HORMONES: Dormancy, Senescence

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HORMONESDormancy, Senescence Stress

• Chapter 10

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Ethylene C2H4

• Senescence hormone which is involved in
• Plant senescence
• Fruit ripening
• Leaf abscission

• Ethylene stimulates flowering in pineapple

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Synthesized from methionine. ACC Synthase is the
limiting enzyme.
Met SAM ACC H2CCH2
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Fruit Ripening

• Caused by
• Increase in membrane permeability which releases
  compartmentalized enzymes
• Increase in protein (enzyme) synthesis
• Low temperature and high CO2 can reduce ethylene
  effects

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Ethylene Effects (continued)
Conversion of Starch to Sugars via Hydrolysis
Synthesis of Pigments and Biochemical Involved in
Flavor
Cell Wall Degradation which Leads to Softening
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Leaf Abscission

• Starts with abscission zone formation
• Triggered by photoperiod
• Auxin and cytokinin production is reduced
• Ethylene increases, causing an increase in
  cellulase synthesis

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Ethylene and Climacteric
Ethylene that is released by many fruit species
is coincident with the increase in respiration
often associated with maturing fruit.
Ethylene released by these fruit may stimulate
climacteric in immature fruits located nearby.
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Overall Plant Senescence

• Less is known about this process
• However, some plants senesce in response to
  completion of the reproductive process
• Annuals
• Biennials

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Synthetic Ethylene
Ethephon (2-chloroethylphosphonic acid, Prep in
cotton) When applied to plants, ethylene is
generated in cytoplasm where physiological pH
exists.
Hopkins, 1999
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Abscisic Acid (ABA)

• a.k.a. the stress hormone
• Plants must anticipate and prepare for
  unfavorable weather
• approaching winter
• drought stress
• wax cuticle thickening
• leaf drop
• stopping of growth
• buildup of anti-freeze

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ABA

• Not involved in leaf abscission as originally
  thought
• Is ABA involved in dormancy? The data are mixed.
• Cold treatment is often required to break
  dormancy
• Reduce ABA ??
• Increase GA, cytokinin

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Seed Maturation

• Cytokinin concentration is high during the early
  stages of embryo development

• During the linear phase of seed growth, GA and
  IAA concentration rises.

• During the final stages of seed growth, GA and
  IAA concentration falls and ABA rises.

Hopkins, 1999
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ABAs Function in the Seed
1. Induce transport of photosynthate to seed.
2. Prevent precocious germination, vivipary
3. Prevent GA-induced ?-amylase synthesis in
cereal grains. This would cause unwanted
digestion of starch.
Hopkins, 1999
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Phenylpropane Derivatives
Coumarin - inhibits auxin-induced elongation and
seed germination trans-cinnamic acid - also acts
as an antiauxin