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M. Sc. BOTANY
Self –instructional material
Paper- IV
PLANT PHYSIOLOGY AND
METABOLISM
Block-III
M.P. BHOJ (OPEN)
UNIVERSITY
Paper-IV
Block – III
Unit – 4 page 2-20
Edited by –
Written by-
Dr. Renu Mishra
Prof. Uday Chitnis
Head, Dept. of Botany
and Microbiology
Sri Sathya Sai College,
Asstt. Prof. of Botany
Govt. V.M.L.B.Girls’ P.G.
College, Kila Bhavan,
Bhopal (M.P.).
Indore (M.P.)
2
UNIT-IV
Structure
4.0 Introduction
4.1 Objectives
4.2 Nitrogen fixation, nitrogen and sulphur metabolism
4.2.1 Overview
4.2.2 Biological nitrogen fixation
4.2.3 Nodule formation and nod factors
4.2.4 Mechanism of nitrate uptake
4.2.5 Reduction of nitrate
4.2.6 Ammonium assimilation
4.2.7 Sulphate uptake, transport and assimilation.
4.3 Sensory Photobiology
4.3.1 History
4.3.2 Phytochromes and cryptochromes
biochemical and photochemical properties
4.3.3 Photophysiology of light induced responses
4.3.4 Cellular localization
4.3.5 Mechanism of action of photomorphogenetic receptors
4.3.6
4.4
4.5
4.6
4.7
Signaling and gene expression
Lets sum up
Check your progress
Activities
References
3
4.0
Introduction
In this unit, we’ll try to understand about the fixation of
atmospheric nitrogen by various micro organisms. The nitrogen which is fixed
symbiotically or assymbiotically is utilized by plants and fixed into organic
combinations such as amino-acids, proteins, nucleic acids, etc. in living
organisms via inorganic forms as ammonia. The amount of nitrogen in the soil
varies from 0.096 to 0.21 per cent. Nitrogen is an essential constituent of
proteins, chlorophyll and protoplasm; it is essential for growth of vegetative
parts but delays reproductive activity.
Sulphur is another macronutrient, which is usually found in the
complex proteins of the plants. It is the main constituent of several
coenzymes, vitamins (thiamine, biotin, CoA) and ferredoxin. The plants take
this element from the soil in the form of calcium and potassium sulphate.
Light is the ultimate source of free energy for all life. All biological
process initiated by light depend, on the absorption of photons by specific
photoreceptive substances which become activated and energized. Their
changed chemical reactivity initiates a chain of metabolic processes, which
leads to developmental changes.
4.1
Objectives
The main aim of this unit is to study the nitrogen and sulfate
metabolism and sensory photobiology. After going through this unit, you
would be able to
Know about biological nitrogen fixation.

Know about nodule formation.

Understand nitrogen metabolism

Understand sulfate metabolism.

Know about phyotochromes and cryoptochromes

Photomorphogenesis.
4
4.2
4.2.1
Nitrogen fixation, nitrogen and sulphur metabolism
Overview
Earth’s atmosphere contains 78 % nitrogen, but this is always in short
supply to the plants as only few microorganisms are capable of converting
molecular nitrogen into forms which can be utilized by plants. These
microorganisms are of four principal types:

Symbiotic microorganisms living in the roots of certain plants,

Free-living heterotrophic soil bacteria,

Photosynthetic bacteria,

Photosynthetic blue-green algae.
They together fix as much as 100,000,000 tons per year of atmospheric
nitrogen.
4.2.2
Biological nitrogen fixation
The first report of fixation of nitrogen by plants was obtained in
1838 by Boussingault.
Symbiotic nitrogen fixationAlthough, many plants as Azolla and cycads contain blue green algae
as symbionts,Pinus contains Mycorrhiza,some members of Rubiaceae contain
nitrogen fixing bacteria in their leaf nodules and Alnus, Myrica and
Hippophaë contain actinomycetes as symbionts, the best known nitrogen
fixing symbiotic bacterium is Rhizobium leguminosarum.
This bacterium
lives in soil to form root nodules in plants of the family Leguminosae such as
gram, pea, groundnut, beans etc. root nodules are little outgrowths on roots.
They are pink in color due to the presence of a pigment called
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Leghaemoglobin.
Leghaemoglobin
is
an
oxygen
scavenger.
The
Leghaemoglobin combines with oxygen present in the nodule and protects
the enzyme nitrogenase (enzyme catalyses the fixation of nitrogen function
under anaerobic conditions.
Nodules act as a site for nitrogen fixation. It contains all the necessary
biochemical compounds, such as nitrogenase and Leghaemoglobin. The
enzyme nitrogenase is a Mo-Fe protein and catalyses the conversion of
atmospheric N2 to NH3.
4.2.3
Nodule formation and nod factors
When a root hair of a leguminous plant comes in contact with Rhizobium
it becomes curved due to some specific chemical substances secreted by
bacteria. Partial destruction of cell wall takes place at the point of contact.
At this site, Rhizobium embedded in a thread of mucilagineous substance
invades in the root tissue and multiplies within the root hair. Some of the
bacteria enlarge to form membrane bound structures called bacteriods. The
bacteriods cannot divide, so, some bacteria remain untransformed, and they
carry out the infection to spread up.
An infection thread made up of plasma membrane is formed, which grows
inward from the infected cell of the plant that separates the infected tissue
from rest of the plant.
Cell division is stimulated in the infected tissue and more bacteria invade
the newly formed tissue. It is believed that a combination of cytokinin
produced by the invading bacteria and auxin produced by plant cells,
promotes cell division and extension, which leads to nodule formation. These
cells have higher DNA content in their nuclei and the chromosome number is
also polyploid.
The nodule thus formed makes direct vascular connection with the host for
exchange of nutrients.
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The nitrogen fixation takes place under the control of plant nod genes and
bacterial nod, nif and fix gene cluster.
The nitrogen fixation genes (nif) in Rhizobium are more or less similar in
structure to the nif genes of free-living bacterium Klebsiella pneumoniae. In
Klebsiella, 17 genes have been identified to be involved in nitrogen fixation.
Rhizobium contains the nifHD and K genes (nitrogenase proteins)
nifA(regulation) and nifB (synthesis of FeMoCo).
Nod-Genes for Nodulation- only a few genes are required for nodulation.
In the R. leguminosarum symbiotic plasmid pRL1JI, less than 10kb is
required for nodulation in peas.
In Klebsiella pneumoniae , the regulation of the nif-genes is carried out by
genes nifA and nif L and nif cluster (gin genes). Some other genes which are
involved in the expression of nif genes are1. nar D = is involved in molybdenum processing
2. unc and a gene in his operon = influence ATP supply
3. nim gene = gene of the uncertain nature near tip.
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Non-symbiotic nitrogen fixationAs stated earlier, Nitrogen is fixed both in photosynthetic and nonphotosynthetic organisms. Photosynthetic blue-green algae such as Anabaena
and Nostoc can fix nitrogen by using light energy and deriving electrons from
water. All nitrogen fixing blue-green algae have long, thick walled colorless
cells called heterocysts. These are the site of nitrogen fixation.
Green
and
purple
non-sulphur
bacteria
as
Rhodospirullum,
Rhodopseudomonas, Cholorobium, Chromatium get electrons from other
reduced electron donors in presence of light energy.
A proper amount of minerals like molybdenum, iron and calcium in the
soil is essential for nitrogen fixation.
Mechanism of nitrogen fixationJ.E. Carnahan and his group worked upon a two enzyme systems from
Clostridium pasteurianum during 1955-60. The reduction of N2 to NH3 is
catalyzed by nitrogenase, a complex enzyme consists of two proteins one of
them contains iron, is known as Heme- Protein or denitrogen reductase and
the other one which contains both molybdenum and iron at its reactive site is
known as iron molybdenum protein or dinitrogenase. The conversion takes
place at the surface of nitrogenase N2 bonds to both metals at the reactive site
and is then reduced to NH3 as followsN2  N2H2  N2H4NH3,
Diimide
hydrazine
These intermediate compounds are extremely poisonous so they do not
occur in free state, but are bound to enzyme systems.
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Nitrogen fixation is inhibited by presence of oxygen, hydrogen, poisons,
organic nitrogen compounds and ammonia.
4.2.4
Mechanism of nitrate uptake
NH3 produced by nitrogen fixation is rapidly converted to nitrate by soil
bacteria. So, the nitrogen is available in the form of highly oxidized form
nitrate NO3, nitrites and ammonia. Of these, the nitrates are absorbed at the
highest pace followed by nitrites and ammonium ions.
Oxidation of ammonia into nitrate is known as nitrification. Many autotrophic
bacteria utilize this oxidation process to derive energy for their metabolic
activities. Nitrification involves two steps1. Ammonia is oxidized into nitrite. This oxidation process is carried out
by bacteria like Nitrosomonas.
2NH3 + O2  2HNO2 + 2H2O + ENERGY
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2. Nitrite is further oxidized into nitrate by bacteria like Nitrobacter
HNO2 + ½ O2  HNO3 + ENERGY
These bacteria excrete nitric acid into the soil, where it dissociates to
hydrogen ion and nitrate ions.
Much of the nitrate is absorbed by roots and is swept upward to leaves.
Many plants, such as tomato, normally reduce nitrates in the roots. Grasses,
normally transport nitrate to leaves and store it there, utilize it when required.
Thus, nitrate assimilation occurs mostly in leaves.
4.2.5
Reduction of nitrate
The reduction of nitrate to ammonium is again takes place in two steps(1)
Reduction of Nitrate to NitriteReduction of Nitrate to Nitrite takes place in the presence of the enzyme
nitrate reductase which requires reduced coenzyme I (NADH) or II (NADPH).
NO3 + NADH + H+
 NO2 + NAD + H2O
This enzyme is a Molybdoflavo protein with a sulfhydryl group. It was first
isolated by Evans and Nason in 1953 from Neurospora and Soybean leaves.
This enzyme contains FAD (Flavin Adenine Dinucleotide) as its prosthetic
group and molybdenum.
(2)
Reduction of Nitrite to AmmoniaIt takes place in the presence of the enzyme nitrite reductase which requires
reduced coenzyme I or II.
3NADH + NO2 + 4H+
 3NAD + NH3 + 2H2O
Manganese seems to be associated with the enzyme. This was first isolated
by Nason et al from Neurospora and Soybean leaves.
Previously it was thought that the reduction of nitrite to ammonia
involved the formation of two intermediate compounds but it is doubtful as-
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NH3  NO2-  N2O22- NH2OH  NH3
Nitrate
nitrite
hyponitrite
hydroxylamine
ammonia
(i) Hyponitrite is quite unstable.
(ii) Hydroxylamine is toxic.
(iii)They were never observed in free state in the cells.
Now, it is believed that Hydroxylamine and Hyponitrite may be formed at
the surface of the enzyme and leave the surface when they are completely reduced
to ammonia.
Nitrate reduction is strongly stimulated by light, by the presence of nitrate,
and in some plants by hormones as gibberellins and cytokinins.
4.2.6
Ammonium assimilation
Inorganic nitrogen in the form of ammonia (produced by reduction of nitrates
or by biological nitrogen fixation or obtained from the soil) is converted into the
amide amino group of glutamine with the help of enzyme glutamine synthetase.
The ammonia rapidly becomes incorporated into carbohydrates to form aminoacids. The amino-acids are compounds that have an amino and an acid group on a
carbon skeleton. They are initial products of nitrogen assimilation. There are two
ways in which nitrogen is converted to amino-acids1. Reductive amination
2. Transamination
1. Reductive amination- The ammonia reacts with -ketoglutaric acid (an
intermediate of Kreb’s cycle) in the presence of enzyme glutamic
dehydrogenase and reduced enzyme NADPH2 to form an amino acid,
the glutamic acid, and the ammonia is converted to organic form.
-ketoglutaric acid + NADPH2 +NH3  Glutamic acid + NADP +H2O
As this process of conversion of the inorganic nitrogen (NH3) into organic
nitrogen (amino acid) is accompanied by Amination and Reduction at the keto
group of the organic acid hence it is called as reductive amination.
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There are three more ways to form amino acidsPyruvic acid + NADPH2 +NH3  Alanine + NADP +H2O
Oxaloacetic acid + NADPH2 +NH3  Aspartic acid + NADP +H2O
Fumaric acid +NH3  Aspartic acid
2. Transamination – other amino acids are produced by Transamination
reactions involving the transfer of amino group from gltamic acid to the
keto position of the other amino acid. Amino groups from other amino
acids except glutamic acid may also be transformed to other keto acids
forming more amino acids.
Transamination reactions require the presence of an enzyme
Transaminase or aminotransferase which needs coenzyme Pyridoxal
phosphate. The coenzyme Pyridoxal phosphate acts as a carrier of amino
group. It picks up the amino group from the donor amino acid and is
converted into Pyridoxamine phosphate. It transfers this amino group to
the acceptor keto acid forming a new amino acid and is itself is converted
into Pyridoxal phosphate.
Amino acids may also be produced by the transformation of acid amides and
other nitrogenous compounds or by the hydrolysis of proteins by proteolytic
enzymes.
These amino acids are then further joining together by peptide bonds and form
proteins which are the building blocks of protoplasm and various metabolic
biomolecules.
4.2.7
Sulphate uptake, transport and assimilation.
Sulphur is present as sulphate of minerals, elemental sulphur and
sulphides of iron in soil. But, the last two forms are not available to plants. A
number of soil microorganisms are capable of oxidizing sulphur or the sulphides
to sulphate and decomposing the organic sulphur compounds.
The importance of sulphur has already been mentioned. It is also found in the
form of oxidizable sulphhydryl (-SH) groups, which form the active site of some
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redox and electron transfer agents. It also forms disulphide (S-S) bonds that are
necessary in forming and stabilizing the tertiary structure of enzymes and other
proteins.
Higher plants obtain their supply of sulphur principally by uptake of sulphate
ions by roots. Although trace amounts of sulphur dioxide gas are absorbed and
assimilated by leaves, but it is also converted to sulphate ions.
Most of the sulphate absorbed by the roots is carried upward in the
transpiration stream to leaves, where it is assimilated. In fact, sulphate
assimilation can occur in all actively growing cells.
The assimilation of sulfate ions begins with its activation. First, a sulphate ion
reacts with ATP to yield adenosine -5’-phosphosulphate (APS) the reaction is
catalyzed by enzyme ATP- sulfurylase. Now, APS reacts with another molecule
of ATP to form 3’-phosphoaenosine-5’-phosphosulfate (PAPS) the reaction is
catalyzed by enzyme APS-kinase.These two forms are interconvertible and afe
referred to as Activated sulfates. The PAPS is the form of storage in plant cells
but APS is the substrate for sulfate reduction.
The APS is reduced through adenosine-5’-phosphosulphite and adenosine 5’phosphosulphide is formed in the catalytic activity of enzyme complex sulphate
reductase.
This sulphide reacts with O-acetyl serine and converts it into cysteine, a
sulphur-containing amino-acid; the reaction is catalyzed by enzyme O-acetyl
serine sulfhydrase. The amino acid cysteine is the starting point of the
biosynthesis of a wide variety of sulphur containing plant constituents.
4.3 Sensory Photobiology
Plants are able to detect the quality, quantity and direction of light and use it
as an external signal to optimize their growth and development. Plants have at
least two types of photoreceptors which acquire the information. These are the
phytochrome(absorbs mainly the red (R) and far-red (FR) regions of spectrum)
and Cryptochrome (blue (B) or UV-A photoreceptors).
13
4.3.1
History
S. B. Hendricks and H.A. Borthwick of United States Department of
Agriculture in Beltsville, Maryland were working on the effect of light on lettuceseed germination. These workers found that germination was promoted by red
light, not by blue or far-red light. They further tried to find out whether the
inhibitory far-red light would reverse the promoting effect of red light and vice
versa.
These experiments show that a light-receptive pigment exists and it has two
forms. One absorbs red light, converted to the other form. The other form absorbs
far-red light, and is converted back to the first form.
For this, a special spectrophotometer of extreme sensitivity was designed by
W. Butler. Experiments were conducted using etiolated tissue to minimize the
interference of chlorophylls and other pigments. The isolation of the pigment was
very difficult as it is present in very small amounts. It was named as Phytochrome.
H.W. Siegelman in early 1960s purified phytochrome from homogenates of
cereal-grain seedlings by using column chromatography and other techniques.
4.3.2
Phytochromes and cryptochromes
biochemical and photochemical properties
OCCURRENCE
Phytochrome is found in green and red algae, desmids, bryophytes,
gymnosperms and angiosperms. It has been reported from roots, stems ,
coleoptiles,hypocotyls, cotyledons, petioles and blades of leaves, ve4getative
buds, floral tissues, seeds and developing fruits.
PHYSICO-CHEMICAL NATURE-
It is made up of a protein and chromophore. Protein is composed of
structural units called amino acids which form polypeptide chain through peptide
linkages. The amino acids composition and their sequence in different proteins of
phytochromes detected from different plants usually differ but more or less
proteins show similar size or configuration.
14
Phytochromes are of two different sizes have been isolated and identified.
The small-sized phytochrome possesses a molecular weight of about 60 kDa. and
is believed to be produced by the degradation of large-sized phytochrome. It does
not exist in vivo. The large-sized natural phytochrome is a dimer. Its each
monomer unit has a molecular weight of about 120 kDa. Each monomer unit is
made up of one globular protein monomer and one chromophore. Due to the
presence of chromophore appears bluish in colour.
Diagrammatic representation of a phytochrome molecule
Globular protein Monomer: Each protein monomer contains three disulphide
(S-S) bonds. Each unit is made up of about 1100 amino-acids. About 46% amino
acids including lysine, histidine, arginine, aspargine, serine, glutamic acid, and
glutamine are polar and the sequence of amino acids in a protein monomer is as
followsLeucine – arginine – alanine – praline – histidine – serine – cysteine – histidine –
leucine –glutamine – tyrosine.
Chromophore:
each chromophore is linked to protein monomer through
cysteine residue by thio-ester linkages to the C-2 side chain of ring A. The basic
structure of chromophore is like the cyanophyceaen algal pigment cphycocyanin.
15
Chemical structure of the chromophore of the phytochrome
Chromophore is made up of four pyrrole rings arranged in alinear row.
These rings posses many double bonds which become rearranged as a result of
absorption of rar-red light. Thus, the inter-conversion of red form(Pr) and far
red form (Pfr)is due to the redistribution or shifting of double bonds in the
pyrrole rings of chromophore.
The phytochromes also contain one phosphate per monomer unit whose
function is unknown.
In darkness PR is produced within the cytoplasm. Equilibrium is always
maintained between synthesis and degradation of phytochrome. The
transformation of PR into Pfr after exposure to red light is a fast process. Pfr is
highly unstable compound .
16
DIFFERENCES BETWEEN PR AND PFR FORMS OF
PHYTOCHROME
PR
PFR Form
Form
1. It is an inactive form
of phytochrome.
2.
It
does
phytochrome
not
1. It is an active form of phytochrome.
show
mediated
2. It shows phytochrome mediated
responses.
responses.
3. Absorption maximum in
3. Absorption maximum in far red
red region (about 680nm)
region (about 730nm)
4. It is diffused throughout
4. It is found in discrete areas of
the cytosol.
the cytosol.
5. It is converted to PFR form
5. It is converted to PR form in
in presence of red light (660-
presence of far red light (730-
665nm).
735nm).
17
6.
at
6. On centrifugation at 20,000x g,
20,000x g, it remains in
it settles down in the form of
supernatant.
pillets.
7.
On
It
centrifugation
shows
activity
in
7. It shows more activity in
presence of urea, metal ions
presence of urea, metal ions Cu2+,
Cu2+, Co2+, Zn2+ etc. and N-
Co2+,
ethyl maleimide.
maleimide.
8. It shows many double
8. It shows rearrangements of
bonds in pyrrole rings.
Zn2+
etc.
and
N-ethyl
double bonds in all four pyrrole
rings.
CryptochromesCryptochromes are photosensory receptors mediating light regulation of
growth and development in plants. Since the isolation of the Arabidopsis CRY1
gene in 1993, cryptochromes have been found in every multicellular eukaryote
examined. Most plant cryptochromes have a chromophore-binding domain that
shares similar structure with DNA photolyase, and a carboxyl terminal extension
that contains a DQXVP-acidic-STAES (DAS) domain conserved from moss, to
fern, to angiosperm. In Arabidopsis, cryptochromes are nuclear proteins that
mediate light control of stem elongation, leaf expansion, photoperiodic flowering,
and the circadian clock. Cryptochromes may act by interacting with proteins such
as phytochromes, COP1, and clock proteins, or/and chromatin and DNA. Recent
studies
suggest
that
cryptochromes
18
undergo
a
blue
light–dependent
phosphorylation that affects the conformation, intermolecular interactions,
physiological activities, and protein abundance of the photoreceptors.
4.3.4 Cellular localization
Phytochromes have been localized within the cell plasma, the nucleus and
the plastids by Indirect immunofluoresence. Not all the cells contain the same
amount. In the epidermis, phytochrome is present in the guard cells only.
In the dark, phytochrome is localized to the cytoplasm. In the light,
phytochrome translocates to the nucleus. PR is oriented in parallel to the cell
surface, PFR is oriented vertically.
Cryptochromes probably exists associated with a cytochrome protein in or
tightly bound to the plasma membrane.
4.3.5 Mechanism of action of photomorphogenetic receptors
It has been assumed that further phytochrome states PRX’, PFRX’,
PRX’’, PFRX’’ exist in addition to PR and PFR and can reversibly be transformed into
each other. They are in equilibrium with each other within the cell. It looks as if
energy is transferred between the sensory pigments thus causing a strong
modulation of the sensitivity towards light. This amplifies the original light
signal.
Cryptochromes action requires the presence of phytochrome as is evident
by studies on mutants.
4.3.6 Signaling and gene expression
Phytochrome plays an important role in regulating plant
development in response to light. The main light-induced genes controlled by
19
phytochromes include those encoding the small sub-units of ribulose-1,5
biphosphate carboxylase, the a-subunit of ATP synthetase, the 32-kDa protein of
photosystem II, the chalcon-syntetase and the chlorophyll a/b binding(CAB)
protein of the light harvesting chlorophyll – protein complex. Mohr and
collaborators suggested the participation of gene activation and repression in
phytochrome-mediated photomorphogenesis. Mohr suggested that the genome of
each cell consists of four classes of genes as characterized by their state of activity
and their potential for reacting with PFR. The four classes are1. Inactive genes- these are the genes not required for the particular either
in presence or absence of Pfr Eg. Genes related to root development
being inactive in shoot apex cells.
2. Active genes- these are the genes required for the particular
irrespective of the availability of Pfr Eg. Genes related to synthesis of
the enzyme of basic metabolism.
3. Inactive genes, which are activated by Pfr
4. Active genes, which are repressed by Pfr
The phytochrome invades with the control of transcription by a specific
DNA recognition sequence. Morelli (1985) showed that a sequence of 33 base
pairs is essential for the light-induced control of gene expression. It includes part
of TATA-Box (a part of the promoter). Phytochromes combine with these
sequences and act as a regulator for production of mRNA transcripts.
The CAB gene is the most studied plant gene family as far as the expression
in phytochrome regulation at the level of transcription is concerned.
For optimal growth and development, plants require the ability to complex
developmental processes and at the same time sense and respond to endogenous
physiological factors and environmental cues. Light is an extremely important
environmental cue as it is non-motile in nature and is needed for photosynthesis.
It also provides signals to optimize growth and development. The perception and
transduction of these signals is carried out through photoreceptors.
20
In Arabdiopsis, the Histidine kinase play key role in sensing and transducing
extracellular signals. This signal transduction on system is mediated by
phosphotransfer between two types of signal transducers and is referred as the
two-compartment system.
4.4 Lets sum up
The main aim of this unit is to study the mechanism of nitrogen and sulphur
metabolism and the sensory photobiology. After going through this unit, you
would be able to know that 
The most essential element for sustenance of life is nitrogen.

Plants cannot use atmospheric nitrogen directly. .

Atmospheric nitrogen is fixed into organic combinations.

Plants absorb nitrates, which again is converted to ammonia by enzymatic
reactions.

Ammonium ions are later on converted to amino acids, proteins and other
nitrogenous compounds.

Sulphur is another major macronutrient.

It is absorbed in the form of sulphates and is important part of proteins.

Light is an important factor in plants life.

Phytochromes and cryptochromes are photoreceptors of the cell.

Phytochromes are found in two interconvertible forms.

Signals received through photoreceptors are later on transferred in the cell.
4.5 Check your progress
Write your answers according to the points given below
Q. 1 Discuss the process of biological nitrogen fixation in plants.
Your answer should include1. Diagrams
2. Microorganisms involved in process
Q.2
Discuss of
sulphate
metabolism.
3. Process
nitrogen
fixation
4. Significance
21
Q.2Describe the sulphate assimilation in plants.
Your answer should include1. Significance of sulphate
2. Process of sulphate uptake
3. Assimilation of sulphate in plants.
Q.3 What are phytochromes? How were they discovered?
Your answer should include1. Discovery of phytochromes
2. Structure
3. Difference between two forms.
Q.4 How the signal received by photomorphoreceptors is
transformed into gene expression?
Your answer should include1. Diagrams
2. Receiving of stimulus
3. Gene expression.
Multiple Choice QuestionsTick the correct answerCompare your answers with the answers given at the endQ.1 Nitrate absorbed by plants is-(a) Converted to nitrate
(b) Reduced to ammonia
(c) Changed to nitrite
(d) Combined with oxygen
Q.2 Which pigment is essential for nitrogen fixation by leguminous plants
22
(a) Phycocyanin
(b) Phycoerythrine
(c ) Leghaemoglobin
(d) Myoglobin.
Q.3 In nitrogen cycle nitrite is converted to nitrate by(a) Azotobacter
(b) Rhizobium
(c) Nitrobacter
(d) Nitrosomonas
Q.4 Mutants of this plant were used for studying cryptochromes(a) Mimosa
(b) Avena
(c) Cocos
(d) Arabdiopsis.
Q.5 Phytochromes were discovered from(a) Petals
(b) Leaves
(c) Etiolated stems
(d) Roots
Hints for multiple choice questions1(b), 2 (c), 3 (c), 4 (d), 5 (c)
4.6 Activities
1. Collect some leguminous plants and observe their root systems.
2. Isolate Rhizobium and observe under microscope.
3. Draw nitrogen cycle.
4. Grow some seeds in dark and observe their behavior in red and far-red
light.
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4.7References
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