BIOL3745 Plant Physiology

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BIOL3745 Plant Physiology

• Unit 3
• Chapter 22
• Ethylene
• The Gaseous Hormone

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Summary

• Ethylene regulates
• fruit ripening and
• process associated with leaf and flower
  senescence and abscission,
• root hair development and nodulation,
• seedling growth and hook opening
• Flowering in family Bromeliaceae

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Figure 22.1 Triple response of etiolated pea
seedlings
10 ppm ethylene
untreated
The treated seedlings show radial swelling,
inhibition of epicotyl elongation, and horizontal
growth of epicotyl (diagravitropism)
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22 In-Text Art, p. 650 Ethylene
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22 In-Text Art, p. 667 Ethephon releases ethylene
slowly by a chemical reaction
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Figure 22.2 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

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Biosynthesis of ethylene

• ACC can be converted to a major conjugated form,
  N-malonylACC (MACC), and a minor conjugated form,
  1-?-L-glutamylamino cyclopropane-1-carboxylic
  acid (GACC).
• ACC deaminase can breakdown ACC to ammonia and
  a-ketobutyrate to regulate ethylene biosynthesis

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Figure 22.3 ACC concentrations, ACC oxidase
activity, and ethylene during ripening of apples
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Figure 22.4 Two inhibitors that block ethylene
binding to its receptor
inactive
Active
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Figure 22.5 The triple response in Arabidopsis
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Ethylene signal transduction pathways

• Two classes of mutants have been identified by
  experiments in which mutagenized Arabidopsis
  seeds were grown on an agar medium in presence or
  absence of ethylene for 3 days in the dark
• mutants fail to respond to ethylene
  ethylene-resistant or insensitive mutants
• mutants that display the response even in
  absence of ethylene (constitutive mutants)

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Figure 22.6 Screen for the etr1 mutant of
Arabidopsisthe mutant is completely insensitive
to ethylene
Grown in dark in ethylene
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Figure 22.9 Screen for Arabidopsis mutants that
constitutively display the triple response (ctr1
mutant)
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Figure 22.7 Schematic diagram of five ethylene
receptor proteins and their functional domains
The GAF domain is a conserved cGMP-binding
domain. H histidine D aspartate residue, both
participate in phosphoralation
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Ethylene receptors

• 5 ethylene receptors are identified, all share at
  least two domains.
• The amino-terminal domain spans the membrane at
  least three times and contains the ethylene
  binding site.
• The carboxyl-terminal half of the ethylene
  receptors contains a domain homologous to
  histidine kinase catalytic domains
• They are all located on endoplasmic reticulum.
  ETR1 may also be localized on the Golgi
  apparatus.
• Ethylene binds to its receptor via a copper
  cofactor

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Ethylene receptors

• Unbound ethylene receptors are negative
  regulators of the response pathway
• Binding of ethylene inactivates the receptors,
  allowing the response pathway to proceed
• ETR1 activates CTR1, a protein kinase that shuts
  off ethylene responses

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Figure 22.8 Model for ethylene receptor action
based on the phenotype of receptor mutants
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Ethylene regulation of gene expression

• Ethylene affects the transcription of numerous
  genes via specific transcription factors
• Analysis of epistatic interactions revealed the
  sequence of action for genes ETR1, EIN2, EIN3,
  and CTR1

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Figure 22.10 Model of ethylene signaling in
Arabidopsis
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Figure 22.11 Ethylene production and respiration
in banana
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Ethylene promotes the ripening of some fruits

• Climatic fruits fruits ripen in response to
  ethylene exhibit a respiratory rise. Apples,
  bananas, avocados, tomatoes.
• Non-climatic fruits do not exhibit the
  respiration and ethylene production rise. Citrus,
  grapes.

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Figure 22.12 Leaf epinasty (downward bending) in
tomato
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Figure 22.13 Amounts of ACC in the xylem sap and
ethylene production in the petiole
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Figure 22.15 Kinetics of effects of ethylene
addition and removal on hypocotyl elongation
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Figure 22.16 Promotion of root hair formation by
ethylene in lettuce seedlings
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Figure 22.17 Inhibition of flower senescence by
inhibition of ethylene action
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Figure 22.18 Formation of the abscission layer
of jewelweed (Impatiens)
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Figure 22.19 Effect of ethylene on abscission in
birch (Betula pendula)
Left wild-type Right etr1 transgenic The
trees are fumigated with 50 ppm ethylene for 3
days
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Figure 22.20 Schematic view of the roles of
auxin and ethylene during leaf abscission
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Developmental and physiological effects of
ethylene

• Ethylene is involved in seeding germination,
  seedling growth, hypocotyle elongation, fruit
  ripening, leaf epinasty
• influences cell expansion and orientation of the
  cellulose microfibrils in the cell wall
• stimulates rapid internode or petiole elongation
  when plants are submerged.
• regulates flowering, sex determination, and
  defense responses in some species
• stimulates root hair formation
• promotes leaf and flower senescence and leaf
  abscission.