Discoveries and Functions of Phytohormones.

Hormones are special chemicals that carry messages for various purposes like growth, development and also calm the stress conditions. Plant hormones are specially called Phytohormones which can promote as well as inhibit the plant's growth. Phytohormones can perform various functions like root formation, shoot formation, flower and fruit production, and abscission of leaves, flowers, and roots and they are also responsible for senescence (old aging). Some of the phytohormones are defensive for plants against pathogens like abscisic acid, jasmonic acid, salicylic acid, etc while some of the hormones are responsible for growth and development in plants like auxin, and gibberellin,s, etc. in shorts, phytohormones are the messenger chemicals that produced in small quantity but performs big functions for plant’s better growth and development.

Auxin hormone.

Auxin is a growth promoter hormone. It is mainly responsible for apical dominance in plants. Auxin is an acidic (in nature) hormone that is a group of different chemicals and can promote growth. It can be produced naturally or can be formed in laboratories. Example of naturally occurring auxin is IAA (indole acetic acid) and IBA (indole butyric acid) while the NAA (naphthalene acetic acid) is a common example of auxin that can be formed in the laboratories.

Discovery of Auxin.

Auxin was discovered by Charles Darwin and Francis Darwin when they were trying to study the process of phototropism.

Now, what is the process of phototropism?
Phototropism is a natural adaptation by plants that helps to maximize the process of photosynthesis by exposing the leaf of the plant to sunlight. This process helps plants for better growth and development.

Charles Darwin was the father of Francis Darwin. Both father and son worked on canary grass for understanding the process of phototropism but they observed that the coleoptile of canary grass produces a special chemical that bends the leave of canary grass towards sunlight and is hence responsible for growth.

Now, what is meant by coleoptile?
A coleoptile is a protective covering around the growing part or tip of shoots and leaves. Its major function is to provide protection but it also produces auxin which is responsible for the growth of the plant.

Now, how they were able to conclude that the special chemical is produced by the coleoptile of the canary grass and plant bends towards sunlight because of this coleoptile?
They were able to say that because they perform experiments on the coleoptile of the canary grass that shows the coleoptile is responsible for the bending of leaves towards sunlight. They placed the canary grass having coleoptile in front of sunlight. The plant shows bending toward sunlight. Then they removed the coleoptile from the plant and this time plant did not show any bending towards sunlight.

To confirm that this bending is because of coleoptile, they take two plants of canary grasses.
They covered the shoot tip (coleoptile part) of the first grass and the stem region of the second plant and were exposed to sunlight.
The plant whose coleoptile region is covered did not show any bending while the plant whose stem other than the coleoptile side is covered show bending towards sunlight.

However, father and son discovered auxin but they were unable to identify and isolate it. This was done by another scientist Frits Went in 1926.

He took the coleoptile of the oat plant and isolate the auxin from it.
He took an agar plate and placed it near the coleoptile region of the oat plant.
He exposed this oat plant to sunlight and hence auxin started to produce and move towards the agar plate. In this way, he collects auxin from plants on an agar plate.
He took another oat plant and removed its shoot tip. This process is also known as decapitation.
He placed this second plant in darkness (i.e. not exposed to sunlight) and took the agar block which had the auxin on it which he collects from the previous oat plant.
He attaches this agar block to this (second plant) plant whose head region (coleoptile) is removed by him. This plant showed bending however it was not exposed to sunlight.
He concluded from his experiment that coleoptile released a special chemical that traveled downwards from the head region of the shoot and resulted in the bending of the plant. This chemical is responsible for the growth of plants.

Functions of Auxin.

Apical dominance.

Auxin performs several functions in plants like it promotes apical growth and inhibits the branching of plants. This function of auxin can be seen normally in plants that grow in gardens and parks. The gardener cuts some plants from its head region. He does so to promote the growth of branching sides of the plants that are inhibited by auxin.

Root growth.

The auxin can promote rooting as well. A common example of this function of auxin is seen in rose plants.

Abscession of older leaves and fruits.

Auxin is normally considered as a growth hormone but it is responsible for the abscession (shedding off leaves and fruits) of older leaves and fruits. When a young green leaf is formed, then the auxin production at this site is very high but with time, the concentration of auxin started to decrease and because of this low concentration of auxin, the leaves and fruits started to fall down from the plant. In this way, auxin controls the shedding-off of the leaves and fruits.

Used as a Parthenocarpic agent.

One of the biggest applications of auxin is its use in parthenocarpy. Parthenocarpy is simply termed an unfertilized fruit. We inject synthetic auxin into the female part of the plant. The auxin promotes growth there and this female part started to convert into fruit without any fertilization with the male part. The seeds of that fruit are unable to develop because there is no fertilization occur. The fruits like grapes, bananas, etc are the best example of it.

Gibberellins hormone.

Like auxin, gibberellins are also a compound of chemicals. It is mainly responsible for the increase of heights of plants, helps in seed germination, and provides a healthy space for fruits to grow well.

Discovery of Gibberellins.

Gibberellins was discovered accidentally. In 1926, a Japanese scientist E. Kurosawa studied a rice plant disease which is known as foolish seedling disease.

Now, what is the foolish seedling disease of rice plants?
The foolish seedling disease is caused by a fungus, Gibberella fujikuroi. The plant affected by this fungus have weak stems, become sterile (not able to reproduce), and the leaves of the plants become yellow but the plant grows bigger in height.

The symptom of the larger size of rice plant in foolish seedling disease seemed strange to E. Kurosawa. He took the fungus and made it unaffected (i.e. means the Gibberella fujikuroi fungus is unable to cause disease). He inserted it in healthy plants.
He observed that the plants grow larger in size.
He concluded from his work that plants release some chemicals that are responsible for their large height and the fungus (Gibberella fujikuroi) has the same thing. When the concentration of this chemical becomes high in plants, they grow well in height.

Another scientist Yabuta, named these chemical gibberellins as it was first discovered in the Gibberella fujikuroi fungus and promotes the height of plants. Gibberellins are also known as gibberellic acid because of their acidic nature. However, more than 100 types of gibberellins have been discovered and the most common example of gibberellin is GA3.

Functions of Gibbrellins.

Helps in Seed germination.

Gibberellins are produced in the seeds under favorable conditions when the seed needs to grow. The seed has an embryo and a food source for this embryo. The embryo gets a continuous supply of food to live but this food is not enough for the embryo to grow. In favorable conditions (conditions on which seed can grow), gibberellins are started to be produced in the seeds and increase the supply of food for the embryo and in this way, it helps in the germination of seeds.

Helps in the growth of Internodes.

It is also present in the internodes of many plants. Many plants have nodes and internodes in their structure. The length of internodes is normally greater than the nodes and in this way, the internodes are involved in the growth of plants. The internodes have a large height than nodes because of gibberellins.

Elongate the axis of the Stem.

Gibberellins are also involved in the elongation of the axis of the stem. As the large size of stems can bear more fruits, leaves, flowers, etc so gibberellins are very beneficial for plants and farmers as well.

Helps in the production of healthy bigger size fruits.

Gibberellins also increase the size of fruits like apples, pears, etc. As we discussed earlier that gibberellins can elongate the axis of stem so it provides a bigger space for fruits. In this way, gibberellins help to increase the quantity as well as the quality of fruits.

Inhibits abscession.

As gibberellins promote growth and can help to increase the fruit’s size but it is also delay the ripening of fruits. It delays the ripening by strengthening the plant to hold its fruits and vegetables. It is very beneficial for the fruits like tomatoes (botanically it is a fruit). In this way, it helps to inhibit the process of abscession.

Cytokinins.

Like auxin and gibberellins, cytokinins are also growth promotor hormones but it is very different from auxin and gibberellins. The word cytokinins is made up of two words i.e. "Cyto" means "Cells" while "kinin" means "division". So the function of cytokinins is to promote cell division. The precursors of cytokinins are purines and because of this cytokinins are basic in nature.

Discovery of Cytokinins.

Cytokinins were discovered by a series experiments of by different scientists at different times. In 1913 Haberlandt, first reported a substance that can cause cell division in the parenchymatic cells of potato tubers.
In 1941, another scientist J.V.Overbeek found the same substance (that causes cell division) in coconut milk.
In 1954, two different scientists Skoog and Miller discovered and identified this substance respectively. Miller named this substance "Kinetin" because of its functions and activity. Both Skoog and Miller were working on the tissue culture technique.

Now, what is the tissue culture technique?
Tissue culture is a technique in which we cultured or grow cells in the laboratory by providing the cells a proper growth medium and nutrient-enriched environment. We can grow cells outside of our natural environment (i.e. organism’s body) by providing such conditions. This technique is very useful to study cancer cells.

Skoog's Experiment.

Skoog carried the work of Haberlandt. He took agar medium in a culture plate.

Now, what is an agar medium?
Agar medium is a gel-like substance that is commonly used to grow microbes in laboratories. A nutrient-rich environment is provided to the microbes so that they can grow well. It is also used to grow cells on it.

He took some pith cells of the tobacco plant and placed them in the culture plate.
He added auxin in the culture plate also because he thinks auxin is the only growth promoter hormone that can help the pith cells to grow.
Pith cells showed some cell division but it was very slow.
He added coconut water (from Haberlandt’s work) and yeast extract to the culture.
He noticed rapid cell division in the pith cells.
Pith cells formed an undifferentiated mass of cells (tissue) which we called Callus. It was very shocking for him because he don’t know why the pith cells are dividing so fast.
He concluded that this kind of rapid cell division must be due to coconut water and yeast extract because auxin is not able to cause such rapid cell division. Some mysterious thing is present in the coconut water and yeast extract which promotes such kind of cell division in tobacco pith cells.
Skoog discovered a cytokinin hormone but was unable to identify it.

Miller’s Experiment.

Miller identified the mysterious substance that was unknown by Skoog with the help of sperm cells of herring fish. Miller took the DNA from the sperm cells of herring fish.

He took the pith cells of the tobacco plant.
He placed both pith cells of the tobacco plant and DNA extracted from the sperm cells of Herring fish in a culture plate.
He noticed rapid cell division in the pith cells.
He concluded that the same substance is present in both coconut milk and the DNA of the sperm cells of herring fish that cause cell division.
He isolated and named the substance “Kinetin” because it promotes the karyokinesis and cytokinesis ability of the cell.

Functions of Cytokinins.

Helps in Cell division.

As cytokinins are responsible for the active cell division in the tobacco pith cells so it is a growth hormone that promotes cell division but for a proper cell division, the optimum concentration of cytokinin and auxin is very essential. If the concentration of cytokinins is very high then the cell division usually forms an undifferentiated group of cells (which may be unable to perform its function).

Inhibits the Apical dominancy.

Cytokinins inhibit the apical dominance of the plant. As we discussed earlier that auxin is responsible for the apical dominance of the plant (which means an increase in the height of plants). Cytokinins inhibit this apical dominance and promote the lateral branching of the plant.

Helps in the growth of roots and shoots of plants.

Cytokinins are responsible for the growth of roots and shoots of plants by increasing their lengths.

Chlorophyll protection.

Cytokinins also protect the chlorophyll of the plant’s leaves. It means as long as the optimum concentration of cytokinins is present in the leaves of the plants, the leaves remain green and perform photosynthesis.

But when the concentration of cytokinins decreases in the leaves, the leaves started to turn a yellow color and plants shed off these leaves. In this way, it inhibits senescence (i.e. old aging). Even if a plant shed its leaves off and then we spray cytokinins on it, the leaves remain green because chlorophyll is protected by cytokinins hormone.

Leaves formation.

Cytokinins are also involved in the formation of leaves. We normally spray cytokinin in the field area where the average number of leaves is very low. Cytokinin can also increase the length of leaves.

Chloroplast formation and promote photosynthesis.

Cytokinins can increase the number of chloroplasts in the cell. As we know that chloroplasts are the sites of photosynthesis and have their own DNA. Cytokinins promote the division of chloroplast to increase its numbers in the cell. More numbers of chloroplast promotes more photosynthesis and it gives plants more food and energy.

Ethylene.

Ethylene is a growth promotor as well as a growth inhibitor hormone. It is a gaseous hormone that is responsible for the ripening of fruits. Ripening is due to the growth promotor activity of ethylene while abscession and senescence of fruit after fully ripped or mature indicates that it is a growth inhibitor hormone. Ethylene is the most used hormone in agricultural fields because of its ripening ability. it is usually provided in the form of Ethapon.

Ethylene is a triple-response hormone. For example, if we give ethylene to the seed then the plumule (i.e. the embryonic part of the seed that forms (shoots) stems, and leaves of the plant) shows three types of responses:

The first response is that it shows horizontal growth because ethylene inhibits longitudinal growth.
The second response is that the plumule of the seed shows a hook-like appearance or growth. This means the new shoot formed a hook-like appearance first and then move forward.
The third response that ethylene shows on the plumule of the seed is the broadening of the shoot axis. This means it increases the width of the shoot.

Discovery of Ethylene.

It seemed that ethylene was first discovered by Johann Joachim Becher when he heated ethanol with sulfuric acid.
In the nineteen century, coal gas (lamp’s light) is used as a source of energy for street lights. It was observed that the plants that grow near this light source lost more leaves than other plants. At that time, it was thought that it may be due to air pollution. But now we know that this was due to ethylene present in the coal gas.
In 1901, Dimitry Neljubov observed that pea plants growing near the coal gas had abnormal horizontal growth and shorten in length. D. Neljuboy concluded that it was due to ethylene.
In 1910, two cousin brothers noticed that something was released from oranges that ripened the bananas. They found that it was ethylene that caused the ripening of bananas and published their work which indicates that ethylene can be produced naturally.
Later in 1934, another scientist R.Gane told that ethylene is a plant’s natural product that caused the ripening of fruits. But still, ethylene did not get such importance until 1959 when gas chromatography was introduced to study ethylene.

Functions of Ethylene.

Senescence and Abscession in plants.

Ethylene is mainly responsible for senescence (old aging) and abscession (shedding off leaves) because it caused ripening. Earlier the ripening of fruits, vegetables, etc, caused the earlier fall of them from plants. Senescence belongs to the maturity of fruit, flower, etc. It is the condition when the rate is photosynthesis decreases while the rate of respiration increases because more food is used or taken by fruits etc. When the rate of photosynthesis decrease then ultimately plant tries to Shedd-off the fruit to save its energy.

Ripening of Fruits.

Many of the fruits release ethylene in the gaseous form. If we take five bananas and one of them is fully ripened (or mature). this ripened banana release ethylene and influence other bananas to get ripened. In this way, ethylene in the gaseous form caused ripening.

Growth of roots and internodes under submerged water conditions.

Ethylene is also involved in the development of roots and root hairs that are submerged in the water. It also grows the internodes of plants like rice because some of the part of the internode I submerged in water and that submerged region secretes ethylene that promotes growth.

Break seed dormancy.

Ethylene breaks the seed dormancy of plants like peanut and also break the bud dormancy as seen in potatoes.

Abscisic Acid.

Abscisic acid is a growth inhibitor hormone.. it usually works in stressful conditions like drought, light stress, temperature, etc. It is acidic in nature that’s why it is called as abscisic acid (or ABA hormone).

Discovery of Abscisic Acid.

Abscisic acid was discovered by many scientists in different forms of compounds. For example, in 1936, an American scientist Kenneth V. Thimann discovered Dormin. It was a special hormone that inhibits the growth of buds. It was normally produced in mature leaves.

In 1959, Dr. Philip discovered β-inhibitors from oats. He observed that β-inhibitor inhibits the elongation of coleoptile (a protective sheath that protects the emerging shoots of the germinating seeds) in oat plants.

Then in 1963, Frederick Addicott discovered Abscisin ॥ from cotton plants. They named it abscisin because this chemical involves in the process of abscission. Later a term i.e. abscisic acid (ABA) is introduced and all of these growth-inhibiting substances are included in this term. it was called abscisic acid because of its acidic nature.

Functions of Abscisic Acid.

Inhibit Seed Germination.

Abscisic acid inhibits seed germination. As we discussed earlier that gibberellins are responsible for seed germination under favorable conditions. But what happens when the conditions are not favorable for seeds?

The seed did not germinate because when a seed is formed it has a high concentration of abscisic acid. This abscisic acid inhibits the seed to germinate. But when the environmental conditions are normal then gibberellins started to produce in seeds and lead to the germination of seeds. In this way, abscisic acid is very important for seeds because it keeps seed alive under unfavorable conditions.

Seeds Dormancy.

Abscisic acid keeps the seed in dormancy that’s why it is also called Dormin. Sometimes it has been observed that seeds did not grow even if we gave them favorable conditions. The reason for this is abscisic acid because gibberellins are not produced in such concentration where it shows its expression or function while abscisic is present in seeds by birth of its and performs its functions well.

Inhibit Metabolism in Plants.

Abscisic acid inhibits the metabolism of plants. In seeds, the embryo needs food for growth but abscisic acid did not allow the embryo to get food. However, a small amount of food is continuously supplied to the embryo but it's not enough for him to grow. In this way, abscisic acid inhibits the metabolism in seeds.

Inhibit the formation of mRNA.

Abscisic acid can inhibit the formation of mRNA. The inhibition of mRNA means that no protein is formed in the cells. As plants parts like stems, roots, shoots, leaves, etc are very dependent on the protein to fulfill their needs, so abscisic acid caused senescence. Plants also shed off their leaves to save energy and minimize their energy needs. Abscisic acid cause senescence and abscession processes in plants.

Helps in Stress Conditions.

Abscisic acid is also known as a stress hormone because it is mostly released by plants in stressful conditions. For example, its concentration will be high in seeds when they faced unfavorable or stressful conditions.

In mature plants, it also helps to deal with stress conditions. Suppose a plant facing drought stress due to a hot environment. The loss of water in this condition is very high by the transpiration process. So to deal with this condition, plants release an abscisic acid hormone which closes the stomata and decreases the rate of transpiration (i.e. water loss). in this way, abscisic acid is very useful to deal with stress conditions.

Discoveries and Functions of Plant Hormones | Auxin, Gibberellins, Cytokinins, Ethylene and Abscisic Acid