This is explained by a nitrogen (N) limitation condition induced by AVG treatment and a N excess condition induced by AVG + Glu treatment. AA levels were mainly impaired by AVG treatment in roots, where levels of Ser, Thr, α-Ala, β-Ala, Val, Asn and His were significantly increased. His was the only amino acid for which no restoration was observed in roots and shoots after glutamate treatment suggesting important control of His homeostasis on aminotransferase network. Results were discussed in light of recent findings on the interconnection between His homeostasis and the general amino acid control system (GAAC) in eukaryotes. Conclusions: These results demonstrate that AVG concentration above 5 μM is a powerful pharmacological tool for unraveling the involvement of GAAC system or new N sensory system in morphological and metabolic changes of the roots in leguminous and non-leguminous plants.

SCC-S2 Facilitates Tumor Proliferation and Invasion via Activating Wnt Signaling and Depressing Hippo Signaling in Colorectal Cancer Cells and Predicts Poor Prognosis of Patients

SCC-S2 overexpression has been implicated in several human cancers, its correlation with prognosis and the mechanism how it reserved biological roles are still uncertain. The current study demonstrated that, in 142 archived colorectal carcinoma (CRC) tissue samples, SCC-S2 expression was significantly correlated with higher histological grade ( p=0.001), tumor invasion ( p=0.001), advanced Dukes staging ( p=0.002), positive regional lymph node metastasis ( p=0.024), and poor overall survival ( p<0.001). MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) and Transwell assays showed that SCC-S2 significantly promoted the proliferation and invasion. SCC-S2 expression was also accompanied by the overexpression CyclinD1, matrix metalloproteinase-7 (MMP-7), active-β-catenin, yes-associated protein (YAP), and connective tissue growth factor (CTGF), as well as the depression of p-large tumor suppressor kinase 1 (p-LATS1) and p-YAP. Moreover, SCC-S2 interacted and colocalized with LATS1, the interaction may interrupt Hippo signaling and thereafter activate canonical Wnt signaling. In conclusion, our data suggested that SCC-S2 was associated with the progression and unfavorable prognosis of CRCs. Meanwhile, SCC-S2 facilitated canonical Wnt signaling and its downstream effectors (CyclinD1, MMP-7) and promoted tumor proliferation and invasion, which depended on the inhibition of Hippo signaling induced by SCC-S2-LATS1 interaction. These results indicated that SCC-S2 might be used as a novel target for the prevention and treatment of colorectal cancer.

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

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

Fine-tuning of root elongation by ethylene: a tool to study dynamic structure-function relationships between root architecture and nitrate absorption

Background Recently developed genetic and pharmacological approaches have been used to explore NO3-/ethylene signalling interactions and how the modifications in root architecture by pharmacological modulation of ethylene biosynthesis affect nitrate uptake. Key Results Structure-function studies combined with recent approaches to chemical genomics highlight the non-specificity of commonly used inhibitors of ethylene biosynthesis such as AVG (l-aminoethoxyvinylglycine). Indeed, AVG inhibits aminotransferases such as ACC synthase (ACS) and tryptophan aminotransferase (TAA) involved in ethylene and auxin biosynthesis but also some aminotransferases implied in nitrogen (N) metabolism. In this framework, it can be assumed that the products of nitrate assimilation and hormones may interact through a hub in carbon (C) and N metabolism to drive the root morphogenetic programme (RMP). Although ethylene/auxin interactions play a major role in cell division and elongation in root meristems, shaping of the root system depends also on energetic considerations. Based on this finding, the analysis is extended to nutrient ion-hormone interactions assuming a fractal or constructal model for root development. Conclusion Therefore, the tight control of root structure-function in the RMP may explain why over-expressing nitrate transporter genes to decouple structure-function relationships and improve nitrogen use efficiency (NUE) has been unsuccessful.

Role of ethylene in responses of plants to nitrogen availability

Ethylene is a plant hormone involved in several physiological processes and regulates the plant development during the whole life. Stressful conditions usually activate ethylene biosynthesis and signaling in plants. The availability of nutrients, shortage or excess, influences plant metabolism and ethylene plays an important role in plant adaptation under suboptimal conditions. Among the plant nutrients, the nitrogen (N) is one the most important mineral element required for plant growth and development. The availability of N significantly influences plant metabolism, including ethylene biology. The interaction between ethylene and N affects several physiological processes such as leaf gas exchanges, roots architecture, leaf, fruits, and flowers development. Low plant N use efficiency (NUE) leads to N loss and N deprivation, which affect ethylene biosynthesis and tissues sensitivity, inducing cell damage and ultimately lysis. Plants may respond differently to N availability balancing ethylene production through its signaling network. This review discusses the recent advances in the interaction between N availability and ethylene at whole plant and different organ levels, and explores how N availability induces ethylene biology and plant responses. Exogenously applied ethylene seems to cope the stress conditions and improves plant physiological performance. This can be explained considering the expression of ethylene biosynthesis and signaling genes under different N availability. A greater understanding of the regulation of N by means of ethylene modulation may help to increase NUE and directly influence crop productivity under conditions of limited N availability, leading to positive effects on the environment. Moreover, efforts should be focused on the effect of N deficiency or excess in fruit trees, where ethylene can have detrimental effects especially during postharvest.

Breaking conceptual locks in modelling root absorption of nutrients: reopening the thermodynamic viewpoint of ion transport across the root

BACKGROUND

The top-down analysis of nitrate influx isotherms through the Enzyme-Substrate interpretation has not withstood recent molecular and histochemical analyses of nitrate transporters. Indeed, at least four families of nitrate transporters operating at both high and/or low external nitrate concentrations, and which are located in series and/or parallel in the different cellular layers of the mature root, are involved in nitrate uptake. Accordingly, the top-down analysis of the root catalytic structure for ion transport from the Enzyme-Substrate interpretation of nitrate influx isotherms is inadequate. Moreover, the use of the Enzyme-Substrate velocity equation as a single reference in agronomic models is not suitable in its formalism to account for variations in N uptake under fluctuating environmental conditions. Therefore, a conceptual paradigm shift is required to improve the mechanistic modelling of N uptake in agronomic models.

SCOPE

An alternative formalism, the Flow-Force theory, was proposed in the 1970s to describe ion isotherms based upon biophysical 'flows and forces' relationships of non-equilibrium thermodynamics. This interpretation describes, with macroscopic parameters, the patterns of N uptake provided by a biological system such as roots. In contrast to the Enzyme-Substrate interpretation, this approach does not claim to represent molecular characteristics. Here it is shown that it is possible to combine the Flow-Force formalism with polynomial responses of nitrate influx rate induced by climatic and in planta factors in relation to nitrate availability.

CONCLUSIONS

Application of the Flow-Force formalism allows nitrate uptake to be modelled in a more realistic manner, and allows scaling-up in time and space of the regulation of nitrate uptake across the plant growth cycle.

Complex phylogeny and gene expression patterns of members of the NITRATE TRANSPORTER 1/PEPTIDE TRANSPORTER family (NPF) in wheat

NPF (formerly referred to as low-affinity NRT1) and 'high-affinity' NRT2 nitrate transporter genes are involved in nitrate uptake by the root, and transport and distribution of nitrate within the plant. The NPF gene family consists of 53 members in Arabidopsis thaliana, however only 11 of these have been functionally characterized. Although homologous genes have been identified in genomes of different plant species including some cereals, there is little information available for wheat (Triticum aestivum). Sixteen genes were identified in wheat homologous to characterized Arabidopsis low-affinity nitrate transporter NPF genes, suggesting a complex wheat NPF gene family. The regulation of wheat NFP genes by plant N-status indicated involvement of these transporters in substrate transport in relation to N-metabolism. The complex expression pattern in relation to tissue specificity, nitrate availability and senescence may be associated with the complex growth patterns of wheat depending on sink/source demands, as well as remobilization during grain filling.

An updated model for nitrate uptake modelling in plants. II. Assessment of active root involvement in nitrate uptake based on integrated root system age: measured versus modelled outputs

Background and Aims An updated version of a mechanistic structural-functional model was developed to predict nitrogen (N) uptake throughout the growth cycle by a crop of winter oilseed rape, Brassica napus, grown under field conditions. Methods The functional component of the model derives from a revisited conceptual framework that combines the thermodynamic Flow-Force interpretation of nitrate uptake isotherms and environmental and in planta effects on nitrate influx. Estimation of the root biomass (structural component) is based upon a combination of root mapping along the soil depth profile in the field and a relationship between the specific root length and external nitrate concentration. The root biomass contributing actively to N uptake was determined by introduction of an integrated root system age that allows assignment of a root absorption capacity at a specific age of the root. Key Results Simulations were well matched to measured data of N taken up under field conditions for three levels of N fertilization. The model outputs indicated that the two topsoil layers (0-30 and 30-60 cm) contained 75-88 % of the total root length and biomass, and accounted for 90-95 % of N taken up at harvest. Conclusions This conceptual framework provides a model of nitrate uptake that is able to respond to external nitrate fluctuations at both functional and structural levels.

An updated model for nitrate uptake modelling in plants. I. Functional component: cross-combination of flow-force interpretation of nitrate uptake isotherms, and environmental and in planta regulation of nitrate influx

BACKGROUND AND AIMS

In spite of major breakthroughs in the last three decades in the identification of root nitrate uptake transporters in plants and the associated regulation of nitrate transport activities, a simplified and operational modelling approach for nitrate uptake is still lacking. This is due mainly to the difficulty in linking the various regulations of nitrate transport that act at different levels of time and on different spatial scales.

METHODS

A cross-combination of a Flow-Force approach applied to nitrate influx isotherms and experimentally determined environmental and in planta regulation is used to model nitrate in oilseed rape, Brassica napus. In contrast to 'Enzyme-Substrate' interpretations, a Flow-Force modelling approach considers the root as a single catalytic structure and does not infer hypothetical cellular processes among nitrate transporter activities across cellular layers in the mature roots. In addition, this approach accounts for the driving force on ion transport based on the gradient of electrochemical potential, which is more appropriate from a thermodynamic viewpoint.

KEY RESULTS AND CONCLUSIONS

Use of a Flow-Force formalism on nitrate influx isotherms leads to the development of a new conceptual mechanistic basis to model more accurately N uptake by a winter oilseed rape crop under field conditions during the whole growth cycle. This forms the functional component of a proposed new structure-function mechanistic model of N uptake.

Modulation of ethylene biosynthesis by ACC and AIB reveals a structural and functional relationship between the K15NO3 uptake rate and root absorbing surfaces

The modification of root traits in relation to nitrate uptake represents a source for improvement of nitrogen uptake efficiency. Because ethylene signalling modulates growth of exploratory and root hair systems more rapidly (minutes to hours) than nitrate signalling (days to weeks), a pharmacological approach was used to decipher the relationships between root elongation and N uptake. Rape seedlings were grown on agar plates supplied with 1mM K(15)NO3 and treated with different concentrations of either the ethylene precursor, ACC (0.1, 1, and 10 μM) or an inhibitor of ethylene biosynthesis, AIB (0.5 and 1 μM). The results showed that rapid modulation of root elongation (up to 8-fold) is more dependent on the ethylene than the nitrate signal. Indeed, ACC treatment induced a partial compensatory increase in (15)N uptake associated with overexpression of the BnNRT2.1 and BnNRT1.1 genes. Likewise, daily root elongation between treatments was not associated with daily nitrate uptake but was correlated with N status. This suggested that a part of the daily root response was modulated by cross talks between ethylene signalling and N and C metabolisms. This was confirmed by the reduction in C allocation to the roots induced by ACC treatment and the correlations of changes in the root length and shoot surface area with the aspartate content. The observed effects of ethylene signalling in the root elongation and NRT gene expression are discussed in the context of the putative role of NRT2.1 and NRT1.1 transporters as nitrate sensors.

In low transpiring conditions, uncoupling the BnNrt2.1 and BnNrt1.1 NO 3(-) transporters by glutamate treatment reveals the essential role of BnNRT2.1 for nitrate uptake and the nitrate-signaling cascade during growth

In plants, the nitrate transporters, NRT1.1 and NRT2.1, are mainly responsible for nitrate uptake. Intriguingly, both nitrate transporters are located in a complementary manner in different cells layers of the mature root suggesting that their coordination should occur during nitrate uptake and plant growth. This hypothesis was examined on 5-d-old rape seedlings grown on agar medium supplemented with 1 or 5mM nitrate. Seedlings were treated with increasing potassium glutamate concentrations in order to uncouple the two nitrate transporters by inhibiting BnNRT2.1 expression and activity specifically. In both nitrate treatments, increasing the glutamate concentrations from 0.5 to 10mM induced a reduction in (15)NO 3(-) uptake and an inhibition of N assimilation. The decrease in (15)NO 3(-) uptake was caused by downregulation of BnNRT2.1 expression but surprisingly it was not compensated by the upregulation of BnNRT1.1. This created an unprecedented physiological situation where the effects of the nitrate signal on shoot growth were solely modulated by nitrate absorption. In these conditions, the osmotic water flow for volumetric shoot growth was mainly dependent on active nitrate transport and nitrate signaling. This behavior was confirmed by the allometric relationships found between changes in the root length with (15)N and water accumulation in the shoot. These findings demonstrate that the BnNRT2.1 transporter is essential for nitrate uptake and growth, and renew the question of the respective roles of the NRT2.1 and NRT1.1 transporters in nitrate uptake and sensing at the whole plant level.