Nitrogen uptake, assimilation and remobilization in plants: challenges for sustainable and productive agriculture
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)
VII.References
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