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Ethylene
Nature of Ethylene
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Ethylene, unlike the rest of the plant
hormone compounds is a gaseous
hormone.
Like abscisic acid, it is the only member of
its class.
Of all the known plant growth substance,
ethylene has the simplest structure.
It is produced in all higher plants
It is usually associated with fruit ripening
and the triple response
Ethylene gas with ripening process
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The ancient Egyptians : gas stimulate ripening.
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lemons are often too green to be acceptable in the
market  lemon growers used to store newlyharvested lemons in sheds kept warm with
kerosene stoves  with a more modern heating
system, the no turned yellow on time.
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the important factor in the ripening process was
not heat but the small amount of ethylene gas
given off by the burning kerosene.
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leaf abscission to the presence of illumination gas
(Doubt, 1917; Fahnestock, 1858)
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In 1864, leaks of gas from street lights showed the triple
response : stunting of growth, twisting of plants, and
abnormal thickening of stems  inhibition of vegetative
tissues (Arteca, 1996; Salisbury and Ross, 1992).
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illumination gas was responsible for the horizontal
growth of etiolated pea seedlings
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the typical symptoms of ethylene action : inhibition of
stem and root growth, leaf abscission, horizontal
growth and plant senescence
Figure 1. the ethyleneinsensitive mutants etr1-1
and ein2, and the
constitutive ethyleneresponse mutant ctr1-2.
Figure 2. The Triple-Response to Ethylene of Dark-Grown Arabidopsis Seedlings. (A) Wildtype seedlings grown in the absence (left) or presence (right) of ethylene. (B) Wild-type
seedling grown in the presence of the ethylene precursor ACC. (C) Close-up of the pronounced
apical hook found with the triple response to ethylene. (D) Close-up of the shortened root found
with the triple response to ethylene.
History of Discovery in Plants
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the active component was ethylene (Dimitry Neljubow,
1901)
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plants synthesize ethylene  an
endogenous growth regulator.
this gas was ethylene was the plant
hormone responsible for fruit ripening (Crocker,
1935)
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plant tissues (ripening apples) naturally
produce ethylene (Gane, 1934)
History of Ethylene Research
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increase in ethylene production was associated with
peak in respiration during tomato ripening
(Zegzouti, 1997).
ethylene is a fruit-ripening agent that acts in very
small amounts
this endogenous growth regulator as a plant
hormone in amounts that can reach 500 nL g-1 h-1
but is active at very low concentrations from 10 to
100 nL L-1 (Abeles et al., 1992)
Ethylene is now known to have many other
functions.
Biosynthesis and metabolism
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Ethylene is produced in higher plants (varies with
the type of tissue, the plant species, and also the
stage of development)
More ethylen in meristematic tissues, nodal region,
dormant buds, flowers senescence, maturity and
ripening fruit
Precursor : methionine
ethylene is synthesized in response to different type
of stress, such as wounding, very low and very
high temperatures, flooding or drought, treatments
with other hormones, heavy metals and attack by
pathogens (Pech et al., 1992).
The ethylene
biosynthesis
pathway
1.
2.
3.
ATP is an essential component
in the synthesis of ethylene
from methionine. ATP and
water are added to
methionine resulting in loss
of the three phosphates and
S-adenosyl methionine (SAM
= AdoMet).
1-amino-cyclopropane-1carboxylic acid synthase
(ACC-synthase) facilitates the
production of ACC from SAM.
Oxygen is then needed in
order re oxidize ACC and
produce ethylene. This
reaction is catalyzed by an
oxidative enzyme called
ethylene forming enzyme.
intermediet
prekursor
Figure 3. Ethylene Biosynthetic Pathway. The enzymes catalyzing each step are shown above the
arrows. AdoMet: S-adenyl-methionine; Met: methionine; ACC: 1-aminocyclopropane-1-carboxylic acid;
MTA: methylthioadenine.
Functions of Ethylene
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Stimulates the release of dormancy.
Stimulates shoot and root growth and differentiation
(triple response)
May have a role in adventitious root formation.
Stimulates leaf and fruit abscission.
Stimulates flower induction.
Induction of femaleness in dioecious flowers.
Stimulates flower opening.
Stimulates flower and leaf senescence.
Stimulates fruit ripening.
Ethylene and fruit ripening
Fruit ripening involves a series of biochemical and
structural changes that make the fruit acceptable for
eating.
Two major groups of fruits based on the intervention
of ethylene during maturation :
 Non-climacteric fruits
 Climacteric fruits
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fruits
Non-climacteric
 are those whose
maturation does not
depend on ethylene,
 such as cherry,
strawberry and
pineapple
Climacteric
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are characterized by an
extraordinary increment in
ethylene production which
accompanies the respiratory
peak during ripening, called
the 'climacteric crisis' (Abeles et al.,
1992)
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The sharp increase of
ethylene production which
occurs at the initiation at the
onset of the climacteric phase
is the key phenomenon
controlling the initiation of
changes in color, aromas,
texture and flavor
tomato, avocado, melon,
apple, pear, peach and
kiwifruit
Climacteric and non-climacteric fruits
Non-climacteric
Climacteric
Bell pepper
Olives
Apple
Melons
Blackberries
Orange
Apricot
Nectarine
Blueberries
Pineapple
Avocado
Papaya
Cacao
Pomegranate
Banana
Passionfruit
Cashew apple
Pumpkin
Breadfruit
Peach
Cherry
Raspberries
Cherimoya
Pear
Cucumber
Strawberries
Feijoa
Persimmon
Eggplant
Summer squash
Fig
Plantain
Grape
Tart cherries
Guanábana
Plum
Lemon
Tree tomato
Jackfruit
Sapodilla
Lime
Kiwifruit
Sapote
Lychee
Mango
Watermelon
Grapefruit
Guava
Quince
Source: Wills, et al., 1982; Kader, 1985
Fruit Ripening
Caused by :
 Increase in membrane
permeability which
releases
compartmentalized
enzymes
 Increase in protein
(enzyme) synthesis
 Low temperature in high
CO2 can reduce
ethylene effects
Ehtylene effect
 Conversion of starch to
sugars via hydrolysis
 Synthesis of pigments
and biochemical
involved in flavor
 Cell wall degradation
which lead to softening
Several structural and biochemical
changes during fruit maturation
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organoleptic qualities, such
as modifications in the external
aspect, texture and flavor of
the fruit (Seymour et al., 1993).
the change in the color of
tomato fruits results from (Gray
et al., 1992) :
- transformation of chloroplasts
into chromoplasts
- the degradation of chlorophyll,
- the accumulation of pigments
such as carotenes and
lycopenes, which are
responsible for the orange and
red color of the fruit
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alterations in the texture of
the fruit, the loss of firmness,
due to structural changes in
the principal cell wall
components (cellulose,
hemicellulose and pectin).
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Finally, the accumulation of
sugars such as glucose and
fructose and organic acids in
vacuoles and the production
of complex volatile
compounds is responsible
for the aroma and flavor of
the fruit (Seymour et al., 1993)
Most physical and biochemical
changes are associated with :
-activity
of enzymes such as invertase (Iki et
al., 1978) and polygalacturonase (Tucker and
Grierson, 1982), which increase during the
ripening of tomato fruits,
- or citrate synthase and malate
dehydrogenase (Jefferey et al., 1984) which
decreases considerably during ripening.
Leaf Epinasty Results When ACC from the
Root Is Transported to the Shoot
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In tomato and other dicots, flooding (waterlogging) or anaerobic
conditions around the roots enhances the synthesis of ethylene in the
shoot, leading to the epinastic response.
Ethylene and high concentrations of auxin induce epinasty,
auxin acts indirectly by inducing ethylene production.
Ethylene Biosynthesis in the Abscission
Zone Is Regulated by Auxin
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The shedding of leaves, fruits, flowers, and
other plant organs is termed abscission.
Abscission place in abscission layers,
which become morphologically and
biochemically differentiated during organ
development.
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Weakening of the cell walls at
the abscission layer depends
on cell wall–degrading
enzymes such as cellulase
and polygalacturonase
Ethylene appears to be the
primary regulator of the
abscission process, with
auxin acting as a suppressor
of the ethylene effect
A model of the hormonal control of leaf abscission describes
the process in three distinct sequential phases