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

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 Absorption Pattern Detection and Expression Analysis of Nitrate Transporters in Flowering Chinese Cabbage

Nitrate transporters (NRTs) play an important role in nitrate absorption and internal distribution in plant roots and other parts. Experiments were carried out to explore the sequences and expression characteristics of NRT genes, and their correlation with the N uptake in flowering Chinese cabbage. We have isolated three important BcNRTs (BcNRT1.1, BcNRT1.2, and BcNRT2.1) from flowering Chinese cabbage. Spatio-temporal expression analysis found that BcNRT1.1 and BcNRT2.1 were mainly expressed in roots, while BcNRT1.2 was more expressed in roots than in leaves during vegetative growth and was mainly expressed in leaves during reproductive growth. The NO3− uptake rate of the entire growth period was significantly correlated with BcNRT1.1 and BcNRT1.2 expression in roots. In addition, the total N content was increased with the increase in NO3− concentration in flowering Chinese cabbage. The NH4+ uptake was slightly induced by NH4+, but the total N content had no significant difference under the NH4+ concentration of 1–8 mmol/L. We also found that lower concentrations of NH4+ promoted the expression of BcNRT1.1 and BcNRT1.2 while inhibiting the expression of BcNRT2.1 in the roots of flowering Chinese cabbage. The amount of total N uptake in the treatment with 25/75 of NH4+/NO3− was significantly higher than that of the other two treatments (0/100 and 50/50). In the mixture of NH4+ and NO3−, total N uptake was significantly correlated with the BcNRT1.2 expression. We concluded that mixed nutrition with an NH4+/NO3− of 25/75 could significantly increase total nitrogen uptake in flowering Chinese cabbage, in which two members of the NRT1 subfamily (BcNRT1.1 and BcNRT1.2) might play a major regulatory role in it. This study is a beneficial attempt to dig deeper into the NRT genes resources and lays the foundation for the ultimate use of genetic improvement methods to increase the NUE with less nitrogen fertilizer in flowering Chinese cabbage.

Ethephon Reduces Maize Nitrogen Uptake but Improves Nitrogen Utilization in Zea mays L

Increasing use of plant density or/and nitrogen (N) application has been introduced to maize production in the past few decades. However, excessive planting density or/and use of fertilizer may cause reduced N use efficiency (NUE) and increased lodging risks. Ethephon application improves maize lodging resistance and has been an essential measure in maize intensive production systems associated with high plant density and N input in China. Limited information is available about the effect of ethephon on maize N use and the response to plant density under different N rates in the field. A three-year field study was conducted with two ethephon applications (0 and 90 g ha^-1), four N application rates (0, 75, 150, and 225 kg N ha^-1), and two plant densities (6.75 plants m^-2 and 7.5 plants m^-2) to evaluate the effects of ethephon on maize NUE indices (N agronomic efficiency, NAE; N recovery efficiency, NRE; N uptake efficiency, NUpE; N utilization efficiency, NUtE; partial factor productivity of N, PFPN), biomass, N concentration, grain yield and N uptake, and translocation properties. The results suggest that the application of ethephon decreased the grain yield by 1.83-5.74% due to the decrease of grain numbers and grain weight during the three experimental seasons. Meanwhile, lower biomass, NO 3 - and NH 4 + fluxes in xylem bleeding sap, and total N uptake were observed under ethephon treatments. These resulted in lower NAE and NUpE under the ethephon treatment at a corresponding N application rate and plant density. The ethephon treatment had no significant effects on the N concentration in grains, and it decreased the N concentration in stover at the harvesting stage, while increasing the plant N concentration at the silking stage. Consequently, post-silking N remobilization was significantly increased by 14.10-32.64% under the ethephon treatment during the experimental periods. Meanwhile, NUtE significantly increased by ethephon.

The functional characterization of LjNRT2.4 indicates a novel, positive role of nitrate for an efficient nodule N2 -fixation activity

Atmospheric nitrogen (N₂₎ -fixing nodules are formed on the roots of legume plants as result of the symbiotic interaction with rhizobia. Nodule functioning requires high amounts of carbon and energy, and therefore legumes have developed finely tuned mechanisms to cope with changing external environmental conditions, including nutrient availability and flooding. The investigation of the role of nitrate as regulator of the symbiotic N₂ fixation has been limited to the inhibitory effects exerted by high external concentrations on nodule formation, development and functioning. We describe a nitrate-dependent route acting at low external concentrations that become crucial in hydroponic conditions to ensure an efficient nodule functionality. Combined genetic, biochemical and molecular studies are used to unravel the novel function of the LjNRT2.4 gene. Two independent null mutants are affected by the nitrate content of nodules, consistent with LjNRT2.4 temporal and spatial profiles of expression. The reduced nodular nitrate content is associated to a strong reduction of nitrogenase activity and a severe N-starvation phenotype observed under hydroponic conditions. We also report the effects of the mutations on the nodular nitric oxide (NO) production and content. We discuss the involvement of LjNRT2.4 in a nitrate-NO respiratory chain taking place in the N₂ -fixing nodules.

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.

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.

Exogenous glycine inhibits root elongation and reduces nitrate-N uptake in pak choi (Brassica campestris ssp. Chinensis L.)

Nitrogen (N) supply, including NO3--N and organic N in the form of amino acids can influence the morphological attributes of plants. For example, amino acids contribute to plant nutrition; however, the effects of exogenous amino acids on NO3--N uptake and root morphology have received little attention. In this study, we evaluated the effects of exogenous glycine (Gly) on root growth and NO3--N uptake in pak choi (Brassica campestris ssp. Chinensis L.). Addition of Gly to NO3--N agar medium or hydroponic solution significantly decreased pak choi seedling root length; these effects of Gly on root morphology were not attributed to the proportion of N supply derived from Gly. When pak choi seedlings were exposed to mixtures of Gly and NO3--N in hydroponic culture, Gly significantly reduced 15NO3--N uptake but significantly increased the number of root tips per unit root length, root activity and 15NO3--N uptake rate per unit root length. In addition, 15N-Gly was taken up into the plants. In contrast to absorbed NO3--N, which was mostly transported to the shoots, a larger proportion of absorbed Gly was retained in the roots. Exogenous Gly enhanced root 1-aminocyclopropane-1-carboxylic acid synthase (ACS) and oxidase (ACO) activities and ethylene production. The ethylene antagonists aminoethoxyvinylglycine (0.5 μM AVG) and silver nitrate (10 μM AgNO3) partly reversed Gly-induced inhibition of primary root elongation on agar plates and increased the NO3--N uptake rate under hydroponic conditions, indicating exogenous Gly exerts these effects at least partly by enhancing ethylene production in roots. These findings suggest Gly substantially affects root morphology and N uptake and provide new information on the specific responses elicited by organic N sources.

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.

A new insight into root responses to external cues: Paradigm shift in nutrient sensing

Higher plants are sessile and their growth relies on nutrients present in the soil. The acquisition of nutrients is challenging for plants. Phosphate and nitrate sensing and signaling cascades play significant role during adverse conditions of nutrient unavailability. Therefore, it is important to dissect the mechanism by which plant roots acquire nutrients from the soil. Root system architecture (RSA) exhibits extensive developmental flexibility and changes during nutrient stress conditions. Growth of root system in response to external concentration of nutrients is a joint operation of sensor or receptor proteins along with several other cytoplasmic accessory proteins. After nutrient sensing, sensor proteins start the cellular relay involving transcription factors, kinases, ubiquitin ligases and miRNA. The complexity of nutrient sensing is still nebulous and many new players need to be better studied. This review presents a survey of recent paradigm shift in the advancements in nutrient sensing in relation to plant roots.

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.

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.