This MCQ module is based on: Development Pgr Discovery
TOPIC 58 OF 90
Development Pgr Discovery
🎓 Class 11
Biology
CBSE
Theory
Ch 13 – Plant Growth and Development
⏱ ~14 min
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Development, Plasticity and the Discovery of Plant Growth Regulators
Growth adds material; differentiation gives it form. Put the two together over the whole life of a plant and you have development — and development, unlike an animal's, is negotiable with the surroundings. This part follows that idea and then turns to the chemical messengers that carry it out.
13.3 Development
Development is a term that includes all changes that an organism goes through during its life cycle, from germination of the seed to senescence. The sequence is applicable to tissues and organs as much as to a single cell.
Plasticity
Plants follow different pathways in response to environment or phases of life to form different kinds of structures. This ability is called plasticity.
Heterophylly — the standard example. Plasticity shows itself as heterophylly in cotton, coriander and larkspur: in such plants, the leaves of the juvenile plant are different in shape from those in mature plants. A second kind appears in buttercup, where the difference in shapes of leaves produced in air and those produced in water represents heterophyllous development due to environment.
Read the two examples together and you can see what plasticity really means. In cotton and larkspur the trigger is the phase of life — the plant is older. In buttercup the trigger is the environment — whether a leaf grows in air or under water. Same genes in both cases; different form. A plant's shape is not a blueprint executed blindly but a running response to circumstances.
Growth, differentiation and development together
Growth, differentiation and development are very closely related events in the life of a plant. Broadly, development is considered as the sum of growth and differentiation.
What controls development. Development in plants — both growth and differentiation — is under the control of intrinsic and extrinsic factors. The intrinsic factors include both intracellular (genetic) and intercellular factors (chemicals such as plant growth regulators). The extrinsic factors include light, temperature, water, oxygen, nutrition and so on.
13.4 Plant Growth Regulators
13.4.1 Characteristics
The plant growth regulators (PGRs) are small, simple molecules of diverse chemical composition. They are variously described as plant growth substances, plant hormones or phytohormones in the literature.
| Chemical class | Example |
|---|---|
| Indole compounds | Indole-3-acetic acid, IAA |
| Adenine derivatives | N⁶-furfurylamino purine, kinetin |
| Derivatives of carotenoids | Abscisic acid, ABA |
| Terpenes | Gibberellic acid, GA₃ |
| Gases | Ethylene, C₂H₄ |
The PGRs can be broadly divided into two groups based on their functions in a living plant body.
Group 1 — growth promoting. Involved in activities such as cell division, cell enlargement, pattern formation, tropic growth, flowering, fruiting and seed formation. These are also called plant growth promoters: auxins, gibberellins and cytokinins.
Group 2 — stress responses and inhibition. These play an important role in plant responses to wounds and stresses of biotic and abiotic origin, and are involved in growth inhibiting activities such as dormancy and abscission. Abscisic acid belongs to this group.
And the awkward one. The gaseous PGR ethylene could fit either of the groups, but it is largely an inhibitor of growth activities. Keep that wording in mind — a question asking you to classify ethylene expects exactly this qualified answer, not a flat placement in one group.
13.4.2 The Discovery of Plant Growth Regulators
The single most quotable fact in the chapter. The discovery of each of the five major groups of PGRs has been accidental. Not one of them was found by someone looking for a plant hormone.
Auxin — from a bending grass seedling
It started with the observation of Charles Darwin and his son Francis Darwin, who observed that the coleoptiles of canary grass responded to unilateral illumination by growing towards the light source (phototropism). After a series of experiments it was concluded that the tip of the coleoptile was the site of a transmittable influence that caused the bending of the entire coleoptile. Auxin was isolated by F. W. Went from tips of coleoptiles of oat seedlings.
Gibberellins — from a disease of rice
The ‘bakanae’ (foolish seedling) disease of rice seedlings was caused by a fungal pathogen, Gibberella fujikuroi. E. Kurosawa (1926) reported the appearance of symptoms of the disease in rice seedlings when they were treated with sterile filtrates of the fungus. The active substances were later identified as gibberellic acid.
Cytokinins — from autoclaved DNA
F. Skoog and his co-workers observed that from the internodal segments of tobacco stems, the callus (a mass of undifferentiated cells) proliferated only if, in addition to auxins, the nutrient medium was supplemented with one of the following: extracts of vascular tissues, yeast extract, coconut milk or DNA. Miller et al. (1955) later identified and crystallised the cytokinesis-promoting active substance that they termed kinetin.
Abscisic acid — three names for one molecule
During the mid-1960s, three independent researches reported the purification and chemical characterisation of three different kinds of inhibitors: inhibitor-B, abscission II and dormin. Later all the three were proved to be chemically identical. It was named abscisic acid (ABA).
Ethylene — from oranges ripening bananas
H. H. Cousins (1910) confirmed the release of a volatile substance from ripened oranges that hastened the ripening of stored unripened bananas. Later this volatile substance was identified as ethylene, a gaseous PGR.
| PGR | Who and when | The accident |
|---|---|---|
| Auxin | Charles & Francis Darwin; isolated by F. W. Went | Canary grass coleoptiles bending towards unilateral light — phototropism; isolated from oat coleoptile tips |
| Gibberellins | E. Kurosawa (1926) | The bakanae or foolish seedling disease of rice, caused by Gibberella fujikuroi; sterile fungal filtrates reproduced the symptoms |
| Cytokinins | F. Skoog; Miller et al. (1955) | Tobacco callus grew only if vascular tissue extract, yeast extract, coconut milk or DNA was added to auxin — kinetin crystallised |
| Abscisic acid | Three independent groups, mid-1960s | Inhibitor-B, abscission II and dormin turned out to be the same molecule |
| Ethylene | H. H. Cousins (1910) | A volatile substance from ripened oranges hastened the ripening of stored unripened bananas |
📐 Activity 13.3 — Repeat the Darwins’ coleoptile experiment
What to do. Germinate oat, wheat or canary grass seeds in four small pots until the coleoptiles are about 2 cm tall. Treat them as follows: (a) leave one intact; (b) snip off the top 2 mm of the second; (c) fit the third with a tiny cap of black paper or foil over the tip only; (d) fit the fourth with a black paper collar around the middle of the shoot, leaving the tip exposed. Place all four in a box open on one side only, so that light reaches them from a single direction, and leave them for a day.
Predict: which seedlings will bend towards the light and which will not? Then state what conclusion each pair of results allows you to draw that a single seedling could not.
Observations. (a) bends towards the light. (b) does not bend, and barely grows. (c) does not bend, although it is physically undamaged. (d) bends normally, even though the region that actually curves was in darkness.
What each comparison proves.
- (a) versus (b): the tip is essential — remove it and the response is abolished. But this alone might mean the tip was merely a source of growth, not of a signal.
- (b) versus (c): the crucial pair. Seedling (c) keeps its tip intact yet still fails to bend, so the effect is not about wounding. The tip must be the light-perceiving region.
- (c) versus (d): the bending region need not see light at all. So the perception happens at the tip while the response happens lower down — which means something must travel between the two.
The conclusion the Darwins reached. Exactly this: the tip of the coleoptile was the site of a transmittable influence that caused the bending of the entire coleoptile. The substance itself was not isolated for another fifty years, when F. W. Went obtained auxin from the tips of coleoptiles of oat seedlings.
A note on method. This is one of the most elegant experimental designs in all of biology, and the reason is that each treatment removes exactly one variable. Notice that no single seedling proves anything; the conclusion lives in the comparisons.
13.4.3 Physiological Effects of Plant Growth Regulators
13.4.3.1 Auxins
Auxins — from the Greek auxein: to grow — were first isolated from human urine. The term is applied to indole-3-acetic acid (IAA) and to other natural and synthetic compounds having certain growth regulating properties.
They are generally produced by the growing apices of the stems and roots, from where they migrate to the regions of their action.
| Type | Examples |
|---|---|
| Isolated from plants (natural) | IAA (indole-3-acetic acid) and IBA (indole butyric acid) |
| Synthetic | NAA (naphthalene acetic acid) and 2,4-D (2,4-dichlorophenoxyacetic acid) |
All these auxins have been used extensively in agricultural and horticultural practices. Their effects:
- They help to initiate rooting in stem cuttings, an application widely used for plant propagation.
- Auxins promote flowering, e.g. in pineapples.
- They help to prevent fruit and leaf drop at early stages but promote the abscission of older, mature leaves and fruits. Note the deliberate contrast — the same hormone does opposite things depending on the age of the organ.
- They induce parthenocarpy, e.g. in tomatoes.
- They are widely used as herbicides. 2,4-D, widely used to kill dicotyledonous weeds, does not affect mature monocotyledonous plants, and is used to prepare weed-free lawns by gardeners.
- Auxin also controls xylem differentiation and helps in cell division.
Apical dominance
In most higher plants, the growing apical bud inhibits the growth of the lateral (axillary) buds — a phenomenon called apical dominance. Removal of shoot tips (decapitation) usually results in the growth of lateral buds.
In-text question — why is decapitation widely applied in tea plantations and hedge-making? Because in both cases what is wanted is many side branches, not one tall stem. In a tea plantation the harvest is the young leaves and buds, so the bushes are repeatedly pruned: removing the apical buds lifts the inhibition on the axillary buds, which then grow out, producing a dense flush of new shoots and a far larger picking surface on a bush kept conveniently low. In hedge-making the aim is a thick, even wall of foliage with no gaps; clipping the tips all over the hedge forces lateral buds everywhere to sprout, and repeated clipping keeps the hedge dense and shapely. The same principle explains why gardeners “pinch out” the tips of young plants to make them bushy.
13.4.3.2 Gibberellins
Gibberellins are another kind of promotory PGR. There are more than 100 gibberellins reported from widely different organisms such as fungi and higher plants. They are denoted as GA₁, GA₂, GA₃ and so on. Gibberellic acid (GA₃) was one of the first gibberellins to be discovered and remains the most intensively studied form. All GAs are acidic.
They produce a wide range of physiological responses in the plants:
- Their ability to cause an increase in length of axis is used to increase the length of grape stalks.
- Gibberellins cause fruits like apple to elongate and improve its shape.
- They also delay senescence. Thus the fruits can be left on the tree longer, so as to extend the market period.
- GA₃ is used to speed up the malting process in the brewing industry.
- Sugarcane stores carbohydrate as sugar in their stems. Spraying sugarcane crop with gibberellins increases the length of the stem, thus increasing the yield by as much as 20 tonnes per acre.
- Spraying juvenile conifers with GAs hastens the maturity period, thus leading to early seed production.
- Gibberellins also promote bolting — internode elongation just prior to flowering — in beet, cabbages and many plants with a rosette habit.
🎯 Interactive: Pick the right PGR application
PGR to use: Auxin — IAA, IBA or NAA
Auxins help to initiate rooting in stem cuttings, an application widely used for plant propagation. The cut end first dedifferentiates into a callus, from which adventitious roots then redifferentiate.
🎯 Competency-Based Questions
Scenario: A horticulturist runs four treatments on identical plants. Plant P has its shoot tip removed. Plant Q is sprayed with GA₃. Plant R, a tomato, is treated with auxin at flowering but its flowers are never pollinated. A lawn of mixed grass and broad-leaved weeds is sprayed with 2,4-D.
Q1. Predict the outcome for P and name the phenomenon involved. L3 Apply
The lateral (axillary) buds of P will grow out into branches, making the plant bushy. The phenomenon is apical dominance: in most higher plants the growing apical bud inhibits the growth of the lateral buds, and removal of shoot tips, called decapitation, usually results in the growth of lateral buds. This is the principle behind pruning in tea plantations and hedge-making.
Q2. Plant R sets fruit although no pollination occurred. Explain, and name the phenomenon. L3 Apply
Auxins induce parthenocarpy, e.g. in tomatoes — the development of fruit without fertilisation, and therefore without seeds. The applied auxin substitutes for the hormonal signal that a developing seed would normally supply, so the ovary continues to grow into a fruit even though no ovule was fertilised.
Q3. Fill in the blanks: Auxin comes from the Greek ______, meaning ______, and was first isolated from ______. IAA and ______ are natural auxins; ______ and ______ are synthetic. L1 Remember
auxein; to grow; human urine; IBA (indole butyric acid); NAA (naphthalene acetic acid); 2,4-D.
Q4. Why does 2,4-D leave the lawn grass alive while killing the weeds, and what does this tell you about the word “hormone”? L4 Analyse
Because 2,4-D, widely used to kill dicotyledonous weeds, does not affect mature monocotyledonous plants. Lawn grasses are monocots; the broad-leaved weeds are dicots. It is therefore a selective herbicide, and is used to prepare weed-free lawns by gardeners. What this reveals is that a PGR's effect is not a property of the molecule alone but of the molecule and the tissue together — the same compound that promotes growth at low concentration in one plant kills another. The chapter makes the same point twice more within auxins alone: they prevent fruit and leaf drop at early stages but promote the abscission of older mature leaves and fruits. “Growth promoter” is a summary, not a rule.
Q5. “All five PGRs were found by accident, so plant hormone research was mostly luck.” Evaluate. L5 Evaluate
The premise is accurate — the chapter states that the discovery of each of the five major groups of PGRs has been accidental, and the histories bear it out: a grass seedling bending towards light, a fungal disease of rice, autoclaved herring sperm DNA in a callus medium, three inhibitors that turned out to be one molecule, and oranges ripening bananas. Not one investigator set out to find a hormone. But “mostly luck” misreads what happened next, in three ways. (i) The accident was only the starting point. The Darwins' observation became knowledge only after a series of experiments with decapitation and opaque caps, and the substance itself was isolated much later by Went. (ii) Recognising an anomaly is a skill. Kurosawa's insight was to test sterile filtrates of the fungus — proving a chemical, not the organism, caused bakanae. Skoog's was to notice that four unrelated supplements all worked, which implied a single common active substance. Cousins' was to suspect a volatile agent rather than contact. (iii) Converging on one identity from three names required careful chemical characterisation, not luck. A fair verdict: the initial clues were accidental; the discoveries were not. This is how much of biology proceeds — chance supplies the anomaly, and controlled experiment converts it into understanding. Pasteur's remark that chance favours the prepared mind is exactly the point.
🧠 Assertion–Reason Questions
For each pair choose: (A) Both A and R are true and R is the correct explanation of A. (B) Both A and R are true but R is not the correct explanation of A. (C) A is true but R is false. (D) A is false but R is true.
Assertion (A): Heterophylly in buttercup is an example of plasticity.
Reason (R): Plants follow different pathways in response to environment or phases of life to form different kinds of structures.
Answer: A. Both are true and the reason is the definition of plasticity. In buttercup the difference between aerial and submerged leaves is heterophyllous development due to environment.
Assertion (A): Removing the shoot tip of a plant makes it grow bushy.
Reason (R): The growing apical bud inhibits the growth of the lateral buds, so decapitation releases them.
Answer: A. Both are true and the reason explains the assertion. This is apical dominance, and its release is the basis of pruning in tea plantations and hedges.
Assertion (A): Ethylene is classified without qualification as a plant growth promoter.
Reason (R): Auxins, gibberellins and cytokinins are the growth promoting PGRs, while abscisic acid is involved in growth inhibiting activities.
Answer: D. The assertion is false — the chapter says ethylene could fit either of the groups, but it is largely an inhibitor of growth activities. The reason correctly lists the membership of the two groups.
Coming next. Part 4 completes Section 13.4.3 — cytokinins, ethylene and abscisic acid with all their agricultural applications, how PGRs act together and against one another, and the role of light and temperature in the initiation of flowering.
Frequently Asked Questions - Development, Plasticity and the Discovery of PGRs
What is development in plants?
Development includes all the changes an organism goes through during its life cycle, from germination of the seed to senescence. Broadly, development is considered the sum of growth and differentiation, and the same sequence applies to a cell, a tissue or an organ.
What is plasticity, and what is heterophylly?
Plasticity is the ability of plants to follow different pathways in response to environment or phases of life, so as to form different kinds of structures. Heterophylly is its standard example: in cotton, coriander and larkspur the juvenile leaves differ in shape from mature leaves, and in buttercup the leaves formed in air differ from those formed in water.
What controls development in plants?
Both intrinsic and extrinsic factors. Intrinsic factors include intracellular genetic factors and intercellular chemical factors, that is the plant growth regulators. Extrinsic factors include light, temperature, water, oxygen and nutrition.
What are plant growth regulators and how are they classified?
PGRs are small, simple molecules of diverse chemical composition, also called plant growth substances, plant hormones or phytohormones. They fall into two functional groups: growth promoters, namely auxins, gibberellins and cytokinins; and those involved in stress responses and growth inhibition, namely abscisic acid. Ethylene could fit either group but is largely an inhibitor.
How were the plant growth regulators discovered?
All five accidentally. Auxin from the Darwins' observation of canary grass coleoptiles bending towards light, later isolated by F. W. Went from oat coleoptile tips. Gibberellins from the bakanae disease of rice caused by Gibberella fujikuroi, reported by E. Kurosawa in 1926. Cytokinins as kinetin, crystallised by Miller and co-workers in 1955 from autoclaved DNA. ABA from three separately named inhibitors that proved identical in the mid-1960s. Ethylene from H. H. Cousins' 1910 observation that ripened oranges hastened the ripening of bananas.
What are the physiological effects of auxins?
They initiate rooting in stem cuttings for propagation, promote flowering as in pineapple, prevent fruit and leaf drop at early stages while promoting abscission of older organs, cause apical dominance, induce parthenocarpy as in tomato, act as herbicides, and control xylem differentiation and cell division.
What is apical dominance and how is it used in practice?
In most higher plants the growing apical bud inhibits the growth of the lateral or axillary buds. Removal of shoot tips, called decapitation, usually results in the growth of lateral buds, which is why tea plantations and hedges are pruned to make the plants dense and bushy.
What are the uses of gibberellins?
They increase the length of grape stalks, elongate apples and improve their shape, delay senescence so fruit can stay on the tree longer, speed up malting in brewing, raise sugarcane yield by as much as 20 tonnes per acre by lengthening the stem, hasten maturity in juvenile conifers for early seed, and promote bolting in beet, cabbages and other rosette plants.
Which auxins are natural and which are synthetic?
IAA, indole-3-acetic acid, and IBA, indole butyric acid, have been isolated from plants. NAA, naphthalene acetic acid, and 2,4-D, 2,4-dichlorophenoxyacetic acid, are synthetic. All have been used extensively in agriculture and horticulture.
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