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CELL SIGNALING MECHANISMS IN PLANTS

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Title: CELL SIGNALING MECHANISMS IN PLANTS


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CELL SIGNALING MECHANISMS IN PLANTS
ARSHAD MAHMOOD KHAN Lecturer (HED
Punjab) arshadbotanist_at_gmail.com PhD Scholar
BOTANY DEPARTMENT UAAR RAWALPINDI
3
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 plants cycle of
  • growth and movements
  • flowering and
  • fruiting

4
Cont
3. Thus plant cells also communicate to
coordinate activities in their roots stems
and leaves, flower and fruits
5
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

6
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
    multi-cellularity independently, each starting
    from a different unicellular eukaryote, sometimes
    between 1.6
  • and 0.6 billion years ago

7
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

8
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.

9
Fig 15.81 Alberts 5th Ed
10
Definition of plant hormone (phytohormone) and
their role
  • 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?

11
Different types of the Plant Hormones
12
Effects of plant hormones on plant growth and
development
Senescence
  • Embryogenesis

(Cell division, expansion, differentiation and
cell death)
13
Chronological events and persons involved in
identification of different hormone receptors
Fawzi A. Razem
2007 (CHLH, GCR2)
14
Currently identified different plant hormone
receptors
Nature 4051071-1078 (2009)
15
Cellular locations of different plant hormone
receptors
Nature 4591071-1078 (2009)
16
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
  • 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

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Various stimuli that produce plant responses
through synthesis of signals
ETHYLENE is a gaseous plant hormone.
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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

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Ethylene Biosynthesis
Cold stress Oxidative stress Osmotic
stress Mechanical stress UV stress Pathogen
attack
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Ethylene biosynthetic pathway and the Yang cycle
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Ethylene biosynthetic pathway and the Yang cycle
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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.

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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
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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 shoots tendency to grow
    away
  • from the direction of gravity, so as to avoid the
    obstacle
  • This triple response is controlled by ethylene


25
Cont
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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
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Apple slices inducing ripening of persimmons
29
Genetic dissection of the ethylene signaling
pathway and receptors
Ethylene
Perception by receptors
Ethylene can reversibly bind to its receptors
present in ER membrane through a transition
metal (Cu)
Signal transduction
Responses
30
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

31
Ethylene signaling pathway
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Arrows and T-bars represents positive and
negative control respectively
34
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)

35
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.

36
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.

37
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

38
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.

39
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.

40
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.

41
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.

42
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 Jun30(3)241-50.
  • Wang ZF, Ying TJ. Zhi Wu Sheng Li Yu Fen Zi Sheng
    Wu Xue Xue Bao. 2004 Dec30(6)601-8.
  • Chang C. Trends Plant Sci.2003 Aug8(8)365-8.
  • Zimmerli L, Stein M, Lipka V, Schulze-Lefert
    P, Somerville S. Plant J. 2004 Dec40(5)633-46.
  • Guo H, Ecker JR. Cell. 2003 Dec 12115(6)667-77.
  • Alonso JM, Stepanova AN. Science. 2004 Nov
    26306(5701)1513-5.
  • Bleecker AB, Kende H Ethylene a gaseous signal
    molecule in plants. Annu Rev Cell Dev Biol 2000,
    161 18.
  • Alonso JM, Ecker JR The ethylene pathway a
    paradigm for plant hormone signaling and
    interaction. Sci STKE 2001, 2001RE1.
  • Wang KL, Li H, Ecker JR Ethylene biosynthesis
    and signaling networks. Plant Cell 2002,
    14(Suppl)S131-S151.
  • Klee HJ Control of ethylene-mediated processes
    in tomato at the level of receptors. J Exp Bot
    2002, 532057-2063.
  • Chang C, Stadler R Ethylene hormone receptor
    action in Arabidopsis. Bioessays 2001,
    23619-627.
  • 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,
    33385-393.
  • Hongwei Guo and Joseph R Ecker1 2004 Current
    Opinion in Plant Biology 2004, 74049
  • 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

43
Abbreviations
  • ACC 1-aminocyclopropane-1-carboxylate
  • AdoMet Adenosyl methionine
  • ChiB chitinaseB
  • CTR1 Constitutive triple response1
    (serine/threonine kinase)
  • EBF1,2 EIN3-Binding F Box protein1,2
  • EDF1,2,3,4 Ethylene response DNA binding
    factor1,2,3,4
  • EIL1 EIN3 like1
  • EIN2,3,4 Ethylene insensitive2,3,4
  • ER Endoplasmic reticulum
  • ERF1 Ethylene Response factor1
  • ERS1,2 Ethylene response sensor1,2
  • ETR1,2 Ethylene receptor1,2
  • His Histidine
  • Hls1 Hookless1
  • JA Jasmonic acid/Jasmonate
  • MAPK Mitogen-activated protein kinase
  • MET Methionine
  • MPK6 Arabidopsis MAPK6
  • PDF1.2 Plant defensin factor1.2 protein

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Questions and Comments?
45
Thanks for your attention!
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