Download Document

1
CELL SIGNALING MECHANISMS IN
PLANTS
ARSHAD MAHMOOD KHAN
Lecturer (HED Punjab)
[email protected]
PhD Scholar
BOTANY DEPARTMENT
UAAR RAWALPINDI
2
General
Introduction
Cells in plants like in animals remain in constant
communication with one an other
1. Plant cells communicate to coordinate their activities in
response to the changing conditions of
• light and dark
• gravity
• temperature
• water
2. Which guide the plant’s cycle of
• growth and movements
• flowering and
• fruiting
3
Cont…
3. Thus plant cells also communicate to coordinate
activities in their
• roots
• stems and
• leaves, flower and fruits
4
PROBLEM
• Less is known about the receptors and intracellular
signaling mechanisms involved in cell communication in
plants than is known in animals
Hypothesis
Multicellularity and Cell Communication Evolved
Independently in Plants and Animals
• Although plants and animals are both eukaryotes, they
have had separate evolutionary histories for more than
a billion years
• Their last common ancestor thought to have been a
unicellular eukaryote that had mitochondria but no
chloroplast
5
Cont…
•
•
•
The plant lineage acquired chloroplasts after plants and
animals diverged (Endo-symbiont Hypothesis)
The earliest fossils of multicellular animals and plants
date from almost 600 million years ago
Thus, it seems that plants and animals evolved multicellularity independently, each starting from a different
unicellular eukaryote, sometimes between 1.6
and 0.6 billion years ago
6
Cont…




If multicellularity evolved independently in plants and
animals, the molecules and mechanisms used for cell
communication will have evolved separately and
would be expected to be different
However, there should be some degree of
resemblance because the genes in both plants and
animal genes diverged from those contained their
last common unicellular ancestor. For example
Like animals, plants make extensive use of cell
surface receptors
Whereas most cell-surface receptors in animals are
G-protein linked, most found so far in plants are
enzyme linked
7
Cont…


Moreover, the largest class of enzyme linked receptors
in animals is tyrosine kinases, this type of receptor is
extremely rare in plants
Whereas plants seem to rely largely on serine/threonine
kinases cell membranes receptors.
8
Fig 15.81 Alberts 5th Ed
9
Definition of plant hormone
(phytohormone) and their role
1.
The word hormone is derived from the Greek verb meaning to
excite.
2. Hormones are organic substances synthesized in one tissue and
transported out where their presence results in physiological
responses ( not always true; may act at or close to synthesis site).
They are required in minute amounts (10-6 to 10 -8M).
3. Each hormone may result in multiple effects -- the particular effect
depending on a number of factors:
(a) The presence of other hormones and the presence of
activator molecules ( calcium, sugars)
(b) The amount of the hormone (dosage or concentration)
(c) The sensitivity of that tissue to the hormone.
(d) The condition of the plant itself is critical: what is the
condition of the plant? its age?
10
Different types of the Plant Hormones
11
Effects of plant hormones on plant growth and development
Embryogenesis
Senescence
(Cell division, expansion, differentiation
and cell death)
12
Chronological events and persons involved in
identification of different hormone receptors
2007 (CHLH, GCR2)
Fawzi A. Razem
13
Currently identified different plant hormone receptors
Nature 405:1071-1078 (2009)
14
Cellular locations of different plant hormone receptors
Nature 459:1071-1078 (2009)
15
ETHYLENE SIGNALING PATHWAY
As the detail discussion about the signaling pathways of
all phyto-hormones is too lengthy; only the ethylene
signaling pathway is discussed here
OUTLINE
1.
2.
3.
Introduction to the ethylene hormone
(history, synthesis, significance)
Genetic dissection of the ethylene signaling pathway
(this provides for the genetic engineering of many
responses to ethylene)
Summary
16
Various stimuli that produce plant responses through synthesis of signals
ETHYLENE is a
gaseous plant
hormone.
17
History
Neljubov (1901):
 Gaseous hydrocarbon olefin
 Triple response in etiolated pea
seedlings
Cousins (1910):
 Orange and banana in the same
shipment
Gane (1934):
 Ethylene as a natural plant product
18
Ethylene Biosynthesis
Wounding
Heat stress
Drought stress
Cold stress
Oxidative stress
Osmotic stress
Mechanical stress
UV stress
Pathogen attack
Biotic stress
Flooding
19
Ethylene biosynthetic pathway and the Yang cycle
20
Ethylene biosynthetic pathway and the Yang cycle
21
Biosynthesis of ethylene






The precursor for ethylene biosynthesis is methionine, which is converted
sequentially to S-adenosylmethionine, ACC, and ethylene. ACC can be
transported and thus can produce ethylene at a site distant from its
synthesis.
Two key enzymes: ACC synthase and ACC oxidase
Ethylene biosynthesis is stimulated by environmental factors, other
hormones (auxin), physical and chemical stimuli
The biosynthesis and perception (action) of ethylene can be antagonized
by inhibitors, some of them have commercial applications
ACC can be converted to a conjugated form, N-malonylACC (MACC) to
avoid over production
Ethylene can travel through diffusion (short transport) or in the form of
ACC when long distance transport is required.
22
Ethylene responses/effects/significance
Developmental processes
Fruit ripening - ethylene is essential
Promotion of seed germination
Root initiation
Bud dormancy release
Inhibition/promotion of flowering
Sex shifts in flowers
Senescence of leaves, flowers
Responses to abiotic and biotic stress
Abscission of leaves, flowers, fruits
Epinasty of leaves
Inhibition/promotion of cell division/elongation
Altered geotropism in roots, stems
Induction of phytoalexins/disease resistance
Aerenchyma formation
23
Constitutive triple response (CTR) by
ethylene
For instance, when the shoot of germinating seedling
encounter an obstacle such as a piece of gravel
underground in the soil, the seedling respond to the
encounter in three ways:
• Firstly, it thickens its stem which can then exert more
force on the obstacle
• Secondly, it shields the tip of the shoot by increasing the
curvature of specialized hook structure
• Thirdly, it reduces the shoot’s tendency to grow away
from the direction of gravity, so as to avoid the obstacle
This triple response is controlled by ethylene
24
Cont…
25
Ethylene has far-reaching consequences for agriculture and
horticulture
Transport and storage of
fruits and vegetables
requires ethylene control
Flood-tolerant rice created by
expression of ethylene response
factor genes
“One bad apple spoils
the whole bunch…”
26
Wounding induces
ethylene production
Ethylene causes senescence
Can block ethylene receptors with silver thiosulfate
27
Apple slices inducing ripening of persimmons
8 days in bag
with apple
slices
Controls, 8
days outside
of bag
28
Genetic dissection of the ethylene signaling
pathway and receptors
Ethylene
Perception by receptors
Signal transduction
Ethylene can reversibly bind
to its receptors present in ER
membrane through a transition
metal (Cu)
Responses
29
Genetic dissection of the ethylene signaling
pathway and receptors
Plants have various ethylene receptors like Ethylene
receptor 1 & 2 (ETR1, ETR2), Ethylene response sensor 1 &
2 (ERS1, ERS2) and Ethylene insensitive protein 4 (EIN4)
which are located in the endoplasmic reticulum and are all
structurally related
They are dimeric, trans membrane proteins with a copper
containing ethylene binding domain and a His-kinase
domain that interacts with protein called CTR1
30
Ethylene signaling pathway
31
32
Arrows and T-bars represents positive and negative control respectively 33
Genetic dissection of the ethylene signaling
pathway and receptors

Most of the ethylene signaling pathways studies was
performed in the model plant Arabidopsis thaliana.
Receptors:


In Arabidopsis ethylene is perceived by a family of five
receptors viz. ETR1,ETR2,ERS1,ERS2 and EIN4. All
these dimeric receptor molecules are integral part of
ER cell membrane.
The receptors family is further divided into
 Type 1 subfamily (It includes ETR1 and ERS1)
 Type 2 subfamily (It includes ETR2,ERS2 and EIN4)
34
Cont…
Components of a receptor:
 Each receptor molecule (e.g. ETR1 or ERS1 of Type1
subfamily) have two domains like
 Amino terminal, also called sensor domain where
ethylene binding can occur.
 Carboxyl terminal, or Histidine kinase domain or
receiver domain
 RAN1 protein transfer or deliver copper ions to the
sensor domain of receptors that acts as cofactor.
 In the absence of ethylene each dimeric receptor
molecule is functional or active (due to
phosphorylation of receiver domain) and hence
negatively control the ethylene responsive genes. 35
Cont…
CTR1:


It is a serine/threonine kinase receptor (commonly
called as constitutive triple response protein)
CTR1 also have two domains;




The sensor domain and the receiver or active kinase domain.
The sensor domain of CTR1 is bonded with the receiver
domains of initial ethylene receptor molecules. (-COOH
terminal of receptor and –NH2 terminal of CTR1)
Hence CTR1 is not transmembrane bounded directly.
In the absence of ethylene both receptor and CTR1 receiver
domain are active and negatively controlling the ethylene
response pathway.
36
Cont…
Signal perception and role of MAPK
cascade family:

Under biotic or abiotic stress, when ethylene binds
with the initial receptors present in the ER
membranes, it causes the following effects
downstream;



Initial receptors becomes inactive, thus causing a
conformational change at the receiver ends.
This will release CTR1 into cytosol and it also become
inactive, as they further phosphorylate MAPK cascade.
This cascade includes SIMKK and MPK6
37
Cont…
Further downward positive regulation of
EIN2:

The positive activation of MAPK cascade family
members in the cytosol causes;



The positive activation of EIN2 that are nuclear membrane
bounded.
Further downward EIN2 positively regulate the concentration
of transcription factors like EIN3 and EIL1.
Ubiquitin-proteosome complex (Ub/26S) and EBF1 & 2
negatively control the concentration of EIN3 within the
nucleoplasm.
38
Cont…
Further downward positive regulation of
EIN3 & EIL1:


EIN3 and EIL1 transcription factors acts on the
immediate target genes (like ERF1, EDF1, EDF2,
EDF3 and EDF4) by binding with promoter called
PERE.
Above mentioned transcription and then translation
results in ERF1, EDF1, EDF2, EDF3 and EDF4 proteins
or secondary transcription factors.
39
Cont…
Further downward positive regulation of ERF1:
 ERF1 a secondary transcription factor then binds with
the GCC box present in the promoter of other genes
(PDF1.2,Hls1 and ChiB).
 The product of above mentioned genes acts as
metabolic protein and control the various responses
in plant body. For example PDF1.2 shows defensive
response against the viral or various microbial
infections whereas Hls1 protein is responsible for
differentiation and growth in plants.
 An unidentified JA transcription factor also binds to
the promoter of ERF1 to activates its expression.
40
Summary
Although ethylene is the simplest of all
plants hormones, it has a strong
influence on many different
developmental processes, from
germination to senescence. In the last
decade, molecular and genetic
investigations have contributed
enormously to the understanding of
ethylene perception and signal
transduction.
41
References
Aaron Santner & Mark Estelle, Nature 459, 1071-1078 (25 June 2009)
Liu Q, Zhou GY, Wen CK. Zhi Wu Sheng Li Yu Fen Zi Sheng Wu Xue Xue Bao. 2004 Jun;30(3):241-50.
Wang ZF, Ying TJ. Zhi Wu Sheng Li Yu Fen Zi Sheng Wu Xue Xue Bao. 2004 Dec;30(6):601-8.
Chang C. Trends Plant Sci.2003 Aug;8(8):365-8.
Zimmerli L, Stein M, Lipka V, Schulze-Lefert P, Somerville S. Plant J. 2004 Dec;40(5):633-46.
Guo H, Ecker JR. Cell. 2003 Dec 12;115(6):667-77.
Alonso JM, Stepanova AN. Science. 2004 Nov 26;306(5701):1513-5.
Bleecker AB, Kende H: Ethylene: a gaseous signal molecule in plants. Annu Rev Cell Dev Biol 2000, 16:1
18.
Alonso JM, Ecker JR: The ethylene pathway: a paradigm for plant hormone signaling and interaction. Sci
STKE 2001, 2001:RE1.
Wang KL, Li H, Ecker JR: Ethylene biosynthesis and signaling networks. Plant Cell 2002, 14(Suppl):S131S151.
Klee HJ: Control of ethylene-mediated processes in tomato at the level of receptors. J Exp Bot 2002,
53:2057-2063.
Chang C, Stadler R: Ethylene hormone receptor action in Arabidopsis. Bioessays 2001, 23:619-627.
Xie C, Zhang JS, Zhou HL, Li J, Zhang ZG, Wang DW, Chen SY: Serine/threonine kinase activity in the
putative histidine kinase like ethylene receptor NTHK1 from tobacco. Plant J 2003, 33:385-393.
Hongwei Guo and Joseph R Ecker1 2004 Current Opinion in Plant Biology 2004, 7:40–49
Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, and Peter Walter. 2002
Molecular Biology of the Cell, 5th edition, Garland science, New York, USA
42
Abbreviations

























ACC
AdoMet
ChiB
CTR1
EBF1,2
EDF1,2,3,4
EIL1
EIN2,3,4
ER
ERF1
ERS1,2
ETR1,2
His
Hls1
JA
MAPK
MET
MPK6
PDF1.2
PERE
RAN1
SAM
SIMK
SIMKK
TF
1-aminocyclopropane-1-carboxylate
Adenosyl methionine
chitinaseB
Constitutive triple response1 (serine/threonine kinase)
EIN3-Binding F Box protein1,2
Ethylene response DNA binding factor1,2,3,4
EIN3 like1
Ethylene insensitive2,3,4
Endoplasmic reticulum
Ethylene Response factor1
Ethylene response sensor1,2
Ethylene receptor1,2
Histidine
Hookless1
Jasmonic acid/Jasmonate
Mitogen-activated protein kinase
Methionine
Arabidopsis MAPK6
Plant defensin factor1.2 protein
Primary ethylene response elements
Responsive to antagonist1
Sulfur- Adenosyl methionine
Salt-stress inducible MAPK
SIMK Kinase
Transcription factor
43
Questions and Comments?
44
Thanks for your attention!
45