Part 3 Plant Mineral Nutrition
3.1 Overview of nitrogen in the biosphere
and in plants
The nitrogen cycle
Nitrogen deficiency phenotype in leaves of sugar beet,
Nitrogen deficiency is often associated with uniform
yellowing of older leaves.
Overview of N uptake by a nonnodulated plant (left),
and by a nodulated plant with N-fixing symbionts (right).
Table 3.1 Rates of natural and anthropogenic nitrogen
fixation
Source Amount of N fixed
Lighting <10 Tg/year
Biological N-fixation in terrestrial systems 90-140 Tg/year
Biological N-fixation in marine systems 30-300 Tg/year
N fertilizer synthesis 80 Tg/year
Fossil fuel combustion >20 Tg/year
* Tg =1012 g,
3.2 Biological nitrogen fixation
3.2.1 Nitrogen fixation reduces nitrogen gas to ammonia,at the cost of
ATP and reducing equivalent.
N2 + 16 ATP + 8 e - + 8 H→ 2 NH 3 + H2 + 16 ADP + 16 Pi
Eukaryotes cannot utilize dinitrogen,but some prokaryotes are able to
catalyze the enzymatic reduction of this compound to ammonia.
3.2.2 Nitrogen fixation is sensitive to oxygen.
3.2.3 Enzymology of nitrogen fixation
Table 3.2 Substrates and products of nitrogenase
Substrate Product
N2 NH3
H+ H2
N2O N2,H2O
CN- NH3,CH4
C2H2 C2H4,C2H6
3.3 Symbiotic nitrogen fixation
3.3.1 Some vascular plants establish nitrogen-fixing
symbioses
There are three major types of nitrogen-fixing symbioses.
1) Gram-negative bacteria,the rhizobia,form associations with
numerous legume host plants (Fabaceae) and at least one non
legume,Parasponia(Ulmaceae).
2) Gram-positive actinomycete genus ( Frankia) and a diverse
group a dicots,generally trees or woody shrubs from about 60
genera in 9 families,including alder(Alnus),myrtle(Myrica),
Casuarina,and Ceanothus.
3) Symbioses exist between cyanobacteria and a diverse array
of plants,dicots (e.g.,Gunnera),cycads,ferns,liverworts,and
hornworts,Azolla,a water fern,associates symbiotically with
the cyanobacterium,
3.3.2 Legumes create root nodules to house their
rhizobial symbionts.
Overview of events leading to formation of legume-
rhizobium symbiosis.
Release of bacteria from a walled infection thread into a
target cell.
Different plant-bacterial combinations After release,
bacteria differentiate to form morphologically yield
distinct bacteroid forms.
3.3.3 Legume roots exude inducers of bacterial
symbiosis genes.
Table 3.3.3 Some bacterial genes used in symbiosis with legumes.
-----------------------------------------------------------------------------------
Stage of symbiosis Rhizobium genes
Gene regulation in response to host plant signal nodD,nolR,nodVW,other
Nodule formation,host recognition nod,nol,noe.
Infection thread growth exo,lps,ndv,others.
Differentiation,bacteroid metabolism bacA,dct,others.
Regulation of bacterial nitrogen-fixation genes fixl,fixj,nifa,,fixk.
Nitrogen fixation nifHDK,other nif,fix.
Compouds released by plants induce bacterial gene expression,
which leads to production by bacteria of signals that modify plant
metabolism.
The plant nucleus encodes a series of genes that are
expressed only during late stages of nodule
development in response to bacterial differentiation..
Regulatory circuit of nodD and plant inducers.
3.3.4 Rhizobia and their plant hosts interact to create a
microaerobic nodule environment conducive to nitrogen
fixation.
Three factors are important in maintaining low oxygen
concentrations in nodules:
1) The entry of oxygen into the nodule is controlled by a
variable-permeability barrier in the nodule parenchyma.
2)Leghemoglobin,an oxygen-binding plant protein,plays
an active role in regulating and dilevring oxygen in the
infected cells.
3) Bacterial respiration constitutes a major oxygen sink.
Mechanisms that maintain ATP production in an
appropriately low oxygen environment in nitrogen-fixing
nodules
3.3.5 The host plant provides carbon to the bacteroids
as dicarboxylic acids,which are used to generate the
ATP and reductant needed for nitrogen fixation in the
bacteroids.
3.3.6 Plant ammonia assimilation occurs in the plant
nodule cytosol and organelles,Plant glutamine synthetase
(GS) and NADH-dependent glutamate synthase ( NADH-
GOGAT) are responsible for the initial assimilation of
ammonia into organic compounds.
3.4 Overview of nitrate uptake and reduction
Nitrate assimilation by plant cells involves transport of
nitrate across the plasma membrane and then reduction to
ammonia in a two-step process,A proton-pumping ATPase
maintains the electro-chemical gradient that drives cellular
uptake of nitrate,
The values shown for electrical potentials and
intracellular nitrate concentrations are typical but can
vary significantly.
3.4.1 Nitrate is reduced in the cytosol of root and shoot
cells.
The first committed step in the nitrate assimilation
pathway is the reduction of nitrate nitrite,The reaction is
catalyzed by NR,a complex metalloenzyme that forms
homodimers and homotetramers.
NO3-+ NAD(P)H +H+→NO 2- + NAD(P)++H2O
Domain structure of nitrate reductase
3.4.2 Nitrate and other compouds seve as signals to
regulate NR gene expression.
Table 3.4.2 Signals that influence the transcription and
activity of NR
Signal Effect on NR
Glutamine Down-regulates transcription
Nitrogen starvation Down-regulates transcription
Circadian rhythm Modulates transcription depending on time of day
Nitrate Up-regulates transcription
Cytokinin Up-regulates transcription
Sucrose Up-regulates transcription
Light Up-regulates transcription
Dark Down-regulates transcription and activty
High (CO2) Up-regulates transcription
Low (CO2) Down-regulates transcription
Oxygen Down-regulates transcription
Anoxia Up-regulates transcription
3.1 Overview of nitrogen in the biosphere
and in plants
The nitrogen cycle
Nitrogen deficiency phenotype in leaves of sugar beet,
Nitrogen deficiency is often associated with uniform
yellowing of older leaves.
Overview of N uptake by a nonnodulated plant (left),
and by a nodulated plant with N-fixing symbionts (right).
Table 3.1 Rates of natural and anthropogenic nitrogen
fixation
Source Amount of N fixed
Lighting <10 Tg/year
Biological N-fixation in terrestrial systems 90-140 Tg/year
Biological N-fixation in marine systems 30-300 Tg/year
N fertilizer synthesis 80 Tg/year
Fossil fuel combustion >20 Tg/year
* Tg =1012 g,
3.2 Biological nitrogen fixation
3.2.1 Nitrogen fixation reduces nitrogen gas to ammonia,at the cost of
ATP and reducing equivalent.
N2 + 16 ATP + 8 e - + 8 H→ 2 NH 3 + H2 + 16 ADP + 16 Pi
Eukaryotes cannot utilize dinitrogen,but some prokaryotes are able to
catalyze the enzymatic reduction of this compound to ammonia.
3.2.2 Nitrogen fixation is sensitive to oxygen.
3.2.3 Enzymology of nitrogen fixation
Table 3.2 Substrates and products of nitrogenase
Substrate Product
N2 NH3
H+ H2
N2O N2,H2O
CN- NH3,CH4
C2H2 C2H4,C2H6
3.3 Symbiotic nitrogen fixation
3.3.1 Some vascular plants establish nitrogen-fixing
symbioses
There are three major types of nitrogen-fixing symbioses.
1) Gram-negative bacteria,the rhizobia,form associations with
numerous legume host plants (Fabaceae) and at least one non
legume,Parasponia(Ulmaceae).
2) Gram-positive actinomycete genus ( Frankia) and a diverse
group a dicots,generally trees or woody shrubs from about 60
genera in 9 families,including alder(Alnus),myrtle(Myrica),
Casuarina,and Ceanothus.
3) Symbioses exist between cyanobacteria and a diverse array
of plants,dicots (e.g.,Gunnera),cycads,ferns,liverworts,and
hornworts,Azolla,a water fern,associates symbiotically with
the cyanobacterium,
3.3.2 Legumes create root nodules to house their
rhizobial symbionts.
Overview of events leading to formation of legume-
rhizobium symbiosis.
Release of bacteria from a walled infection thread into a
target cell.
Different plant-bacterial combinations After release,
bacteria differentiate to form morphologically yield
distinct bacteroid forms.
3.3.3 Legume roots exude inducers of bacterial
symbiosis genes.
Table 3.3.3 Some bacterial genes used in symbiosis with legumes.
-----------------------------------------------------------------------------------
Stage of symbiosis Rhizobium genes
Gene regulation in response to host plant signal nodD,nolR,nodVW,other
Nodule formation,host recognition nod,nol,noe.
Infection thread growth exo,lps,ndv,others.
Differentiation,bacteroid metabolism bacA,dct,others.
Regulation of bacterial nitrogen-fixation genes fixl,fixj,nifa,,fixk.
Nitrogen fixation nifHDK,other nif,fix.
Compouds released by plants induce bacterial gene expression,
which leads to production by bacteria of signals that modify plant
metabolism.
The plant nucleus encodes a series of genes that are
expressed only during late stages of nodule
development in response to bacterial differentiation..
Regulatory circuit of nodD and plant inducers.
3.3.4 Rhizobia and their plant hosts interact to create a
microaerobic nodule environment conducive to nitrogen
fixation.
Three factors are important in maintaining low oxygen
concentrations in nodules:
1) The entry of oxygen into the nodule is controlled by a
variable-permeability barrier in the nodule parenchyma.
2)Leghemoglobin,an oxygen-binding plant protein,plays
an active role in regulating and dilevring oxygen in the
infected cells.
3) Bacterial respiration constitutes a major oxygen sink.
Mechanisms that maintain ATP production in an
appropriately low oxygen environment in nitrogen-fixing
nodules
3.3.5 The host plant provides carbon to the bacteroids
as dicarboxylic acids,which are used to generate the
ATP and reductant needed for nitrogen fixation in the
bacteroids.
3.3.6 Plant ammonia assimilation occurs in the plant
nodule cytosol and organelles,Plant glutamine synthetase
(GS) and NADH-dependent glutamate synthase ( NADH-
GOGAT) are responsible for the initial assimilation of
ammonia into organic compounds.
3.4 Overview of nitrate uptake and reduction
Nitrate assimilation by plant cells involves transport of
nitrate across the plasma membrane and then reduction to
ammonia in a two-step process,A proton-pumping ATPase
maintains the electro-chemical gradient that drives cellular
uptake of nitrate,
The values shown for electrical potentials and
intracellular nitrate concentrations are typical but can
vary significantly.
3.4.1 Nitrate is reduced in the cytosol of root and shoot
cells.
The first committed step in the nitrate assimilation
pathway is the reduction of nitrate nitrite,The reaction is
catalyzed by NR,a complex metalloenzyme that forms
homodimers and homotetramers.
NO3-+ NAD(P)H +H+→NO 2- + NAD(P)++H2O
Domain structure of nitrate reductase
3.4.2 Nitrate and other compouds seve as signals to
regulate NR gene expression.
Table 3.4.2 Signals that influence the transcription and
activity of NR
Signal Effect on NR
Glutamine Down-regulates transcription
Nitrogen starvation Down-regulates transcription
Circadian rhythm Modulates transcription depending on time of day
Nitrate Up-regulates transcription
Cytokinin Up-regulates transcription
Sucrose Up-regulates transcription
Light Up-regulates transcription
Dark Down-regulates transcription and activty
High (CO2) Up-regulates transcription
Low (CO2) Down-regulates transcription
Oxygen Down-regulates transcription
Anoxia Up-regulates transcription
