2. Nitrogen Assimilation
Nitrogen assimilation is a vital process controlling plant growth and
development.
Inorganic nitrogen is assimilated into the amino acids glutamine,
glutamate, asparagine, and aspartate, which serve as important
nitrogen carriers in plants.
The enzymes glutamine synthetase (GS), glutamate synthase
(GOGAT), glutamate dehydrogenase (GDH), aspartate
aminotransferase (AspAT), and asparagine synthetase (AS) are
responsible for the biosynthesis of these nitrogen- carrying amino
acids.
3. Recent molecular analyses demonstrate that each enzyme is encoded by a
gene family wherein individual members encode distinct isoenzymes that are
differentially regulated by environmental stimuli, metabolic control,
developmental control, and tissue/cell-type specificity.
The assimilation of inorganic nitrogen onto carbon skeletons has marked
effects on plant productivity, biomass, and crop yield. Nitrogen deficiency in
plants has been shown to cause a decrease in the levels of photosynthetic
structural components such as chlorophyll and ribulose bisphosphate
carboxylase (rubisco), with resulting reductions in photosynthetic capacity
and carboxylation efficiency. Because enzymes involved in the
assimilation of nitrogen into organic form in plants are crucial to plant
growth, they are also effective targets for herbicide development.
Nitrogen Assimilation
4. Ammonia assimilation in plants is a complex process since it is produced in plants from
different sources. These sources can be classified as the primary or secondary.
1. Primary source refers to the ammonia which is produced from the inorganic nitrogen,
such as ammonium ions absorbed by the plants and ammonia generated due to nitrite
reduction or due to nitrogen fixation by the nitrogen-fixing bacteria in symbiotic
association with plants.
2. Secondary sources refer to the ammonia generated from the organic compounds
during metabolism. These include ammonia generated due to
(i) oxidation of glycine to serine during photorespiration;
(ii) degradation of nitrogenous compounds, such as asparagine, arginine, and ureides; and
(iii) protein degradation and deamination of amino acids. Since ammonia is produced at
different sites in plants, the enzymes catalyzing ammonia assimilation function in
different cellular conditions. As a result, various isozymes of the enzymes are involved
in assimilation of ammonia.
Nitrogen Assimilation in the form of Ammonia
5. Ammonia produced in nodules by the bacteroids is released through the
bacteroid membrane. Since ammonia is nonpolar, it can easily diffuse out
of the membrane. Once in the peribacteroid space, ammonia accepts up
proton and is converted to NH4
+. Ammonium ions are transported to the
cytosol of the infected cell of the nodule through the channels present in
the peribacteroid membrane of the symbiosome by facilitated diffusion.
NH4
+ transport is mediated by an aquaporin-related channel (NOD 26)
across the peribacteroid membrane in soybean nodules. NH4
+ is
assimilated in the cytosol of the infected cells of the host plant. Two
enzymes responsible for the initial assimilation of NH4
+ are glutamine
synthetase (GS) and glutamate synthase (which is also known as GOGAT,
glutamine 2-oxoglutarate aminotransferase).
Nitrogen Assimilation in the form of Ammonia
7. Glutamine synthetase (GS) catalyzes the synthesis of glutamine
from NH4
+ and glutamate. This ATP-requiring reaction is
Glutamate + NH4
þ + ATP Glutamine + ADP + Pi
A divalent cation such as Mg2+ or Mn2+ or Co2+ is also required as
the cofactor for the enzyme. There are two classes of glutamine
synthetase (GS) in plants: cytosolic (GS1) and plastidial (GS2).
GS1 is also present in leaves in low concentrations, while it is
present in higher concentrations in roots. Role of GS1 is important
in primary assimilation of NH4
+. GS1 is expressed in germinating
seeds or in vascular bundles of roots or shoots and generates
glutamine for intercellular transport.
Nitrogen Assimilation in the form of Ammonia
8. GS2 is responsible for assimilating NH4
+ produced during
photorespiration. GS2 is the primary enzyme of glutamine
synthesis in the leaves. Gene encoding GS2 is expressed in
mesophyll cells, while gene for GS1 is expressed in phloem,
indicating its primary role in producing glutamine for long-
distance transport. Mutants of GS2 do not survive in conditions
which favor photorespiration, while they survive in conditions
which suppress photorespiration. Glutamate synthase transfers
amino group from glutamine to 2-oxoglutarate, resulting in the
synthesis of two molecules of glutamate. One of the glutamate
molecules serves as the substrate for GS, while the other one is
available for further metabolism. The n for ammonium metabolism
is known as GS/GOGAT reaction.
Nitrogen Assimilation in the form of Ammonia
9. Nitrate and ammonium ions are two sources of nitrogen in nonluminous
plants. Most of the nitrogen taken up by the plants is derived from the soil
in the form of nitrate (NO3-), which is then converted into nitrite by nitrate
reductase in the cytosol. Nitrite is transported to plastids and is then
converted into NH4
+, by the action of nitrite reductase. Soluble forms of
nitrogen are transported as amines and amides. Nitrogen is a constituent of
all proteins, enzymes, and various metabolic processes involved in the
synthesis and transfer of energy. It is also a constituent of many other
important biomolecules, such as hormones (indole-3-acetic acid and
cytokinin's) and chlorophyll. Some plants, such as corn (Zea mays),
require very high dosage of nitrogen as compared to other plants. It
facilitates rapid plant growth and helps increase seed and fruit production.
Nitrogen Assimilation in the form of Nitrate
10. Nitrogen is the most abundant gas (80%) in the
atmosphere, but only certain bacteria and cyanobacteria
can utilize gaseous nitrogen directly. It exists in a number
of oxidized and reduced forms and cycles in the
atmosphere between organic and inorganic pools. A
number of plant species can fix nitrogen by having
symbiotic association with diazotrophic microorganisms.
Importance of Nitrogen
11. Nitrogen is a mobile
element. Therefore, older
leaves exhibit chlorosis
and necrosis earlier than
younger leaves during
nitrogen deficiency.
During severe nitrogen
deficiency, leaves become
completely yellow and fall
off.
Importance of Nitrogen
12. Nitrogen deficiency causes stunted
and slow growth because cell division
is inhibited and lateral buds become
dormant. Chlorosis and purple
appearance on the stems as well as
petiole and underside of leaves are
also caused by nitrogen deficiency.
Some plants, such as tomato and
maize, also accumulate anthocyanins
which accompany nitrogen deficiency.
Importance of Nitrogen
13. Plants grown in the presence of
excess nitrogen produce dark green
leaves and vigorous foliage as root
system is highly reduced resulting in
high shoot/ root ratio. In nitrogen
deficiency, reverse situation is
evident, i.e., low shoot/root ratio.
Potato plants grown in the presence
of abundant nitrogen exhibit more
foliage and small tubers. Flower and
seed formation are highly reduced
due to high nitrogen in the soil.
Importance of Nitrogen
14. Excess nitrogen also results in the
splitting of tomato fruits as they
ripen. The crops become susceptible
to disease, insect infestation, and
drought stress, leading to lodging,
when nitrogen content is high. Soils
are usually deficient in nitrogen as
compared to other elements. In
addition to atmosphere, the primary
source of nitrogen is often provided
to cultivated plants from fertilizer
application.
Importance of Nitrogen
