Transcript NO 2

Chapter 12
Assimilation of Mineral Nutrients
Nutrient assimilation
Incorporation of mineral nutrients into
organic substances such as amino acids,
nucleic acids, enzyme cofactors, lipids,
and pigments
The Nitrogen Cycle
Thiobacillus
Denitrificans
(Denitrification)
Lightning
Bacteria or
cyanobacteria
75%
15%
N2 + H2
Fertilizer
Nitrosomonas
Nitrococcus
10%
Thiobacillus
denitrificans
Nitrobacter
(Nitrification)
Plants
Microorganisms
Food chain
Flag leaf
NH4+ toxicity can dissipate pH gradient
ammonium
ammonia
Nitrate assimilation at roots
NO3-
Nitrate-proton
cotransporter
Root cell
NO3- (Nitrate)
NADH
(or NADPH)
NAD+
(or NADP+)
GS/GOGAT
Nitrate
reductase
NO2(Nitrite, toxic)
Glutamine
Plastids
NO2-
Glutamate
NH4+
NADPH nitrite (Ammonium)
reductase
Nitrate reductase
 Homodimers, two identical subunits
 Polypeptide sequences are similar in eukaryotes
(except the hinge regions)
 Each subunit with 100kD
 Each subunit contains 3 prosthetic groups:
FAD, heme, and molybdenum complexed to
pterin (an organic molecule).
 Nitrate reductase is the main molybdenumcontaining protein in vegetative tissues.
The nitrate reductase dimer
NAD+, H+
NO2+
NO2+
(+3)
(+5)
NAD+, H+
Three binding domains
Addition of nitrate stimulates nitrate reductase mRNA
accumulation and enzyme activity in roots and shoots of
barley
Other factors regulate nitrate reductase
 Nitrate reductase-Pi
(inactive form)
Phosphatase
Nitrate reductase
(active form)
Light
[carbohydrate]
 Nitrate reductase
Nitrate reductase -Pi
(active form) Serine/Threonine (inactive form)
protein kinase
Dark
[Mg+2]
Regulation of nitrate reductase activity
through phosphorylation and
dephosphorylation provides more rapid
control than can be achieved through
synthesis or degradation of the enzyme
(minutes versus hours).
 In many plants, when the roots receive small
amounts of nitrate, nitrate is reduced primarily in
the roots. As the supply of nitrate increases, a
greater proportion of the absorbed nitrate is
translocated to the shoot and assimilated there.
 Generally, plants native to temperate region
assimilate NO3- at roots.
 Plants native to tropical or subtropical region
assimilate NO3- at shoots.
Nitrite reductase converts nitrite to ammonium in roots
NO3-
Root cell
NO3NADH
or NADPH
NAD+
or NADP+
Nitrate
reductase
Plastids
NO2-
NO2-
NADPH-nitrite
reductase
NH4+
Nitrite reductase converts nitrite to ammonium in leaves
NO3-
Mesophyll cell
NO3NADH
Nitrate
reductase
Chloroplasts
NAD+
NO2-
NO2-
Ferredoxinnitrite
reductase
NH4+
Nitrite reductase
 Nitrite (NO2-) is a highly reactive, potentially toxic ion.
 Plant cells immediately transport the nitrite into chloroplasts
or plastids.
 The enzyme nitrite reductase reduces nitrite to ammonium.
 Leaf chloroplasts and root plastids contain different forms of
nitrite reductase.
 Nitrite reductase is synthesized in cytoplasm with an Nterminal transit peptide.
 Both nitrite reductases consist of a single 63kD polypeptide.
 Each polypeptide contains two prosthetic groups, an ironsulfur group and a specialized heme.
 NO3-, high sucrose conc, and light induce the transcription of
nitrite reductase mRNA.
 Asparagine and glutamine repress the induction.
A. Chloroplasts
2 eNADP+, H+
Ferredoxin
- a protein of 12 ~ 24 kD
- containing a FeS group
- e- carried by the iron
NADPH
Pentose phosphate pathway
B. Root plastids
Relative amounts of nitrate and other nitrogen
compounds in the xylem exudate of various plant species
R-CO-NH2
Ammonium assimilation
 NH3 is almost certainly protonated to form NH4+.
 NH4+ is toxic to plants.
 Inhibit dinitrogenase
 Interfere energy metabolism by dissociating ATP
formation from ETC in both mitochondria and
chloroplasts.
 To avoid these problems, NH4+ generated from nitrate
assimilation or photorespiration is rapidly converted into
amino acids.
 Conversion of ammonium to amino acids requires two
enzymes, glutamine synthetase (GS) and glutamate
synthase (GOGAT).
Ammonium can be assimilated by one of
several processes
GS
GOGAT
Glutamine synthetase (GS)
 Glutamine synthetase
Glutamate + NH4+ -----------------> Glutamine
GS
 Two classes of glutamine synthetase (GS)
 in cytosol
 The cytosolic form GS express in germinating seeds or
in the vascular bundles of roots and shoots, and
produce glutamine for intracellular nitrogen transport
 in root plastids and in shoot chloroplasts
 GS in root plastids : generating amide nitrogen
(glutamine & asparagine) for local consumption.
 The GS in shoot chloroplast reassimilates
photorespiratory NH4+.
Ammonium assimilation via GS/GOGAT
Germinating
seeds/vascular bundles
of roots and shoots
2-oxoglutarate
2 Glutamate
GOGAT
Glutamine
GS
Plastids
NO2-
Nitrite reductase
NH4+
NH4+
Glutamate
Ammonium assimilation in roots
NO3-
Root cell
NO3NADH
or NADPH
NAD+
or NADP+
Nitrate
reductase
Plastids
NO2-
NO2-
NH4+
NADPH nitrite (Ammonium)
reductase
Glutamate
GS/NADHGOGAT
Ammonium assimilation in leaves
NO3-
Mesophyll cell
NO3NADH
Nitrate
reductase
Chloroplasts
NAD+
NO2-
NO2-
Ferredoxin-nitrite
reductase
Glutamate
NH4+
GS/Fd-GOGAT
 Light and carbohydrate levels alter the expression
of the plastid forms of the enzyme, but they have
little effect on the cytosolic forms.
Glutamate synthase (glutamine:2-oxoglutarate aminotransferase, GOGAT)
 Plants contain two types of GOGAT.
 NADH-GOGAT
 Ferredoxin-GOGAT (Fd-GOGAT)
 NADH-GOGAT
 NADH-GOGAT is located in plastids of roots or
vascular bundles of leaves.
 In roots, NADH-GOGAT is involved in the assimilation
of NH4+ absorbed from the rhizosphere.
 Fd-GOGAT
 Fd-GOGAT is found in chloroplasts and serves in
photorespiratory nitrogen metabolism.
Ammonium can be assimilated via an
alternative pathway
Alternative pathway for assimilating
ammonium
 Two types of Glutamate dehydrogenase (GDH) :
NADH-GDH is found in mitochondria.
NADPH-GDH is found in chloroplasts.
 Although both forms are relatively abundant, they
cannot substitute for the GS-GOGAT pathway for
assimilation of ammonium, and their primary
function is in deaminating glutamate during the
reallocation of nitrogen.
Transamination reactions transfer nitrogen
 Once assimilated into glutamine and glutamate,
nitrogen is incorporated into other amino acids via
transamination reactions.
 Aminotransferase catalyze the transamination
reaction.
 Aminotransferase are found in the cytoplasm,
chloroplasts, mitochondria, glyoxysomes, and
peroxisomes.
An example of aminotransferase
Ammonium Assimilation via GS/GOGAT
Root plastids/shoot chloroplast
NO2-
NH4+
Nitrite reductase
Glutamate
GS
Glutamate
2 Glutamate
GOGAT
Glutamine
2-oxoglutarate
Oxaloacetate
Aspartate
aminotransferase
2-oxoglutarate +
Aspartate
Transamination reactions
Asparagine synthetase
(AS)
Glutamine
Asparagine
Glutamate
Asparagine and glutamine link carbon and
nitrogen metabolism
 Asparagine serves not only as a protein precursor,
but as a key compound for nitrogen transport and
storage because of its stability and high nitrogen-tocarbon ratio (1:2).
 The major pathway for asparagine synthesis involves
the transfer of the amide nitrogen from glutamine to
asparagine.
 Asparagine synthetase (AS) catalyzes the asparagine
synthesis reaction.
 AS is found in the cytosol of leaves and roots, and in
nitrogen-fixing nodules.
amide
Biosynthetic pathways for the carbon skeletons of the
20 amino acids in plants
Amino acid biosynthesis
 The nitrogen-containing amino groups derives
from transamination reactions with glutamine or
glutamate.
 The carbon skeleton for amino acids derive from
3-phosphoglycerate, phosphoenolpyruvate, or
pyruvate generated during glycolysis, or from aketoglutarate or oxaloacetate generated in the
citric acid cycle.
 Human and most animals cannot synthesize
certain amino acids.
Histidine, isoleucine, leucine, lysine, methionine,
phenylalanine, threonine, tryptophan, valine
and arginine => essential amino acids
 Obtain these essential amino acids from their diet.
 In contrast, plants synthesize all the 20 amino
acids.
 High levels of light and carbohydrates stimulate
GS and GOGAT, inhibit AS, and thus favor
nitrogen assimilation into glutamine and glutamate,
compounds that are rich in carbon and participate
in the synthesis of new plant materials.
 In contrast, energy-limiting conditions inhibit GS
and GOGAT, stimulate AS, and thus favor nitrogen
assimilation into asparagine, a compound that is
rich in nitrogen and sufficient stable for longdistance transport or long-term storage.
N:C ratio
Allantoin (尿囊素 )
Allantoic acid
Asparagine
Citrulline
Glutamine
Glutamate
= 1:1
= 1:1
= 1:2
= 1:2
= 1:2.5
= 1:5
The more nitrogen ratio the molecule contains, the
higher efficiency the molecule transports nitrogen.
(Synthesized in
peroxisome)
(ER)
(amide : glutamine & asparagine )
p.310 Fig.12-21
Biological nitrogen fixation
Biological nitrogen fixation
 Biological nitrogen fixation accounts for most of the
fixation of atmospheric N2 into ammonium.
 N2 + 8H+ + 16ATP ---------------> 2NH3 + H2 + 16ADP + 16Pi
nitrogenase
 Two major types of nitrogen fixation :
 Prokaryotes that are free-living in the soil, lake, and
ocean.
 Symbiotic bacteria
(Nonsymbiotic N2 fixation)
N2 fixation
 Symbiotic N2 fixation
 Rhizobia – Legume
Unicellular
G(-)
Rhizobial symbiosis
 Actinomycetes – Nonleguminous plants
Frankia spp, Alnus (alder), Myrica (bayberry),
Casuarina (Australian pine)
Filamentous bacteria – G(+)
Live freely in soil, but fix nitrogen in symbiotic
association with host.
Actinorhizal symbiosis
 Nonsymbiotic N2 fixation
N2 fixation
 Symbiotic N2 fixation
 Nonsymbiotic N2 fixation
Cyanobacteria (Blue-green algae) : Nostoc,
Anabaena, Calothrix, etc.
Other bacteria
Aerobic : Azotobacter, Beijerinckia, Derxia, etc.
Facultative : Bacillus, Klebsiella, etc.
Anaerobic : Nonphotosynthetic - Clostridium,
Methanococcus
Photosynthetic – Rhodospirillum,
Chromatium
Cyanobacterium Anabaena
Heterocysts
 Thick-walled cells
 Differentiate when filamentous cyanobacteria are
deprived of NH4+.
 Heterocysts lack photosystem II, so they do not
generate oxygen.
 Exist among aerobic cyanobacteria that fix
nitrogen.
Root nodules on soybean
黃豆, 大豆
紫花苜蓿
扁豆
豌豆
蠶豆
苜蓿
菜豆
Symbiosis
 The symbiosis between legumes and rhizobia is
not obligatory.
 Legume seedlings can be unassociated with
rhizobia throughout their life cycle.
 Rhizobia also occur as free-living organisms in
the soil.
 Under nitrogen-limited conditions, the symbionts
seek out one another through an elaborate
exchange of signals.
Nodule formation
 Recognition and Colonization
 Chemotaxis (response to flavonoids and betaine)
pea roots – Rhizobium leguminosarum
 Flavonoids (host) -> activate expression of nod genes -> Nod
factor (lipo-chito-oligosaccharide)
 Lectin – Nod factor -------------------------> initiate nodule
formation
e.g. pea lectin gene -> white clover -> nodulated by R.
leguminosarum bv. viciae (pea)
 Rhicadhesin : a calcium-binding protein (of all rhizobia)
 Invasion of root hairs
 Nodulation
Flavonoids
Nod factor
Lectin
Root hair
Constitutive
expression
nod D
Nod factor
nod box nodA nodB nodC
Rhizobia
The common nod genes – nodA, nodB, and nodC – are found in all
rhizobial strains and are required for synthesizing the basic structure.
Root
Flavonoids
Nod factor
Lectin
Root hair
Constitutive
expression
nod D
Nod factor
nod box nodA nodB nodC
Rhizobia
The common nod genes – nodA, nodB, and nodC – are found in all
rhizobial strains and are required for synthesizing the basic structure.
Root
Nod Factors
 NodA is an N-acyltransferase that catalyzes the
addition of a fatty acyl chain.
 NodB is a chitin-oligosaccharide deacetylase that
removes the acetyl group from the terminal
nonreducing sugar.
 NodC is a chitin-oligosaccharide synthase that
links N-acetyl-D-glucosamine monomers.
General structure for Nod factors
NodP
NodQ
NodH
etc
NodC
NodB
NodA
18C NodE
16C NodF
20C
(varied
unsaturation
NodL
= 1-4
(n=2~3)
b-1,4-linked N-acetyl-D-glucosamine backbone
Flavonoids
Nod factor
Nod factor
nod D
Lectin
nod box nodF nodE nodL
nod box nodP nodQ nodH
Rhizobiz
Root
The host-specific nod genes – nodP, nodQ, and nodH, or nodF, nodE, and nodL differ among rhizobial species and determine the host range.
 NodE and NodF determine the length and
degree of saturation of the fatty acyl chain.
 NodL adds specific substitutions at the reducing
or nonreducing sugar moieties of the chitin
backbone.
 The host-specific nod genes – nodP, nodQ, and
nodH, or nodF, nodE, and nodL - differ among
rhizobial species and determine the host range.
General structure for Nod factors
NodP
NodQ
NodH
etc
NodC
NodB
NodA
18C NodE
16C NodF
20C
(varied
unsaturation
NodL
= 1-4
(n=2~3)
b-1,4-linked N-acetyl-D-glucosamine backbone
Determination of host specificity
 The binding of flavonoids to the NodD gene product is
one of the major determinants of rhizobial host
specificity because each plant species produces its own
specific set of flavonoid molecules and each rhizobial
species recognizes only a limited number of flavonoid
structures.
 Some strains, such as R. leguminosarum bv. trifolii,
have a very narrow host range, responding to only a
few kinds of flavonoids, while others, such as
Rhizobium sp. strain NGR234, have broad host range
and respond to a much larger number of different
flavonoids.
Recognition
Flavonoids
Nod factor
Lectin
Root hair
Constitutive
expression
nod D
Nod factor
nod box nodA nodB nodC
Rhizobia
Root
 A particular legume host responds to a specific Nod
factor.
 The legume receptors for Nod factor appear to
involve special lectins (sugar binding proteins)
produced in the root hairs.
 Nod factors activate these lectins, increasing their
hydrolysis of phosphoanhydride bonds of
nucleoside di- and triphosphates.
 This lectin activation directs particular rhizobia to
appropriate hosts and facilitates attachment of the
rhizobia to the cell walls of a root hair.
 A major goal of agricultural biotechnology research
is the development, by genetic manipulation, of
Rhizobium strains that can increase plant
productivity more than naturally occurring strains
can.
 However, to date, no simple genetic means has been
devised to use nod genes to enable inoculated strains
of Rhizobium to outcompete indigenous strains.
 The indigenous strains might be more efficient at
nodulation and, as a consequence, might prevent an
inoculated strain from becoming established.
 Nevertheless, host specificity can be altered by the
transfer of a nodD gene from a broad-specificity
rhizobial strain to one with a narrow specificity.
Bacteroid
Stages in the initiation and development of a soybean root nodule. (A) Events involved in the initiation of the
nodule: (1) the root excretes substances; (2) these substances attract rhizobia and stimulate them to produce celldivision factors; (3) cells in the root cortex divide to form the primary nodule meristem. (B) Stages of infection and
nodule formation: (4) bacteria attach to the root hair; (5) cells in the pericycle near the xylem poles are stimulated
to divide; (6) the infection thread forms and extends inward as the primary nodule meristem and the pericylce
continue to divide; (7) the two masses of dividing cells fuse into a single clump while the infection thread continues
to grow; (8) the nodule elongates and differentiates, including the vascular connection to the root stele. Bacteroids
are released into the cells in the center.
Nitrogen fixation by the nitrogenase enzyme complex
Nitrogen fixation by the nitrogenase enzyme complex
N2 + 8H+ + 16ATP ---------------> 2NH3 + H2 + 16ADP + 16Pi
Nitrogenase
Nitrogenase (Dinitrogenase)




Only prokaryotes have this enzyme.
~20% of the total protein in the cell
contains two components – Fe protein and MoFe protein
Fe protein
 contains 2 identical subunits (dimer) – nif H
 Each subunit has MW of 30 ~ 72 kD.
 Each subunit contains an Fe-S cluster (4Fe and 4S2-).
 MoFe protein
 2 pairs of identical subunits (tetramer, nif D & nif K)
 A total of MW 180 ~ 235kD
 Each subunit has 2 Mo-Fe-S clusters.
 Both Fe proteins and MoFe proteins are sensitive to O2.
The nitrogenase reaction in bacteroids
TCA cycle
Fe+2
Fe+3
NADH
Fe+3
NAD+, H +
Ferredoxin
- 12 ~ 24 kD
- a FeS group
- e- carried by the iron
- nif F
Fe+2
Fe+3
nif H
Fe+2
nif D, nif K
Fe+2
Fe+3
Fe+2
Fe+3
Fe+2
Fe+3
Proposed structure of the iron-molybdenum
cofactor bound to a molecule of dinitrogen (N2)
MoFe cofactor of MoFe protein
MoFe proteins can reduce numerous substrates
Under natural conditions it reacts only with N2 and H+.
Dinitrogenase is sensitive to O2
How to reconcile the conflicting demands of O2 for respiratory
(producing ATP for nitrogen fixation) and the sensitivity of
dinitrogenase to O2 ?
 Live anaerobically
Bacteroids
 Nitrogen-fixation vesicles (multilayered envelope
consisting of hopanoids-steroid lipids)
 Leghemoglobin
 O2-binding proteins
 located in the cytoplasm of the bacteroid-infected cells
(nodule cells) at high concentration (~ 30% of total
proteins or 700mM).
 Control the release of O2 in the region of bacteroids.
Leghemoglobin
 An oxygen-binding heme protein that help transport only
enough oxygen to the respiring symbiotic bacterial cells.
 Leghemoglobin has a high affinity for oxygen, about ten times
higher than the b chain of human hemoglobin.
 Present in the cytoplasm of infected nodule cells at high
concentration (~ 30% of total proteins or 700mM in soybean
nodules)
 Leghemoglobin contains two parts : globin portion and heme
portion.
 The host plant produces the globin portion in response to
infection by bacteria; the bacterial symbiont produces the
heme portion.
O2
leghemoglobin-O2
Peribacteroid
membrane
H2O
In soybean, 12 g of carbon are required to fix 1 g of N2 => Nitrogen
fixation is energy demanding.
Genetic engineering of the nitrogenase gene
cluster
 The first nif genes identified by complementation were
isolated from klebsiella pneumoniae.
 The entire set of the nif genes are arranged in a single cluster
that occupies approximately 24 Kb.
 The cluster contains of 7 operons that encode 20 proteins.
 Alfalfa plants that were inoculated by the Rhizobium meliloti
contain an extra copy of the nifA gene (a positive activator)
grew larger and produced more biomass than plants treated
with the nontransformed strain.
 Genetic modification of plants with the entire nif genes
cluster would not be effective because the normal level of
oxygen in the host cell would inactivate nitrogenase.
pLAFR1
Isolation of nif
genes by genetic
complementation
The nif gene cluster
(Fe protein)
Ferredoxin
(MoFe protein)
N2 + 8H+ + 8e- +16ATP ---------------> 2NH3 + H2 + 16ADP + 16Pi
Nitrogenase
 Accompany of H2 envolving during N2 fixation is
energy consuming.
 N2 fixer contains O2-dependent “uptake
hydrogenase” which recovers some of the energy
lost to H2 production by coupling H2 oxidation to
ATP production.
Recycling of the hydrogen gas produced by
nitrogen fixation
H+
H2
e-
Some strains of B. japonicum could use hydrogen as an energy
source for growth under low-oxygen concentration condition.
O2
leghemoglobin-O2
H2O
 Amides and ureides are the major transported forms of
nitrogen.
 Amide (醯胺, R-CO-NH2) : glutamine, asparagine
 Ureides (醯脲): allantoic acid (尿囊酸), allantoin (尿囊
素), and citrulline (瓜氨酸),
(Synthesized in
peroxisome)
(ER)
The major ureide compounds used to transport
nitrogen
NH4+ is reassimilated via
GS/GOGAT systems
Release NH4+
Other tissue
allantoinase
peroxisome
Urate
oxidase
Legume
 Tropical legume
Soybean, cowpeas, Southern peas, kidney beans,
peanut, and mung beans
Use ureides for nitrogen transport => ureide
exporters
 Temperate legume
Peas, lupins (羽扇豆), alfalfa, clovers, broad
bean, and lentil (扁豆)
Use glutamine or asparagine (amide) for
nitrogen translocation => amide exporters
N:C ratio
Allantoin (尿囊素 )
Allantoic acid
Asparagine
Citrulline
Glutamine
Glutamate
= 1:1
= 1:1
= 1:2
= 1:2
= 1:2.5
= 1:5
The more nitrogen ratio the molecule contains, the
higher efficiency the molecule transports nitrogen.
Importance of sulfur in cells
 Confer disulfide bridge in proteins
 Participate in Fe-S clusters for electron transport
 Several enzymes need sulfur.
 Some secondary metabolites contain sulfur.
Sulfur assimilation
 Most of the sulfur in higher plants derives from
sulfate (SO4-2) absorbed from the soil solution via an
H+-SO4-2 symporter.
 The first step in the synthesis of sulfur-containing
organic compounds is the reduction of sulfate (+6) to
cysteine (-4).
 The enzymes involved in cysteine synthesis have been
found in the cytosol, plastids and mitochondria.
 Sulfate assimilation occurs mainly in leaves.
 Sulfur assimilated in leaves is exported as glutathione
via the phloem to sites of protein synthesis.
Glutathione : r-glutamylcysteinylglycine
Reduced
Oxidized
Phosphate assimilation
 Plant roots absorb phosphate (H2PO4-) via H+H2PO4- symporter from the soil.
 The main entry point of phosphate into
assimilatory pathway is ATP.
 The phosphate is subsequently incorporated into
a variety of organic compounds, including sugar
phosphates, phospholipids, and nucleotides.
Cation assimilation
 Plants assimilate macronutrient cations (P, Mg,
and Ca) and micronutrient cations (Cu, Fe, Mn,
Co, Na, and Zn).
 Cations taken up by plant cells form complexes
with organic compounds.
 Cations form coordination bonds and electrostatic
bonds with carbon compounds.
Examples of coordination complexes
A major constituent
of pectins
Examples of electrostatic complexes
Uptake of iron from soil
 Most of the iron in the plant is found in the heme
molecule of cytochromes within the chloroplasts
and mitochondria.
 In addition, iron also exists in iron-sulfur proteins
(ferredoxin).
The ferrochelatase reaction
A precursor of heme group in cytochrome
Uptake of iron from soil
 Supplied FeSO4 or Fe(NO3)2, iron is present
primarily as ferric iron (Fe+3) in oxides such as
Fe(OH)3 and can precipitate out of solution. At
neutral pH, ferric iron is highly insoluble, and the
Ksp is 2X10-39.
 If phosphate salts are present, insoluble iron
phosphate Fe2(PO4)3 will form.
 How to increase iron solubility in soil and its
availability for plants?
Soil acidification increases the solubility of ferric
iron.
Reduction of ferric iron (Fe+3) to the more soluble
ferrous form (Fe+2) at the root surface and release
the iron from the chelator.
(continued)
Problem in maintaining availability of iron
 Root cells may reduce the Fe+3 to Fe+2.
 After uptake into the root, iron is kept soluble by
chelation with organic chelator citric acid.
 Chelator agents such as citric acid, caffeic acid,
tartaric acid, EDTA, and DTPA are used to solve the
problems.
The chelator DTPA and chelated to iron
DTPA
DTPA-Fe
p.78 Fig. 5.2
Two strategies for plants to uptake Fe
 Strategy I
Nongrain dicot (leafy vegetable)
Pea, tomato, soybean
 Strategy II
Monocot grain
Barley, maize, oat
Strategy I : nongrain dicot (leafy vegetable)
ATP
rhizosphere
Soil
colloid
Fe+3
H+
Proton
pump
Cytoplasm
H+
ADP + Pi
Fe+3
Fe+3
NADH
Fe+3
Ferric
reductase
Chelator
Fe+2
NAD+
Ferritin
Fe+2
Iron
transporter
Plasma membrane
Fe+2
Fe+2
In soil or nutrient solution
 Fe+2[SO4-2 or (NO3-)2] -> Fe(OH)3 or Fe2(PO4)3
(precipitate)
 Fe+3 + chelator (EDTA, citric acid, caffeic acid, and
tartaric acid) -> chelator-Fe+3 -> root surface ->
chelator-Fe +3 -> chelator + Fe+2
Fe+2 is absorbed by roots (Fe+2 + citric acids) ->
xylem
chelator diffuse back to soil or solution
Chelators
The chelator DTPA and chelated to iron
DTPA
DTPA-Fe
p.78 Fig. 5.2
增加米中鐵的含量
 從黃豆(Soybean)選殖ferritin
Control
Fk1
FK11
Seed
14
38
35
gene並轉殖入米中
Leaf Stem Root
119
170
956 (mg-Fe/g-DW)
104
162
962
98
175 966
Fe+3
Strategy I
nongrain dicot (leafy vegetable)
Phytosiderophores (PS)
Strategy II
Monocot grain (Graminae)
Phytosiderophores (PS)
 PS are nitrogen containing compounds.
 PS are found in members of family, Graminae.
 PS are synthesized and released under iron
stress.
 PS have high affinity for Fe+3 and can scavenge
iron from rhizosphere.
 Iron-PS complexes (ferrisiderophore) is then
reabsorbed into the roots.
End