Gibberellins (GAs): Plant Hormone Functions & Signaling Pathway (AP Biology Guide)

 

Master the Foundations of  the ​Gibberellins (GAs): Plant Hormone Functions & Signaling Pathway (AP Biology Guide) ( Aligned with College Board Standards)

Our study guides align perfectly with the advanced AP Biology curriculum taught at Stuyvasant high school, Illinois mathmatics and science Academy , Gwinnett School of Mathmatics Technology , Basis Chandler, Basis Peoria and Maggie L. Walker Governor's School  ensuring ensuring high scores in AP biology assessments."

Before diving into the Gibberellins (GAs): Plant Hormone Functions & Signaling Pathway (AP Biology Guide) ensure you have gone through comprehensive guide on Auxin Signal Transduction Pathway in AP Biology: Cell Communication, Phototropism, and Practice Questions 

Table of content 

• Introduction to Gibberellins (GAs)
• Key Physiological Effects & Commercial Applications
• ​Chemical Structure and Biosynthesis Site
• ​​Gibberellin Signaling Pathway (The DELLAs Repressor)
• ​Commercial Applications in Agriculture
• ​Auxin vs. Gibberellin: Quick Comparison Table
• ​​​​Your Understanding  Practice Questions
• Advanced Thinking: Critical  Questions
• Data Analysis: Interpreting Graphs

Introduction to Gibberellins (GAs)

• Gibberellins (GAs) are a class of weakly acidic plant growth hormones that play a crucial role in regulating various developmental processes. 
• The active substance was first discovered by E. Kurosawa from the filtrate of the fungus Gibberella fujikuroi, which causes the famous "Bakanae" (foolish seedling) disease in rice plants.

 

Bakanae disease rice plants Gibberella fujikuroi

• While more than 100 structurally distinct gibberellins (GA1, GA2, GA3. up to GA136 have been isolated from different organisms like fungi and higher plants, GA3 (Gibberellic Acid) was the first to be isolated in its pure form and remains the most extensively studied.

​​​ 🧪 Key Bioassays to Remember

The Barley Endosperm Bioassay (alpha-amylase induction test) and the Dwarf Maize/Pea Elongation Test (Drop Test) are widely used to detect the presence and activity of gibberellins.

Major Sites of Synthesis & Transport

• ​In higher plants, gibberellins are primarily synthesized in actively growing tissues, including ​Apical shoot buds and young leaves near the shoot tips, Root tips, Developing seeds and embryos.

Biosynthesis Precursor: 

• Gibberellic acid is synthesized via the terpenoid pathway. Mevalonic acid (MVA), which is derived from Acetyl-CoA (acetyl coenzyme A) , serves as the metabolic precursor for the synthesis of gibberellins.

​Transport Mechanism: 

• Unlike Auxins, gibberellin transport is non-polar. It moves throughout the plant through simple cellular diffusion as well as through vascular conducting channels (Phloem and Xylem).

Key Physiological Effects & Commercial Applications

​Seed Germination & Overcoming Dormancy

• ​Gibberellins play a master role in breaking seed and bud dormancy. 
• It enhances seed germination by triggering the de novo synthesis of alpha-amylase and other hydrolytic enzymes in the aleurone layer, which break down stored starch into glucose for the developing embryo.

​Photoblastic Seeds: 

• In light-sensitive seeds like Lettuce (Lactuca sativa), application of gibberellin can trigger germination even in complete darkness, replacing the natural requirement for red light.

​ Bolting in Rosette Plants

• ​When gibberellin is sprayed on rosette plants (like cabbage), it induces rapid internodal elongation just prior to flowering, producing flowers with long stalks. 
• This phenomenon is highly utilized in agriculture and is known as Bolting.

​Overcoming Genetic Dwarfism

• ​If gibberellin is applied to single-gene dwarf mutants (such as dwarf varieties of maize or peas), it overcomes genetic dwarfism, causing the plants to grow to normal phenotypic heights.

​Sex Expression & Parthenocarpy

• ​In monoecious or dioecious plants (like cucumber), the application of GA promotes the formation of male flowers on genetically female plants.
• Gibberellins are potent agents for inducing seedless fruit production (parthenocarpy) in tomatoes, apples, and pears.

​Industrial & Agricultural Uses

• ​GA3 is extensively used to speed up the malting process in Brewing Industryby accelerating barley germination.
• Spraying sugarcane crops with gibberellins increases the length of the internodes, thereby boosting the total biomass yield by up to 20 tons per acre.
• Spraying young conifers with GA shortens their juvenile phase, leading to early maturity and advanced seed production.

​💡 Related study to understand the  Photosynthesis: Light & Dark Reactions, Pigments, and PAR | AP Biology Guide

Chemical Structure and Biosynthesis Site of Gibberellins

• The Tetracyclic Ring: Structurally, all gibberellins are derived from a basic skeleton called the ent-gibberellane ring system. 
• It is a complex tetracyclic diterpenoid structure (consisting of four interconnected rings).

​Carbon Skeleton: 

• Based on the number of Carbon atoms, gibberellins are divided into two main categories:
• ​C20-Gibberellins contain the full 20 carbon atoms of the diterpene skeleton (e.g., GA 12)
• ​C19 Gibberellins lose one carbon atom (usually Carbon-20) during metabolism and have only 19 carbon atoms. GA3 (Gibberellic Acid) belongs to this C19 group, which is physiologically the most active form in plants.
• ​All active gibberellins possess a free Carboxylic acid group (-COOH) at the Carbon-7 position, which gives them their weakly acidic nature.

​🏭 Biosynthesis Pathway and Sites in Plants

• ​Gibberellin synthesis in plants is a highly coordinated, three-step process that takes place across three different cellular compartments:

Geranylgeranyl Pyrophosphate( GGPP formation)   ➡️ Plastid ( ent-kaurene)   ➡️ Endoplasmic reticulum  (GA12 aldehyde) ➡️ Cytoplasm (Active GA3 ) 

The Starting Material (Precursor): 

• The primary precursor for gibberellin biosynthesis is Mevalonic Acid (MVA). Through the terpenoid pathway, it is converted into Geranylgeranyl Pyrophosphate (GGPP), which contains 20 carbon atoms.

​The Three Cellular Compartments:

• In ​Plastids (Chloroplasts),  The initial steps occur here, where GGPP is converted into a molecule called ent-kaurene.
• ​In Endoplasmic Reticulum (ER),  Ent-kaurene is then transported to the ER, where it undergoes a series of oxidation reactions driven by P450 monooxygenases to form GA12 aldehyde (the base for all other GAs).
• ​Finally, in the cytoplasm, soluble dioxygenase enzymes convert GA 12 into active forms like GA1, GA3, and GA4.

Location of Synthesis

• ​As you know, gibberellins are manufactured in tissues that are undergoing rapid growth and development. 
• The peak synthesis sites are - Shoot Apical Meristem and young expanding leaves,  ​Root Apical Meristem  (though to a lesser extent than shoots) and  Developing Seeds and Fruits (the highest concentration of gibberellins is often found in immature seeds).

​💡 Related study to understand the. AP Biology: Transpiration Process Explained | Plant Transport Mechanisms

Gibberellin Signaling Pathway (The DELLAs Repressor)

• Gibberellin (GA) doesn't  directly grow  the plant.  Instead, it works by a "Turn OFF mechanism of Repressor  because  repressor is security guard who  blocks  the  door for Gibberellin's to smooth conduction of the cellular machinery. 
• Let's break down this molecular mechanism into two simple states: When GA is Absent vs. When GA is Present.

​State  1 :  When GA is Absent (The "OFF" State)

• ​When the plant cell has low or no Gibberellin, plant growth genes are kept tightly locked.
• ​The DELLA Proteins , inside the nucleus,  is a family of nuclear proteins called DELLA proteins  which act as master growth repressors.

​​​🔬 Did You Know?

📝The DELLA protein gets its unique name from a highly conserved sequence of five amino acids at its N-terminus: Aspartic acid (D), Glutamic acid (E), Leucine (L), Leucine (L), and Alanine (A). 

📝This specific region is crucial because it acts as the "docking site" where the GA-GID1 complex binds to target the protein for destruction. 

• These DELLA proteins physically bind to growth-promoting transcription factors like - Phytochrome Interacting Factors.
• ​Because the transcription factors are blocked by DELLA, they cannot bind to the DNA. Therefore, growth-responsive genes remain turned OFF, and the plant exhibits dwarfism or remains dormant.

​

Gibberellin Signaling Pathway (The DELLAs Repressor)

State  2  :  When GA is Present  (The "OFF" State)

• ​When Gibberellin (GA) levels rise in the cell, it triggers a cellular chain reaction that destroys the repressor.
• Active gibberellin enters the nucleus and binds to its soluble receptor called  Gibberellin Insensitive Dwarf 1 ( GID1)
• The binding of GA changes the shape of the GID1 receptor. This newly shaped GA-GID1 complex now acts like a magnet and binds directly to the DELLA repressor protein, forming a GA-GID1-DELLA complex.
• This complex recruits a specific enzyme complex called E3 Ubiquitin Ligase . This enzyme acts like a cellular marker and attaches a chain of ubiquitin molecules to the DELLA protein. In the cell, a ubiquitin tag is a "death sentence" for a protein.
• The tagged DELLA protein is immediately recognized and dragged into the 26S Proteasome (the cellular garbage disposal unit). 
• The proteasome completely breaks down and destroys the DELLA repressor.
• With the DELLA protein destroyed, the growth-promoting transcription factors are finally set free! 
• They instantly bind to the plant's DNA, turning ON the transcription of genes responsible for Cell elongation and division , alpha-amylase synthesis (leading to seed germination).

​🚜 Commercial Applications of Gibberellins in Agriculture

• ​Gibberellins (GA3) are widely used as plant growth regulators in modern agriculture and horticulture to increase crop yield, improve quality, and manage harvesting time. Here are the most important commercial uses:

​🍇 Increasing Fruit Size and Cluster Length in Grapes

• ​The Problem: Grapes often grow too tightly packed in a bunch, leading to poor air circulation, fungal infections, and small fruit size.
• ​The GA3 Solution: Spraying gibberellins causes the pedicels (fruit stalks) to elongate. 
• This loosens the grape clusters, gives each grape more space to grow, and significantly increases the size and weight of individual grapes. 
• It is also used to produce seedless varieties (like Thompson Seedless).

🍺 Speeding Up the Malting Process in Brewing Industry

• ​The Process: In the beer-making (brewing) industry, barley seeds must be germinated to release alpha-amylase, an enzyme that converts starch into maltose sugar (Malting).
• ​The GA3 Solution: Instead of waiting for seeds to naturally germinate slowly, brewers spray GA3 directly onto barley grains. 
• This speeds up the production of alpha-amylase, cutting down the malting time by several days and saving huge amounts of energy and money.

​💡 Related study to understand the Pyruvate Oxidation: The Link Reaction Between Glycolysis and Krebs Cycle 

🎋 Boosting Sugarcane Yield (Stem Elongation)

• ​The Logic: In sugarcane, the economic yield (sugar) is stored inside the stem (internodes). Longer stems mean more juice and more sugar.
• ​The GA3 Solution: Spraying sugarcane crops with gibberellins triggers rapid internode elongation. 
• This single application can increase the sugarcane yield by up to 20 tonnes per acre!

​🍏 Elongating Fruit Shape (Apple and Pear Production)

• ​Market Demand: Consumers prefer apples that are large and have a specific elongated shape with prominent lobes at the bottom (like the 'Red Delicious' variety).
• ​The GA3 Solution: A mixture of Gibberellins (GA4 + GA7) and Benzyladenine (Promalin) is sprayed on apple trees. 
• This improves the shape, size, and market value of the fruit.

​🍊 Delaying Senescence (Shelf-Life Extension in Citrus)

• ​The Benefit: Spraying GA3 on citrus fruits (like oranges and lemons) delays senescence (aging/ripening).
• ​The Application: This keeps the fruits fresh on the tree for a longer period, effectively extending the harvesting season and preventing the fruits from rotting quickly during transport.

Crop / Industry	Agricultural Problem	Role of Gibberellin (GA₃)	Commercial Benefit
Grapes	Tightly packed clusters, small fruit size, fungal risk.	Elongates pedicels (fruit stalks) to loosen the bunch.	Increases individual fruit size; produces seedless grapes.
Sugarcane	Short internodes limit sugar storage space.	Triggers rapid elongation of stem internodes.	Boosts crop yield by up to 20 tonnes per acre.
Brewing	Slow natural seed germination for malting process.	Speeds up the synthesis and release of α-amylase enzyme.	Saves time and energy costs in commercial malt production.
Apple & Pears	Round, less appealing fruit shapes in market.	Promotes fruit elongation and proper shape development.	Improves premium look and increases market value.
Citrus Fruits	Early ripening and rotting on trees or during transport.	Delays senescence (aging/ripening) of the fruit skin.	Extends harvesting season and improves shelf-life.

🥬Overcoming Dwarfism and Promoting Bolting

• ​In crops like cabbage and lettuce, GA3 is used commercially to induce bolting (rapid stem elongation before flowering). 
• This is extremely useful for seed production companies that need seeds quickly.

​Auxin vs. Gibberellin: Quick Comparison Table

Property / Feature	Auxin	Gibberellin
Chemical Nature	Derived from Indole ring (e.g., Indole-3-acetic acid / IAA).	Derived from Gibberellane ring skeleton (Tetracyclic diterpenoids).
Primary Precursor	Tryptophan (Amino acid).	Mevalonic acid / Acetyl-CoA (Terpenoid pathway).
Stem Elongation Mechanism	Promotes cell elongation via the **Acid Growth Hypothesis** (loosening cell wall).	Promotes cell elongation by activating **XET enzymes** and induces cell division.
Apical Dominance	Strongly promotes apical dominance (blocks lateral buds).	Has no direct role in apical dominance.
Seed Germination	No major role in seed germination.	Crucial for breaking dormancy and inducing germination (α-amylase).
Transport Mode	**Polar transport** (Basipetal - from apex to base via PIN proteins).	**Non-polar transport** (Moves smoothly via vascular tissues - Xylem & Phloem).
Major Bioassay	Avena curvature test, Root growth inhibition test.	Dwarf maize/pea elongation test, Barley endosperm bioassay.

To understand   the  detail  information about the  Cytokinins – Plant Hormone Functions, Discovery, and Signaling Pathway, read my next detailed guide

📝 Test Paper : 1  Gibberellins (GAs): Plant Hormone Functions & Signaling Pathway (AP Biology Guide)

Total Marks: 30 | Time: 1.5 Hours

Section  A : Multiple Choice Questions (8 Marks)

​Q1. The basic carbon skeleton of all gibberellins is derived from which of the following systems?

(A) Indole ring system

(B) Ent-gibberellane skeleton

(C) Purine ring system

(D) Porphyrin ring system

​

Q2. Which cellular compartment is responsible for the final steps of gibberellin biosynthesis where active forms like GA3 are synthesized?

(A) Plastids / Chloroplasts

(B) Endoplasmic Reticulum

(C) Cytoplasm / Cytosol

(D) Golgi Apparatus

​

Q3. Structurally, Gibberellic Acid (GA3) belongs to which specific group of gibberellins based on its carbon number?

(A) C20 gibberellins

(B) C18 gibberellins

(C) C19 gibberellins

(D) C21 gibberellins

​Q4. In the absence of gibberellin, plant growth genes remain turned OFF because:

(A) GID1 receptors directly block the DNA.

(B) DELLA proteins physically bind to and repress growth-promoting transcription factors.

(C) Ubiquitin destroys the transcription factors.

(D) The cell wall is too acidic for growth.

​Q5. The molecular mechanism of gibberellin signaling is often compared to the Lac Operon system because both function via:

(A) Positive feedback loops

(B) Direct activation by a repressor

(C) Negative regulation via derepression (removing a repressor)

(D) Continuous synthesis of enzymes without regulation

​

Q6. Which enzyme is rapidly synthesized in the aleurone layer of barley seeds when treated with Gibberellin during the malting process?

(A) Protease

(B) alpha-amylase

(C) Lipase

(D) XET (Xyloglucan endotransglycosylase)

​Q7. What chemical modification serves as a "death sentence" for the DELLA repressor protein during GA signaling?

(A) Phosphorylation

(B) Methylation

(C) Ubiquitination (Tagging with Ubiquitin)

(D) Acetylation

​Q8. A sugarcane farmer wants to increase his crop yield. Spraying gibberellins helps him achieve this primarily by promoting:

(A) Apical dominance and lateral root formation

(B) Internode elongation in the stem

(C) Fruit ripening and senescence

(D) Abscission of older leaves

​

Section 2: Short Answer Questions (12 Marks)

​

Q9. Briefly explain why the amino acid sequence sequence D-E-L-L-A is highly significant for the DELLA repressor protein. Name the actual amino acids represented by 'D' and 'E'.

Q10. What is the "Bakanae Disease" (Foolish Seedling Disease)? Name the fungal pathogen responsible for it and the chemical substance isolated from it.

Q11. Contrast the transport mode of Auxin with that of Gibberellin inside the plant body.

Q12. Why do commercial brewers use GA_3 in the beer industry? Explain the physiological benefit.

​

Section 3: Long Answer/Free Response Questions (10 Marks)

​

Q13. Describe the step-by-step molecular mechanism of the Gibberellin Signaling Pathway when active GA3 is present in the cell. Highlight the roles of GID1, DELLA, and the 26S Proteasome.

Q14. Explain how Gibberellins are used commercially in modern agriculture to manage Citrus fruits (Senescence delay) and Grape cultivation (Cluster modification).

📝 Test Paper : 2  Gibberellins (GAs): Plant Hormone Functions & Signaling Pathway (AP Biology Guide)

Total Marks: 30 | Time: 1.5 Hours

Section  A : Multiple Choice Questions (8 Marks)

Q1. Gibberellin was first discovered from which group of organisms?

(A) Bacteria

(B) Fungus

(C) Virus

(D) Algae

​Q2. What is the full form of the precursor molecule "GGPP" used in Gibberellin biosynthesis?

(A) Geranylgeranyl Pyrophosphate

(B) Glucose Glycogen Pyrophosphate

(C) Glycerol Glucagon Phosphate

(D) Geranyl Glycol Phosphate

​

Q3. Which active form of Gibberellin is most commonly used and studied in laboratories and agriculture?

(A) GA1

(B) GA2

(C) GA3 (Gibberellic Acid)

(D) GA12

​

Q4. In the DELLA signaling pathway, what is the role of the DELLA protein?

(A) It promotes growth directly.

(B) It acts as a Repressor (Security Guard) that blocks plant growth.

(C) It absorbs sunlight for the plant.

(D) It helps in water absorption.

​

Q5. When Gibberellin (GA) binds with its receptor GID1, it forms a GA-GID1 complex that acts like a magnet to bind with:

(A) Auxin protein

(B) DELLA repressor protein

(C) Chlorophyll molecule

(D) Cell wall cellulose

​

Q6. The process of rapid stem elongation just before flowering in rosette plants like cabbage is called:

(A) Senescence

(B) Bolting

(C) Abscission

(D) Photoperiodism

​

Q7. Spraying Gibberellin on sugarcane crops is highly profitable for farmers because it increases the length of:

(A) Roots

(B) Leaves

(C) Internodes (Stems)

(D) Flowers

​

Q8. Which plant hormone is primarily responsible for breaking seed dormancy and promoting seed germination by releasing enzymes?

(A) Abscisic Acid (ABA)

(B) Auxin

(C) Gibberellin

(D) Ethylene

​

Section 2: Short Answer Questions (12 Marks)

​

Q9. What is the name of the disease in rice seedlings that led to the discovery of Gibberellins? What does its name mean in English?

Q10. Name the three cellular compartments (parts of the cell) where the biosynthesis of Gibberellin takes place in order.

Q11. How does spraying Gibberellin (GA3) help grape farmers improve their crop quality?

Q12. In the brewing (beer) industry, why is Gibberellin sprayed on barley grains? Name the enzyme it activates.

​

Section 3: Long Answer/Free Response Questions (10 Marks)

​

Q13. Explain the simple "ON/OFF" mechanism of Gibberellin. What happens to the DELLA repressor when Gibberellin is Absent vs. when Gibberellin is Present?

Q14. List any three commercial applications of Gibberellins in modern agriculture and briefly explain how each application benefits farmers or industries.

🚀 Join the Community! ​Get access to FREE Worksheets, PDF Notes, and discuss Lessons with experts and fellow students

📝   Advanced Thinking: Critical  Application  Questions

Question: If a plant geneticist creates a mutant plant with a "loss-of-function" mutation in the GID1 receptor gene, and another mutant with a "loss-of-function" mutation in the DELLA repressor gene, how will the growth phenotypes of these two mutant plants differ in the complete absence of Gibberellin (GA)? Explain the molecular reasoning.

​

Answer:  GID1 Loss-of-Function Mutant: This plant will be a severe dwarf  Without a functional GID1 receptor, the plant can never detect Gibberellin, meaning the DELLA repressor will permanently remain bound to the DNA, keeping the growth genes turned OFF forever, even if you spray external GA.

​

DELLA Loss-of-Function Mutant: This plant will show a "slender" or hyper-elongated growth phenotype  Since the DELLA protein (the security guard) itself is missing or non-functional, there is nothing left to block the transcription factors. Therefore, the growth genes will remain permanently turned ON, causing uncontrolled growth even in the complete absence of Gibberellin.

​

Question: In a laboratory experiment, barley seeds were treated with a metabolic inhibitor that specifically blocks the functioning of the 26S Proteasome. When Gibberellic Acid (GA3) was later applied to these seeds, no alpha-amylase secretion was observed in the aleurone layer. Why did the signaling pathway fail despite the presence of active GA3?

​

Answer: The 26S Proteasome acts as the cell's "molecular paper shredder." In normal GA signaling, after the GA-GID1 complex binds to the DELLA repressor, the DELLA protein must be tagged with ubiquitin and destroyed by the 26S Proteasome to free up the growth genes.

Because the inhibitor blocked the 26S Proteasome, the GA-GID1-DELLA complex was formed, but the DELLA repressor could not be degraded (destroyed). As a result, DELLA kept blocking the transcription factors, and the transcription of the \alpha-amylase gene could not take place.

​Question: Both Auxin and Gibberellin promote stem elongation, but their physiological mechanisms are completely different. If you block the cell wall Proton Pumps (H+-ATPase) using a chemical inhibitor, Auxin-induced elongation stops immediately, but Gibberellin-induced elongation continues for a significant period. Explain why.

​

Answer: Auxin relies heavily on the Acid Growth Hypothesis. It activates proton pumps to pump H+ ions into the cell wall, making it acidic to activate expansin enzymes that loosen the wall quickly. Inhibiting these pumps completely stops Auxin's action.

​

Gibberellin, on the other hand, does not rely on acidifying the cell wall. Instead, it works by upregulating genes for enzymes like XET (Xyloglucan endotransglycosylase) and hydrolases, which mechanically rearrange the cell wall polymers. Furthermore, GA promotes cell division (mitosis) in the intercalary meristems, which is independent of the immediate proton pump activity.

​

​Question: The fungus Gibberella fujikuroi causes "Bakanae Disease" by producing massive amounts of Gibberellins inside the host rice plant. From an evolutionary and ecological perspective, why does the fungus spend its own energy synthesizing a plant hormone that causes the host plant to grow excessively tall and weak? How does this benefit the pathogen?

​

Answer: This is a classic evolutionary strategy of a necrotrophic pathogen. By flooding the rice plant with Gibberellins, the fungus forces the plant to undergo rapid, uncontrolled cell elongation. This makes the stems extremely thin, weak, and structurally unstable.

Eventually, the top-heavy seedling collapses (falls over) due to its own weight or wind. Once the weakened plant falls and dies, the fungus easily colonizes the dead tissue to absorb nutrients and complete its life cycle. The excessive growth is a pathological manipulation, not healthy development.

📝  Data Analysis: Interpreting Graphs

Context: A researcher treated dwarf pea seedlings (Pisum sativum) with different concentrations of Active Gibberellin (GA3) and measured the average stem elongation over a period of 10 days. The data collected is shown in the table below:

Group	GA₃ Concentration (μM)	Initial Plant Height (cm)	Final Plant Height (10 Days) (cm)	Total Stem Elongation (cm)
Control (A)	0 (Distilled Water)	12.0	14.5	2.5
Group B	1	12.2	19.2	7.0
Group C	10	12.1	28.1	16.0
Group D	100 (Saturation)	11.9	36.9	25.0
Group E	500	12.0	37.1	25.1

​Analyze the data provided in the table and answer the following three questions:

​Question: 1  Describe the relationship between the concentration of GA3 and the total stem elongation.

​

Question: 2 Why is there almost no difference in total stem elongation between Group D 100 micro  M and Group E 500 micro  M What cellular/molecular limitation explains this phenomenon?

​

Question: 3 What is the purpose of Group A (Control), and why did it still show a minor elongation of 2.5 cm without any external GA3 treatment?

​

​Answer: 1  The data shows a direct/positive correlation between GA3 concentration and stem elongation up to a certain limit 100 micro  M As the concentration increases from 0 micro  M to 100 micro  M.  the total stem elongation increases significantly from 2.5 cm to 25.0 cm. This proves that Gibberellin works in a dose-dependent manner to trigger cell division and elongation.

​

Answer: 2  This is due to Receptor Saturation. In plant cells, Gibberellin must bind to the GID1 receptor to trigger the degradation of DELLA proteins.

​Once the concentration reaches 100 micro  M all available GID1 receptors on the cell membrane and inside the cytoplasm become completely occupied (saturated). Increasing the dose further to 500 micro  M provides no extra benefit because there are no free receptors left to bind with the excess hormone. Therefore, the growth response reaches a plateau (maximum limit).

​

Answer:  3 Purpose: Group A serves as a Baseline/Standard to prove that the extra growth in other groups is purely due to the added GA3, and not because of environmental factors like water, light, or temperature.

​Reason for minor growth (2.5 cm): Even though no external GA3 was sprayed, the plant still produces its own endogenous (natural) hormones in very small amounts within its apical tissues. This baseline concentration of natural Gibberellins and Auxins is responsible for the minor 2.5 cm elongation.

Graph Interpretation 

​Context: The bar graph below illustrates the experimental data of seed germination percentage (%) when treated with different concentrations of Gibberellic Acid (GA3) ranging from 0 micro M to 200 micro M.  Analyze the graph carefully to answer the questions.

​Questions: 1  Optimal Concentration: At which concentration of GA3 does the seed germination percentage reach its peak (maximum)? What is the approximate germination percentage at this point?

​

​Questions: 2 When the concentration is increased from 50 micro M to 100 micro M , the seed germination percentage drops significantly. What physiological phenomenon or cellular feedback loop explains why a higher dose of a growth hormone can sometimes reduce its effectiveness?

​

​Questions: 3 What does the bar at  0 micro M  represent, and what conclusion can you draw from the fact that more than 50% of seeds still g

germinated without any external GA3?

​

💡 Detailed Answer Key & Explanation:

​

​​Answer 1 : The seed germination percentage reaches its maximum (peak) at 50 micro M  of GA3, where the germination rate is approximately 79%. This represents the optimal physiological dose for this specific plant species to activate maximum \alpha-amylase synthesis.

 

​Answer 2 : This drop is caused by Supra-optimal Inhibition / Negative Feedback Regulation.

​Inside plant cells, when a hormone concentration goes beyond the required threshold, it triggers defense mechanisms. Excess GA3 can induce the expression of GA-2-oxidases (enzymes that degrade active GA) or trigger the synthesis of Abscisic Acid (ABA), which is a growth inhibitor and GA-antagonist. This restores the physiological balance, resulting in a temporary dip in germination at 100 micro M .

​

​Answer 3 :  The 0 micro M  bar represents the Control Group (seeds treated only with water).

​The fact that ~52% of seeds still germinated proves that these seeds already contained a sufficient amount of endogenous (natural) Gibberellins stored inside them. External GA3 is not completely mandatory for germination, but applying it (up to  500 micro M ) significantly accelerates and boosts the overall success rate.

🚀 Agla Kadam (Next Steps)

Biology ki taiyari ko aur mazboot banayein!

🏠 Library Home 💬 Ask Your Doubt

Doston ke saath **Share** karein aur comment mein batayein agla topic kya ho!