1. Unit 3 Chapter 22 Ethylene: The Gaseous Hormone
BIOL3745 Plant Physiology
Unit 3
Chapter 22
Ethylene:
The Gaseous Hormone

2. process associated with leaf and flower senescence and abscission,
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

3. Figure 22.1 Triple response of etiolated pea seedlings
10 ppm ethylene
untreated
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The treated seedlings show: radial swelling, inhibition of epicotyl elongation, and horizontal growth of epicotyl (diagravitropism)

4. 22 In-Text Art, p. 650 Ethylene
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5. 22 In-Text Art, p. 667 Ethephon releases ethylene slowly by a chemical reaction
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6. Figure 22.2 Ethylene biosynthetic pathway and the Yang cycle
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7. 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

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

9. Figure 22.3 ACC concentrations, ACC oxidase activity, and ethylene during ripening of apples
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10. Figure 22.4 Two inhibitors that block ethylene binding to its receptor
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inactive
Active

11. Figure 22.5 The triple response in Arabidopsis
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12. 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)

13. Grown in dark in ethylene
Figure Screen for the etr1 mutant of Arabidopsis the mutant is completely insensitive to ethylene
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Grown in dark in ethylene

14. Figure Screen for Arabidopsis mutants that constitutively display the triple response (ctr1 mutant)
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15. Figure 22.7 Schematic diagram of five ethylene receptor proteins and their functional domains
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The GAF domain is a conserved cGMP-binding domain. H: histidine; D: aspartate residue, both participate in phosphoralation

16. 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

17. 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

18. Figure 22.8 Model for ethylene receptor action based on the phenotype of receptor mutants
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19. 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

20. Figure 22.10 Model of ethylene signaling in Arabidopsis
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21. Figure 22.11 Ethylene production and respiration in banana
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22. 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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24. Figure 22.12 Leaf epinasty (downward bending) in tomato
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25. Figure 22.13 Amounts of ACC in the xylem sap and ethylene production in the petiole
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26. Figure 22.15 Kinetics of effects of ethylene addition and removal on hypocotyl elongation
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27. Figure 22.16 Promotion of root hair formation by ethylene in lettuce seedlings
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28. Figure 22.17 Inhibition of flower senescence by inhibition of ethylene action
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29. Figure 22.18 Formation of the abscission layer of jewelweed (Impatiens)
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30. The trees are fumigated with 50 ppm ethylene for 3 days
Figure 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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31. Figure 22.20 Schematic view of the roles of auxin and ethylene during leaf abscission
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32. 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.