GIBBERLINS
Introduction
Gibberellic Acid (GA) is a natural hormone in plant that
promotes and regulates growth and development in plants. It belongs to the
family of hormone known as Gibberellins. Scientists are exploring the benefits
it can offer in enhancing and maximizing fruit and vegetable yield. There is
also evidence that it helps in the seed germination process in some plants. Gibberellins
(GAs) are a large family of tetracyclic, diterpenoid compounds, some of which
function as endogenous plant growth regulators. Through phenotypic analyses of
mutants with reduced GA production, it has been revealed that bioactive GAs
play an essential role in many aspects of plant growth and development, such as
stem elongation, flower and fruit development, and seed germination. Despite
considerable effort, it has not yet been determined precisely where the
bioactive GAs are synthesized in plants, or which cells/tissues are targeted by
the bioactive GAs to initiate GA-mediated biologic actions. GAs is primarily
synthesized at the site of their action. There is some evidence for the
presence of GAs in xylem and phloem exudates, indicating a long-distance
transport of GAs through these tissues.
Gibberllins
The nature of gibberlins
Unlike the classification of auxins which are classified on
the basis of function, gibberellins are classified on the basis of structure as
well as function. All gibberellins are derived from the ent-gibberellane
skeleton. The structures of this skeleton derivative along with the structure
of a few of the active gibberellins are shown above. The gibberellins are named
GA1....GAn in order of discovery. Gibberellic acid,
which was the first gibberellins to be structurally characterized, is GA3.
There are currently 136 GAs identified from plants, fungi and bacteria. The most widely available compound is GA3, or
gibberellic acid, which is a fungal product. The most important GA in plants is
GA1, which is the GA primarily responsible for stem elongation.
Gibberellin Biosynthesis and Metabolism
GAs are synthesized from mevalonic acid
in young tissues of the shoot (exact location uncertain) and developing seed.
GAs are probably transported in the phloem and xylem. Gibberellins are diterpenes synthesized from acetyl CoA via
the mevalonic acid pathway. They all have either 19 or 20 carbon units grouped
into either four or five ring systems. The fifth ring is a lactone ring as
shown in the structures above attached to ring A. Gibberellins are believed to
be synthesized in young tissues of the shoot and also the developing seed. It
is uncertain whether young root tissues also produce gibberellins. There is
also some evidence that leaves may be the source of some biosynthesis .3 acetyl
CoA molecules are oxidized by 2 NADPH molecules to produce 3 CoA molecules as a
side product and mevalonic acid. Mevalonic acid is then Phosphorylated by
ATP and decarboxylated to form isopentyl pyrophosphate. 4 of these
molecules form geranylgeranyl pyrophosphate which serves as the donor for all
GA carbon atoms. This compound is then converted to copalylpyrophosphate
which has 2 ring systems Copalylpyrophosphate is then converted to kaurene
which has 4 ring systems Subsequent oxidations reveal kaurenol (alcohol
form), kaurenal (aldehyde form), and kaurenoic acid respectively. Kaurenoic
acid is converted to the aldehyde form of GA12 by decarboxylation. GA12 is the
1st true gibberellane ring system with 20 carbons. From the aldehyde form
of GA12 arise both 20 and 19 carbon gibberellins but there are many mechanisms
by which these other compounds arise. Certain commercial chemicals which
are used to stunt growth do so in part because they block the synthesis of
gibberellins. Some of these chemicals are Phosphon D, Amo-1618, Cycocel (CCC),
ancymidol, and paclobutrazol. During active growth, the plant will metabolize
most gibberellins by hydroxylation to inactive conjugates quickly with the
exception of GA3. GA3 is degraded much slower which helps to explain why the
symptoms initially associated with the hormone in the disease bakanae are
present. Inactive conjugates might be stored or translocated via the phloem and
xylem before their release (activation) at the proper time and in the proper
tissue.
Functions of Gibberellins
Active gibberellins show many physiological effects, each
depending on the type of gibberellins present as well as the species of plant.
Some of the physiological processes stimulated by gibberellins are outlined
below Stimulate stem elongation by stimulating cell division and
elongation.
- Stimulates bolting/flowering in
response to long days.
- Breaks seed dormancy in some
plants which require stratification or light to induce germination.
- Stimulates enzyme production
(a-amylase) in germinating cereal grains for mobilization of seed
reserves.
- Induces maleness in dioecious
flowers (sex expression).
- Can cause parthenocarpic
(seedless) fruit development.
- Can delay senescence in leaves
and citrus fruits.
Physiological effect
of gibberellins:
Germination:
This
hormone also plays the central role in breaking seed dormancy in plant species
that require their seeds to be exposed to light or their seed coats to be
cracked before germination. During the
germination process, seeds initially take water, which triggers the germ of the
plant to begin growing. The seed begins to produce hormones and enzymes that
stimulate the plant to grow. Among these hormones are the gibberellins, which
regulate the synthesis of proteins and the growth of stem tissues. Introducing
additional gibberellins in the form of gibberellic acid can greatly accelerate
seed germination. A standard solution is typically used for most seeds and is
effective for both breaking the dormancy of seeds and accelerating their
sprouting and initial growth. Gibberellins are responsible for
promoting growth in the embryo of a seed. Gibberellin
released by the embryo travels towards the aleurone layer, its target tissue
situated in the endosperm region of the seed (alongside the embryo). Gibberellin
acts as the inducer, as its presence allows the enzyme induction of amylase,
which can break down starch INTO a sugar to be used in the embryo. Sugar is
used in the plant to synthesize proteins and break out of dormancy.
.
Increased Growth:
Gibberellins are a group of plant hormones that influence
growth and development. Gibberellic acid occurs naturally in the plant and is
used commercially to manipulate plant growth. Stem growth - GA1 causes
hyperelongation of stems by stimulating both cell division and cell elongation. The
hormone gibberellic acid is used by plants to induce growth .Growth is regarded
as the elongation of plant cells, which is most profound at the tips of stem
shoots. As days lengthen, gibberellic acid acts to induce flowering, as in the
case of plants in the mustard family like radish, cauliflower and broccoli.
Thus, the new seedling begins growth and breaks out of the seed. Gibberellins initiate this process in Summer, when the
external environment exhibits favorable conditions for plant growth. This
produces tall, as opposed to dwarf, plants. Bolting in long day plants - GAs
cause stem elongation in response to long days.
Flowering:
If Gibberellic acid is applied to young plants,
the result is often premature flowering of the plant. According to the
California Rare Fruit Growers, “formation of male flowers is generally promoted
by concentrations of 10 to 200 ppm.; female flowers by concentrations of 200 to
300 ppm.” These dosage amounts should be taken into consideration when using
Gibberellic acid for premature flowering.
Increased Yield from Fruit Plants:
In fruit plants that have not been properly
pollinated, Gibberellic acid is used to increase the fruit yield from these
plants. Often, the resulting fruit is seedless.
Gibberellic acid is applied to plants to increase overall plant growth,
and it is often used on grapes. Gibberellic acid not only increases the number
of grapes per bunch but also the average size of each grape.
Frost
Protection:
Gibberellic acid, if sprayed on plants in
full-bloom, can counteract the deadly effects of frost when the weather gets
cold.
Commercial
Use of gibberlin:
Commercially,
gibberellins are produced by fungal cultures, and it is the purified natural
products that are applied to plants. Generally, gibberellic acid (GA3) is used,
because this is the only gibberellin obtainable in commercial quantities,
although an expensive mixture of GA4 and GA7 is now commercially available for
specific purposes. Dwarf landscape plants and potted flowering plants
are treated with a hormone that inhibits the synthesis of gibberellic acids.
The absence of gibberellic acid results in smaller, more compact plants.The
most popularly used gibberlins are written as follows:
Seed
Treatment:
Seeds are protected by a thick seed coat that
must be permeated before germination can take place. Seeds that are difficult
to germinate are often treated with giberellic acid to increase germination
rates. Gibberellic acid is frequently used as a substance to stimulate
germination in seeds that have become dormant. This is one of the most common
and widespread uses of Gibberellic acid on plants.
Sex Expression
Presence
of gibberellic acid is needed for male flower parts, the anther with its
stamen, pollen and filament, to be formed in the developing flower bud. This is
the case for plants with perfect flowers, those that have both male and female
parts in the same blossom, as well as for plants that form single-sexed male
flowers.
Secondary Effects
The
presence of gibberellic acid, depending on specific plant species, can have
secondary effects. One is the prevention of seeds in fruits, or the fleshy
swelling of the ovary without forming seeds, such as in modern bananas. An
abundance of this hormone can retard the dropping of dying leaves or the ripe
fruits of citrus, a process called senescence.
Enhanced production of seedless grapes
Bigger, more uniform bunches with larger fruit
are produced. Among other effects, the gibberellin causes lengthening of the
peduncle (stalk) attaching each grape to the cluster, thus permitting larger
grapes to form. Virtually all the grapes that go to market are now treated with
gibberellin.
The production of hybrid cucumber seed:
Most high-producing cucumbers are F1 hybrids.
GA sprays induce the production of male flowers on cucumber plants that
normally produce only female flowers. The seed from neighboring all-female
plants of a different strain is then exclusively hybrid.
Increased malt production:
The addition of GA to germinating barley during
beer production enhances a-amylase production so that more malt is produced
more quickly. As the malt is the raw material for fermentation, a greater production
of beer is made possible by this technique.
Increased sugarcane yield:
GA promotes the elongation of sugarcane stalks
with no change in the sugar concentration, so that the net yield of sugar is
increased.
Mukesh RamjaliNepali
MSc.Ag first sem
Article unpublished
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