Growth Hormones Ethylene and Abscisic Acid Plant Physiology 751

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Growth HormonesEthylene and Abscisic Acid
Plant Physiology 751
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Ethylene response
http//www.youtube.com/watch?vKCUceQulHdw
Pineapple flowering www.pbase.com
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Ethylene Biosynthesis
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Some physiological effects of ethylene
Triple response
Epinasty (downward bending of leaves)
Air
C2H4
Air
C2H4
Air
C2H4
Tomato
Arabidopsis
Pea
Promotion of root hair formation
Inhibition of flower senescence
Lettuce
Air
C2H4
STS Silver thiosulfate
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Ethylene and fruit ripening
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Role of Auxin and Ethylene during leaf abscission
formation and senescence
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Ethylene signal transduction mechanism
Ethylene Response Factor 1
Transcription factors are degraded by F box
proteins in the absence of ethylene
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The pathway summary The RAN1 protein is
essential for assembling the Cu cofactor with the
receptor for Ethylene binding. In the absence of
Ethylene, the receptor activates the downstream
kinase CTR1 that in turn inhibits the
response/gene expression. On the contrary in
the presence of Ethylene, the receptor is
inhibited so is the CTR1 kinase. The response
is now on. The kinase cascade in Ethylene
response serve as a negative regulator of the
gene expression.
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Ethylene Things to Remember

• Major hormone regulating fruit ripening
• Two types of fruit based on amount of ethylene
  produced and respiration (definitions and example
  plant names mentioned in HANDOUT)
• Climacteric fruits
• Non-climacteric fruits
• Induce flowering in Pineapple
• Promotes root hair growth
• Induces triple response (inhibition and swelling
  of hypocotyl, inhibition of root elongation and
  exaggeration of the curvature of the apical hook
• S-Adenosyl Methionine derived from Methionine is
  the precursor for its biosynthesis
• ACC-Synthase and ACC-Oxidase are the two rate
  limiting enzymes in its pathway
• The Ethylene response factors contributes
  submergence tolerance in rice and Tomato
• Induce leaf senescence and abscission tissue
  formation

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Abscisic Acid (ABA)
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Abscisic acid biosynthesis a sesquiterpenoid
isopentenyl PP (C5)
farnesyl PP (C15)
geranylgeranyl PP (C20)
phytoene
vp2, vp5, vp7, vp9 corn mutants
zeaxanthin
aba1
C40
violaxanthin
9-cis neoxanthin
vp14
xanthoxal
aba2
ABA aldehyde
flacca, sitiens tomato droopy potato aba3
At nar2a barley
ABA
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ABA pool size is regulated by both biosynthesis
and inactivation
synthesis
Developmental and physiological effects of ABA

• seed maturation
• seed dormancy (ABA/GA ratio)
• stomatal closure
• promotes root growth/inhibits shoot growth at low
  water potentials
• promotes leaf senescence (but not abscission
  directly ethylene)
• accumulates in dormant buds of perennials

ABAs principle role is in maintaining water
balance and osmotic stress tolerance
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Roles of ABA in plants

• Under Abiotic stress conditions such as
  drought/water stress it promotes root growth and
  suppresses shoot growth
• Plays role in closing stomata in response to
  water stress
• Role in senescence and Abscission layer formation

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ABA Things to Remember

• Regulates seed dormancy
• Seed dormancy is is of two types namely the
  primary and secondary (see handout)
• Vivipary in seeds
• ABA/GA ratio controls seed dormancy
• Induces Abscission tissue formation leading to
  senescence and shattering of leaf and fruits.
• Similar to Gibberellic Acid the precursor
  molecule for Biosynthesis is Gerany-Geranyl-pyroph
  osphate (GGPP)
• Under Abiotic stress conditions such as
  drought/water stress it promotes root growth and
  suppresses shoot growth
• Inhibits flowering by interacting with Flowering
  time control gene (FCA) in Arabidopsis