Signal Transduction and Gene Regulation in Plant Development
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Transcript of Signal Transduction and Gene Regulation in Plant Development
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Cellular Signal Transduction
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Signal Transduction and gene
regulation in plant development
and stress responsesSignal transduction is the process by which anextracellular signaling molecule activates amembrane receptor, that in turn alters intracellular
molecules creating a response.There are two stages in this process: a signallingmolecule activates a certain receptor on the cellmembrane, causing a second messenger to
continue the signal into the cell and elicit aphysiological response. In either step, the signalcan be amplified, meaning that one signallingmolecule can cause many responses.
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Signaling molecule
Receptor of target cell
Intracellular molecule
biological effect
Signaltransduction
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Pathway
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Receptor
Receptors are specific membrane
proteins, which are able to recognize
and bind to corresponding ligandmolecules, become activated, and
transduce signal to next signaling
molecules.
Glycoprotein or Lipoprotein
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ligandA small molecule that binds
specifically to a larger one; for
example, a hormone is the ligand for
its specific protein receptor.
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Membrane receptors
membrane
Glycoprotein
Intracellular receptors
Cytosol or nuclei
DNA binding protein
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Signalling molecules
Signal transduction involves the binding of
extracellular signalling molecules and ligands
to cell-surface receptors that trigger events
inside the cell.
Intracellular signaling cascades can be startedthrough cell-substratum interactions.
Receptors- membrane proteins, membrane
potential,proteinaceous pores,channels
C- terminal region of transmembrane protein
receptor is phosphorylated by protein kinases
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Effect bymembrane
receptors
Effect by
intracellular
receptors
Intracellular
molecules
Extracellular
molecules
Signal
molecules
cAMP, cGMP, IP3, DG, Ca2+
Proteins and peptides:
Hormones, cytokinesAmino acid derivatives:
Catecholamines
Fatty acid derivatives:
Prostaglandins
Steroid hormones,Thyroxine, VD3
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Transduction of abiotic signals to altered
gene expression at the cellular level
Ozone,extreme temperatures,flooding, drought,salt
Plants
Physiological & developmental events
Sress recognitionSignal transductionAltered cellularmetabolism
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Gene expression patterns often change
in response to stress:
Changes in metabolism & development leads to alteredgene pattern
These changes are integrated into a response by thewhole plant that may modify growth ,development andeven influence reproductive capabilities
Clues from yeast & bacterial proteins show that theseproteins initiate signal transduction in response toabiotic stress (e.g. low osmotic potential) ,and also
involves hormones (ABA,JA,ethylene) & secondarymessengers (Ca 2+)
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Model depicting various possible
events involved in abiotic stresses
(1)stress perceival,
(2) stress signal
transduction
(3) transcriptional
activation of stress genes(4)synthesis and
accumulation of stress
proteins, resulting finally in
(5) biochemical,(6) cellular and
(7) Physiological
manifestations
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Elements of signal transduction
Intracellular Ca++(Second messenger)-
information from extracellular source to target
with in the cell
Protein kinases (enzyme that phosphorylate &
alter the activity of target protein)-
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Many enzymes are regulated by covalent
attachment of phosphate, in ester linkage, to the
side-chain hydroxyl group of a particular amino acid
residue (serine, threonine, or tyrosine).
H3N+
C COO
CH OH
CH3
H
threonine(Thr)
H3N+
C COO
CH2
OH
H
serine(Ser)
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A protein kinasetransfers the terminal phosphate of
ATP to a hydroxyl group on a protein. A protein phosphatasecatalyzes removal of the Piby
hydrolysis.
Protein OH + ATP Protein O P
O
O
O
+ ADP
Pi H2O
Protein Kinase
Protein Phosphatase
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Phosphorylationmay directly alter activity of an
enzyme, e.g., by promoting a conformational change.
Alternatively, altered activity may result from binding
another proteinthat specifically recognizes a
phosphorylated domain.
E.g., 14-3-3proteins bind to domains that include
phosphorylated Ser or Thrin the sequence
RXXX[pS/pT]XP, where X can be different amino acids.
Binding to 14-3-3is a mechanism by which someproteins (e.g., transcription factors) may be retained
in the cytosol, & prevented from entering the
nucleus.
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Protein kinases and phosphatases are themselves
regulated by complex signal cascades. For example:
Some protein kinases are activated by Ca++
-calmodulin.
Protein Kinase Ais activated by cyclic-AMP(cAMP).
Protein OH + ATP Protein O P
O
O
O
+ ADP
Pi H2O
Protein Kinase
Protein Phosphatase
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Protein Kinase A(cAMP-Dependent Protein Kinase)
transfers Pifrom ATP to OH of a Ser or Thr in a
particular 5-amino acid sequence.
Protein Kinase A in the resting stateis a complex of:
2 catalytic subunits(C)
2 regulatory subunits(R).R2C2
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R2C2
Each regulatory subunit (R) of Protein Kinase A
contains a pseudosubstratesequence, like thesubstrate domain of a target protein but with Ala
substituting for the Ser/Thr.
The pseudosubstrate domain of (R), which lacks ahydroxylthat can be phosphorylated, binds to the
active site of (C), blocking its activity.
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R2C2+ 4 cAMP R2cAMP4+ 2C
When each (R) binds 2 cAMP, a conformational
change causes (R) to release (C).
The catalytic subunits can then catalyzephosphorylation of Ser or Thr on target proteins.
PKIs, Protein Kinase Inhibitors, modulate activity of
the catalytic subunits (C).
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Phosphodiesteraseenzymes
catalyze:
cAMP + H2O AMP
The phosphodiesterase that
cleaves cAMP is activated byphosphorylation catalyzed by
Protein Kinase A.
Thus cAMP stimulates its owndegradation, leading to rapid
turnoff of a cAMP signal.
N
N
N
N
NH2
O
OHO
HH
H
H2C
HO
PO
O-
1'
3'
5' 4'
2'
cAMP
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Signal amplificationis an important feature of signalcascades:
One hormone molecule can lead to formation of
many cAMP molecules.
Each catalytic subunit of Protein Kinase A catalyzes
phosphorylation of many proteins during the life-
time of the cAMP.
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Phosphatidylinositol Signal Cascades
Some hormones activate a signal cascade based on
the membrane lipid phosphatidylinositol.
O P
O
O
H2C
CH
H2C
OCR1
O O C
O
R2
OH
H
OH
H
H
OHH
OH
H
O
H OH
1 6
5
43
2
phosphatidyl-inositol
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Kinases sequentially catalyze transfer of Pifrom ATP toOH groups at positions 5 & 4 of the inositol ring, to yield
phosphatidylinositol-4,5-bisphosphate(PIP2).
PIP2is cleaved by the enzyme Phospholipase C.
O P
O
O
H2C
CH
H2C
OCR1
O O C
O
R2
OH
H
OPO32
H
H
OPO32H
OH
H
O
H OH
1 6
5
43
2
PIP2
phosphatidylinositol-
4,5-bisphosphate
O
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When a particular GPCR (receptor) is activated, GTP
exchanges for GDP. Gq-GTP activates Phospholipase C.
Ca++, which is required for activity of Phospholipase C,
interacts with () charged residues & with Pimoieties of
the phosphorylated inositol at the active site.
O P
O
O
H2C
CH
H2C
OCR1
O O C
O
R2
OH
H
OPO32
H
H
OPO32H
OH
H
O
H OH
1 6
5
43
2
PIP2
phosphatidylinositol-
4,5-bisphosphate
cleavage by
Phospholipase C
Different isoforms
of Phospholipase C
have different
regulatory domains,
& thus respond to
different signals.
A G-protein, Gqactivates one form
of Phospholipase C.
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Cleavage of PIP2, catalyzed by Phospholipase C, yields 2
second messengers:
inositol-1,4,5-trisphosphate(IP3)
diacylglycerol(DG).
Diacylglycerol, with Ca++, activates Protein Kinase C,
which catalyzes phosphorylation of several cellular
proteins, altering their activity.
OHH2C
CH
H2C
OCR1
O O C
O
R2
diacylglycerol
OH
H
OPO32
H
H
OPO32H
OH
H
H OH
OPO32
1 6
5
43
2
IP3
inositol-1,4,5-trisphosphate
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IP3activates Ca++-release channels in ER membranes.
Ca++stored in the ER is released to the cytosol, where itmay bind calmodulin, or help activate Protein Kinase C.
Signalturn-offincludes removal of Ca++from the cytosol
via Ca++-ATPase pumps, & degradation of IP3.
Ca++
ATP ADP + Pi
Ca++
IP3
calmodulin
endoplasmicreticulum
Ca++
Ca++-ATPase
Ca -release channel
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Sequential dephosphorylationof IP3by enzyme-catalyzed
hydrolysis yields inositol, a substrate for synthesis of PI.
IP3may instead be phosphorylatedvia specific kinases, to
IP4, IP5or IP6. Some of these have signal roles.
E.g., the IP4inositol-1,3,4,5-tetraphosphate in some cells
stimulates Ca++entry, perhaps by activating plasma
membrane Ca++channels.
OH
H
OH
H
H
OHH
OH
H
H OH
OH
OH
H
OPO32
H
H
OPO32
H
OH
H
H OH
OPO32
(3 steps) +3 Pi
IP3 inositol
O
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The kinasesthat convert PI (phosphatidylinositol) to PIP2
(PI-4,5-P2) transfer Pifrom ATP to OH at positions 4 & 5of the inositol ring.
PI 3-Kinasesinstead catalyze phosphorylation of
phosphatidylinositol at the 3 position of the inositol ring.
O P
O
O
H2C
CH
H2C
OCR1
O O C
O
R2
OH
H
OH
HH
OHH
OPO3
2
H
O
H OH
1 6
52
3 4
phosphatidyl-inositol-
3-phosphate
O
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Head-groups of these transiently formed lipids are ligands
for particular pleckstrin homology(PH) & FYVEproteindomainsthat bind proteins to membrane surfaces.
Other protein domains called MARKSare (+) charged, and
their binding to () charged head-groups of lipids like PIP2
is antagonized by Ca++.
O P
O
O
H2C
CH
H2C
OCR1
O O C
O
R2
OH
H
OH
H
H
OHH
OPO32
H
O
H OH
1 6
52
3 4
phosphatidyl-inositol-
3-phosphate
PI-3-P, PI-3,4-P2,
PI-3,4,5-P3, and
PI-4,5-P2havesignaling roles.
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Protein Kinase B(also called Akt) becomes activated when
it is recruited from the cytosol to the plasma membrane
surface by bindingto products of PI-3 Kinase, e.g., PI-3,4,5-P3.
Other kinases at the cytosolic surface of the plasma
membrane then catalyze phosphorylation of Protein
Kinase B, activating it.
Activated Protein Kinase B catalyzes phosphorylationof
Ser or Thr residues of many proteins, with diverse
effects on metabolism, cell growth, and apoptosis.
Downstream metabolic effectsof Protein Kinase B
include stimulation of glycogen synthesis, stimulation
of glycolysis, and inhibition of gluconeogenesis.
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Gene regulation in plant growth
and development
A variety of external & internal signals modify
plant cell metabolism , growth, &
development
The ability of cell s to respond to these signals
is not confined to cells that are still growing
and developing
Mature cells too, can initiate metabolicresponses and can even reinitiate growth and
division in response to signal information
Characteristics of Signal transduction
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Characteristics of Signal transduction
in Plants The stream of signals to which plant cell react
is continuous and complex
Signal transduction uses a net work of
interactions with in the cells, among the cells
and through out the plant
Plant cells contain two information systems;
genetic & epigenetic
Different signals affect the transduction
network in different ways and at different
places, but most modify gene expressson
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Types of signal transduction pathway in
Plants
The bacterial two component system in whicha receptor and an effector interact through
phosphorylation of histidine and aspartate
residueAutophosphorylation of receptor---
Phosphorylation of response regulator---
dephosphorylation of response regulator
(e.g.-Arabdopsis ethylene receptor-ETR1,
&cytokinin sensing CKI1/GCR1)
Plant hormones as a receptor---ethylene,
ABA,GA,IAA,JA,phytochromes etc.
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Saline stress inhibits sucrose synthesis and
promotes accumulation of mannitol
Fructose- 6-Po4
Phosphomannose isomerase
Mannose-6-Po4Mannose -6 Po4 reductase NADPH----NADP
Mannitol-1 po4
Mannitol-1 Po4 phosphatse -Pi
Mannitol
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Accumulation of pinitol in response to
salt stress
Glucose-6 Po4
NAD+ myo- Inositol-1 PO4 synthase
myo-Inositol- 1- PO4
-Pi myo-Inositol- 1- PO4-phosphatsemyo-inositol
S-adenosylmethionine--- myo-inositol-6-O-methyltransferase
S-adenosylhomocysteineOnonitol
Ononitol epimerasePinitol
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SOS signaling pathway for ion homeostasis
under salt stress inArabidopsis.
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Salt stress elicited Ca2+signals are perceived by SOS3, which
activates the protein kinase SOS2.
Activated SOS2 phosphorylates SOS1, a plasma membrane Na+/H+
antiporter, which then transports Na+out of the cytosol.
The transcript level of SOS1 is regulated by the SOS3-SOS2 kinase
complex.
SOS2 also activates the tonoplast Na+/H+antiporter that
sequesters Na+into the vacuole. Na entry into the cytosol through
the Na+transporter HKT1 may also be restricted by SOS2.
ABI1 regulates the gene expression of NHX1, while ABI2 interactswith SOS2 and negatively regulates ion homeostasis either by
inhibiting SOS2 kinase activity or the activities of SOS2 targets.
Double arrow indicates SOS3-independent and SOS2-dependent
pathway.
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ABA-dependent and ABA-
independent signal transduction.
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Environmental factors that increase
concentration of ROS
Ozone- UV radiations +SO2,NO,NO2,--------------Stratasphoric/Troposphoric-
Drought,
Senesence,
Herbicides,(paraquat dichloride)Wounding,
Intense light, --OXIDATIVE STRESS
Pathogens,
Heat&Cold,
Heavy metals,
Root nodules-----------------------------------------
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Generation and scavenging of superoxide radical and
hydrogen peroxide, and hydroxyl radical-induced lipid
peroxidation and glutathione
peroxidase-mediated lipid (fatty acid) stabilization
R l f l i l
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Role of osmolytes in plant
protection against abiotic stress Some compatible solutes may serve protective
functions in addition to OA called osmoprotector.
Glycine betain prevents salt- induced inactivation of
Rubisco & destabilization of the oxygen evolvingcomplex of PSII
Sorbitol,mannitol,myo-inositol, and proline can also
scavenge hydroxyl radicals in addition to OA.
Transgenic plants can be used to test the acclimative
functions of specific osmolytes
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Fatty acids signalling molecules
Protein kinases (PK): primary elements in
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Protein kinases (PK): primary elements in
the signal transduction These are ubiquitous enzymes and are signal specific
Catalyses reversible transfer of-PO4 from ATP toSerine,threonine or tyrosine amino acid chains on target
proteins
Activity is counter balanced by the specific protein
phosphatases Activation of PK has been implicated in response to
light,pathogen attack,GR,salt,heat temp.stress,nutrient
deprivation etc.
Several protein kinases are concerned with regulation ofmetabolic pathways
Hundereds of plant genes fdor different PKs have been
identified but atleast 1000 must exist(Tab,:various groups of
PKs identified in plants
Kinase cascade involved in biotic stress
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Kinase cascade involved in biotic stressresponses in plants
The sensing of stress signals and their transduction into
appropriate responses is crucial for the adaptation andsurvival of plants.
Kinase cascades of the mitogen-activated protein kinase
(MAPK) class play a remarkably important role in plant
signalling of a variety of abiotic and biotic stresses. MAPK
cascade-mediated signalling is an essential step in the
establishment of resistance to pathogens. Here, we describe
the most recent insights into MAPK-mediated pathogen
defence response regulation with a particular focus on the
cascades involving MPK3, MPK4 and MPK6.
We also discuss the strategies developed by plant pathogens
to circumvent, inactivate or even hijack MAPK-mediated
defence responses.
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PAMP- induced MAPK cascade in the plant
defence to the bacterial & fungal
pathogens
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SALT AND DROUGHT STRESS SIGNAL
TRANSDUCTION IN PLANTS
Salt and drought stress signal transduction consists of ionic and osmotic
homeostasis signaling pathways, detoxification (i.e., damage control and
repair) response pathways, and pathways for growth regulation. The ionic
aspect of salt stress is signaled via the SOS pathway where a calcium-
responsive SOS3-SOS2 protein kinase complex controls the expression and
activity of ion transporters such as SOS1. Osmotic stress activates severalprotein kinases including mitogen-activated kinases, which may mediate
osmotic homeostasis and/or detoxification responses. A number of
phospholipid systems are activated by osmotic stress, generating a diverse
array of messenger molecules, some of which may function upstream of
the osmotic stressactivated protein kinases. Abscisic acid biosynthesis isregulated by osmotic stress at multiple steps. Both ABA-dependent and -
independent osmotic stress signaling first modify constitutively expressed
transcription factors, leading to the expression of early response
transcriptional activators, which then activate downstream stress
tolerance effector genes.
I & O ki f
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Inputs & Out puts:making sense fordrought &salt stress signalling
pathways
Figure 1 Functional demarcation of salt and drought stress signaling pathways.
The inputs for ionic and osmotic signaling pathways are ionic (excess NaC) and osmotic (e.g.,
turgor) changes. The output of ionic and osmotic signaling is cellular and plant homeostasis.
Direct input signals for detoxification signaling are derived stresses (i.e., injury), and the signaling
output is damage control and repair (e.g., activation of dehydration tolerance genes). Interactions
between the homeostasis, growth regulation, and detoxification pathways are indicated
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THE SOS REGULATORY PATHWAY FOR ION
HOMEOSTASIS AND SALT TOLERANCE
High NaC stress initiates a calcium
signal that activates the SOS3-
SOS2 protein kinase complex,
which then stimulates the NaC/HC
exchange activity of SOS1 and
regulates transcriptionally
and posttranscriptionally theexpression of some genes. SOS3-
SOS2 may also stimulate or
suppress the activities of other
transporters involved in ion
homeostasis under salt stress,
such as vacuolar HC-ATPases and
pyrophosphatases (PPase),vacuolar NaC/HC exchanger
(NHX), and plasma membrane KC
and NaC transporters.
Figure 2 Regulation of ion (e.g., NaC and KC) homeostasis by
the SOS pathway.
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PROTEIN KINASE PATHWAYS FOR
OSMOTIC STRESS SIGNALING
Figure 3 Activation of protein kinases by hyperosmotic stress.
The MAP kinase cascade shown is also activated by other stresses.
Currently, the functional significance of the kinase activation is unclear
(hence the unknownoutput). SIPK, SIMK, and ATMPK6 are homologous
MAP kinases from tobacco, alfalfa, and Arabidopsis, respectively.
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OSMOTIC STRESSACTIVATED
PHOSPHOLIPID SIGNALING
Figure 4 Phospholipid signalingunder salt stress, drought, cold, or
ABA.
Osmotic stress,cold, and ABA activate
several types of phospholipases that
cleave phospholipids to generate lipid
messengers (e.g., PA, DAG, and IP3),which regulate stress tolerance partly
through modulation of stress-
responsive gene expression.FRY1(a 1-
phosphatase) and 5-
phosphatasemediated
IP3 degradation attenuates the stressgene regulation by helping to control
cellular IP3 levels. PLC, phospholipase
C; PLD, phospholipase D; PLA2,
phospholipase A2; PA, phosphatidic
acid; DAG, diacyglycerol.
S d l d G
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Stress- and ABA-Regulated Gene
Expression
Figure 5 ABA
metabolism is regulated
by osmotic stress at
multiple steps.
The ABA biosynthesis
genes ZEP, NCED,
LOS5/ABA3, and AAO are
upregulated by salt and
drought stresses. ABA
degradation is also
important in controlling
cellular ABA content, and
biochemical evidence
suggests osmotic stress
inhibition of the first step
of catabolism
ABA Dependent and ABA
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ABA-Dependent and ABA-
Independent Signaling
Figure 6 Model showing osmotic
stress regulation of early-
response and delayedresponse
genes. (A) Model integrating stress
sensing, activation of phospholipid
signaling and MAP kinase cascade,
and transcription cascade leading to
the expression of delayed-response
genes. (B) Examples of early-response
genes encoding inducible transcription
activators and their downstreamdelayed-response genes encoding
stress tolerance effector proteins.
Question marks denote unknown
transcription factors that activate the
early-response genes.
F tt id d i d i l i l t
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Fatty acid - derived signals in plants
e.g. Jasmonates
Its mutant opr3 in biosynthetic pathway,
allow the dissection of cyclopentanone &
cyclopentenone signalling
In addition, keto,hydroxy & hyperhydroxy FA
involved in cell death & stress related gene
expression.
Bruchins & volicitin as a signal molecule frominsects show FA 0derived signalling in plant
defence