Gibberellin: Discovery, Functions and Uses, Practice Problems and FAQs

We all grow. Sometimes we check our height and weight to understand whether growth is happening in the proper way or not. We know that the same is applicable to plants also. But have you ever wondered how plant length increases? Which factors are responsible for this? 

 GIF: Growing plant

In humans, along with nutrients, chemicals called hormones are responsible for growth and development. Similarly, in plants also there are some chemicals called phytohormones that are responsible for growth and development. These chemicals are also known as plant growth regulators. They either promote or inhibit several functions. For example, gibberellin is considered as a growth promoting hormone that stimulates stem growth and seed germination.

Let’s discuss more about the gibberellin in detail in this article.

Table of contents:

• Gibberellins
• Discovery of gibberellins
• Functions of gibberellins
• Uses of gibberellins
• Practice Problems
• FAQs

Gibberellins

Gibberellin is a type of growth promoting phytohormone. More than 100 types of gibberellins are identified in different organisms such as fungi and higher plants. They are commonly denoted as GA1, GA2, GA3 and so on. All GAs are acidic. They are transported throughout the plant body by simple diffusion and vascular tissues. They are manufactured in the plastids through a terpenoid pathway. After formation, they are transferred to the ER and cytosol until they become biologically active.

Structure of gibberellins

Gibberellins possess a gibberellane ring structure. They are tetracyclic diterpenes. They possess 19 - 20 carbon atoms in their structure. 

                                                     Fig: Gibberellin

Discovery of Gibberellins

Rice farmers in Asia in the early part of the 20th century were aware of a disease. This disease makes the rice plants grow tall but declines seed production. The affected plants had weak, elongated stems. They called it the ‘foolish seedling’ or bakanae (baka means fool) disease in Japan.

                                                             Fig: Rice plants

In 1926, E. Kurosawa reported the symptoms of this particular disease. He reported that the symptoms appeared in seedlings on treatment with a fungus filtrate of Gibberella fujikuroi.

           Fig: Fungal infection in rice

The chemical obtained from the sterile filtrate of those tall parts was named as gibberellin (after Gibberella fujikuroi, the name of the fungus). The chemical compound is actually gibberellic acid which induces tallness. Gibberellic acid (GA3) is considered as one of the first gibberellins to be discovered. It is the most intensively studied form of gibberellins.

Functions of gibberellins

Gibberellins perform several functions. Some of them are given below:

• Stimulates stem growth
• Promotes seed germination
• Breaks bud dormancy
• Delays senescence

Stimulates stem and leaf growth

Gibberellin stimulates stem elongation in dwarf plants after external application of GA3. These plants resemble the tallest varieties of the same species. Accompanying this, the stem thickness and size of the leaves become reduced. Leaves turn to pale green too. 

Elongation in dwarf plants

Gibberellins cause elongation of internodes in genetically dwarf plants. Examples include peas and maize plants. Dwarfness in these occurs mainly due to deficiency of gibberellins.

             Fig: Stem elongation in dwarf plants

Bolting in plants

Gibberellins promote stem elongation in rosette plants. Rosette is described as a circular arrangement of leaves. Rosette plants normally show retarded internodal growth and profuse leaf development. Gibberellins promote bolting just prior to flowering or reproductive phase. Bolting is an internode elongation. It is seen in beet, cabbages etc.

                  Fig: Bolting in plants

Promotes seed germination

During the process of seed germination, the embryo imbibes water and swells. The embryo produces gibberellins that triggers the synthesis of hydrolytic enzymes like alpha-amylase, proteases, lipases etc. These enzymes digest the starch, proteins and lipids present in the endosperm. This will release nutrients that are required for the synthesis of new cells.

                                                       Fig: Seed germination

Breaks bud dormancy

Gibberellins reactivate the growth of buds. The buds that are formed in autumn stay dormant until next spring. Thus bud dormancy can be overcome by treating them with gibberellins. 

Delays senescence

Senescence is the process of aging in plants. It occurs due to loss of RNAs, proteins, lipids and chlorophylls. Due to this, the leaves turn reddish yellow. These leaves fall off eventually. Gibberellin delays the aging process and is considered as one of the anti-aging hormones for plants. 

                          Fig: Senescence

Development and ripening of fruits

Gibberellins used along with auxins normally to control fruit growth and development. It is able to induce parthenocarpy also. Fruit ripening is also affected by gibberellin. Ripening of Citrus fruits and apples can be delayed with the help of gibberellins. In this way the fruits can be left on the plant for a longer period to extend the market period. This can also be used for safe and prolonged storage of the fruits. Treatment with gibberellins elongate and improve the shape of apples.

                      Fig: Effect of gibberellin on apples

Uses of gibberellins

The gibberellins are commonly used for the following purposes:

In grapes

Gibberellin increases the length of a grape stalk. This will increase the yield of grapes.

            Fig: Effect of gibberellin in grapes

In sugarcane

It increases the stem length of sugarcane (Saccharum officianarum) and thus increases the yield. In sugarcane the carbohydrates are stored in the form of sugar in the stem.

                        Fig: Sugarcane 

In malting process

Gibberellin accelerates the malting process in the brewing industry. Malting is considered as the process of steeping, germinating and drying grain, so that it gets converted into malt. The malt is then used for brewing or making whisky. 

                                           Fig: Malting

In conifers

Treatment of GA on juvenile conifers hasten the maturity period. This leads to early seed production.

                                       Fig: Conifers

Practice Problems

Q 1. From the given options, identify the person who discovered gibberellin?

a. E. Kurosawa
b. Miller
c. Francis Darwin
d. F.W. Went

Answer: E. Kurosawa in 1926 reported the symptoms of foolish seedling disease in rice. He reported that the symptoms appeared in seedlings on treatment with a fungus filtrate. The chemical obtained from the sterile filtrate of those tall parts was named as gibberellin (after Gibberella fujikuroi, the name of the fungus). The chemical compound is actually gibberellic acid which induces tallness. Hence, the correct option is a.

Q 2. From the given options, identify the hormone which is responsible for elongation of genetically dwarf plants?

a. Cytokinin
b. IAA
c. Gibberellin
d. Abscisic Acid

Answer: Gibberellin stimulates stem elongation in dwarf plants after external application of GA3. These plants resemble the tallest varieties of the same species. Examples include peas and maize. Hence, the correct option is c.

Q 3. Which hormone is used to accelerate the malting process in the brewing industry?

a. NAA
b. GA3
c. Kinetin
d. Abscisic acid

Answer: Malting is the process of soaking barley, wheat, or sorghum in water and allowing it to germinate. In the brewing industry, GA3 is used to speed up the malting or seed germination process. Hence, the correct option is b.

Q 4. How do gibberellins promote seed germination?

Answer: During the process of seed germination, the embryo imbibes water and swells. The embryo produces gibberellins that triggers the synthesis of hydrolytic enzymes like amylases, proteases and lipases. These enzymes digest the starch, proteins and lipids present in the endosperm. This will release nutrients that are required for the synthesis of new cells.

FAQs

Q 1. How does gibberellin increase the yield of grapes?
Answer: Gibberellin increases the length of the grape stalk. It is also responsible to increase the grape size and induce seedlessness in some varieties of grapes. This will give high yield in grapes.

Q 2. How is gibberellic acid used by farmers?
Answer: Farmers use gibberellic acid in different ways. For example, the yield of grapes increases after the treatment with gibberellins. It promotes stalk elongation and grape size. Gibberellin also promotes the stem length in sugarcane and thus increases the yield. The treatment with gibberellins elongate and improve the shape of apples. 

Q 3. How is gibberellin formed in the plants?
Answer: Gibberellins are manufactured in the plastids through a terpenoid pathway. After formation, they are transferred to the ER and cytosol until they become biologically active.

Q 4. Which are the common natural sources of gibberellins?
Answer: Gibberellins are naturally present in plants, some species of fungus and bacteria. This hormone is seen in all flowering plants and many non-flowering plants like ferns.

Q 5. Can we say that gibberellic acid is harmful to humans?
Answer: Gibberellins are relatively harmless for animals and humans. But they have shown some adverse effects on various animal tissues. People normally exposed to residues of GA3 when they have fruits and vegetables treated with GA3.

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