In the early 1900s, F. W. Went made the profound statement that translates as, тАЬWithout growth substances, there is no growth.тАЭ Plant hormone shave an important role throughout the plant kingdom. A considerable amount of experiment shows that extremely low concentrations of a plant hormone can regulate many aspects of plant growth and there development from seed germination through senescence to death of the plant.
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Auxins were the 1st class of plant hormones discovered more than 80 years ago. Since this time, five additional classes of plant hormones have been recognized:-
тАв Gibberellins
тАв Cytokinins
тАв Abscisic acid (ABA)
тАв Ethylene and
тАв The newest - Brassino Steroids.
Numerous advances have been made in the use of plant growth regulating substances on a practical scale, along with basic research at the biochemical, physiological, and molecular levels. Today, the use of plant growth regulating compounds in agriculture is on the rise and will continue to increase as more scientific information becomes available.
This Article will provide you a survey of Plant Growth Regulating compounds and their many effects on plant growth and development.
The main difference between a plant hormone and plant growth regulator can be summed by remembering:-
тАЬAll plant hormones are plant growth regulators but all plant growth regulators are not plant hormones.тАЭ
This means that plant hormones are naturally occurring and so are plant growth regulators, but that plant growth regulators can be both naturally occurring and synthetic and therefore cannot fall into the plant hormone category.
The only active auxin found in plants today is Indole-3-aceticacid (IAA) and is involved in many physiological processes in plants including:-
тАв Cellular elongation
тАв Phototropism
тАв Gravitropism
тАв Apical dominance
тАв Root initiation
тАв Ethylene production
тАв Fruit growth
тАв Sex expression and
тАв Weed control.
Auxins promote a dramatic stimulation in cell elongation in detached plant parts. Typically in these detached tissues a little or no IAA is found therefore when these tissues are placed in contact with IAA, there are large increases in growth.
Phototropism is the movement of a plant organ in response to gradients in light. When higher concentrations of IAA occur on the shaded side of the coleoptile in response to light, there is accelerated growth in this region. Growth proceeds normally on the illuminated side, and this differential in growth causes bending toward the light.
Gravitropism is the movement of a plant organ in response to gravity. Studying plant growth and development in response to gravity is important for many reasons, such as better understanding of how a seed knows which way is up or down during the germination process. When a seedling is placed horizontally, it responds to EarthтАЩs gravitational force. The stem bends upward against gravity, which is called negative gravitropism, whereas the roots bend downward with gravity, which is called positive gravitropism.
Auxins stimulate root initiation and inhibit root elongation. Auxins are the only class of plant hormones that majorly affect rooting, and they are used commercially to stimulate adventitious rooting.
The 2 most commonly used auxins for the acceleration of adventitious root formation are indole-3-butyric acid (IBA) and naphthalene acetic acid (NAA).
Auxins promote increase in fruit size.
Auxins alter sex expression in plants. When it is sprayed on cucurbits, okra, pepper,and Cannabis, there is a dramatic increase in the number of female flowers.
Auxins play a part in weed control.
Gibberellins are involved in numerous physiological processes in plants like :-
тАв Stem Growth Elongation
тАв Bolting and flowering
тАв Seed germination
тАв Dormancy
тАв Sex expression
тАв Fruit growth and
тАв Parthenocarpy.
More than 100 gibberellins are known today. The most commonly used name for Gibberellic acid is GA3. There are no synthetic forms of gibberellins. The reason behind dwarfness of many landscape plants is that the plant either cannot produce gibberellins, or there cells are not receptive to the hormone. Dwarf trees, like the bonsai seen have highly reduced amounts of gibberellin. The stem starts to grow when it is applied in low concentration to a bush. The Gibberellins also defeat the genetic limitations in different dwarf varieties.
There are ample amount of evidences which supports that gibberellins are directly involved in controlling and promoting seed germination.
Some seeds that are sensitive to light such as tobacco exhibit poor germination in the absence of sunlight.
Germination begins rapidly if the seeds are exposed to the sunlight.
Dependency of light can be decreased if the seeds are treated with gibberellic acid.
Gibberellins cause the conversion of starch to sugar in the endosperm. This sugars is used for energy generation during the germination process.
While auxins stimulated production of female flowers, gibberellins stimulate the production of male flowers in some plants.
Gibberellins have been shown to affect fruit shape.
Gibberellins have also been shown to promote the elongation of apple fruits to make the fruit look more appealing.
Gibberellins are commonly used to induce parthenocarpy ( development of fruits without pollination or fertilization, resulting in seed less ness ) in grapes.
Same as auxins and gibberellins, Cytokinins were also not initially isolated from a plant source.
In 1963, the first naturally occurring cytokinin was found in plants.
Cytokinins are involved in numerous physiological processes in plants, including :-
тАв Cell division
тАв Organ formation
тАв Senescence
тАв Stomatal opening
тАв Lateral bud break and
тАв Sex expression.
The major function of cytokinins in plants is to promote cell division by stimulating the production of proteins needed for mitosis.
Cytokinins promote callus ( an undifferentiated mass of cells and it is grown for tissue culture) formation in most plant tissues. The capability to regenerate plants from callus is a biotechnological tool commonly used today for selecting plants that are resistant to drought, salt stress, pathogens, herbicides, and others.
Cytokinins can delay the senescence process in plants. When cytokinins are sprayed on a single leaf, the plant diverts nutrients to that particular leaf. This is called creating a sink, which delays senescence.
Foliar applications of cytokinins have been reported to stimulate photosynthetic rates in a variety of plants. Some says that application of cytokinins on plant leaves results in wider opening of stomates and stay open for longer period which further leads to an increase rates of photosynthesis.
When cytokinins are applied to the whole plant it will result in overcoming apical dominance, i.e promoting lateral bud break.
A commonly used synthetic cytokinin is kinetin, and a naturally occurring form is zeatin.
Abscisic acid (ABA) has been shown to be widely distributed within the plant kingdom, and it is the only inhibitor in this category.
A important effective feature of abscisic acid in plants is its capability to realize that a plant is undergoing water stress by which ABA levels are increases dramatically. The increase in ABA levels leads to stomatal closure, thereby preventing water loss via transpiration.
In addition to being a plant stress signal, abscisic acid is also involved in :-
тАв Dormancy
тАв Abscission
тАв Delaying seed germination and
тАв Retardation of plant growth and development.
In 1935, William Crocker and coworkers were the first to report that ethylene was the fruit-ripening hormone.
Other roles of ethylene in physiological processes in plants are its effects on:-
тАв Seedling growth
тАв Senescence
тАв Abscission, and
тАв Sex expression.
Ethylene is the only gaseous plant hormone.
When ethylene is applied to fruits, it causes them to ripen. For example, green tomatoes are shipped from farms to supermarkets, where they are treated with ethylene to cause them to turn red.
In 1901, a Russian scientist was the first to show that ethylene has an effect on seedling growth.
Ethylene can inhibit or promote elongation of stems, roots, or other organs.
The inhibition of elongation is rapid and reversible. The triple response promoted by ethylene can act as a survival mechanism in seedlings.
Many synthetic reactions generally results in Senescence ( characterized by chlorophyll, protein, and nucleic acid degradation )
Ethylene promotes abscission in plants. The process of abscission is very important in agriculture because abscission or no abscission of flowers, fruits, and leaves influences yields, efficiency, and harvesting operations.
The newest class of plant hormone was identified in 1979 and called Brassinolide. Brassinosteroids (BRs)are a class of growth-promoting steroidal phytohormones.
BRs control almost all activities of plant growth and development, and also play significant role in plant adaptation for there survival under various biotic and abiotic stresses.
The major role of brassinosteroids in plants is the promotion of stem elongation that is independent of gibberellins.
Brassinosteroids also affect :-
тАв Crop yields
тАв Assimilate uptake
тАв Enhance xylem differentiation
тАв Improve resistance to disease, herbicide, and salt stress
тАв Promote germination and
тАв Decrease abortion and fruit drop.
They also have the potential to be used as an insecticide and in plant tissue culture.
In conclusion, now we have a better understanding of plant growth regulators and their many uses in agriculture today.
We learned the definitions for a Plant hormone, plant growth regulators :- Auxin, Gibberellin, Cytokinin, Abscisic acid, Ethylene, and Brassinosteroids.
Each of the six classes of plant hormones is involved in key physiological processes in plants.
Plant growth regulators can be used in combination or separately to manipulate nature for agricultural production.
- TAPSENDRA PATEL
тАв Gibberellins
тАв Cytokinins
тАв Abscisic acid (ABA)
тАв Ethylene and
тАв The newest - Brassino Steroids.
Numerous advances have been made in the use of plant growth regulating substances on a practical scale, along with basic research at the biochemical, physiological, and molecular levels. Today, the use of plant growth regulating compounds in agriculture is on the rise and will continue to increase as more scientific information becomes available.
This Article will provide you a survey of Plant Growth Regulating compounds and their many effects on plant growth and development.
The main difference between a plant hormone and plant growth regulator can be summed by remembering:-
тАЬAll plant hormones are plant growth regulators but all plant growth regulators are not plant hormones.тАЭ
This means that plant hormones are naturally occurring and so are plant growth regulators, but that plant growth regulators can be both naturally occurring and synthetic and therefore cannot fall into the plant hormone category.
Auxins
AuxinsтАФthe first class of plant hormones discoveredтАФwere initially found in human urine. Since this time, they have been isolated from numerous plants and are thought to be ubiquitous in the plant kingdom. The auxin term is derived from the Greek word тАЬauxeinтАЭ means тАЬto grow.тАЭThe only active auxin found in plants today is Indole-3-aceticacid (IAA) and is involved in many physiological processes in plants including:-
тАв Cellular elongation
тАв Phototropism
тАв Gravitropism
тАв Apical dominance
тАв Root initiation
тАв Ethylene production
тАв Fruit growth
тАв Sex expression and
тАв Weed control.
Auxins promote a dramatic stimulation in cell elongation in detached plant parts. Typically in these detached tissues a little or no IAA is found therefore when these tissues are placed in contact with IAA, there are large increases in growth.
Phototropism is the movement of a plant organ in response to gradients in light. When higher concentrations of IAA occur on the shaded side of the coleoptile in response to light, there is accelerated growth in this region. Growth proceeds normally on the illuminated side, and this differential in growth causes bending toward the light.
Gravitropism is the movement of a plant organ in response to gravity. Studying plant growth and development in response to gravity is important for many reasons, such as better understanding of how a seed knows which way is up or down during the germination process. When a seedling is placed horizontally, it responds to EarthтАЩs gravitational force. The stem bends upward against gravity, which is called negative gravitropism, whereas the roots bend downward with gravity, which is called positive gravitropism.
Auxins stimulate root initiation and inhibit root elongation. Auxins are the only class of plant hormones that majorly affect rooting, and they are used commercially to stimulate adventitious rooting.
The 2 most commonly used auxins for the acceleration of adventitious root formation are indole-3-butyric acid (IBA) and naphthalene acetic acid (NAA).
Auxins promote increase in fruit size.
Auxins alter sex expression in plants. When it is sprayed on cucurbits, okra, pepper,and Cannabis, there is a dramatic increase in the number of female flowers.
Auxins play a part in weed control.
Gibberellins
Gibberellins are a group of plant hormones which aims for growth and development of plants essential organs. It was later found that the active component secreted by fungus was a gibberellin.Gibberellins are involved in numerous physiological processes in plants like :-
тАв Stem Growth Elongation
тАв Bolting and flowering
тАв Seed germination
тАв Dormancy
тАв Sex expression
тАв Fruit growth and
тАв Parthenocarpy.
More than 100 gibberellins are known today. The most commonly used name for Gibberellic acid is GA3. There are no synthetic forms of gibberellins. The reason behind dwarfness of many landscape plants is that the plant either cannot produce gibberellins, or there cells are not receptive to the hormone. Dwarf trees, like the bonsai seen have highly reduced amounts of gibberellin. The stem starts to grow when it is applied in low concentration to a bush. The Gibberellins also defeat the genetic limitations in different dwarf varieties.
There are ample amount of evidences which supports that gibberellins are directly involved in controlling and promoting seed germination.
Some seeds that are sensitive to light such as tobacco exhibit poor germination in the absence of sunlight.
Germination begins rapidly if the seeds are exposed to the sunlight.
Dependency of light can be decreased if the seeds are treated with gibberellic acid.
Gibberellins cause the conversion of starch to sugar in the endosperm. This sugars is used for energy generation during the germination process.
While auxins stimulated production of female flowers, gibberellins stimulate the production of male flowers in some plants.
Gibberellins have been shown to affect fruit shape.
Gibberellins have also been shown to promote the elongation of apple fruits to make the fruit look more appealing.
Gibberellins are commonly used to induce parthenocarpy ( development of fruits without pollination or fertilization, resulting in seed less ness ) in grapes.
Cytokinins
Same as auxins and gibberellins, Cytokinins were also not initially isolated from a plant source.
In 1963, the first naturally occurring cytokinin was found in plants.
Cytokinins are involved in numerous physiological processes in plants, including :-
тАв Cell division
тАв Organ formation
тАв Senescence
тАв Stomatal opening
тАв Lateral bud break and
тАв Sex expression.
The major function of cytokinins in plants is to promote cell division by stimulating the production of proteins needed for mitosis.
Cytokinins promote callus ( an undifferentiated mass of cells and it is grown for tissue culture) formation in most plant tissues. The capability to regenerate plants from callus is a biotechnological tool commonly used today for selecting plants that are resistant to drought, salt stress, pathogens, herbicides, and others.
Cytokinins can delay the senescence process in plants. When cytokinins are sprayed on a single leaf, the plant diverts nutrients to that particular leaf. This is called creating a sink, which delays senescence.
Foliar applications of cytokinins have been reported to stimulate photosynthetic rates in a variety of plants. Some says that application of cytokinins on plant leaves results in wider opening of stomates and stay open for longer period which further leads to an increase rates of photosynthesis.
When cytokinins are applied to the whole plant it will result in overcoming apical dominance, i.e promoting lateral bud break.
A commonly used synthetic cytokinin is kinetin, and a naturally occurring form is zeatin.
Abscisic acid (ABA)
Abscisic acid (ABA) has been shown to be widely distributed within the plant kingdom, and it is the only inhibitor in this category.
A important effective feature of abscisic acid in plants is its capability to realize that a plant is undergoing water stress by which ABA levels are increases dramatically. The increase in ABA levels leads to stomatal closure, thereby preventing water loss via transpiration.
In addition to being a plant stress signal, abscisic acid is also involved in :-
тАв Dormancy
тАв Abscission
тАв Delaying seed germination and
тАв Retardation of plant growth and development.
Ethylene
In 1935, William Crocker and coworkers were the first to report that ethylene was the fruit-ripening hormone.
Other roles of ethylene in physiological processes in plants are its effects on:-
тАв Seedling growth
тАв Senescence
тАв Abscission, and
тАв Sex expression.
Ethylene is the only gaseous plant hormone.
When ethylene is applied to fruits, it causes them to ripen. For example, green tomatoes are shipped from farms to supermarkets, where they are treated with ethylene to cause them to turn red.
In 1901, a Russian scientist was the first to show that ethylene has an effect on seedling growth.
Ethylene can inhibit or promote elongation of stems, roots, or other organs.
The inhibition of elongation is rapid and reversible. The triple response promoted by ethylene can act as a survival mechanism in seedlings.
Many synthetic reactions generally results in Senescence ( characterized by chlorophyll, protein, and nucleic acid degradation )
Ethylene promotes abscission in plants. The process of abscission is very important in agriculture because abscission or no abscission of flowers, fruits, and leaves influences yields, efficiency, and harvesting operations.
Brassinosteroids
The newest class of plant hormone was identified in 1979 and called Brassinolide. Brassinosteroids (BRs)are a class of growth-promoting steroidal phytohormones.
BRs control almost all activities of plant growth and development, and also play significant role in plant adaptation for there survival under various biotic and abiotic stresses.
The major role of brassinosteroids in plants is the promotion of stem elongation that is independent of gibberellins.
Brassinosteroids also affect :-
тАв Crop yields
тАв Assimilate uptake
тАв Enhance xylem differentiation
тАв Improve resistance to disease, herbicide, and salt stress
тАв Promote germination and
тАв Decrease abortion and fruit drop.
They also have the potential to be used as an insecticide and in plant tissue culture.
In conclusion, now we have a better understanding of plant growth regulators and their many uses in agriculture today.
We learned the definitions for a Plant hormone, plant growth regulators :- Auxin, Gibberellin, Cytokinin, Abscisic acid, Ethylene, and Brassinosteroids.
Each of the six classes of plant hormones is involved in key physiological processes in plants.
Plant growth regulators can be used in combination or separately to manipulate nature for agricultural production.
- TAPSENDRA PATEL
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