How do plants uptake nitrogen

The NH 3 in the soil then reacts with water to form ammonium, NH 4. This ammonium is held in the soils and is available for use by plants that do not get nitrogen through the symbiotic nitrogen fixing relationship described above. The third stage, nitrification, also occurs in soils. Nitrates can be used by plants and animals that consume the plants. Some bacteria in the soil can turn ammonia into nitrites. Although nitrite is not usable by plants and animals directly, other bacteria can change nitrites into nitrates—a form that is usable by plants and animals.

This reaction provides energy for the bacteria engaged in this process. The bacteria that we are talking about are called nitrosomonas and nitrobacter. Nitrobacter turns nitrites into nitrates; nitrosomonas transform ammonia to nitrites. Both kinds of bacteria can act only in the presence of oxygen, O 2 [ 7 ].

The process of nitrification is important to plants, as it produces an extra stash of available nitrogen that can be absorbed by the plants through their root systems. The fourth stage of the nitrogen cycle is immobilization, sometimes described as the reverse of mineralization. These two processes together control the amount of nitrogen in soils. Just like plants, microorganisms living in the soil require nitrogen as an energy source.

These soil microorganisms pull nitrogen from the soil when the residues of decomposing plants do not contain enough nitrogen. Immobilization, therefore, ties up nitrogen in microorganisms.

However, immobilization is important because it helps control and balance the amount of nitrogen in the soils by tying it up, or immobilizing the nitrogen, in microorganisms. In the fifth stage of the nitrogen cycle, nitrogen returns to the air as nitrates are converted to atmospheric nitrogen N 2 by bacteria through the process we call denitrification.

This results in an overall loss of nitrogen from soils, as the gaseous form of nitrogen moves into the atmosphere, back where we began our story.

The cycling of nitrogen through the ecosystem is crucial for maintaining productive and healthy ecosystems with neither too much nor too little nitrogen. Plant production and biomass living material are limited by the availability of nitrogen. Understanding how the plant-soil nitrogen cycle works can help us make better decisions about what crops to grow and where to grow them, so we have an adequate supply of food. Knowledge of the nitrogen cycle can also help us reduce pollution caused by adding too much fertilizer to soils.

As you have seen, not enough nitrogen in the soils leaves plants hungry, while too much of a good thing can be bad: excess nitrogen can poison plants and even livestock! Pollution of our water sources by surplus nitrogen and other nutrients is a huge problem, as marine life is being suffocated from decomposition of dead algae blooms.

Farmers and communities need to work to improve the uptake of added nutrients by crops and treat animal manure waste properly. We also need to protect the natural plant buffer zones that can take up nitrogen runoff before it reaches water bodies.

Evidence showed that ammonium itself regulates GLN genes at the transcriptional level. In soybean, co-operation between three distinct promoter regions is necessary for ammonium-stimulated expression of the GS15 gene. The interaction among these regions may be facilitated by an HMG A high-mobility group A -like protein that binds to the proximal and distal promoter regions of the soybean GS15 gene Reisdorf-Cren et al.

Global transcriptome studies after nitrate induction Scheible et al. Using NR mutants, it was shown that much of this regulation is exerted by nitrate itself Wang et al. The stimulation of N uptake and N assimilation by photosynthesis for a review see Lillo, ensures that N uptake is correlated with C status. For example, nitrate uptake and reduction are co-ordinately regulated by a circadian control.

This control has often been attributed to the regulatory action on gene expression of sugars produced by photosynthesis and transported downward to the roots. This has been shown for the ammonium and nitrate transporters, NR and NiR.

The regulation of nitrate uptake and transporters seems to be independent of the known sugar regulation pathways, such as hexokinase signalling Lillo, Wirth et al. In contrast, the diurnal regulation of Nia transcripts is governed not only by sugars but also by light regulation via phytochrome Lillo, In addition, it was observed that Nia expression is controlled by signals from photosynthetic electron flow, which adds a new facet to the intracellular cross-talk between chloroplasts and the nucleus Lillo, Castaings et al.

NLP7 belongs to a gene family with nine different members, but the functions of the other NLP proteins are still unknown. In Arabidopsis , compelling evidence shows that SnRK1s Snf1-related protein kinases are central integrators of transcription networks in plant stress and energy signalling that are inactivated by sugars Baena-Gonzalez et al. Rapid post-translational regulation such as protein modification is the second mechanism that controls nitrogen uptake and assimilation.

The best studied post-translational regulation in N metabolism is the regulation of NR in higher plants. NR is inactivated by a two-step process that involves the phosphorylation of ser , as shown in spinach, followed by the binding of an inhibitory protein kinase. When a modified form of NR, no longer susceptible to post-translational dark inactivation, was over-expressed, the resulting protein did not decline in the second part of the photoperiod. The inactive phosphorylated form is re-activated by dephosphorylation, probably by PP2A.

Moreover, evidence showed that there is a correlation between the phosphorylation state or the activation state of NR and the rate at which NR protein decreases. Plastidic glutamine synthetase from Medicago truncatula is also regulated through phosphorylation and interactions. The GS2 phosphorylation site ser 97 , critical for the interaction with and subsequent proteolysis, was identified by directed mutagenesis.

Cytosolic glutamine synthetases from M. Phosphorylation occurs at more than one residue and increases affinity for the substrate glutamate.

With the aim of improving NUE, many critical candidate genes have been manipulated, over-expressing them or using knockout mutations, in order to test their effects on biomass and plant nitrogen status. Several good reviews have been written on this subject that provide more detail than mentioned in this section Andrews et al. NR has long been considered to be the rate-limiting step in nitrate assimilation. Nicotiana tabaccum plants constitutively expressing NR from N.

Then, under field conditions of fluctuating water availability, constitutive NR expression may confer some physiological advantage. Over-expressing NR or NiR in Arabidopsis , potato or tobacco reduced nitrate levels in plant tissues but did not increase biomass yield, tuber numbers or seed yields. Over-expression of Nia or Nii genes in plants increased mRNA levels and often affected N uptake without modifying yield or plant growth regardless of the nitrogen source available.

This is believed to be due in part to the complex post-transcriptional regulation of NR reviewed by Pathak et al. Over-expression of cytosolic glutamine synthetase GS2 genes was performed in N. Effects on plant biomass and grain yield were also more successful. For example, over-expression of the Phaseolus vulgaris GS1 gene under the control of the rbcS promoter in wheat resulted in significantly higher root and grain yield with higher N content in grain in some lines Habash et al.

In summary, several studies have demonstrated a direct correlation between an enhanced GS activity in transgenic plants and biomass or yield Good et al.

In conclusion, studies show that over-expression of GS or GOGAT genes can improve biomass and grain yields depending on which gene allele and which promoters are used. This indicates that further characterization is required to demonstrate the beneficial effects of such strategies for crops and in field conditions. Attempts to over-express AS were carried out in tobacco and Arabidopsis for a review see Good et al. Interestingly, over-expression of ASN1 in Arabidopsis enhanced soluble seed protein content and total protein and increased fitness of plants grown under nitrogen-limiting conditions Lam et al.

Alanine is a major amino acid for nitrogen storage under anaerobic stress such as flooding. Over-expression of barley alanine amino transferase under the control of root promoters in canola and rice had interesting effects, considerably increasing plant biomass, seed yield, NUE and shoot nitrogen concentration when plants were grown at low nitrate supply Good et al. These results are of particular interest, showing that it is possible to improve NUE by manipulating downstream steps in N-remobilization.

In addition to manipulating enzymes involved in nitrogen assimilation of amino acid metabolism, the generation of plants modified for the expression of transcription factors has also been attempted.

For example, ectopic expression of the maize Dof1 transcription factor, which regulates the expression of genes involved in organic acid metabolism, led in Arabidopsis to the accumulation of amino acids and to an increase of growth under N-limiting conditions. These effects suggest that NUE could also be improved by manipulating carbon metabolism pathways.

PII-like, NLP7 and TOR target of rapamycin proteins, which are potentially linked to C and N sensing in plants, are other candidates for further engineering as shown by the increased plant growth, yield and stress resistance acquired by TOR-overexpressing plants Ferrario-Mery et al.

Hibberd et al. The Rubisco protein is known to be used as a storage protein in C 3 herbaceous plants and trees Millard et al. In elevated atmospheric CO 2 , Rubisco carboxylase activity is increased and Rubisco protein content is decreased. The selective loss of Rubisco enzyme under elevated CO 2 thus benefits NUE without necessarily significantly changing the leaf C assimilation rate.

More than accessions of Arabidopsis , originating from various locations worldwide, are available in stock centres. Probably due to a selective adaptation to original edaphic and climatic environments, they show natural variation of their development and they constitute large genetic and phenotypic resources McKhann et al.

Several recent papers have presented the first evidence that natural variation exists for nitrogen metabolism, including nitrogen uptake and nitrogen remobilization. The first clue was provided by the analysis of root plasticity. Studies in Arabidopsis have shown the stimulation of root growth by a localized source of nitrate Robinson, ; Forde and Lorenzo, Walch-Liu and Forde assayed the extent of root stimulation using a small collection of six accessions.

This observed variation of root adaptation to nitrogen availability should have consequences for nitrogen uptake in plants. Our recent investigation of natural variation in nitrate uptake and nitrogen remobilization in Arabidopsis gives the second clue Fig. Masclaux-Daubresse and F. Chardon, unpubl. A core-collection of 18 accessions of Arabidopsis was grown under limiting and ample nitrogen nutritive conditions. The aim of this study was to collect data allowing us to monitor the natural variation of N uptake at the vegetative stage and the N-remobilization to the seeds at the reproductive stage depending of nitrogen availability, and also to measure traits related to NUE such as biomass of rosettes at the vegetative stage, seed yield, harvest index and nitrogen concentrations in the different plant material collected at vegetative and reproductive stages.

Using all the data collected at low and high nitrogen supply, groups of accessions can be clustered, assembling plants that have similar responses depending on nitrate availability. Surprisingly, for most of the traits measured or computed the variation was higher at high nitrate supply when N uptake and N remobilization are not forced by nitrate limitation.

Figure 5 shows schematically the large variation observed between the classes of accessions. The differences between the lowest and highest performing accessions were three-fold for N uptake and six-fold for N remobilization. We also noted interesting correlations between how plants manage N uptake and N metabolism and their biomass. Nitrogen absorption and nitrogen remobilization profiling of five Arabidopsis accessions.

A core collection of 18 accessions of Arabidopsis grown with 10 m m nitrate was used to measure traits related to biomass, N uptake, N remobilization and NUE C. Five accessions representative of the main classes found are presented. The small sample of accessions highlights the variation of performances. Plants, such as Col0 or Sha, have a relatively good N uptake. The highest N remobilization score was found in Stw-0 while plants with high N percentage and high biomass were Bur-0 and Tsu The last clue is provided by the Arabidopsis eFP-Browser database, which combines microarray analyses Winter et al.

Some experiments included in this database used several accessions Lempe et al. It is possible to select specifically the pattern of genes involved in primary N metabolism. Although plants were cultivated in the same conditions 4-day-old seedlings grown in soil in the glasshouse , the signal intensities of some N genes indeed varied among these accessions Fig.

Whether such variation is correlated with N-dependence and NUE remains to be determined. Heat map illustrating the natural variation in expression of genes involved in nitrogen metabolism. For each gene, high expression is depicted as dark shading, and low expression is depicted as light shading. Experiments presented above provide ideas about the various traits that can be measured to explore NUE N-gene transcription levels, N uptake efficiency, NRE, nitrogen content, enzyme activities, biomass and encourage computing data according to a systems biology approach, in order to reveal the functioning of the different modules that constitute nitrogen metabolism adaptation in plants.

A better characterization of edaphic environments and the metabolism of different Arabidopsis accessions would consequently allow a better understanding of how these modules are regulated according to the nitrogen availability in soil.

Natural variation also exists in crops. Approaches currently performed by breeders to improve varieties for many agronomical traits include QTL mapping and marker-assisted selection. However, it is worth noting that the experiments performed on Arabidopsis presented above have been carried out on wild plants, meaning that the plants studied have not been modified for agronomic criteria and that no adaptive selection has decreased differences between them.

Using Arabidopsis rather than highly selected plants such as crops for such approaches should then be more informative. Natural variation of N uptake and N remobilization identified in the model plant Arabidopsis is a source of knowledge that can be useful to transfer to crops.

QTL mapping for nitrogen-related enzyme activities such as nitrate reductase or glutamine synthetase are rarer. Even more rare is the mapping of N-remobilization or N-influx QTLs because of the difficulty in performing 15 N tracing on large populations. QTLs explaining the variation of this trait were mapped on chromosomes 6 and 7.

Mickelson et al. However, the most prominent QTL for grain protein content on barley chromosome 6 appeared to be a potential homologue of the grain protein QTL from durum wheat mapped by Joppa et al. Recently, a wheat QTL was cloned through positional cloning and fine mapping Uauy et al.

The locus encodes an NAC transcription factor, NAM-B1, which accelerates leaf senescence and increases nutrient filling in developing grains. The ancestral wild wheat allele is functional whereas modern wheat varieties carry a non-functional NAM-B1 allele. The effect of the chromosome Gpc-B1 region including the NAM-B1 gene was studied further by introgressing the Gcp-B1 locus in hexaploid near-isogenic lines.

As a result of Gcp-B1 introgression, significantly lowered straw N concentration at maturity and higher nitrogen harvest index NHI were measured, suggesting that the functional Gcp-B1 allele improves N remobilization and diminishes the amount of nitrogen lost in residual dry remains Brevis and Dubcovsky, University of California, Davis, CA, unpubl. In the same barley population used by Mickelson et al.

QTL co-localization strongly suggested that the major endo- or amino-peptidases were not involved in leaf N remobilization or in the control of grain protein content. By contrast, QTL co-localization suggested that vacuolar carboxy-peptidase isoenzymes are involved in leaf N remobilization. The authors monitored an impressive number of traits for NUE, leaf senescence, enzyme activities, yield, biomass, N uptake and N remobilization, which, together with the former data from Hirel et al.

The study by Coque et al. QTL clustering showed an antagonism between N remobilization and N uptake at several loci. Positive coincidences between N uptake, root system architecture and leaf greenness were also found in eight clusters, while N-remobilization QTLs mainly coincided with leaf senescence QTLs.

Co-localization with N-related genes showed that the two NR loci chromosomes 1 and 4, see Hirel et al. Grain yield-related traits coincided with the three GS1 loci corresponding to Gln , Gln and Gln Interestingly, Habash et al.

This finding was confirmed by Fontaine et al. Unlike in maize, there was no correlation between GS activity and yield components in wheat. Improving global plant productivity and product quality together with taking care of environmental quality and human wellbeing are the main challenges for the immediate future Vitousek et al.

Such a goal depends on agricultural development and policy and can be achieved by providing the right nutrient source at the right rate, the right time and the right place. To improve sustainable agricultural production, it is also necessary to grow crops that can remove the nutrient applied to soil efficiently, and therefore require less fertilizer. This review gives an overview of the different metabolic and physiological clues that agronomical research has provided.

The enzymes and regulatory processes that can be manipulated to control NUE are presented. The last results obtained from natural variation and QTL studies show the complexity of NUE and open new perspectives. With regard to the complexity of the challenge we have to face and with regard to the numerous approaches available, the integration of data coming from transcriptomic studies, functional genomics, quantitative genetics, ecophysiology and soil science into explanatory models of whole-plant behaviour in the environment has to be encouraged.

We are grateful to Dr Heather I. Google Scholar. In an agricultural setting, nitrogen deficiency can be combated by the addition of nitrogen-rich fertilizers to increase the availability of nutrients and thereby increase crop yield.

However, this can be a dangerous practice since excess nutrients generally end up in ground water, leading to eutrophication and subsequent oxygen deprivation of connected aquatic ecosystems. Plants are able to directly acquire nitrate and ammonium from the soil. However, when these nitrogen sources are not available, certain species of plants from the family Fabaceae legumes initiate symbiotic relationships with a group of nitrogen fixing bacteria called Rhizobia.

These interactions are relatively specific and require that the host plant and the microbe recognize each other using chemical signals. The interaction begins when the plant releases compounds called flavanoids into the soil that attract the bacteria to the root Figure 4. In response, the bacteria release compounds called Nod Factors NF that cause local changes in the structure of the root and root hairs. Specifically, the root hair curls sharply to envelop the bacteria in a small pocket.

The plant cell wall is broken down and the plant cell membrane invaginates and forms a tunnel called an infection thread that grows to the cells of the root cortex. The bacteria become wrapped in a plant derived membrane as they differentiate into structures called bacteroids. These structures are allowed to enter the cytoplasm of cortical cells where they convert atmospheric nitrogen to ammonia, a form that can be used by the plants.

Mycorrhizal interactions with plants. In addition to symbiotic relationships with bacteria, plants can participate in symbiotic associations with fungal organisms as well. There are several classes of mycorrhiza, differing in structural morphology, the method of colonizing plant tissue, and the host plants colonized.

However, there are two main classes that are generally regarded as the most common and therefore, the most ecologically significant. The endomycorrhizae are those fungi that establish associations with host plants by penetrating the cell wall of cortical cells in the plant roots. By contrast, ectomycorrizae develop a vast hyphae network between cortical cells but do not actually penetrate the cells.

The most common endomycorrhizal interaction occurs between arbuscular mycorrhizal fungi AMF; also called Vesicular-Arbuscular Mycorrhiza or VAM and a variety of species of grasses, herbs, trees and shrubs. When phosphate is available in the soil, plants are able to acquire it directly via root phosphate transporters.

However, under low phosphate conditions, plants become reliant on interactions with mycorrhizal fungi for phosphorus acquisition. Mycorrhizal spores present in the soil are germinated by compounds released from the plant. Hyphae extend from the germinating spore and penetrate the epidermis of the plant root. Inside the root, the hyphae branch and penetrate cortical cells, where highly branched structures called arbuscules develop Figure 5.

Externally, hyphae extend into the soil beyond the area accessible to the root. This kind of symbiosis facilitates plant phosphorus uptake from the soil by increasing the root's absorptive surface area. Since plants take up phosphorus at a much higher rate than phosphorus diffuses into the soil surrounding the root, a phosphorus depletion zone is quickly established, limiting uptake of phosphorus by the plant.

Figure 5: Plant-mycorrhizal fungus interactions. Diagram of arbuscular mycorrhizae colonization of a plant root showing the extension of hyphae beyond the phosphorus depletion zone and the presence of arbuscules in cells of the root cortex. Diagram of Ectomycorrhizal fungi showing growth of hyphae around cortical cells, a mantle sheath on the outside of the root, and hyphae that extend into soil around the root.

Although plants are non-motile and often face nutrient shortages in their environment, they utilize a plethora of sophisticated mechanisms in an attempt to acquire sufficient amounts of the macro- and micronutrients required for proper growth, development and reproduction. These mechanisms include changes in the developmental program and root structure to better "mine" the soil for limiting nutrients, induction of high affinity transport systems and the establishment of symbioses and associations that facilitate nutrient uptake.

Together, these mechanisms allow plants to maximize their nutrient acquisition abilities while protecting against the accumulation of excess nutrients, which can be toxic to the plant.

It is clear that the ability of plants to utilize such mechanisms exerts significant influence over crop yields as well as plant community structure, soil ecology, ecosystem health, and biodiversity. References and Recommended Reading Beyer P. Share Cancel. Revoke Cancel. Keywords Keywords for this Article. Save Cancel. Flag Inappropriate The Content is: Objectionable. Flag Content Cancel.

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Sorry, a shareable link is not currently available for this article. Provided by the Springer Nature SharedIt content-sharing initiative. Skip to main content. Search SpringerLink Search. Abstract In the biosphere plants are exposed to different forms of N, which comprise mineral and organic N forms in soils as well as gaseous NH3, NOx, and molecular N2 in the atmosphere.

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