NITROGEN AND SULPHUR ASSIMILATION IN PLANTS.pptx

PLANT NUTRITION AND CROP PRODUCTIVITY
SAID HUSSEIN MARZOUK
NITROGEN AND SULPHUR ASSIMILATION IN PLANTS
4/6/2022 1
binmarzouk@gmail.com

NITROGEN
 Found in both inorganic and organic form in plant tissue
 Combine C,H,O and sometime sulphur to form amino
acids, enzymes, nucleic acids, chlorophyll, alkaloids, and
purine bases.
 Organic N mostly are high molecular weight proteins
 Inorganic N can accumulate in plants, primarily in stem in
form of NO3
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Contents and distribution of N in plants
• Consist of 1.5% to 6.0% of the dry weight of many crops
with sufficiency value of 2.5 and 3.5% in leaf tissue
• A lower ranges of 1.5% to 2.2% is found in most fruits
plants and higher range of 4.8% to 5.5% in legumes
species.
• Critical values varies and depends on plant species, stage
of growth and plant parts.
• Highest concentration are found in new leaves and the
concentration decrease with age of the plants and or parts
of the plant ( due to mobility from source to sink)
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Available forms for root absorption
• Exists in the soil solution as either the NO3– anion or the NH4+
cation.
• The uptake of either form influenced by soil pH, temperature, and
the presence of other ions in the soil solution.
• The effect of external pH on nitrogen uptake depends on whether
nitrogen is supplied as NH4+ or NO3–, lowering the external pH
increases the uptake of NO3–, but decreases uptake of NH4+
(Zsoldos and Haunold, 1982).
• The NH4+ cation participates in cation exchange in the soil.
• Nitrite (NO2–) may be present in the soil solution under anaerobic
conditions and is toxic to plants at very low levels (<5 ppm).
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Movement in soil and root absorption
• The NO3- anion moves in the soil primarily by mass flow
with most of the NO3- absorbed when it reaches the root
surface.
• Nitrate ions can be readily leached from the rooting zone
by irrigation water and/or rainfall, or lifted into the
rooting zone by upward moving water .
• The NH4+ cation acts much like the K+ cation in the
soil, and its movement in the soil solution is primarily by
diffusion
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Nutrients Assimilation
• Assimilation is the conversion of inorganic form
of nutrients into organic form of different
molecular weights, eg enzymes, proteins, nucleic
acids, amino acids, cofactors and lipids.
• Assimilation of some nutrients especially
nitrogen and sulphur requires a complex series of
biochemical reactions that require a lot of energy
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Nitrogen assimilation
• N is absorbed by the plants through xylem as NH4, or
NO3 by Amt-protein and Nitrate transport protein.
• Plants cannot use inorganic N as such so it has to be
reduced.
• Plant available NH4 form can be directly incorporated
into amino acids.
• Whereas NO3 is readily mobile in the xylem and can also
be stored in the vacuoles of roots, shoots, and storage
organs
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• Nitrate accumulation in vacuoles can be of
considerable importance for cation-anion balance
• However, in order to be incorporated into organic
and as essential functions in plant nutrient, nitrate
has to be reduced to ammonia.
• Most of Nitrate reduction take place in the shoots
while small amount of nitrate reduced in the root
system
NITRATE NITRITES AMMONIUM AMINO ACID
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Nitrate Reduction and Assimilation
• The reduction of nitrate to ammonia is mediated by two enzymes:
nitrate reductase (NR), which involves the two-electron reduction
of nitrate to nitrite, and nitrite reductase (NiR) which transforms
nitrite to ammonia in a six-electron reduction.
• First step nitrate converted to nitrite by nitrate reductase (NR)
• (NR) Contains three prosthetic groups, flavine adenine
dinucleotide (FAD), cytochrome 557 (Cytc) and molybdenum
cofactor (MoCo). NB. (Function as electron donor from reductant NAD(P)H via FAD,
haem and Moco to nitrate)
• Molybdenum deficiency in plants results in large accumulation of
NO3 in the plant.
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• (NR) use NAD(P)H or
NADH as an electron
donor.
• The reaction take
place in the cytosol
• Light, nitrates and
carbohydrates
dependent
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Nitrite reduction
• Nitrite is highly reactive and potentially toxic ion.
• Plant cell immediately transport nitrites from cytosol into
chloroplasts of leaves and plastids in the roots
• The enzyme responsible is nitrite reductase, reduces
nitrites to ammonium according the following equation
NO2- + Fdred + 6e + 8H+ NH4+ + 6Fdox + 2H2O
NIR
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Assimilation of nitrite cont..
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• In contrast to nitrate reductase which is localized in the
cytoplasm, nitrite reductase is localized in the
chloroplasts in leaves and in the proplastids of roots and
other nongreen tissue (Oaks and Hirel, 1985).
• In green leaves, the electron donor is reduced feredoxin,
generated in the light by photosystem I.
• In the dark and particularly in the roots and other
nongreen tissue a protein similar to feredoxin may serve
in this function (Rentsch et al., 2007)
• Energy for production of reducing equivalents is
provided by glycolysis (Couturier et all., 2007).

Assimilation of Ammonium
• Conversion of ammonia generated from nitrate
assimilation or photorespiration into amino acid.
• Plants cells avoid ammonium toxicity by rapidly
converting the ammonium generated from nitrate
assimilation or photorespiration into amino acids
• This requires the action of two enzymes
• Glutamine synthetase – combines ammonium with
glutamate to form glutamine
• Glutamate synthase – stimulated by elevated levels of
glutamine synthetase
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• 2 enzymes involved :
a. Glutamine synthetase (GS)
a. Glutamate synthase (also known as GOGAT)
• GOGAT – Glutamine 2-oxo-glutarate aminotransferase
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Glutamine
synthetase action
Glutamate are carboxylic
acid , an amino acids used
in synthases of protein.
In human it is non
essential amino acids.
Glutamine is non essential
amino acids = amide
charge neutral polar amino
acid
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Glutamate
synthase action
Transfers amide
group of glutamine
to 2-oxoglutarate,
and yields two
glutamate molecules.
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Types of GOGAT
• NADH-GOGAT
accepts electrons from NADH.
located in plastids of non-photosynthetic tissue like roots
Glutamine + 2-oxoglutarate + NADH + H+ 2 Glutamate + NAD+
• Fd-GOGAT
accepts electron from ferredoxin.
located in chloroplast
Glutamine + 2-oxoglutarate + Fdred 2 Glutamate + Fdox
4/6/2022 18

4/6/2022 19

Amino Acid and Protein Biosynthesis
• The organically bound nitrogen of glutamate and
glutamine can be used for the synthesis of other amides,
as well as ureides, amino acids, amines, peptides, and
high molecular- weight peptides such as proteins.
• Although plants may contain up to 200 different amino
acids, only about 20 of them are required for protein
synthesis.
• The carbon skeletons for these different amino acids are
derived mainly from intermediates of photosynthesis,
glycolysis, and the tricarboxylic acid cycle.
4/6/2022 20

• In protein synthesis the individual amino acids are
coupled by peptide bonds (RrCONH-R2) in a
condensation reaction.
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• Summary of nitrogen assimilation
4/6/2022 22
N2,
NH4,
NO3,
Amino
acid,
amide,
peptides,
amines
Proteins,
nucleic
acids,
enzymes,
membrane
constituents

Deficiency symptoms:
• Plants deficient in N are very slow growing, weak, and
stunted.
• Typically, plants are light green to yellow in foliage color,
starting with the older or mature foliage.
• Initial and more severe symptoms of yellow-leaf
deficiency are seen in older leaves, as N is mobilized in
the older tissue for transport to the actively growing
portions of the plant.
• N-deficient plants will mature early with yield and quality
significantly reduced.
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Excess (toxicity) symptoms:
• Plants with an excess of N are dark green in color with
succulent foliage, which is easily susceptible to disease
and insect invasion.
• Produced fruit and grain will be of poor quality.
• If NH4 is the only or major form of N available for plant
uptake, a toxicity condition may develop that results in a
breakdown of vascular tissue, thereby restricting water
uptake.
• Fruiting crops (e.g., tomato, pepper, cucumber) may
develop blossom-end rot symptoms, or fruit set may be
poor.
• Symptoms of Ca deficiency may occur if NH4 is the
primary source of N.
4/6/2022 24

sulphur
• Sulfur is an essential macronutrient required for plant
growth. It is primarily used to
• Involved in protein synthesis.
• Is part of the amino acids cysteine and thiamine.
• Is present in peptide glutathione, coenzyme A, and vitamin
B1, and in glycosides such as mustard oil and thiols,
which contribute the characteristic odor and taste to plants.
• Reduces the incidence of disease in many plants.
4/6/2022 25

Content and distribution in plants:
• Content in leaf tissue ranges from 0.15% to 0.50% of the
dry weight.
• Total S content varying with plant species and stage of
growth.
• Some plants like cereals, grasses, and potato may
contain from 11 to 90 kg S/ha
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Available forms for root absorption:
• The sulfate (SO4
2–) anion is the primary available form found
in the soil solution.
• In general, most of the available SO4
2– is found in the subsoil
as the anion and can be easily leached from the surface
horizon.
• Availability may depend on S fertilization, deposited by acid
rain and/or that released from organic matter decomposition.
• At high soil pH (>7.0), S may be precipitated as calcium
sulfate (CaSO4), while at lower pH levels (<4.0), the SO4
2–
anion may be adsorbed by Al and Fe oxides.
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Sulphur assimilation
• Conversion of inorganic sulphur compounds such as
SO4
2- into sulfur-containing organic compounds.
• The first step in sulfate assimilation in plants is
conversion of sulfate to cysteine.
• Sulfur in sulfate is present in highly oxidised state with
six positive charges while in cysteine it is present in
reduced state with four negative charges. Therefore,
conversion of sulfate into cysteine is a reduction process
that is energy dependent and requires ATP.
4/6/2022 28

• Reduction of sulfate to cystine is a multiple process
i) In the first step, sulfate is activated by ATP in the
presence of the enzyme ATP- sulfurylase to form APS
(Adenosine-5′-phosphosulfate) and pyrophosphate
(PPi). Mg++ ions are required in this reaction.
SO4
2- + Mg-ATP → APS + Ppi
• PPi so formed is quickly hydrolyzed by the enzyme
inorganic pyrophosphatase to yield two molecules of
inorganic phosphaie (2Pi).
PPi + H2O → 2Pi
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• ii) The APS (Adenosine-5′-phosphosulfate) is reduced by
different process in the plastids
• The enzyme APS-reductase transfers two electrons from two
molecules of reduced glutathione (2GSH) to produce sulfite
(SO3
2-). Glutathione is oxidised (GSSG).
• NB. GSH (tripeptide of glutamic acid, cysteine and glycine),
most antioxidant thiols = R-S-H ( R- from OH)
APS + 2 GSH → SO3
2- + 2 H+ + GSSG + AMP
• (iii) Sulfite (SO3
2-) is now reduced to form sulfide (S2-) in the
presence of the enzyme sulfite reductase.
• This reduction requires six electrons which are provided by 6
mols of reduced ferredoxin (Fd.red).
SO3
2- + 6 Fd.red → S2- + 6 Fd.oxi
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• (iv) In the last step, sulfide reacts with O-acetylserine (OAS)
under the influence of the enzyme OAS-thiolyase to form the
amino acid cysteine and acetic acid. N.B oas = alpha amino acid intermediate
• There is transfer of two electrons in this reduction, from OAS
to sulfide.
OAS + S2- → cysteine + acetate
• (The OAS is formed from serine and acetyl-CoA in the
presence of the enzyme serine acetyl transferase.)
• Sulfate (Sulfur) assimilation takes place chiefly in leaves in
chloroplasts.
• The sulfate absorbed by roots from soil solution is
translocated through xylem to shoots for assimilation.
• To some extent sulfate assimilation may also occur in roots in
proplastids.
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Formation of other Sulphur-containing Organic
Compounds:
• After the synthesis of cysteine, another sulfur-containing
amino acid methionine is synthesized from it.
• Thereafter, other sulfur-containing organic compounds
are synthesized from these two amino acids.
4/6/2022 32

Deficiency symptoms:
• Light yellow-green in foliage color initially over the entire plant.
• Roots are longer than normal and stems become woody; also, root
nodulation in legumes is reduced and delayed maturity occurs in
grains.
• Interestingly, S deficiency is desired in tobacco in order to obtain
proper leaf color.
• S-deficiency symptoms can sometimes be confused with N-
deficiency symptoms, although S symptoms normally affect the
whole plant, while N-deficiency symptoms occur initially on the
older portions of the plant.
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• Frequently on sandy and/or acid soils, symptoms of S
deficiency may occur in newly emerging plants only
to disappear as the plant roots enter the subsoil
because S as SO4
2– tends to accumulate in the subsoil
under such soil conditions; drought conditions may
reduce the uptake of S, thereby inducing a S
deficiency.
Excess (toxicity) symptoms:
• Premature senescence of leaves may occur.
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References
• Couturier J, Montanini B, Martin F, Brun A, Blaudez D, Chalot M.
The expanded family of ammonium transporters in the perennial
poplar plant. New Phytologist. 2007;174:137–150.
• Rentsch D, Schmidt S, Tegeder M. Transporters for uptake and
allocation of organic nitrogen compounds in plants. FEBS
Letters. 2007;581:2281–2289.
• Oaks, A. (1991). Nitrogen assimilation in roots: a re-evaluation.
Bio Science 42, 103- 111.
• Oaks, A. and Hirel, B. (1985). Nitrogen metabolism in roots. Annu.
Rev. Plant Physiol. 36, 345-365.
• Robinson, D. and Rorison, I. H. (1987). Root hairs and plant
growth at low nitrogen availabilities. New Phytol. 107, 681-693.
4/6/2022 35

THANK YOU
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