Saturday, 2 April 2016

Nitrogen uptake, assimilation and remobilization in plants: challenges for sustainable and productive agriculture

By : Risky Anggraeni Puspitasari

Nitrogen uptake, assimilation and remobilization in plants: challenges for sustainable and productive agriculture

I.         Introduction
Nitrogen is one of the most expensive nutrients to supply and commercial fertilizers represent the major cost in plant production. Productive agriculture needs a large amount of expensive nitrogenous fertilizers. Improving nitrogen use efficiency (NUE) of crop plants is thus of key importance. NUE definitions differ depending on whether plants are cultivated to produce biomass or grain yields. However, for most plant species, NUE mainly depends on how plants extract inorganic nitrogen from the soil, assimilate nitrate and ammonium, and recycle organic nitrogen.
Furthermore, there is serious concern regarding nitrogen loss in the field, giving rise to soil and water pollution. Incomplete capture and poor conversion of nitrogen fertilizer also causes global warming through emissions of nitrous oxide. NUE in plants is complex and depends on nitrogen availability in the soil and on how plants use nitrogen throughout their life span. As a concept, NUE is expressed as a ratio of output (total plant N, grain N, biomass yield, grain yield) and input (total N, soil N or N-fertilizer applied). Increasing NUE and limiting nitrogen fertilizer use are both important and challenges to preserve the environment and improve a sustainable and productive agriculture.
II.      Nitrogen Source and Uptake
The preferred form in which N is taken up depends on plant adaptation  to soil conditions. Generally, plants adapted to low pH and reducing soils as found in mature forests or arctic tundra tend to take up ammonium or amino acids, whereas plants adapted to higher pH and more aerobic soils prefer nitrate. Nitrate uptake occurs at the root level and two nitrate transport systems have been shown to coexist in plants and to act co-ordinately to take up nitrate from the soil solution and distribute it within the whole plant.
Once taken up by root cells, nitrate must be transported across several cell membranes and distributed in various tissues. Electrophysiological studies together with the pH-dependent equilibrium between the uncharged NH3 and charged NH4+ forms suggest that the ion is predominant under all physiological conditions and is the dominant species for controlled membrane transport.
Thus far, putative plant amino acid transporters have been identified as members of at least five gene families. In Arabidopsis these comprise at least 67 genes. Substrate specificities as well as gene expression patterns or subcellular localization of the protein may give a good indication of the function of each protein. Forward and reverse genetic approaches were used to identify transporters involved in root amino acid uptake. The precise localization of these transporter mRNAs within different cell types in the root led to propose a hypothetic mode of root amino acid uptake in non-mycorrhizal plants.
III.   Nitrogen Assimilation
The nitrogen sources taken up by higher plants are nitrate or ammonium as inorganic nitrogen sources and amino acids under particular conditions of soil composition. Nitrogen assimilation requires the reduction of nitrate to ammonium, followed by ammonium assimilation into amino acids.
Nitrate reduction takes place in both roots and shoots but is spatially separated between the cytoplasm where the reduction takes place and plastids/chloroplasts where nitrite reduction occurs. Nitrate reduction into nitrite is catalysed in the cytosol by the enzyme nitrate reductase (NR).
After nitrate reduction, nitrite is translocated to the chloroplast where it is reduced to ammonium by the second enzyme of the pathway, the nitrite reductase (NiR). The Nii genes encoding the NiR enzyme have been cloned from various species, the number of genes varying from one to two copies.
NR, NiR and GOGAT require reducing power as either NADH or ferredoxin (Fdx) according to the enzyme. Glutamine synthetase and asparagine synthetase need ATP. In addition, carbon skeletons and especially keto-acids are essential to form organic nitrogen as amino acids. The availability of carbon skeletons for ammonium condensation and the supply of ATP, Fdx and NADH as products of photosynthesis, respiration and photorespiration pathways are thus essential for nitrogen assimilation.
IV.   Nitrogen Remobilization
Nitrogen remobilization has been studied in several plant species through the ‘apparent remobilization’ method, which is the determination of the amount of total nitrogen present in the different plant organs at different times of development and through 15N long-term labelling, which allows the determination of fluxes. Experiments of 15N tracing at the reproductive stage showed that the rate of nitrogen remobilization from the rosettes to the flowering organs and to the seeds was similar in early- and late-senescing lines. At the reproductive stage, NRE is mainly related to harvest index.
N remobilization is also environment dependent and favoured under limiting nitrate supplies. Although 15N remobilization is a step-by-step mechanism that involves the different plant organs, evidence shows that grain nitrogen content is correlated with flag leaf senescence in maize, wheat and barley. Leaf senescence is not only essential for nitrogen mobilization. Breeding plants have then to cope with the dilemma that delayed senescence could lead to higher yields but also to a decrease in NRE and to a decrease in grain protein content. On the other hand, increasing nitrogen remobilization has the advantage of re-using nitrogen from the vegetative parts and of lowering nitrogen loss in the dry remains.
Chloroplasts are the main source of nutrients used during senescence. Together with other photosynthesis-related proteins, Rubisco is a major source of nitrogen for remobilization. Over-investment in Rubisco is thus important for N-source management at the whole-plant level. Although chloroplasts show the first symptoms of deterioration during senescence, whereas other organelles are degraded later, the mechanisms responsible for chloroplast degradation are largely unknown. Chloroplast dismantling does not mean chaotic decay. Controlled and coordinated degradation is needed to prevent cell damage due to the highly photodynamic nature of some of the breakdown products and to maintain export capacity and remobilization. The initial steps of chlorophyll and chloroplast protein degradation are likely to take place within the plastid itself.
Depending on the species, nitrogen uptake could be negatively regulated or even in some cases totally inhibited during seed production. There is evidence that plants share common N remobilization mechanisms whether they are monocotyledonous, dicotyledonous, C3 or C4 photosynthesis types.
Prior to phloem loading the central vacuole of mesophyll cells might be a site for transient storage of amino acids released from protein degradation. N-storage and remobilization potential are important for both annual and perennial plants. For annual plants, as mentioned above, nitrogen remobilization is important for seed production and seed nitrogen content. Nitrogen content in the seeds further determines germination efficiency and survival of young seedlings. Nitrogen remobilization is also important for perennial plant survival. Trees, which grow in low nitrogen environments most of the time, have two phases of nitrogen remobilization. Nitrogen is remobilized from the senescing leaves in autumn to be stored in trunks during winter. N is remobilized a second time from trunks to developing organs in spring before root N uptake becomes the main process to meet tree N needs. As trees age, the internal cycling of N becomes more and more important in the whole-tree N-budget. Both nitrogen withdrawal from senescing leaves and root N uptake contribute to the build-up of N storage pools and to the efficient nitrogen management that are essential for plant survival over years. Forage grasses are subject to frequent defoliation by herbivores or mechanical harvesting. Recovery of grasses after defoliation is related to the availability of carbon and nitrogen reserves in the remaining tissues. Decreasing mineral N supply before defoliation was shown to decrease the availability of N reserves in leaves and as a result the absolute amount of N subsequently remobilized to roots.
V.      Regulation of Nitrogen Uptake, Assimilation and Remobilization by Nitrate and Carbon Availibilities
N uptake by the roots and further N assimilation are integrated in the plant to match the nutrient demand of the whole organism. External stimuli or stresses as well as nutritional status of the plant modulate the expression and/or the activity of transport systems and enzymes by various regulatory mechanisms. The first mechanism operates at the transcriptional level and includes the induction by the substrates and the repression exerted by endogenous N assimilates. The stimulation of N uptake and N assimilation by photosynthesis 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 signaling showed that upregulation of nitrate transporters was related to the concentration of glucose 6-phosphate. In contrast, the diurnal regulation of Nia transcripts is governed not only by sugars but also by light regulation via phytochrome 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.
VI.   Conclusion
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. Such global ‘resource use efficiency’ necessitates having a global view of plant physiology, plant uptake capacity, plant metabolism and plant response to restrictions, as well as a view of soil physical and chemical properties. The enzymes and regulatory processes that can be manipulated to control NUE (Nitrogren Use Efficiency)

Bertheloot J, Martre P, Andrieu B. 2008. Dynamics of light and nitrogen distribution during rain filling within wheat canopy. Plant Physiology 148: 1707–1720.
Beuve N, Rispail N, Laine P, Cliquet J-B, Ourry A, Le Deunff E. 2004. Putative role of gamma-aminobutyric acid (GABA) as a longdistance signal in up-regulation of nitrate uptake in Brassica napus L. Plant Cell Environment 27: 1035–1046.
Buchanan-Wollaston V, Earl S, Harrison E, et al. 2003. The molecular analysis of leaf  senescence – a genomics approach. Plant Biotechnology Journal 1: 3–22.
Lillo C. 2008. Signalling cascades integrating light-enhanced nitrate metabolism. Biochemical Journal 415: 11–19.

Masclaux-Daubresse C, Reisdorf-Cren M, Orsel M. 2008. Leaf nitrogen remobilisation for plant development and grain filling. Plant Biology 10 (Suppl. 1): 23–36.

Related Articles


Post a Comment

pangestu.yuda. Powered by Blogger.