Elongation changes of exploratory and root hair systems induced by aminocyclopropane carboxylic acid and aminoethoxyvinylglycine affect nitrate uptake and BnNrt2.1 and BnNrt1.1 transporter gene expression in oilseed rape

Integrated BSA-seq and RNA-seq analysis to identify candidate genes associated with nitrogen utilization efficiency (NUtE) in rapeseed (Brassica napus L.)

Rapeseed (Brassica napus L.) is one of the important oil crops, with a high demand for nitrogen (N). It is essential to explore the potential of rapeseed to improve nitrogen utilization efficiency (NUtE). Rapeseed is an allotetraploid crop with a relatively large and complex genome, and there are few studies on the mapping of genes related to NUtE regulation. In this study, we used the combination of bulk segregant analysis sequencing (BSA-Seq) and RNA sequencing (RNA-Seq) to analyze the N-efficient genotype 'Zheyou 18' and N-inefficient genotype 'Sollux', to identify the genetic regulatory mechanisms. Several candidate genes were screened, such as the high-affinity nitrate transporter gene NRT2.1 (BnaC08g43370D) and the abscisic acid (ABA) signal transduction-related genes (BnaC02g14540D, BnaA03g20760D, and BnaA05g01330D). BnaA05g01330D was annotated as ABA-INDUCIBLE bHLH-TYPE TRANSCRIPTION FACTOR (AIB/bHLH17), which was highly expressed in the root. The results showed that the primary root length of the ataib mutant was significantly longer than that of the wild type under low N conditions. Overexpression of BnaA5.AIB could reduce the NUtE under low N levels in Arabidopsis (Arabidopsis thaliana). Candidate genes identified in this study may be involved in the regulation of NUtE in rapeseed, and new functions of AIB in orchestrating N uptake and utilization have been revealed. It is indicated that BnaA5.AIB may be the key factor that links ABA to N signaling and a negative regulator of NUtE. It will provide a theoretical basis and application prospect for resource conservation, environmental protection, and sustainable agricultural development.

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.

Interplay between ethylene and nitrogen nutrition: How ethylene orchestrates nitrogen responses in plants

The stress hormone ethylene plays a key role in plant adaptation to adverse environmental conditions. Nitrogen (N) is the most quantitatively required mineral nutrient for plants, and its availability is a major determinant for crop production. Changes in N availability or N forms can alter ethylene biosynthesis and/or signaling. Ethylene serves as an important cellular signal to mediate root system architecture adaptation, N uptake and translocation, ammonium toxicity, anthocyanin accumulation, and premature senescence, thereby adapting plant growth and development to external N status. Here, we review the ethylene-mediated morphological and physiological responses and highlight how ethylene transduces the N signals to the adaptive responses. We specifically discuss the N-ethylene relations in rice, an important cereal crop in which ethylene is essential for its hypoxia survival.

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.

Comparative Transcriptome Analysis of Two Peach Rootstocks Uncovers the Effect of Gene Differential Expression on Nitrogen Use Efficiency

Nitrogen is an important nutrient element that limits plant growth and yield formation, but excessive nitrogen has negative effects on plants and the environment. It is important to reveal the molecular mechanism of high NUE (nitrogen use efficiency) for breeding peach rootstock and variety with high NUE. In this study, two peach rootstocks, Shannong–1 (S) and Maotao (M), with different NUE were used as materials and treated with 0.1 mM KNO₃ for transcriptome sequencing together with the control group. From the results of comparison between groups, we found that the two rootstocks had different responses to KNO₃, and 2151 (KCL_S vs. KCL_M), 327 (KNO₃_S vs. KCL_S), 2200 (KNO₃_S vs. KNO₃_M) and 146 (KNO₃_M vs. KCL_M) differentially expressed genes (DEGs) were identified, respectively, which included multiple transcription factor families. These DEGs were enriched in many biological processes and signal transduction pathways, including nitrogen metabolism and plant hormone signal transduction. The function of PpNRT2.1, which showed up-regulated expression under KNO₃ treatment, was verified by heterologous expression in Arabidopsis. The plant height, SPAD (soil and plant analyzer development) of leaf and primary root length of the transgenic plants were increased compared with those of WT, indicating the roles of PpNRT2.1 in nitrogen metabolism. The study uncovered for the first time the different molecular regulatory pathways involved in nitrogen metabolism between two peach rootstocks and provided gene reserve for studying the molecular mechanism of nitrogen metabolism and theoretical basis for screening peach rootstock or variety with high NUE.

Nitrogen use efficiency in cotton: Challenges and opportunities against environmental constraints

Nitrogen is a vital nutrient for agricultural, and a defieciency of it causes stagnate cotton growth and yield penalty. Farmers rely heavily on N over-application to boost cotton output, which can result in decreased lint yield, quality, and N use efficiency (NUE). Therefore, improving NUE in cotton is most crucial for reducing environmental nitrate pollution and increasing farm profitability. Well-defined management practices, such as the type of sources, N-rate, application time, application method, crop growth stages, and genotypes, have a notable impact on NUE. Different N formulations, such as slow and controlled released fertilizers, have been shown to improve N uptake and, NUE. Increasing N rates are said to boost cotton yield, although high rates may potentially impair the yield depending on the soil and environmental conditions. This study comprehensively reviews various factors including agronomic and environmental constraints that influence N uptake, transport, accumulation, and ultimately NUE in cotton. Furthermore, we explore several agronomic and molecular approaches to enhance efficiency for better N uptake and utilization in cotton. Finally, this objective of this review to highlight a comprehensive view on enhancement of NUE in cotton and could be useful for understanding the physiological, biochemical and molecular mechanism of N in cotton.

Identification and Expression Analysis of SLAC/SLAH Gene Family in Brassica napus L

Slow type anion channels (SLAC/SLAHs) play important roles during anion transport, growth and development, abiotic stress responses and hormone responses in plants. However, there is few report on SLAC/SLAHs in rapeseed (Brassica napus). Genome-wide identification and expression analysis of SLAC/SLAH gene family members were performed in B. napus. A total of 23 SLAC/SLAH genes were identified in B. napus. Based on the structural characteristics and phylogenetic analysis of these members, the SLAC/SLAHs could be classified into three main groups. Transcriptome data demonstrated that BnSLAH3 genes were detected in various tissues of the rapeseed and could be up-regulated by low nitrate treatment in roots. BnSLAC/SLAHs were exclusively localized on the plasma membrane in transient expression of tobacco leaves. These results will increase our understanding of the evolution and expression of the SLAC/SLAHs and provide evidence for further research of biological functions of candidates in B. napus.

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.

Transcriptome and Hormone Analyses Revealed Insights into Hormonal and Vesicle Trafficking Regulation among Olea europaea Fruit Tissues in Late Development

Fruit ripening and abscission are the results of the cell wall modification concerning different components of the signaling network. However, molecular-genetic information on the cross-talk between ripe fruit and their abscission zone (AZ) remains limited. In this study, we investigated transcriptional and hormonal changes in olive (Olea europaea L. cv Picual) pericarp and AZ tissues of fruit at the last stage of ripening, when fruit abscission occurs, to establish distinct tissue-specific expression patterns related to cell-wall modification, plant-hormone, and vesicle trafficking in combination with data on hormonal content. In this case, transcriptome profiling reveals that gene encoding members of the α-galactosidase and β-hexosaminidase families associated with up-regulation of RabB, RabD, and RabH classes of Rab-GTPases were exclusively transcribed in ripe fruit enriched in ABA, whereas genes of the arabinogalactan protein, laccase, lyase, endo-β-mannanase, ramnose synthase, and xyloglucan endotransglucosylase/hydrolase families associated with up-regulation of RabC, RabE, and RabG classes of Rab-GTPases were exclusively transcribed in AZ-enriched mainly in JA, which provide the first insights into the functional divergences among these protein families. The enrichment of these protein families in different tissues in combination with data on transcript abundance offer a tenable set of key genes of the regulatory network between olive fruit tissues in late development.

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.

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.

Nitrogen fertility and abiotic stresses management in cotton crop: a review

This review outlines nitrogen (N) responses in crop production and potential management decisions to ameliorate abiotic stresses for better crop production. N is a primary constituent of the nucleotides and proteins that are essential for life. Production and application of N fertilizers consume huge amounts of energy, and excess is detrimental to the environment. Therefore, increasing plant N use efficiency (NUE) is important for the development of sustainable agriculture. NUE has a key role in crop yield and can be enhanced by controlling loss of fertilizers by application of humic acid and natural polymers (hydrogels), having high water-holding capacity which can improve plant performance under field conditions. Abiotic stresses such as waterlogging, drought, heat, and salinity are the major limitations for successful crop production. Therefore, integrated management approaches such as addition of aminoethoxyvinylglycine (AVG), the film antitranspirant (di-1-p-menthene and pinolene) nutrients, hydrogels, and phytohormones may provide novel approaches to improve plant tolerance against abiotic stress-induced damage. Moreover, for plant breeders and molecular biologists, it is a challenge to develop cotton cultivars that can tolerate plant abiotic stresses while having high potential NUE for the future.

Regulation of seedling growth by ethylene and the ethylene-auxin crosstalk

This review highlights that the auxin gradient, established by local auxin biosynthesis and transport, can be controlled by ethylene, and steers seedling growth. A better understanding of the mechanisms in Arabidopsis will increase potential applications in crop species. In dark-grown Arabidopsis seedlings, exogenous ethylene treatment triggers an exaggeration of the apical hook, the inhibition of both hypocotyl and root elongation, and radial swelling of the hypocotyl. These features are predominantly based on the differential cell elongation in different cells/tissues mediated by an auxin gradient. Interestingly, the physiological responses regulated by ethylene and auxin crosstalk can be either additive or synergistic, as in primary root and root hair elongation, or antagonistic, as in hypocotyl elongation. This review focuses on the crosstalk of these two hormones at the seedling stage. Before illustrating the crosstalk, ethylene and auxin biosynthesis, metabolism, transport and signaling are briefly discussed.

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.

Roots Withstanding their Environment: Exploiting Root System Architecture Responses to Abiotic Stress to Improve Crop Tolerance

To face future challenges in crop production dictated by global climate changes, breeders and plant researchers collaborate to develop productive crops that are able to withstand a wide range of biotic and abiotic stresses. However, crop selection is often focused on shoot performance alone, as observation of root properties is more complex and asks for artificial and extensive phenotyping platforms. In addition, most root research focuses on development, while a direct link to the functionality of plasticity in root development for tolerance is often lacking. In this paper we review the currently known root system architecture (RSA) responses in Arabidopsis and a number of crop species to a range of abiotic stresses, including nutrient limitation, drought, salinity, flooding, and extreme temperatures. For each of these stresses, the key molecular and cellular mechanisms underlying the RSA response are highlighted. To explore the relevance for crop selection, we especially review and discuss studies linking root architectural responses to stress tolerance. This will provide a first step toward understanding the relevance of adaptive root development for a plant's response to its environment. We suggest that functional evidence on the role of root plasticity will support breeders in their efforts to include root properties in their current selection pipeline for abiotic stress tolerance, aimed to improve the robustness of crops.

Non-specificity of ethylene inhibitors: 'double-edged' tools to find out new targets involved in the root morphogenetic programme

In the last decade, genetic and pharmacological approaches have been used to explore ethylene biosynthesis and perception in order to study the role of ethylene and ethylene/auxin interaction in root architecture development. However, recent findings with pharmacological approaches highlight the non-specificity of commonly used inhibitors. This suggests that caution is required for interpreting these studies and that the use of pharmacological agents is a 'double-edged' tool. On one hand, non-specific effects make interpretation difficult unless other experiments, such as with different mutants or with multiple diversely acting chemicals, are conducted. On the other hand, the non-specificity of inhibitors opens up the possibility of uncovering some ligands or modulators of new receptors such as plant glutamate-like receptors and importance of some metabolic hubs in carbon and nitrogen metabolism such as the pyridoxal phosphate biosynthesis involved in the regulation of the root morphogenetic programme. Identification of such targets is a critical issue to improve the efficiency of absorption of macronutrients in relation to root the morphogenetic programme.

Consequences of waterlogging in cotton and opportunities for mitigation of yield losses

Climatic variability, typified by erratic heavy-rainfall events, causes waterlogging in intensively irrigated crops and is exacerbated under warm temperature regimes on soils with poor internal drainage. Irrigated cotton is often grown in precisely these conditions, exposing it to waterlogging-induced yield losses after substantial summer rainfall. This calls for a deeper understanding of mechanisms of waterlogging tolerance and its relevance to cotton. Hence this review suggests possible causes of waterlogging-induced yield loss in cotton and approaches to improvement of waterlogging tolerance, drawing upon the slight body of published data in cotton and principles from other species. The yield penalty depends on soil type, phenological stage and cumulative period of root exposure to air-filled porosities below 10 %. Events in the soil include O2 deficiency in the root zone that changes the redox state of nutrients, making them unavailable (e.g. nitrogen) or potentially toxic for plants. Furthermore, root-derived hormones that are transported in the xylem have long been associated with oxygen deficits. These belowground effects (impaired root growth, nutrient uptake and transport, hormonal signalling) affect the shoots, interfering with canopy development, photosynthesis and radiation-use efficiency. Compared with the more waterlogging-tolerant cereals, cotton does not have identified adaptations to waterlogging in the root zone, forming no conspicuous root aerenchyma and having low fermentative activity. We speculate that these factors contribute substantially to the sensitivity of cotton to sustained periods of waterlogging. We discuss the impact of these belowground factors on shoot performance, photosynthesis and yield components. Management practices, i.e. soil aeration, scheduling irrigation and fertilizer application, can reduce waterlogging-induced damage. Limiting ethylene biosynthesis using anti-ethylene agents and down-regulating expression of genes controlling ethylene biosynthesis are strong candidates to minimize yield losses in waterlogged cotton crops. Other key pathways of anoxia tolerance are also cited as potential tools towards waterlogging-tolerant cotton genotypes.

Nitrogen availability regulates proline and ethylene production and alleviates salinity stress in mustard (Brassica juncea)

Proline content and ethylene production have been shown to be involved in salt tolerance mechanisms in plants. To assess the role of nitrogen (N) in the protection of photosynthesis under salt stress, the effect of N (0, 5, 10, 20 mM) on proline and ethylene was studied in mustard (Brassica juncea). Sufficient N (10 mM) optimized proline production under non-saline conditions through an increase in proline-metabolizing enzymes, leading to osmotic balance and protection of photosynthesis through optimal ethylene production. Excess N (20 mM), in the absence of salt stress, inhibited photosynthesis and caused higher ethylene evolution but lower proline production compared to sufficient N. In contrast, under salt stress with an increased demand for N, excess N optimized ethylene production, which regulates the proline content resulting in recovered photosynthesis. The effect of excess N on photosynthesis under salt stress was further substantiated by the application of the ethylene biosynthesis inhibitor, 1-aminoethoxy vinylglycine (AVG), which inhibited proline production and photosynthesis. Without salt stress, AVG promoted photosynthesis in plants receiving excess N by inhibiting stress ethylene production. The results suggest that a regulatory interaction exists between ethylene, proline and N for salt tolerance. Nitrogen differentially regulates proline production and ethylene formation to alleviate the adverse effect of salinity on photosynthesis in mustard.

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.

Ethylene negatively regulates aluminium-induced malate efflux from wheat roots and tobacco cells transformed with TaALMT1

An important mechanism for Al(3+) tolerance in wheat is exudation of malate anions from the root apex through activation of malate-permeable TaALMT1 channels. Here, the effect of ethylene on Al(3+)-activated efflux of malate was investigated using Al(3+)-tolerant wheat genotype ET8, which has high expression of TaALMT1. Exposure of ET8 plants to Al(3+) enhanced ethylene evolution in root apices. Treatment with the ethylene synthesis precursor 1-aminocyclopropane-1-carboxylic acid (ACC) and ethylene gas suppressed Al(3+)-induced malate efflux from root apices, whereas the intracellular malate concentrations in roots were not affected. Malate efflux from root apices was enhanced in the presence of Al(3+) by two antagonists of ethylene biosynthesis, aminoethoxyvinylglycine (AVG) and 2-aminoisobutyric acid (AIB). An increase in Al accumulation in root apices was observed when treated with ACC, whereas AVG and AIB suppressed Al accumulation in root apices. Al(3+)-induced inhibition of root elongation was ameliorated by pretreatment with AIB. In addition, ethylene donor (Ethrel) also inhibited Al(3+)-induced malate efflux from tobacco cells transformed with TaALMT1. ACC and the anion-channel blocker niflumate had a similar and non-additive effect on Al-induced malate efflux from root apices. Treatment of ET8 plants with ACC enhanced expression of TaALMT1, suggesting that the inhibitory effect of ethylene on Al-induced malate efflux is unlikely to occur at the transcriptional level. These findings indicate that ethylene may behave as a negative regulator of Al(3+)-induced malate efflux by targeting TaALMT1-mediated malate efflux by an unknown mechanism.

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.

Current understanding on ethylene signaling in plants: the influence of nutrient availability

The plant hormone ethylene is involved in many physiological processes, including plant growth, development and senescence. Ethylene also plays a pivotal role in plant response or adaptation under biotic and abiotic stress conditions. In plants, ethylene production often enhances the tolerance to sub-optimal environmental conditions. This role is particularly important from both ecological and agricultural point of views. Among the abiotic stresses, the role of ethylene in plants under nutrient stress conditions has not been completely investigated. In literature few reports are available on the interaction among ethylene and macro- or micro-nutrients. However, the published works clearly demonstrated that several mineral nutrients largely affect ethylene biosynthesis and perception with a strong influence on plant physiology. The aim of this review is to revisit the old findings and recent advances of knowledge regarding the sub-optimal nutrient conditions on the effect of ethylene biosynthesis and perception in plants. The effect of deficiency or excess of the single macronutrient or micronutrient on the ethylene pathway and plant responses are reviewed and discussed. The synergistic and antagonist effect of the different mineral nutrients on ethylene plant responses is critically analyzed. Moreover, this review highlights the status of information between nutritional stresses and plant response, emphasizing the topics that should be further investigated.

Linking ethylene to nitrogen-dependent leaf longevity of grass species in a temperate steppe

BACKGROUND AND AIMS

Leaf longevity is an important plant functional trait that often varies with soil nitrogen supply. Ethylene is a classical plant hormone involved in the control of senescence and abscission, but its role in nitrogen-dependent leaf longevity is largely unknown.

METHODS

Pot and field experiments were performed to examine the effects of nitrogen addition on leaf longevity and ethylene production in two dominant plant species, Agropyron cristatum and Stipa krylovii, in a temperate steppe in northern China.

KEY RESULTS

Nitrogen addition increased leaf ethylene production and nitrogen concentration but shortened leaf longevity; the addition of cobalt chloride, an ethylene biosynthesis inhibitor, reduced leaf nitrogen concentration and increased leaf longevity. Path analysis indicated that nitrogen addition reduced leaf longevity mainly through altering leaf ethylene production.

CONCLUSIONS

These findings provide the first experimental evidence in support of the involvement of ethylene in nitrogen-induced decrease in leaf longevity.

Salicylic acid alleviates adverse effects of heat stress on photosynthesis through changes in proline production and ethylene formation

We investigated the potential of salicylic acid (SA) in alleviating the adverse effects of heat stress on photosynthesis in wheat (Triticum aestivum L.) cv WH 711. Activity of ribulose 1,5-bisphosphate carboxylase (Rubisco), photosynthetic-nitrogen use efficiency (NUE), and net photosynthesis decreased in plants subjected to heat stress (40 °C for 6 h), but proline metabolism increased. SA treatment (0.5 mM) alleviated heat stress by increasing proline production through the increase in γ-glutamyl kinase (GK) and decrease in proline oxidase (PROX) activity, resulting in promotion of osmotic potential and water potential necessary for maintaining photosynthetic activity. Together with this, SA treatment restricted the ethylene formation in heat-stressed plants to optimal range by inhibiting activity of 1-aminocyclopropane carboxylic acid (ACC) synthase (ACS). This resulted in improved proline metabolism, N assimilation and photosynthesis. The results suggest that SA interacts with proline metabolism and ethylene formation to alleviate the adverse effects of heat stress on photosynthesis in wheat.

The nitrate transporter NRT2.1 functions in the ethylene response to nitrate deficiency in Arabidopsis

The ethylene signalling pathway is closely associated with complex environmental stresses. Previous studies have reported impact of high nitrate (HN) availability on ethylene biosynthesis and regulation of ethylene on nitrate transporter 2.1 (NRT2.1) expression. However, molecular interaction between NRT2.1 transcript levels and the ethylene signalling pathway under nitrate deficiency is still elusive. Here, we report a low nitrate (LN) treatment-induced rapid burst of ethylene production and regulated expression of ethylene signalling components CTR1, EIN3 and EIL1 in wild-type Arabidopsis thaliana (Col-0) seedlings, and enhanced ethylene response reporter EBS:GUS activity in both Col-0 and the ethylene mutants ein3-1eil1-1 and ctr1-1. LN treatment also caused up-regulation of NRT2.1 expression, which was responsible for an enhanced high-affinity nitrate uptake. Comparison of ethylene production and EBS:GUS activity between nrt1.1, nrt2.1 mutants and Col-0 indicated that this up-regulation of NRT2.1 expression caused a positive effect on ethylene biosynthesis and signalling under LN treatment. On the other hand, ethylene down-regulated NRT2.1 expression and reduced the high-affinity nitrate uptake. Together, these findings uncover a negative feedback loop between NRT2.1 expression and ethylene biosynthesis and signalling under nitrate deficiency, which may contribute to finely tuning of plant nitrate acquisition during exploring dynamic soil conditions.

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

We analyzed how changes in BnNrt nitrate transporter gene expression induced by nitrate are associated with morphological changes in plantlets and osmotic water flow for growth. We hypothesized that in a Petri dish system, reduction in transpiration should induce conditions where nitrate and water fluxes for growth depend directly on nitrate transporter activity and nitrate signaling. Rape seedlings growing on agar plates were supplied with increasing external K (15)NO 3 concentrations from 0.05 to 20 mM. After 5 d of treatment, morphological switches in plantlet growth were observed between 0.5 and 5 mM nitrate supply. Root elongation was reduced by 50% while the cotyledon surface area was doubled. These morphological switches were strongly associated with increases in (15)NO 3(-) and water uptake rates as well as (15)N and water allocation to the shoot. These switches were also highly correlated with the upregulation of BnNrt1.1 and BnNrt2.1 in the root. However, while root expression of BnNrt2.1 was correlated linearly with a shoot growth-associated increase in (15)N and water uptake, BnNrt1.1 expression was correlated exponentially with both (15)N and water accumulation. In low transpiring conditions, the tight control exercised by nitrate transporters on K (15)NO 3 uptake and allocation clearly demonstrates that they modulated the nitrate-signaling cascade involved in cell growth and as a consequence, water uptake and allocation to the growing organs. Deciphering this signaling cascade in relation to acid growth theory seems to be the most important challenge for our understanding of the nitrate-signaling role in plant growth.

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.

Ethylene's role in phosphate starvation signaling: more than just a root growth regulator

Phosphate (Pi) is a common limiter of plant growth due to its low availability in most soils. Plants have evolved elaborate mechanisms for sensing Pi deficiency and for initiating adaptive responses to low Pi conditions. Pi signaling pathways are modulated by both local and long-distance, or systemic, sensing mechanisms. Local sensing of low Pi initiates major root developmental changes aimed at enhancing Pi acquisition, whereas systemic sensing governs pathways that modulate expression of numerous genes encoding factors involved in Pi transport and distribution. The gaseous phytohormone ethylene has been shown to play an integral role in regulating local, root developmental responses to Pi deficiency. Comparatively, a role for ethylene in systemic Pi signaling has been more circumstantial. However, recent studies have revealed that ethylene acts to modulate a number of systemically controlled Pi starvation responses. Herein we highlight the findings from these studies and offer a model for how ethylene biosynthesis and responsiveness are integrated into both local and systemic Pi signaling pathways.

Predicting biological functions of compounds based on chemical-chemical interactions

Given a compound, how can we effectively predict its biological function? It is a fundamentally important problem because the information thus obtained may benefit the understanding of many basic biological processes and provide useful clues for drug design. In this study, based on the information of chemical-chemical interactions, a novel method was developed that can be used to identify which of the following eleven metabolic pathway classes a query compound may be involved with: (1) Carbohydrate Metabolism, (2) Energy Metabolism, (3) Lipid Metabolism, (4) Nucleotide Metabolism, (5) Amino Acid Metabolism, (6) Metabolism of Other Amino Acids, (7) Glycan Biosynthesis and Metabolism, (8) Metabolism of Cofactors and Vitamins, (9) Metabolism of Terpenoids and Polyketides, (10) Biosynthesis of Other Secondary Metabolites, (11) Xenobiotics Biodegradation and Metabolism. It was observed that the overall success rate obtained by the method via the 5-fold cross-validation test on a benchmark dataset consisting of 3,137 compounds was 77.97%, which is much higher than 10.45%, the corresponding success rate obtained by the random guesses. Besides, to deal with the situation that some compounds may be involved with more than one metabolic pathway class, the method presented here is featured by the capacity able to provide a series of potential metabolic pathway classes ranked according to the descending order of their likelihood for each of the query compounds concerned. Furthermore, our method was also applied to predict 5,549 compounds whose metabolic pathway classes are unknown. Interestingly, the results thus obtained are quite consistent with the deductions from the reports by other investigators. It is anticipated that, with the continuous increase of the chemical-chemical interaction data, the current method will be further enhanced in its power and accuracy, so as to become a useful complementary vehicle in annotating uncharacterized compounds for their biological functions.

New insights into the mechanism of development of Arabidopsis root hairs and trichomes

Epidermis cell differentiation in Arabidopsis thaliana is a model system for understanding the mechanisms leading to the developmental end state of plant cells. Both root hairs and trichomes differentiate from epidermal cells and molecular genetic analyses using Arabidopsis mutants have demonstrated that the differentiation of root hairs and trichomes is regulated by similar molecular mechanisms. Molecular-genetic approaches have led to the identification of many genes that are involved in epidermal cell differentiation, most of which encode transcription factors that induce the expression of genes active in both root hair and trichome development. Control of cell growth after fate determination has also been studied using Arabidopsis mutants.

Ethylene is involved in nitrate-dependent root growth and branching in Arabidopsis thaliana

*Here, we investigated the role of ethylene in high nitrate-induced change in root development in Arabidopsis thaliana using wild types and mutants defective in ethylene signaling (etr1, ein2) and nitrate transporters (chl1, nrt2.1). *The length and number of visible lateral roots (LRs) were reduced upon exposure of wild-type seedlings grown on low (0.1 mM) to high nitrate concentration (10 mM). There was a rapid burst of ethylene production upon exposure to high nitrate concentration. *Ethylene synthesis antagonists, cobalt (Co(2+)) and aminoethoxyvinylglycine (AVG), mitigated the inhibitory effect of high nitrate concentration on lateral root growth. The etr1-3 and ein2-1 mutants exhibited less reductions in LR length and number than wild-type plants in response to high nitrate concentration. Expression of nitrate transporters AtNRT1.1 and AtNRT2.1 was upregulated and downregulated in response to high nitrate concentration, respectively. A similar upregulation and downregulation of AtNRT1.1 and AtNRT2.1 was observed by ethylene synthesis precursor aminocyclopropane carboxylic acid (ACC) and AVG in low and high nitrate concentration, respectively. Expression of AtNRT1.1 and AtNRT2.1 became insensitive to high nitrate concentration in etr1-3 and ein2-1 plants. *These findings highlight the regulatory role that ethylene plays in high nitrate concentration-regulated LR development by modulating nitrate transporters.

Transcriptome analysis of nitrogen-efficient rice over-expressing alanine aminotransferase

Crop plants require nitrogen for key macromolecules, such as DNA, proteins and metabolites, yet they are generally inefficient at acquiring nitrogen from the soil. Crop producers compensate for this low nitrogen utilization efficiency by applying nitrogen fertilizers. However, much of this nitrogen is unavailable to the plants as a result of microbial uptake and environmental loss of nitrogen, causing air, water and soil pollution. We engineered rice over-expressing alanine aminotransferase (AlaAT) under the control of a tissue-specific promoter that showed a strong nitrogen use efficiency phenotype. In this study, we examined the transcriptome response in roots and shoots to the over-expression of AlaAT to provide insights into the nitrogen-use-efficient phenotype of these plants. Transgenic and control rice plants were grown hydroponically and the root and shoot gene expression profiles were analysed using Affymetrix Rice GeneChip microarrays. Transcriptome analysis revealed that there was little impact on the transgenic transcriptome compared with controls, with 0.11% and 0.07% differentially regulated genes in roots and shoots, respectively. The most up-regulated transcripts, a glycine-rich cell wall (GRP) gene and a gene encoding a hypothetical protein (Os8823), were expressed in roots. Another transgenic root-specific up-regulated gene was leucine rich repeat (LRR). Genes induced in the transgenic shoots included GRP, LRR, acireductone dioxygenase (OsARD), SNF2 ATP-translocase and a putative leucine zipper transcription factor. This study provides a genome-wide view of the response to AlaAT over-expression, and elucidates some of the genes that may play a role in the nitrogen-use-efficient phenotype.

Ethylene modifies architecture of root system in response to stomatal opening and water allocation changes between root and shoot

Ethylene plays a key role in the elongation of exploratory and root hair systems in plants, as demonstrated by pharmacological modulation of the activity of ethylene biosynthesis enzymes: ACC synthase (ACS) and ACC oxidase (ACO). Thus, treatments with high concentrations (10 microM) of aminoethoxyvinylglycine (AVG, inhibitor of ACS) and 1-aminocyclopropane carboxylic acid (ACC, ethylene precursor, ACO activator) severely decrease the elongation of the exploratory root system but induce opposite effects on the root hair system: root hair length and numbers were increased in seedlings treated with ACC, whereas they were reduced in seedlings treated with AVG. Until now, such elongation changes of root architecture had not been questioned in terms of nitrate uptake. In the march issue of Plant Physiology we report that N uptake and nitrate transporter BnNrt2.1 transcript level were markedly reduced in ACC treated seedlings, but were increased in AVG treated seedlings compared to the control.1 Because recent studies have revealed that ethylene can also modulate stomatal opening as well as root hair cell elongation, we have examined whether pharmacological modulation of ethylene biosynthesis could affect, in an integrated manner, and at a whole-plant level, the exploratory and root hair systems, through changes of stomatal conductance and water allocation between the root and shoot.