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discovery of gibberellins is credited to Ewiti Kurosawa
who found that a fungus was responsible for abnormal rice
seedling growth, called the "foolish seedling"
disease. The fungus secreted a chemical that caused the
rice plants to grew abnormally long, and then collapse
from weakness. The fungus was Gibberella fujikuroi,
hence the hormone name. |
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| Many
seeds contain a variety of different gibberellins. Over
100 different gibberellins (organic acids synthesized
from mevalonic acid) are known. Gibberellins are produced
in roots and younger leaves. Most effects of gibberellins
are shown only in concert with auxins. |
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Gibberellins
work with auxins to promote rapid elongation and
division of stem tissue. It is thought that gibberellins
particularly promote expansin activity. This is
seen in: |
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Bolting
of biennials |
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Reversal
of genetic dwarfism |
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Gibberellins
promote flowering in biennials during the first
growing season, a process called bolting. |
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Gibberellins
are important to breaking dormancy after imbibition
of water by the seed coat. Gibberellins signal germination
activities. In particular application of gibberellins
to seeds will counter the normal environmental cues,
such as exposure to low temperatures. They stimulate
RNA to promote synthesis of enzymes that convert
stored nutrients (starches) to sugars needed for
rapid cell respiration during germination. Absiscic
acid can counter the effect of gibberllin to keep
seeds dormant. |
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Gibberellins
stimulate some fruit enlargement (e.g., grapes with
longer internodes), and may counter the effects
of herbicides. |
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| Abscisic
acid (ABA) is a hormone that functions by inhibiting growth
activities in times of environmental stress rather than
by promoting growth. It often serves as an antagonist
to the other growth promoting hormones in plants. Abscisic
acid, which is also synthesized from mevalonic acid, got
its name from the erroneous belief that it promoted the
formation of abscission layers in leaves and fruits. It
does not, although leaf abscission accompanies dormancy
in many plants. |
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ABA promotes seed dormancy activities. ABA levels
are high when seeds mature, promoting lowered metabolism
and synthesis of proteins needed to withstand the
dehydration associated with dormancy. Seeds germinate
when ABA is degraded by some environmental action.
Desert seeds must have the ABA washed out of the
seed coat; temperate area plants have ABA degraded
by light-stimulated enzymes. In other cases breaking
dormancy is relative to the ratio of ABA (which
keeps seeds in dormancy) and gibberellins (which
promote germination).Low levels of ABA in maturing
seeds promotes premature germination. |
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ABA is also referred to as the stress activity hormone.
For example, ABA promotes stomata closure during
leaf water deficit conditions by activating K+ ion
transport out guard cells. This involves signal
transduction pathways with Calcium secondary messengers.
ABA in this case originates in roots, and detects
low water level in root tissues. ABA moves upward
into leaves and activates stomatal closure. |
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ABA derivatives, called dormins, are used in commercial
nurseries to keep materials to be shipped in dormant
conditions. The dormancy can be reversed with gibberellins. |
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| Ethylene
is the sole growth regulator known which is a gas. (It
is a small hydrocarbon molecule, easily synthesized in
chemistry labs). |
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benefits of ethylene as a fruit ripener were known for
centuries prior to it being identified as a plant product
in the early 1900's. Chinese gardeners knew centuries
ago that fruits ripened better in rooms with burning incense.
Citrus growers used kerosene stoves in the rooms in which
they ripened their fruit. During the era of gas lamps,
leaking lamps along city streets often promoted leaf abscission.
Today, grocer warehouses have ethylene rooms that are
used for ripening most of our produce, which is shipped
unripe. Immature fruits are firmer and less subject to
damage. Thanks to ethylene rooms, we have "mature
green tomatoes". |
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| Ethylene
is produced in many plant organs, though best studied
in fruits. Ethylene affects many aspects of growth and
development in tissues throughout the plant, but emphasis
is on fruit maturation, leaf abscission and senescence.
Ethylene is synthesized from the amino acid methionine.
High auxin concentrations promote an intermediate step
in this pathway which activates an enzyme in the tonoplast
to convert the intermediate into ethylene. Toxic substances
such as air pollutants also trigger the intermediate step.
Ethylene is also produced by bruised tissues. Although
the benefit of this is unknown, it's probably an artifact
of the wounding response. |
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Ethylene
promotes ripening in fruits by triggering chemical
reactions that degrade the pectins of the middle
lamella, softening fruit, and promoting the conversion
of stored starches and/or oils into sugars that
attract seed dispersers. Some fruit ripening involves
a great increase in the rate of cell respiration
with concurrent high O2 uptake. This function of
ethylene (fruit ripening) has extensive agricultural
impact. Biotechnology has been used to alter sensitivity
to ethylene in some crops to facilitate harvest
and shipping of fruits. |
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Ethylene
promotes female flower production in some members
of the Cucurbitaceae, whereas high gibberellins
may promote formation of male flowers. |
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An
unusual function of ethylene occurs when germinating
seeds encounter mechanical stress. A shoot tip that
encounters an immovable object will grow around
the object by changing its growth direction. This
occurs through differential elongation of cell walls.
When the shoot tip can not penetrate the mechanical
obstacle, ethylene is synthesized, slowing cell
wall expansion. The walls thicken (more resistant
to the pressure) and a stem curvature to bypass
the obstacle via horizontal growth. (Essentially
the shoot is growing around the obstacle.) The stem
tip pushes upward periodically to test for the obstacle.
If the obstacle is encountered, ethylene starts
another cycle for horizontal growth. If no obstacle
is encountered, the shoot resumes negative geotropic
growth. |
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Ethylene
is responsible for initiating the programmed death
(apoptosis) of sclerenchyma and xylem vessels and
tracheids. Death involves intense cellular activity
to degrade and salvage materials of the cytoplasm. |
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Ethylene
is the direct cause of leaf and fruit abscission.
Ethylene promotes the degradation of the cell walls
in the abscission zone cells. The production of
ethylene in the abscission zone cells is triggered
by the declining levels of auxin in leaves as summer
ends. This auxin/ethylene interaction is also used
commercially. Fruit growers may spray auxins on
fruits to prevent the fruits from falling to the
ground prior to harvest. Concentration is important;
very high auxin level promotes ethylene production.
High concentrations of CO2 inhibit ethylene production
so CO2 is used to prevent fruit ripening in grocer
warehouses. |
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| Brassinolides
(Brassinosteroids) |
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| Brassinolides
is a plant steroid discovered in pollen of members of
the mustard family, and best studied in Arabadopsis. Chemically
they are very similar to animal steroid hormones. |
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| Brassinosteoroids
activate signal transduction pathways the promote cell
elongation and cell division. Brassinosteroids promote
differentiation of xylem tissue, and perhaps other tissues,
too. Brassinosteroids can also retard leaf abscission.
Absence of brassinolides results in dwarf plants. It is
difficult to study them because their effects overlap
those of auxin and gibberellins. |
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| Salicylic
acid is known to activate defense genes against pathogen
invaders. Salicylic acid, a phenolic extract from willow
bark, was long used as an analgesic. It is now prepared
commercially and is the active ingredient of aspirin. |
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| Jasmonates
are a group of fatty acid derivatives. They appear to
have a role in seed germination, root growth, and the
storage of protein (especially in seeds). Synthesis of
defense proteins may be triggered by jasmonates. |
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| Systemin
is a small peptide found in wound tissue. It may stimulate
defense activities in other parts of the plant to prevent
more wounding. |
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| Oligosaccharins
are short chain sugars in cell walls that may have a role
in defense against pathogens. They may also help regulate
growth, differentiation and flower development all by
activating signal transduction pathways. |
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