fruit ripening and ethylene hort 301: plant physiology december 9 th, 2009 dr. michael van oosten
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Skotomorphogenesis
Seed germination
Genes and enzymes
Embryo and Seed development
Plant life cycle
Photomorphogenesis
Photoreceptors
Phytochrome
Cells and cell growth
Phytochrome: regulation oflight responses
Photosynthesis: light reaction
Photosynthesis: carbon fixationPhotorespiration
Fruit ripening
RespirationPrimary metabolism
Secondary metabolismNitrogen fixation
Mineral nutritionWater transport
Phloem translocation
Abiotic stress
Biotic stress
Flowering
Fertilization and embryogenesis
Seed development
The Ripening Process
• Process of becoming edible
• Fruit becomes sweeter – (accumulation of sugars)
• Fruit becomes softer – (more palatable)
• Fruit becomes less green– (accumulation of pigments/decrease
of chlorophyll)• Fruit generally becomes more
acidic
Fruit ripening
•Partial digestion of cell walls and middle lamella
•Degradation of chlorophyll and starch
•Synthesis of anthocyanins and carotinoids
•Respiration of organic acids
Respiration and Ripening
• Normally when a tissue reaches maturity, respiration rates drop off
• Climacteric fruits show a rise in respiration during onset of ripening– Apples, Bananas, Aavocados
• Climacteric ripening is triggered by Ethylene
• Nonclimacteric do have a ethylene/respiration rise– Oranges, Grapes, Strawberries
Changes in the Cell Wall
• Much of the cell wall is degraded
• Expansins are produced to “loosen” cell wall
• Middle lamella can be selectively degrade to allow cells to become “unglued” from each other
Abscission
Triple response
Pathogen defense
Root hairformation
Fruit ripening/Senescence
Elongationinhibiation
Increased Radial growth
Ethylene effects
Ethylene: it’s a gas!!
Biologically active at less than 0.1ppm
Transported as ACC
Synthesized in ripening fruit and senescing tissues
Induced by auxin, draught, wounding, cold, stress,fruit ripening, senescence, pathogen attack
1864 illuminating gas powered street lights defoliate trees1901 Russian Dimitry Neljubov identifies ethylene as phytohormone1917 Doubt identifies ethylene as defoliant1934 ethylene biosynthesis in plants detected1935 ethylene is proposed as the “ripening hormone”
Ethylene: it’s a gas!!
C C
H
H
H
H
Biologically active at less than 0.1ppm
Transported as ACC
Synthesized in ripening fruit and senescing tissues
Induced by auxin, draught, wounding, cold, stress,fruit ripening, senescence, pathogen attack
Ethylene biosynthesis from methionine
H2C CH2 Ethylene
O2
ACC-Oxidase
Methionine
S-Adenosyl-Methionine
1-Aminocyclopropan- 1-carbonic acid
NH3
COO-
H2C
H2CC
+
(ACC)
(SAM)
ACC-Synthase
SAM Synthetase
Auxin prevents abscission
However: unphysiological auxin concentrations have herbicide effects (agent orange)
Genetic epistasis
Phenotype of first gene is masked by phenotype of a second gene
ETR1, EIN4, ETR2
EIN2, EIN3, EIN5
CTR1
etr1-3 ctr1-1ein4-1 ctr1-1etr2-1 ctr1-1ein2-1 ctr1-1ein3-1 ctr1-1ein5-1 ctr1-1
constitutive triple responseconstitutive triple responseconstitutive triple response ethylene insensitive ethylene insensitive ethylene insensitive
Ethylene Mutants
• Ethylene insensitive (EIN)
• Ethylene resistant (ETR)
• Constitutive Triple Response (CTR)
• Ethylene Response Sensor (ERS)
• Ethylene Overproducer (ETO)
Ethylene: triple response
Triple response:- thickening of hypocotyl, radial growth- reduction of cell elongation in hypocotyl and root- exaggerated curvature of apical hook, reduced geotropism
Understanding the hormone:Searching for ethylene mutants
(CTR1)
ctr1
Constitutive tripleresponse
air
Wild-typectr1
air ethyleneethylene
Recessive loss-of-function ctr1 mutations:- Constitutive activation of ethylene response- Ethylene induced genes are always “on”- Constitutive triple response
CTR1 leads to inhibition of ethylene response in absense of ethylene
Searching for ethylene mutants
etr1
ctr1
Ethylene resistant (exogenous ethylene added)
Constitutive tripleResponse (no exogenous ethylene)
Air
Ethylene
Wild-type
Understanding the hormone:Searching for ethylene mutants
(ETR1)
Ethylene resistant
Air Ethylene
Air Ethylene
Wild-typeEtr1
etr1 Etr (ethylene resistant)Ein (ethylene insensitive)
Ethylene receptors
Understanding the hormone:Searching for ethylene mutants (ETO1)
eto1
Ethylene overproducer
- Ethylene overproducer show same phenotype as ctr1
- ctr1 not reversible by inhibitors of ethylene biosynthesis
- eto phenotype is reversed by ethylene synthesis inhibitors
ETO1 has de-regulated ethylene biosynthesis
Air
Ethylene
Wild-type
Horticultural Question
• Why does blocking ethylene perception work much better than blocking ethylene production within the plant?
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