NCERT Grounding
NCERT Class 11 Biology, Chapter 13 (Plant Growth and Development), Section 13.4.3.2 introduces gibberellins as a group of promotory plant growth regulators. The text anchors the topic to the bakanae (foolish seedling) disease of rice, the discovery by E. Kurosawa in 1926, and lists seven distinct physiological effects — each of which has appeared in NEET at least once. The NIOS Biology Chapter 20 supplements with the role of gibberellins in breaking seed dormancy, promoting parthenocarpy, and restoring height in genetically dwarf plants.
"Gibberellins cause fruits like apple to elongate and improve its shape. They also delay senescence. GA₃ is used to speed up the malting process in the brewing industry."
NCERT Class 11 Biology, Chapter 13 — verbatim
Discovery and History
The discovery of gibberellins followed an unusual path — from a crop disease to a class of plant hormones. Japanese plant pathologist E. Kurosawa (1926) was studying an economically damaging condition of rice paddy known as bakanae disease (Japanese: "foolish seedling" or "bakanae" disease). Infected seedlings grew abnormally tall and slender, with pale leaves, and usually died before setting grain. The causal organism was identified as the fungal pathogen Gibberella fujikuroi.
Kurosawa demonstrated that when healthy rice seedlings were treated with sterile filtrates of the fungus (i.e., the pathogen itself was absent), they exhibited the same exaggerated elongation. This ruled out direct infection as the sole cause and pointed to a diffusible chemical secreted by the fungus. Subsequent work by Japanese and later British and American researchers led to the isolation and chemical characterisation of the active substances, collectively termed gibberellins, with the primary compound named gibberellic acid (GA₃).
Year of Discovery
E. Kurosawa reported that sterile filtrates of Gibberella fujikuroi reproduced the symptoms of bakanae ("foolish seedling") disease in healthy rice seedlings — establishing the existence of a diffusible growth-promoting substance.
Chemical Nature and Types
Gibberellins are terpene-derived acidic compounds — a fact the NCERT text notes explicitly ("terpenes (gibberellic acid, GA₃)"). All gibberellins are acidic in nature. They are designated sequentially as GA₁, GA₂, GA₃, and so on, in the order of their characterisation.
Total GAs Known · Most Active Form
More than 100 gibberellins have been reported from fungi and higher plants. GA₃ (gibberellic acid) was the first to be discovered and remains the most intensively studied and commercially important form.
Gibberellins are synthesised principally in young leaves, embryos, and developing seeds, from where they are transported to target tissues. Unlike auxins, which show polar transport, gibberellins move bidirectionally through the phloem and xylem.
Figure 1. Principal sites of gibberellin biosynthesis in a vascular plant. Young leaves and developing seeds are the dominant sources; transport is bidirectional via phloem and xylem — a key distinction from the polar transport of auxins.
Physiological Effects
NCERT lists seven distinct physiological effects of gibberellins. Each has been tested or could be tested in NEET. The table below organises them with the relevant NEET PYQ year where applicable.
| Effect | Target / Mechanism | Example / Application | NEET PYQ |
|---|---|---|---|
| Stem elongation | Promotes internode elongation by stimulating cell division and elongation in the sub-apical meristem | Spraying sugarcane increases stem length and sugar yield by up to 20 t/acre | 2020 Q.54; 2024 Q.150 |
| Bolting | Rapid internode elongation prior to flowering in rosette-habit plants | Beet, cabbage — plants that normally grow as flat rosettes bolt and flower when treated with GA | Concept card |
| Malting acceleration | Stimulates alpha-amylase synthesis in aleurone layer of cereal grains | GA₃ speeds up malting (starch-to-sugar conversion) in brewing industry | 2022 Q.108 |
| Seed germination / dormancy breaking | Promotes alpha-amylase in aleurone; mobilises endosperm reserves for embryo | Cereal grain germination; can induce germination in darkness | Concept card |
| Flowering in long-day plants | Substitutes for long-day photoperiod to induce flowering | Induces flowering in some LDPs under non-inductive (short-day) conditions | Concept card |
| Parthenocarpy | Stimulates fruit development in the absence of fertilisation | Seedless grape and apple production | Concept card |
| Hastening maturity in conifers | Shortens the juvenile phase; promotes early cone and seed formation | Spraying juvenile conifers with GA₃ leads to early seed production | 2023 Q.108 |
Stem Elongation: the Molecular Basis
Gibberellins promote elongation of internodes primarily by stimulating both cell division in the sub-apical intercalary meristems and cell elongation in the zone of elongation. The mechanism involves GA binding to its receptor (GID1 in rice), which then targets DELLA proteins (growth repressors) for proteasomal degradation, thereby de-repressing GA-responsive growth genes.
The practical consequence is most dramatically seen in genetically dwarf plants. Dwarf pea or dwarf maize plants lack functional GA biosynthesis genes; exogenous GA treatment fully restores their height to that of tall wild-type plants. This was one of the first lines of evidence linking a specific PGR to a genetic phenotype.
Bolting in Rosette Plants
Plants with a rosette growth habit — such as cabbage (Brassica oleracea) and beet (Beta vulgaris) — keep their internodes compressed, so leaves appear to emerge from a tight cluster at ground level. Under natural conditions, bolting occurs only after a period of cold (vernalisation) or under long-day photoperiods. Exogenous gibberellin application mimics both signals, causing rapid internode elongation (bolting) followed by flower initiation even in the absence of the required environmental cues. This makes bolting a classic demonstration of GA action.
Seed Germination: the Aleurone Layer Mechanism
The role of gibberellins in seed germination is mechanistically the most detailed and most NEET-tested biochemical pathway associated with GAs. In cereal grains (e.g., barley, wheat, rice), the seed is composed of a starchy endosperm surrounded by a protein-rich aleurone layer, with the embryo at one end.
GA-Mediated Germination in Cereal Grains — Step-by-Step
-
Step 1
Water Imbibition
Seed absorbs water; seed coat ruptures; embryo becomes metabolically active.
-
Step 2
GA Secretion
Embryo (scutellum) secretes gibberellins (GA₁, GA₃) into surrounding tissue.
Key: embryo is the source -
Step 3
Aleurone Response
GA signals the aleurone layer cells to synthesise and secrete alpha-amylase (and other hydrolytic enzymes).
NEET-tested step -
Step 4
Starch Hydrolysis
Alpha-amylase diffuses into the starchy endosperm and converts starch to fermentable sugars (maltose, glucose).
-
Step 5
Embryo Growth
Sugars and amino acids are absorbed by the embryo to fuel cell division and elongation — the seedling emerges.
This pathway is also the basis for the commercial application of GA₃ in the malting industry. Malting is the controlled germination of barley grains to generate amylases naturally. Treating grain with GA₃ accelerates alpha-amylase production, shortens the malting time, and reduces production costs. NEET 2022 Q.108 specifically tested that it is gibberellin — not ethylene — that accelerates malting.
Agricultural Applications
Key principle: Gibberellins are commercially applied wherever internode elongation, early maturity, or starch mobilisation is economically beneficial. Each application below maps directly to a physiological effect tested in NEET.
Sugarcane Yield
20 t
per acre increase in yield
Mechanism: internode elongation increases the mass of sugar-storing stem tissue without changing cane number.
Spray timing: applied during active stem elongation phase.
NEET 2020 Q.54 · 2024 Q.150Malting (Brewing)
GA₃
speeds up malting process
Mechanism: stimulates alpha-amylase synthesis in aleurone layer of barley grain.
Effect: faster starch hydrolysis; shorter malting time.
NEET 2022 Q.108 — not ethylene!Conifer Maturity
Early
seed production in conifers
Mechanism: shortens the juvenile phase; promotes transition to reproductive maturity.
Application: tree breeding programmes — faster seed lot production.
NEET 2023 Q.108Fruit Quality
Shape
improvement & senescence delay
Apple: GA causes elongation, improves fruit shape; delays senescence to extend market shelf-life.
Grapes: increases stalk (rachis) length, reducing bunch compaction.
NCERT explicitWorked Examples
A farmer wants to increase the sugar yield from his sugarcane crop without increasing the number of plants. Which plant growth regulator should he spray, and what is the mechanism of action?
Answer: Gibberellin (GA₃). Sugarcane stores carbohydrate as sucrose in the parenchyma cells of its stem internodes. Spraying gibberellin promotes internode elongation — each internode becomes longer, increasing the total volume of sugar-storing tissue per plant. NCERT states this can increase yield by up to 20 tonnes per acre. This was the exact question tested in NEET 2020 (Q.54) and NEET 2024 (Q.150).
A barley grain is treated with GA₃ in a laboratory experiment. Trace the sequence of molecular events that leads to the release of sugars from the endosperm.
Answer: (1) GA₃ diffuses from the embryo (scutellum) to the aleurone layer cells surrounding the endosperm. (2) In aleurone cells, GA₃ activates the transcription of the gene encoding alpha-amylase. (3) Alpha-amylase is secreted into the starchy endosperm. (4) Alpha-amylase hydrolyses starch chains into maltose and glucose. (5) The sugars are absorbed by the scutellum and transported to the embryonic axis for growth. This pathway is also exploited industrially in malting: GA₃ shortens the time required to activate amylases, accelerating the brewing process.
A cabbage plant growing as a compact rosette at ground level is sprayed with gibberellic acid. Describe the visible change observed and name the phenomenon.
Answer: The compressed internodes of the rosette elongate rapidly, causing the stem to grow tall and erect — a phenomenon called bolting. Bolting is the internode elongation that occurs just prior to flowering in rosette-habit plants (e.g., beet, cabbage). Gibberellins substitute for the vernalisation or long-day photoperiod normally required to trigger this transition, causing premature bolting and subsequently flowering.
Common Confusion and NEET Traps
Gibberellin (GA₃)
Malting
Speeds up malting — CORRECT
- Stimulates alpha-amylase synthesis in the aleurone layer of barley grains
- Accelerates starch-to-sugar hydrolysis in the endosperm
- Used commercially to reduce malting time in the brewing industry
- Effect: promotes germination and mobilisation of seed reserves
Ethylene
Ripening
Speeds up fruit ripening — NOT malting
- Promotes fruit ripening by enhancing respiration (respiratory climacteric)
- Promotes senescence and abscission of leaves and flowers
- Does NOT stimulate alpha-amylase in cereal grains
- Confusing because it is also produced during germination-like processes