The Arabidopsis NRT1.1 transporter participates in the signaling pathway triggering root colonization of nitrate-rich patches

Knockdown of the partner protein OsNAR2.1 for high-affinity nitrate transport represses lateral root formation in a nitrate-dependent manner

The morphological plasticity of root systems is critical for plant survival, and understanding the mechanisms underlying root adaptation to nitrogen (N) fluctuation is critical for sustainable agriculture; however, the molecular mechanism of N-dependent root growth in rice remains unclear. This study aimed to identify the role of the complementary high-affinity NO₃⁻ transport protein OsNAR2.1 in NO₃⁻-regulated rice root growth. Comparisons with wild-type (WT) plants showed that knockdown of OsNAR2.1 inhibited lateral root (LR) formation under low NO₃⁻ concentrations, but not under low NH₄⁺ concentrations. ¹⁵N-labelling NO₃⁻ supplies (provided at concentrations of 0–10 mM) demonstrated that (i) defects in LR formation in mutants subjected to low external NO₃⁻ concentrations resulted from impaired NO₃⁻ uptake, and (ii) the mutants had significantly fewer LRs than the WT plants when root N contents were similar between genotypes. LR formation in osnar2.1 mutants was less sensitive to localised NO₃⁻ supply than LR formation in WT plants, suggesting that OsNAR2.1 may be involved in a NO₃⁻-signalling pathway that controls LR formation. Knockdown of OsNAR2.1 inhibited LR formation by decreasing auxin transport from shoots to roots. Thus, OsNAR2.1 probably functions in both NO₃⁻ uptake and NO₃⁻-signalling.

Nitrogen signaling and use efficiency in plants: what's new?

Crop productivity is heavily dependent on the application of nitrogen (N) fertilizers. Increasing N fertilization levels, however, are subject to diminishing returns, quite apart from their deleterious impact on the environment. Improving N use efficiency (NUE) is therefore crucial for development of sustainable agriculture. Plant NUE is a complex trait determined by quantitative trait loci and influenced by environmental changes. The natural supply of soil N varies and is frequently limiting for plant growth and crop yield: unraveling the molecular basis of how plants sense and respond to changes in N availability should enable the development of new strategies to increase NUE. This review discusses the latest advances in our understanding of N signaling and crosstalk with other signaling pathways.

Genetic approaches to enhancing nitrogen-use efficiency (NUE) in cereals: challenges and future directions

Over 100million tonnes of nitrogen (N) fertiliser are applied globally each year to maintain high yields in agricultural crops. The rising price of N fertilisers has made them a major cost for farmers. Inefficient use of N fertiliser leads to substantial environmental problems through contamination of air and water resources and can be a significant economic cost. Consequently, there is considerable need to improve the way N fertiliser is used in farming systems. The efficiency with which crops use applied N fertiliser - the nitrogen-use efficiency (NUE) - is currently quite low for cereals. This is the case in both high yielding environments and lower yielding environments characteristic of cereal growing regions of Australia. Multiple studies have attempted to identify the genetic basis of NUE, but the utility of the results is limited because of the complex nature of the trait and the magnitude of genotype by environment interaction. Transgenic approaches have been applied to improve plant NUE but with limited success, due, in part, to a combination of the complexity of the trait but also due to lack of accurate phenotyping methods. This review documents these two approaches and suggests future directions in improving cereal NUE with a focus on the Australian cereal industry.

Genetic diversity for root plasticity and nitrogen uptake in wheat seedlings

Enhancing nitrogen use efficiency (NUE) of wheat is a major focus for wheat breeding programs. NUE may be improved by identifying genotypes that are competitive for nitrogen (N) uptake in early vegetative stages of growth and are able to invest that N in grain. Breeders tend to select high yielding genotypes under conditions of medium to high N supply, but it is not known whether this influences the selection of root plasticity traits or whether, over time, breeders have selected genotypes with higher N uptake efficiency. To address this, genotypes were selected from CIMMYT (1966-1985) and Australian (1999-2007) breeding programs. Genotypes from both programs responded to low N supply by expanding their root surface area through increased total root number and/or length of lateral roots. Australian genotypes were N responsive (accumulated more N under high N than under low N) whereas CIMMYT genotypes were not very N responsive. This could not be explained by differences in N uptake capacity as shown by 15N flux analysis of two representative genotypes with contrasting N accumulation. Expression analysis of nitrate transporter genes revealed that the high-affinity transport system accounted for the majority of root nitrate uptake in wheat seedlings under both low and high N conditions.

Transcriptional networks in the nitrate response of Arabidopsis thaliana

Nitrogen is an essential macronutrient for plants and its availability is a key determinant of plant growth and development and crop yield. Besides their nutritional role, N nutrients and metabolites are signals that activate signaling pathways that modulate many plant processes. Because the most abundant inorganic N source for plants in agronomic soils is nitrate, much of the work to understand plant N-signaling has focused on this nutrient. Over the last years, several studies defined a comprehensive catalog of nitrate-responsive genes, involved in nitrate transport, metabolism and a variety of other processes. Despite significant progress in recent years, primarily using Arabidopsis thaliana as a model system, the molecular mechanisms by which nitrate elicits changes in transcript abundance are still not fully understood. Here we highlight recent advancements in identifying key transcription factors and transcriptional mechanisms that orchestrate the gene expression response to changes in nitrate availability in A. thaliana.

A systems biology approach identifies new regulators of poplar root development under low nitrogen

In Populus, low nitrogen (LN) elicits rapid and vigorous lateral root (LR) proliferation, which is closely mirrored by corresponding transcriptomic changes. Using transcriptomic data, we built a genetic network encompassing a large proportion of the differentially regulated transcriptome. The network is organized in a hierarchical fashion, centered on 11 genes. Transgenic manipulations of only three of the 11 genes had a strong impact on root development under LN. These three genes encoded an F-box protein similar to Hawaiian Skirt (PtaHWS) and two transcription factors (PtaRAP2.11 and PtaNAC1). Up- and downregulation of the three genes caused increased and decreased root proliferation under LN conditions, respectively. The transgenic manipulations had a strong positive effect on growth under greenhouse conditions including increased shoot and root biomass. The three genes appeared to encompass a putative yet-unknown mechanism that underlies root development under LN. Specifically, the genes are predominantly expressed in roots and have a similar temporal response to LN. More importantly, transgenic manipulation for each of the three genes had a highly significant impact on the expression of the other two. The transgenic manipulations appear to also affect the expression of the regulatory miRNA (PtamiRNA164e) of one of the transcription factors (PtaNAC1), albeit in an opposite fashion. Consistent with a putative function of PtaHWS in proteasome degradation, treatment with proteasome inhibitor reversed the expression changes in the transgenic plants. The insights from this study will allow genetic modifications of root architecture for more efficient and dynamic nitrogen foraging in biofuel crops like poplar.

High-resolution quantification of root dynamics in split-nutrient rhizoslides reveals rapid and strong proliferation of maize roots in response to local high nitrogen

The plant's root system is highly plastic, and can respond to environmental stimuli such as high nitrogen (N) in patches. A root may respond to an N patch by selective placement of new lateral roots, and therewith increases root N uptake. This may be a desirable trait in breeding programmes, since it decreases NO3(-) leaching and N2O emission. Roots of maize (Zea mays L.) were grown without N in split-nutrient rhizoslides. One side of the slides was exposed to high N after 15 d of root development, and root elongation was measured for another 15 d, described in a time course model and parameterized. The elongation rates of crown axile roots on the N-treated side of the plant followed a logistic increase to a maximum of 5.3cm d(-1); 95% of the maximum were reached within 4 d. At the same time, on the untreated side, axile root elongation dropped linearly to 1.2cm d(-1) within 6.4 d and stayed constant thereafter. Twice as many lateral roots were formed on the crown axis on the N side compared to the untreated side. Most strikingly, the elongation rates of laterals of the N side increased linearly with most of the roots reaching an asymptote ~8 d after start of the N treatment. By contrast, laterals on the side without N did not show any detectable elongation beyond the first day after their emergence. We conclude that split-nutrient rhizoslides have great potential to improve our knowledge about nitrogen responsiveness and selection for contrasting genotypes.

Impact of domestication on the phenotypic architecture of durum wheat under contrasting nitrogen fertilization

The process of domestication has led to dramatic morphological and physiological changes in crop species due to adaptation to cultivation and to the needs of farmers. To investigate the phenotypic architecture of shoot- and root-related traits and quantify the impact of primary and secondary domestication, we examined a collection of 36 wheat genotypes under optimal and nitrogen-starvation conditions. These represented three taxa that correspond to key steps in the recent evolution of tetraploid wheat (i.e. wild emmer, emmer, and durum wheat). Overall, nitrogen starvation reduced the shoot growth of all genotypes, while it induced the opposite effect on root traits, quantified using the automated phenotyping platform GROWSCREEN-Rhizo. We observed an overall increase in all of the shoot and root growth traits from wild emmer to durum wheat, while emmer was generally very similar to wild emmer but intermediate between these two subspecies. While the differences in phenotypic diversity due to the effects of primary domestication were not significant, the secondary domestication transition from emmer to durum wheat was marked by a large and significant decrease in the coefficient of additive genetic variation. In particular, this reduction was very strong under the optimal condition and less intense under nitrogen starvation. Moreover, although under the optimal condition both root and shoot traits showed significantly reduced diversity due to secondary domestication, under nitrogen starvation the reduced diversity was significant only for shoot traits. Overall, a considerable amount of phenotypic variation was observed in wild emmer and emmer, which could be exploited for the development of pre-breeding strategies.

The plasma membrane H(+) -ATPase AHA2 contributes to the root architecture in response to different nitrogen supply

In this study the role of the plasma membrane (PM) H(+) -ATPase for growth and development of roots as response to nitrogen starvation is studied. It is known that root development differs dependent on the availability of different mineral nutrients. It includes processes such as initiation of lateral root primordia, root elongation and increase of the root biomass. However, the signal transduction mechanisms, which enable roots to sense changes in different mineral environments and match their growth and development patterns to actual conditions in the soil, are still unknown. Most recent comments have focused on one of the essential macroelements, namely nitrogen, and its role in the modification of the root architecture of Arabidopsis thaliana. As yet, not all elements of the signal transduction pathway leading to the perception of the nitrate stimulus, and hence to anatomical changes of the root, which allow for adaptation to variable ion concentrations in the soil, are known. Our data demonstrate that primary and lateral root length were shorter and lower in aha2 mutant lines compared with wild-type plants in response to a variable nitrogen source. This suggests that the PM proton pump AHA2 (Arabidopsis plasma membrane H(+) -ATPase isoform 2) is important for root growth and development during different nitrogen regimes. This is possible by controlling the pH homeostasis in the root during growth and development as shown by pH biosensors.

Nitrate sensing and uptake in Arabidopsis are enhanced by ABI2, a phosphatase inactivated by the stress hormone abscisic acid

Living organisms sense and respond to changes in nutrient availability to cope with diverse environmental conditions. Nitrate (NO3-) is the main source of nitrogen for plants and is a major component in fertilizer. Unraveling the molecular basis of nitrate sensing and regulation of nitrate uptake should enable the development of strategies to increase the efficiency of nitrogen use and maximize nitrate uptake by plants, which would aid in reducing nitrate pollution. NPF6.3 (also known as NRT1.1), which functions as a nitrate sensor and transporter; the kinase CIPK23; and the calcium sensor CBL9 form a complex that is crucial for nitrate sensing in Arabidopsis thaliana. We identified two additional components that regulate nitrate transport, sensing, and signaling: the calcium sensor CBL1 and protein phosphatase 2C family member ABI2, which is inhibited by the stress-response hormone abscisic acid. Bimolecular fluorescence complementation assays and in vitro kinase assays revealed that ABI2 interacted with and dephosphorylated CIPK23 and CBL1. Coexpression studies in Xenopus oocytes and analysis of plants deficient in ABI2 indicated that ABI2 enhanced NPF6.3-dependent nitrate transport, nitrate sensing, and nitrate signaling. These findings suggest that ABI2 may functionally link stress-regulated control of growth and nitrate uptake and utilization, which are energy-expensive processes.

Nitric oxide generated by nitrate reductase increases nitrogen uptake capacity by inducing lateral root formation and inorganic nitrogen uptake under partial nitrate nutrition in rice

Increasing evidence shows that partial nitrate nutrition (PNN) can be attributed to improved plant growth and nitrogen-use efficiency (NUE) in rice. Nitric oxide (NO) is a signalling molecule involved in many physiological processes during plant development and nitrogen (N) assimilation. It remains unclear whether molecular NO improves NUE through PNN. Two rice cultivars (cvs Nanguang and Elio), with high and low NUE, respectively, were used in the analysis of NO production, nitrate reductase (NR) activity, lateral root (LR) density, and (15)N uptake under PNN, with or without NO production donor and inhibitors. PNN increased NO accumulation in cv. Nanguang possibly through the NIA2-dependent NR pathway. PNN-mediated NO increases contributed to LR initiation, (15)NH₄(+)/(15)NO₃(-) influx into the root, and levels of ammonium and nitrate transporters in cv. Nanguang but not cv. Elio. Further results revealed marked and specific induction of LR initiation and (15)NH₄(+)/(15)NO₃(-) influx into the roots of plants supplied with NH₄(+)+sodium nitroprusside (SNP) relative to those supplied with NH₄(+) alone, and considerable inhibition upon the application of cPTIO or tungstate (NR inhibitor) in addition to PNN, which is in agreement with the change in NO fluorescence in the two rice cultivars. The findings suggest that NO generated by the NR pathway plays a pivotal role in improving the N acquisition capacity by increasing LR initiation and the inorganic N uptake rate, which may represent a strategy for rice plants to adapt to a fluctuating nitrate supply and increase NUE.

Disruption of the rice nitrate transporter OsNPF2.2 hinders root-to-shoot nitrate transport and vascular development

Plants have evolved to express some members of the nitrate transporter 1/peptide transporter family (NPF) to uptake and transport nitrate. However, little is known of the physiological and functional roles of this family in rice (Oryza sativa L.). Here, we characterized the vascular specific transporter OsNPF2.2. Functional analysis using cDNA-injected Xenopus laevis oocytes revealed that OsNPF2.2 is a low-affinity, pH-dependent nitrate transporter. Use of a green fluorescent protein tagged OsNPF2.2 showed that the transporter is located in the plasma membrane in the rice protoplast. Expression analysis showed that OsNPF2.2 is nitrate inducible and is mainly expressed in parenchyma cells around the xylem. Disruption of OsNPF2.2 increased nitrate concentration in the shoot xylem exudate when nitrate was supplied after a deprivation period; this result suggests that OsNPF2.2 may participate in unloading nitrate from the xylem. Under steady-state nitrate supply, the osnpf2.2 mutants maintained high levels of nitrate in the roots and low shoot:root nitrate ratios; this observation suggests that OsNPF2.2 is involved in root-to-shoot nitrate transport. Mutation of OsNPF2.2 also caused abnormal vasculature and retarded plant growth and development. Our findings demonstrate that OsNPF2.2 can unload nitrate from the xylem to affect the root-to-shoot nitrate transport and plant development.

Reduced frequency of lateral root branching improves N capture from low-N soils in maize

Suboptimal nitrogen (N) availability is a primary constraint for crop production in developing countries, while in developed countries, intensive N fertilization is a primary economic, energy, and environmental cost for crop production. We tested the hypothesis that under low-N conditions, maize (Zea mays) lines with few but long (FL) lateral roots would have greater axial root elongation, deeper rooting, and greater N acquisition than lines with many but short (MS) lateral roots. Maize recombinant inbred lines contrasting in lateral root number and length were grown with adequate and suboptimal N in greenhouse mesocosms and in the field in the USA and South Africa (SA). In low-N mesocosms, the FL phenotype had substantially reduced root respiration and greater rooting depth than the MS phenotype. In low-N fields in the USA and SA, the FL phenotype had greater rooting depth, shoot N content, leaf photosynthesis, and shoot biomass than the MS phenotype. The FL phenotype yielded 31.5% more than the MS phenotype under low N in the USA. Our results are consistent with the hypothesis that sparse but long lateral roots improve N capture from low-N soils. These results with maize probably pertain to other species. The FL lateral root phenotype merits consideration as a selection target for greater crop N efficiency.

Alanine aminotransferase variants conferring diverse NUE phenotypes in Arabidopsis thaliana

Alanine aminotransferase (AlaAT, E.C. 2.6.1.2), is a pyridoxal-5'-phosphate-dependent (PLP) enzyme that catalyzes the reversible transfer of an amino group from alanine to 2-oxoglutarate to produce glutamate and pyruvate, or vice versa. It has been well documented in both greenhouse and field studies that tissue-specific over-expression of AlaAT from barley (Hordeum vulgare, HvAlaAT) results in a significant increase in plant NUE in both canola and rice. While the physical phenotypes associated with over-expression of HvAlaAT have been well characterized, the role this enzyme plays in vivo to create a more N efficient plant remains unknown. Furthermore, the importance of HvAlaAT, in contrast to other AlaAT enzyme homologues in creating this phenotype has not yet been explored. To address the role of AlaAT in NUE, AlaAT variants from diverse sources and different subcellular locations, were expressed in the wild-type Arabidopsis thaliana Col-0 background and alaat1;2 (alaat1-1;alaat2-1) knockout background in various N environments. The analysis and comparison of both the physical and physiological properties of AlaAT over-expressing transgenic plants demonstrated significant differences between plants expressing the different AlaAT enzymes under different external conditions. This analysis indicates that the over-expression of AlaAT variants other than HvAlaAT in crop plants could further increase the NUE phenotype(s) previously observed.

Nutritional regulation of root development

Mineral nutrients such as nitrogen (N), phosphorus (P), and iron (Fe) are essential for plant growth, development, and reproduction. Adequate provision of nutrients via the root system impacts greatly on shoot biomass and plant productivity and is therefore of crucial importance for agriculture. Nutrients are taken up at the root surface in ionic form, which is mediated by specific transport proteins. Noteworthy, root tips are able to sense the local and internal concentrations of nutrients to adjust growth and developmental processes, and ultimately, to increase or decrease the exploratory capacity of the root system. Recently, important progress has been achieved in identifying the mechanisms of nutrient sensing in wild- and cultivated species, including Arabidopsis, bean, maize, rice, lupin as well as in members of the Proteaceae and Cyperaceae families, which develop highly sophisticated root clusters as adaptations to survive in soils with very low fertility. Major findings include identification of transporter proteins and transcription factors regulating nutrient sensing, miRNAs as mobile signals and peptides as repressors of lateral root development under heterogeneous nutrient supply. Understanding the roles played by N, P, and Fe in gene expression and biochemical characterization of proteins involved in root developmental responses to homogeneous or heterogeneous N and P sources has gained additional interest due to its potential for improving fertilizer acquisition efficiency in crops.

Multiple mechanisms of nitrate sensing by Arabidopsis nitrate transceptor NRT1.1

In Arabidopsis the plasma membrane nitrate transceptor (transporter/receptor) NRT1.1 governs many physiological and developmental responses to nitrate. Alongside facilitating nitrate uptake, NRT1.1 regulates the expression levels of many nitrate assimilation pathway genes, modulates root system architecture, relieves seed dormancy and protects plants from ammonium toxicity. Here, we assess the functional and phenotypic consequences of point mutations in two key residues of NRT1.1 (P492 and T101). We show that the point mutations differentially affect several of the NRT1.1-dependent responses to nitrate, namely the repression of lateral root development at low nitrate concentrations, and the short-term upregulation of the nitrate-uptake gene NRT2.1, and its longer-term downregulation, at high nitrate concentrations. We also show that these mutations have differential effects on genome-wide gene expression. Our findings indicate that NRT1.1 activates four separate signalling mechanisms, which have independent structural bases in the protein. In particular, we present evidence to suggest that the phosphorylated and non-phosphorylated forms of NRT1.1 at T101 have distinct signalling functions, and that the nitrate-dependent regulation of root development depends on the phosphorylated form. Our findings add to the evidence that NRT1.1 is able to trigger independent signalling pathways in Arabidopsis in response to different environmental conditions.

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.

The nitrate transporter MtNPF6.8 (MtNRT1.3) transports abscisic acid and mediates nitrate regulation of primary root growth in Medicago truncatula

Elongation of the primary root during postgermination of Medicago truncatula seedlings is a multigenic trait that is responsive to exogenous nitrate. A quantitative genetic approach suggested the involvement of the nitrate transporter MtNPF6.8 (for Medicago truncatula NITRATE TRANSPORTER1/PEPTIDE TRANSPORTER Family6.8) in the inhibition of primary root elongation by high exogenous nitrate. In this study, the inhibitory effect of nitrate on primary root elongation, via inhibition of elongation of root cortical cells, was abolished in npf6.8 knockdown lines. Accordingly, we propose that MtNPF6.8 mediates nitrate inhibitory effects on primary root growth in M. truncatula. pMtNPF6.8:GUS promoter-reporter gene fusion in Agrobacterium rhizogenes-generated transgenic roots showed the expression of MtNPF6.8 in the pericycle region of primary roots and lateral roots, and in lateral root primordia and tips. MtNPF6.8 expression was insensitive to auxin and was stimulated by abscisic acid (ABA), which restored the inhibitory effect of nitrate in npf6.8 knockdown lines. It is then proposed that ABA acts downstream of MtNPF6.8 in this nitrate signaling pathway. Furthermore, MtNPF6.8 was shown to transport ABA in Xenopus spp. oocytes, suggesting an additional role of MtNPF6.8 in ABA root-to-shoot translocation. (15)NO3(-)-influx experiments showed that only the inducible component of the low-affinity transport system was affected in npf6.8 knockdown lines. This indicates that MtNPF6.8 is a major contributor to the inducible component of the low-affinity transport system. The short-term induction by nitrate of the expression of Nitrate Reductase1 (NR1) and NR2 (genes that encode two nitrate reductase isoforms) was greatly reduced in the npf6.8 knockdown lines, supporting a role of MtNPF6.8 in the primary nitrate response in M. truncatula.

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.

Nitrate foraging by Arabidopsis roots is mediated by the transcription factor TCP20 through the systemic signaling pathway

To compete for nutrients in diverse soil microenvironments, plants proliferate lateral roots preferentially in nutrient-rich zones. For nitrate, root foraging involves local and systemic signaling; however, little is known about the genes that function in the systemic signaling pathway. By using nitrate enhancer DNA to screen a library of Arabidopsis transcription factors in the yeast one-hybrid system, the transcription factor gene TEOSINTE BRANCHED1/CYCLOIDEA/PROLIFERATING CELL FACTOR1-20 (TCP20) was identified. TCP20, which belongs to an ancient, plant-specific gene family that regulates shoot, flower, and embryo development, was implicated in nitrate signaling by its ability to bind DNA in more than 100 nitrate-regulated genes. Analysis of insertion mutants of TCP20 showed that they had normal primary and lateral root growth on homogenous nitrate media but were impaired in preferential lateral root growth (root foraging) on heterogeneous media in split-root plates. Inhibition of preferential lateral root growth was still evident in the mutants even when ammonium was uniformly present in the media, indicating that the TCP20 response was to nitrate. Comparison of tcp20 mutants with those of nlp7 mutants, which are defective in local control of root growth but not in the root-foraging response, indicated that TCP20 function is independent of and distinct from NLP7 function. Further analysis showed that tcp20 mutants lack systemic control of root growth regardless of the local nitrate concentrations. These results indicate that TCP20 plays a key role in the systemic signaling pathway that directs nitrate foraging by Arabidopsis roots.

The primary nitrate response: a multifaceted signalling pathway

Nitrate (NO3(-)) application strongly affects gene expression in plants. This regulation is thought to be crucial for their adaptation in response to a changing nutritional environment. Depending on the conditions preceding or concomitant with nitrate provision, the treatment can affect up to a 10th of genome expression in Arabidopsis thaliana. The early events occurring after NO3(-) provision are often called the Primary Nitrate Response (PNR). Despite this simple definition, PNR is a complex process that is difficult to properly delineate. Here we report the different concepts related to PNR, review the different molecular components known to control it, and show, using meta-analysis, that this concept/pathway is not monolithic. We especially bring our attention to the genome-wide effects of LBD37 and LBD38 overexpression, NLP7, and CHL1/NRT1.1 mutations.

Root nutrient foraging

During a plant's lifecycle, the availability of nutrients in the soil is mostly heterogeneous in space and time. Plants are able to adapt to nutrient shortage or localized nutrient availability by altering their root system architecture to efficiently explore soil zones containing the limited nutrient. It has been shown that the deficiency of different nutrients induces root architectural and morphological changes that are, at least to some extent, nutrient specific. Here, we highlight what is known about the importance of individual root system components for nutrient acquisition and how developmental and physiological responses can be coupled to increase nutrient foraging by roots. In addition, we review prominent molecular mechanisms involved in altering the root system in response to local nutrient availability or to the plant's nutritional status.

Finding a nitrogen niche: a systems integration of local and systemic nitrogen signalling in plants

The ability of plants to sense their nitrogen (N) microenvironment in the soil and deploy strategic root growth in N-rich patches requires exquisite systems integration. Remarkably, this new paradigm for systems biology research has intrigued plant biologists for more than a century, when a split-root framework was first used to study how plants sense and respond to heterogeneous soil nutrient environments. This systemic N-signalling mechanism, allowing plants to sense and forage for mineral nutrients in resource-rich patches, has important implications for agriculture. In this review, we will focus on how advances in the post-genomic era have uncovered the gene regulatory networks underlying systemic N-signalling. After defining how local and systemic N-signalling can be experimentally distinguished for molecular study using a split-root system, the genetic factors that have been shown to mediate local and/or systemic N-signalling are reviewed. Second, the genetic mechanism of this regulatory system is broadened to the whole genome level. To do this, publicly available N-related transcriptomic datasets are compared with genes that have previously been identified as local and systemic N responders in a split-root transcriptome dataset. Specifically, (i) it was found that transcriptional reprogramming triggered by homogeneous N-treatments is composed of both local and systemic responses, (ii) the spatio-temporal signature of local versus systemic responsive genes is defined, and (iii) the conservation of systemic N-signalling between Arabidopsis and Medicago is assessed. Finally, the potential mediators, i.e. metabolites and phytohormones, of the N-related long-distance signals, are discussed.

Signal interactions in the regulation of root nitrate uptake

In most aerobic soils, nitrate (NO3(-)) is the main nitrogen source for plants and is often limiting for plant growth and development. To adapt to a changing environment, plants have developed complex regulatory mechanisms that involve short and long-range signalling pathways in response to both NO3(-) availability in the soil and other physiological processes like growth or nitrogen (N) and carbon (C) metabolisms. Over the past decade, transcriptomic approaches largely contributed to the identification of molecular elements involved in these regulatory mechanisms, especially at the level of root NO3(-)uptake. Most strikingly, the data obtained revealed the high level of interaction between N and both hormone and C signalling pathways, suggesting a strong dependence on growth, development, and C metabolism to adapt root NO3(-) uptake to both external NO3(-) availability and the N status of the plant. However, the signalling mechanisms involved in the cross-talk between N, C, and hormones for the regulation of root NO3(-) uptake remain largely obscure. The aim of this review is to discuss the recent advances concerning the regulatory pathways controlling NO3(-) uptake in response to N signalling, hormones, and C in the model plant Arabidopsis thaliana. Then, to further characterize the level of interaction between these signalling pathways we built on publicly available transcriptome data to determine how hormones and C treatments modify the gene network connecting root NO3(-) transporters and their regulators.

Phenotypic plasticity of the maize root system in response to heterogeneous nitrogen availability

Mineral nutrients are distributed in a non-uniform manner in the soil. Plasticity in root responses to the availability of mineral nutrients is believed to be important for optimizing nutrient acquisition. The response of root architecture to heterogeneous nutrient availability has been documented in various plant species, and the molecular mechanisms coordinating these responses have been investigated particularly in Arabidopsis, a model dicotyledonous plant. Recently, progress has been made in describing the phenotypic plasticity of root architecture in maize, a monocotyledonous crop. This article reviews aspects of phenotypic plasticity of maize root system architecture, with special emphasis on describing (1) the development of its complex root system; (2) phenotypic responses in root system architecture to heterogeneous N availability; (3) the importance of phenotypic plasticity for N acquisition; (4) different regulation of root growth and nutrients uptake by shoot; and (5) root traits in maize breeding. This knowledge will inform breeding strategies for root traits enabling more efficient acquisition of soil resources and synchronizing crop growth demand, root resource acquisition and fertilizer application during crop growing season, thereby maximizing crop yields and nutrient-use efficiency and minimizing environmental pollution.

Nitrogen signalling pathways shaping root system architecture: an update

Root system architecture is a fundamentally important trait for resource acquisition in both ecological and agronomic contexts. Because of the plasticity of root development and the almost infinite complexity of the soil, root system architecture is shaped by environmental factors to a much greater degree than shoot architecture. In attempting to understand how roots sense and respond to environmental cues, the striking effects of nitrate and other forms of nitrogen on root growth and branching have received particular attention. This minireview focuses on the latest advances in our understanding of the diverse nitrogen signalling pathways that are now known to act at multiple stages in the process of lateral root development, as well as on primary root growth.

Inhibition of nitrate transporter 1.1-controlled nitrate uptake reduces cadmium uptake in Arabidopsis

Identification of mechanisms that decrease cadmium accumulation in plants is a prerequisite for minimizing dietary uptake of cadmium from contaminated crops. Here, we show that cadmium inhibits nitrate transporter 1.1 (NRT1.1)-mediated nitrate (NO3 (-)) uptake in Arabidopsis (Arabidopsis thaliana) and impairs NO3 (-) homeostasis in roots. In NO3 (-)-containing medium, loss of NRT1.1 function in nrt1.1 mutants leads to decreased levels of cadmium and several other metals in both roots and shoots and results in better biomass production in the presence of cadmium, whereas in NO3 (-)-free medium, no difference is seen between nrt1.1 mutants and wild-type plants. These results suggest that inhibition of NRT1.1 activity reduces cadmium uptake, thus enhancing cadmium tolerance in an NO3 (-) uptake-dependent manner. Furthermore, using a treatment rotation system allowing synchronous uptake of NO3 (-) and nutrient cations and asynchronous uptake of cadmium, the nrt1.1 mutants had similar cadmium levels to wild-type plants but lower levels of nutrient metals, whereas the opposite effect was seen using treatment rotation allowing synchronous uptake of NO3 (-) and cadmium and asynchronous uptake of nutrient cations. We conclude that, although inhibition of NRT1.1-mediated NO3 (-) uptake by cadmium might have negative effects on nitrogen nutrition in plants, it has a positive effect on cadmium detoxification by reducing cadmium entry into roots. NRT1.1 may regulate the uptake of cadmium and other cations by a common mechanism.

Analysis of nitrate reductase mRNA expression and nitrate reductase activity in response to nitrogen supply

Nitrate is one of the major sources of nitrogen for the growth of plants. It is taken up by plant roots and transported to the leaves where it is reduced to nitrite in the. The main objective of this research was to investigate stimulatory effects of sodium nitrate, potassium nitrate, ammonia and urea on the production/generation of the nitrate reductase mRNA in Triticum aestivum plants. The plants were grown in standard nutrient solution for 21 days and then starved in a media without nitrate for seven days. Starved plants were stimulated with various concentrations of sodium nitrate, potassium nitrate, ammonia and urea, and the expression of nitrate reductase mRNA was analyzed by real-time PCR. Our results indicated that starvation caused significant decrease in the production of nitrate reductase mRNA in the plant leaf. Sodium and potassium nitrate were capable of restoring the production of nitrate mRNA in a dose-dependent manner, since 50 mM of each produced the highest level of the mRNA. The stimulatory effect of potassium nitrate was higher than sodium nitrate, while ammonia and urea did not show such activity. At low concentrations, sodium nitrate and potassium nitrate caused significant increase in the nitrate/nitrite mRNA production, whereas high concentrations of these salts suppressed the expression of this gene considerably.

Unexpectedly low nitrogen acquisition and absence of root architecture adaptation to nitrate supply in a Medicago truncatula highly branched root mutant

To complement N2 fixation through symbiosis, legumes can efficiently acquire soil mineral N through adapted root architecture. However, root architecture adaptation to mineral N availability has been little studied in legumes. Therefore, this study investigated the effect of nitrate availability on root architecture in Medicago truncatula and assessed the N-uptake potential of a new highly branched root mutant, TR185. The effects of varying nitrate supply on both root architecture and N uptake were characterized in the mutant and in the wild type. Surprisingly, the root architecture of the mutant was not modified by variation in nitrate supply. Moreover, despite its highly branched root architecture, TR185 had a permanently N-starved phenotype. A transcriptome analysis was performed to identify genes differentially expressed between the two genotypes. This analysis revealed differential responses related to the nitrate acquisition pathway and confirmed that N starvation occurred in TR185. Changes in amino acid content and expression of genes involved in the phenylpropanoid pathway were associated with differences in root architecture between the mutant and the wild type.

Beyond cellular detoxification: a plethora of physiological roles for MDR transporter homologs in plants

Higher plants possess a multitude of Multiple Drug Resistance (MDR) transporter homologs that group into three distinct and ubiquitous families—the ATP-Binding Cassette (ABC) superfamily, the Major Facilitator Superfamily (MFS), and the Multidrug And Toxic compound Extrusion (MATE) family. As in other organisms, such as fungi, mammals, and bacteria, MDR transporters make a primary contribution to cellular detoxification processes in plants, mainly through the extrusion of toxic compounds from the cell or their sequestration in the central vacuole. This review aims at summarizing the currently available information on the in vivo roles of MDR transporters in plant systems. Taken together, these data clearly indicate that the biological functions of ABC, MFS, and MATE carriers are not restricted to xenobiotic and metal detoxification. Importantly, the activity of plant MDR transporters also mediates biotic stress resistance and is instrumental in numerous physiological processes essential for optimal plant growth and development, including the regulation of ion homeostasis and polar transport of the phytohormone auxin.

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.

Auxin biosynthetic gene TAR2 is involved in low nitrogen-mediated reprogramming of root architecture in Arabidopsis

In plants, the plasticity of root architecture in response to nitrogen availability largely determines nitrogen acquisition efficiency. One poorly understood root growth response to low nitrogen availability is an observed increase in the number and length of lateral roots (LRs). Here, we show that low nitrogen-induced Arabidopsis LR growth depends on the function of the auxin biosynthesis gene TAR2 (tryptophan aminotransferase related 2). TAR2 was expressed in the pericycle and the vasculature of the mature root zone near the root tip, and was induced under low nitrogen conditions. In wild type plants, low nitrogen stimulated auxin accumulation in the non-emerged LR primordia with more than three cell layers and LR emergence. Conversely, these low nitrogen-mediated auxin accumulation and root growth responses were impaired in the tar2-c null mutant. Overexpression of TAR2 increased LR numbers under both high and low nitrogen conditions. Our results suggested that TAR2 is required for reprogramming root architecture in response to low nitrogen conditions. This finding suggests a new strategy for improving nitrogen use efficiency through the engineering of TAR2 expression in roots.

Analysis of the Root System Architecture of Arabidopsis Provides a Quantitative Readout of Crosstalk between Nutritional Signals

As plant roots forage the soil for food and water, they translate a multifactorial input of environmental stimuli into a multifactorial developmental output that manifests itself as root system architecture (RSA). Our current understanding of the underlying regulatory network is limited because root responses have traditionally been studied separately for individual nutrient deficiencies. In this study, we quantified 13 RSA parameters of Arabidopsis thaliana in 32 binary combinations of N, P, K, S, and light. Analysis of variance showed that each RSA parameter was determined by a typical pattern of environmental signals and their interactions. P caused the most important single-nutrient effects, while N-effects were strongly light dependent. Effects of K and S occurred mostly through nutrient interactions in paired or multiple combinations. Several RSA parameters were selected for further analysis through mutant phenotyping, which revealed combinations of transporters, receptors, and kinases acting as signaling modules in K-N interactions. Furthermore, nutrient response profiles of individual RSA features across NPK combinations could be assigned to transcriptionally coregulated clusters of nutrient-responsive genes in the roots and to ionome patterns in the shoots. The obtained data set provides a quantitative basis for understanding how plants integrate multiple nutritional stimuli into complex developmental programs.

NO signaling is a key component of the root growth response to nitrate in Zea mays L

Roots are considered to be a vital organ system of plants due to their involvement in water and nutrient uptake, anchorage, propagation, storage functions, secondary metabolite (including hormones) biosynthesis, and accumulation. Crops are strongly dependent on the availability of nitrogen in soil and on the efficiency of nitrogen utilization for biomass production and yield. However, knowledge about molecular responses to nitrogen fluctuations mainly derives from the study of model species. Nitric oxide (NO) has been proposed to be implicated in plant adaptation to environment, but its exact role in the response of plants to nutritional stress is still under evaluation. Recently a novel role for NO production and scavenging, thanks to the coordinate spatio-temporal expression of nitrate reductase and non-symbiotic hemoglobins, in the maize root response to nitrate has been postulated. This control of NO homeostasis is preferentially accomplished by the cells of the root transition zone (TZ) which seem to represent the most nitrate responsive portion of maize root. The TZ is already known to function as a sensory center able to gather information from the external environment and to re-elaborate them in an adequate response. These results indicate that it could play a central role also for nitrate sensing by roots. A lot of work is still needed to identify and characterize other upstream and downstream signals involved in the "nitrate-NO" pathway, leading to root architecture adjustments and finally to stress adaptation.

It's time to make changes: modulation of root system architecture by nutrient signals

Root growth and development are of outstanding importance for the plant's ability to acquire water and nutrients from different soil horizons. To cope with fluctuating nutrient availabilities, plants integrate systemic signals pertaining to their nutritional status into developmental pathways that regulate the spatial arrangement of roots. Changes in the plant nutritional status and external nutrient supply modulate root system architecture (RSA) over time and determine the degree of root plasticity which is based on variations in the number, extension, placement, and growth direction of individual components of the root system. Roots also sense the local availability of some nutrients, thereby leading to nutrient-specific modifications in RSA, that result from the integration of systemic and local signals into the root developmental programme at specific steps. An in silico analysis of nutrient-responsive genes involved in root development showed that the majority of these specifically responded to the deficiency of individual nutrients while a minority responded to more than one nutrient deficiency. Such an analysis provides an interesting starting point for the identification of the molecular players underlying the sensing and transduction of the nutrient signals that mediate changes in the development and architecture of root systems.

CLE-CLAVATA1 peptide-receptor signaling module regulates the expansion of plant root systems in a nitrogen-dependent manner

Morphological plasticity of root systems is critically important for plant survival because it allows plants to optimize their capacity to take up water and nutrients from the soil environment. Here we show that a signaling module composed of nitrogen (N)-responsive CLE (CLAVATA3/ESR-related) peptides and the CLAVATA1 (CLV1) leucine-rich repeat receptor-like kinase is expressed in the root vasculature in Arabidopsis thaliana and plays a crucial role in regulating the expansion of the root system under N-deficient conditions. CLE1, -3, -4, and -7 were induced by N deficiency in roots, predominantly expressed in root pericycle cells, and their overexpression repressed the growth of lateral root primordia and their emergence from the primary root. In contrast, clv1 mutants showed progressive outgrowth of lateral root primordia into lateral roots under N-deficient conditions. The clv1 phenotype was reverted by introducing a CLV1 promoter-driven CLV1:GFP construct producing CLV1:GFP fusion proteins in phloem companion cells of roots. The overaccumulation of CLE2, -3, -4, and -7 in clv1 mutants suggested the amplitude of the CLE peptide signals being feedback-regulated by CLV1. When CLE3 was overexpressed under its own promoter in wild-type plants, the length of lateral roots was negatively correlated with increasing CLE3 mRNA levels; however, this inhibitory action of CLE3 was abrogated in the clv1 mutant background. Our findings identify the N-responsive CLE-CLV1 signaling module as an essential mechanism restrictively controlling the expansion of the lateral root system in N-deficient environments.

Two young MicroRNAs originating from target duplication mediate nitrogen starvation adaptation via regulation of glucosinolate synthesis in Arabidopsis thaliana

Nitrogen is an essential macronutrient required for plant growth and development. A number of genes respond to nitrogen starvation conditions. However, the functions of most of these nitrogen starvation-responsive genes are unclear. Our recent survey suggested that many microRNAs (miRNAs) are responsive to nitrogen starvation in Arabidopsis thaliana. Here, we identified a new miRNA (miR5090) from the complementary transcript of the MIR826 gene. Further investigation uncovered that both miRNA genes recently evolved from the inverse duplication of their common target gene, ALKENYL HYDROXALKYL PRODUCING2 (AOP2). Similar to miR826, miR5090 is induced by nitrogen starvation. By contrast, the AOP2 transcript level was negatively correlated with miR826 and miR5090 under nitrogen starvation. GUS-fused AOP2 expression suggested that AOP2 was posttranscriptionally suppressed by miR826 and miR5090. miRNA transgenic plants with significantly low AOP2 expression accumulated fewer Met-derived glucosinolates, phenocopying the aop2 mutants. Most glucosinolate synthesis-associated genes were repressed under nitrogen starvation conditions. Furthermore, miRNA transgenic plants with less glucosinolate displayed enhanced tolerance to nitrogen starvation, including high biomass, more lateral roots, increased chlorophyll, and decreased anthocyanin. Meanwhile, nitrogen starvation-responsive genes were up-regulated in transgenic plants, implying improved nitrogen uptake activity. Our study reveals a mechanism by which Arabidopsis thaliana regulates the synthesis of glucosinolates to adapt to environmental changes in nitrogen availability.

Systemic regulation of sulfur homeostasis in Medicago truncatula

Sulfur (S) is an essential macronutrient for plants, and deficiency in soil S availability limits plant growth. Adaptive strategies have been evolved by plants to respond to S deficiency by coordinating systemic regulatory mechanism. A split-root experiment using legume model plant Medicago truncatula Gaertn. was conducted to investigate the systemic response to S deficiency. Plant growth, root morphology and S contents under varying conditions of S supply were determined, and the expression of genes encoding sulfate transporter (MtSULTRs) and MtAPR1 encoding an enzyme involved in S assimilation was monitored. Our results demonstrated that there was an apparent systemic response of M. truncatula to heterogeneous S supply in terms of root length, S contents, and S uptake and assimilation at the transcriptional level. When exposed to heterogeneous S supply, M. truncatula plants showed proliferation of lateral roots in S-rich medium and reduction in investment to S-depleted roots. Growth was stimulated with half-part of roots exposed to S-deficient medium. There were different expression patterns of MtSULTRs and MtAPR1 in response to heterogeneous S supply both in roots and shoots of M. truncatula. Expression of MtSULTR1.1 and MtSULTR1.3 was systemically responsive to S deficiency, leading to an enhancement of S uptake in roots exposed to S-sufficient medium. In addition, the response of S-deprived seedlings to re-supply of sulfate and Cys was also analyzed. It was shown that sulfate, but not Cys, may serve as a systemic signal to regulate the expression of genes associated with S absorption and assimilation in M. truncatula. These findings provide a comprehensive picture of systemic responses to S deficiency in leguminous species.

Auxin-mediated nitrate signalling by NRT1.1 participates in the adaptive response of Arabidopsis root architecture to the spatial heterogeneity of nitrate availability

To optimize their nitrogen nutrition, plants are able to direct root growth in nitrate-rich patches. This depends in Arabidopsis on the NRT1.1 nitrate transporter/sensor. NRT1.1 was shown to display on homogenous medium, an auxin transport activity that lowers auxin accumulation in lateral roots and inhibits their growth at low nitrate. Using a split-root system, we explored the hypothesis that preferential lateral root growth in the nitrate-rich side involves the NRT1.1-dependent repression of lateral root growth in the low nitrate side. Data show that NRT1.1 acts locally to modulate both auxin levels and meristematic activity in response to the low nitrate concentration directly experienced by lateral roots leading to a repression of their growth. A stimulatory role of NRT1.1 in the high nitrate side, which does not rely on changes in auxin levels, is also observed. Altogether, our data suggest that NRT1.1 allows preferential root colonization of nitrate-rich patches by both preventing root growth in response to low nitrate, through modulation of auxin traffic, and stimulating root growth in response to high nitrate, through a yet uncharacterized mechanism. In addition, transcriptional regulation of NRT1.1 affects both mechanisms allowing plants to modulate the effect of nitrate on root branching.

CLE peptides regulate lateral root development in response to nitrogen nutritional status of plants

CLE (CLAVATA3/embryo surrounding region (ESR)) peptides control meristem functions in plants. Our recent study highlights the critical role of a peptide-receptor signaling module composed of nitrogen (N)-responsive CLE peptides and the CLAVATA1 (CLV1) leucine-rich repeat receptor-like kinase in controlling lateral root development in Arabidopsis thaliana. CLE1, -3, -4 and -7 are expressed in root pericycle cells in Arabidopsis roots under N-limited growth conditions. Overexpression of these CLE genes inhibits lateral root emergence from the primary root. The inhibitory action of N-responsive CLE peptides on lateral root development requires the function of CLV1 expressed in phloem companion cells in roots, suggesting that downstream signals are transferred through phloem for systemic regulation of root system architecture. An additional mechanism downstream of CLV1 feedback-regulates transcript levels of N-responsive CLE genes in roots for fine-tuning the signal amplitude.

Nitrate regulation of AFB3 and NAC4 gene expression in Arabidopsis roots depends on NRT1.1 nitrate transport function

Nitrogen is an essential macronutrient for plants and its availability is a major limiting factor for plant growth and crop production. Nitrate is the main source of inorganic N for plants in aerobic soils and can act as a potent signal to control global gene expression. We found that gene expression in response to nitrate treatment of the AFB3 auxin receptor and its target, the NAC4 transcription factor depends on the nitrate transport function of NRT1.1. This gene regulatory function of NRT1.1 on AFB3 and NAC4 differs from the previously described signaling function controlling NRT2.1, NIA1 and NIA2 transcript levels and root colonization of nitrate-rich patches. Our work suggests two different signaling pathways may exist to control gene expression in response to nitrate downstream of NRT1.1.

Aberrant gene expression in the Arabidopsis SULTR1;2 mutants suggests a possible regulatory role for this sulfate transporter in response to sulfur nutrient status

Sulfur is required for the biosynthesis of cysteine, methionine and numerous other metabolites, and thus is critical for cellular metabolism and various growth and developmental processes. Plants are able to sense their physiological state with respect to sulfur availability, but the sensor remains to be identified. Here we report the isolation and characterization of two novel allelic mutants of Arabidopsis thaliana, sel1-15 and sel1-16, which show increased expression of a sulfur deficiency-activated gene β-glucosidase 28 (BGLU28). The mutants, which represent two different missense alleles of SULTR1;2, which encodes a high-affinity sulfate transporter, are defective in sulfate transport and as a result have a lower cellular sulfate level. However, when treated with a very high dose of sulfate, sel1-15 and sel1-16 accumulated similar amounts of internal sulfate and its metabolite glutathione (GSH) to wild-type, but showed higher expression of BGLU28 and other sulfur deficiency-activated genes than wild-type. Reduced sensitivity to inhibition of gene expression was also observed in the sel1 mutants when fed with the sulfate metabolites Cys and GSH. In addition, a SULTR1;2 knockout allele also exhibits reduced inhibition in response to sulfate, Cys and GSH, consistent with the phenotype of sel1-15 and sel1-16. Taken together, the genetic evidence suggests that, in addition to its known function as a high-affinity sulfate transporter, SULTR1;2 may have a regulatory role in response to sulfur nutrient status. The possibility that SULTR1;2 may function as a sensor of sulfur status or a component of a sulfur sensory mechanism is discussed.

New insight into the strategy for nitrogen metabolism in plant cells

Nitrogen (N) is one of the most important mineral nutrients required by higher plants. Primary N absorbed by higher plants includes nitrate (NO3(-)), ammonium (NH4(+)), and organic N. Plants have developed several mechanisms for regulating their N metabolism in response to N availability and environmental conditions. Numerous transporters have been characterized and the mode of N movement within plants has been demonstrated. For further assimilation of N, various enzymes are involved in the key processes of NO3(-) or NH4(+) assimilation. N and carbon (C) metabolism are tightly coordinated in the fundamental biochemical pathway that permits plant growth. As N and C metabolism are the fundamental constituents of plant life, understanding N regulation is essential for growing plants and improving crop production. Regulation of N metabolism at the transcriptional and posttranscriptional levels provides important perceptions in the complex regulatory network of plants to adapt to changing N availability. In this chapter, recent advances in elucidating molecular mechanisms of N metabolism processes and regulation strategy, as well as interactions between C and N, are discussed. This review provides new insights into the strategy for studying N metabolism at the cellular level for optimum plant growth in different environments.

A novel morphological response of maize (Zea mays) adult roots to heterogeneous nitrate supply revealed by a split-root experiment

Approximately 35-55% of total nitrogen (N) in maize plants is taken up by the root at the reproductive stage. Little is known about how the root of an adult plant responds to heterogeneous nutrient supply. In this study, root morphological and physiological adaptations to nitrate-rich and nitrate-poor patches and corresponding gene expression of ZmNrt2.1 and ZmNrt2.2 of maize seedlings and adult plants were characterized. Local high nitrate (LoHN) supply increased both lateral root length (LRL) and density of the treated nodal roots of adult maize plants, but only increased LRL of the treated primary roots of seedlings. LoHN also increased plant total N acquisition but not N influx rate of the treated roots, when expressed as per unit of root length. Furthermore, LoHN markedly increased specific root length (m g(-1)) of the treated roots but significantly inhibited the growth of the lateral roots outside of the nitrate-rich patches, suggesting a systemic carbon saving strategy within a whole root system. Surprisingly, local low nitrate (LoLN) supply stimulated nodal root growth of adult plants although LoLN inhibited growth of primary roots of seedlings. LoLN inhibited the N influx rate of the treated roots and did not change plant total N content. The gene expression of ZmNrt2.1 and ZmNrt2.2 of the treated roots of seedlings and adult plants was inhibited by LoHN but enhanced by LoLN. In conclusion, maize adult roots responded to nitrate-rich and nitrate-poor patches by adaptive morphological alterations and displayed carbon saving strategies in response to heterogeneous nitrate supply.

NO homeostasis is a key regulator of early nitrate perception and root elongation in maize

Crop plant development is strongly dependent on nitrogen availability in the soil and on the efficiency of its recruitment by roots. For this reason, the understanding of the molecular events underlying root adaptation to nitrogen fluctuations is a primary goal to develop biotechnological tools for sustainable agriculture. However, knowledge about molecular responses to nitrogen availability is derived mainly from the study of model species. Nitric oxide (NO) has been recently proposed to be implicated in plant responses to environmental stresses, but its exact role in the response of plants to nutritional stress is still under evaluation. In this work, the role of NO production by maize roots after nitrate perception was investigated by focusing on the regulation of transcription of genes involved in NO homeostasis and by measuring NO production in roots. Moreover, its involvement in the root growth response to nitrate was also investigated. The results provide evidence that NO is produced by nitrate reductase as an early response to nitrate supply and that the coordinated induction of non-symbiotic haemoglobins (nsHbs) could finely regulate the NO steady state. This mechanism seems to be implicated on the modulation of the root elongation in response to nitrate perception. Moreover, an improved agar-plate system for growing maize seedlings was developed. This system, which allows localized treatments to be performed on specific root portions, gave the opportunity to discern between localized and systemic effects of nitrate supply to roots.

Auxin and the integration of environmental signals into plant root development

BACKGROUND

Auxin is a versatile plant hormone with important roles in many essential physiological processes. In recent years, significant progress has been made towards understanding the roles of this hormone in plant growth and development. Recent evidence also points to a less well-known but equally important role for auxin as a mediator of environmental adaptation in plants.

SCOPE

This review briefly discusses recent findings on how plants utilize auxin signalling and transport to modify their root system architecture when responding to diverse biotic and abiotic rhizosphere signals, including macro- and micro-nutrient starvation, cold and water stress, soil acidity, pathogenic and beneficial microbes, nematodes and neighbouring plants. Stress-responsive transcription factors and microRNAs that modulate auxin- and environment-mediated root development are also briefly highlighted.

CONCLUSIONS

The auxin pathway constitutes an essential component of the plant's biotic and abiotic stress tolerance mechanisms. Further understanding of the specific roles that auxin plays in environmental adaptation can ultimately lead to the development of crops better adapted to stressful environments.