Plant Growth Regulators

Plant growth regulators (PGRs) are small, simple molecules of diverse chemical composition that control growth, differentiation and development. This page covers the chemical nature of plant growth regulators, the promoter and inhibitor groups, the accidental discovery of the five major groups, and the physiological effects and uses of auxins, gibberellins, cytokinins, ethylene and abscisic acid. It follows the NCERT Class 11 chapter Plant Growth and Development. NEET asks PGRs mostly as match-the-list and statement questions on effects, uses and discoveries.

1. Plant Growth Regulators: Characteristics

• PGRs are also described as plant growth substances, plant hormones or phytohormones.
• ★ Exam imp They can be of five chemical kinds, for example:

1.1 Two groups based on function

• ★ Exam imp The gaseous PGR ethylene could fit either group, but it is largely an inhibitor of growth activities.
• Abscission: the shedding of plant organs such as leaves, flowers and fruits.
• Dormancy: a state of suspended growth of seeds or buds.

2. Discovery of Plant Growth Regulators

• ★ Exam imp The discovery of each of the five major groups of PGRs was accidental.

2.1 Auxin

• Charles Darwin and his son Francis Darwin observed the coleoptiles of canary grass.
• The coleoptiles responded to unilateral illumination by growing towards the light source; this is phototropism.
• ★ Exam imp After a series of experiments, they 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 the tips of coleoptiles of oat seedlings.

Reading the experiment

1. (a) Intact coleoptile: bends towards the light.
2. (b) Tip removed: no bending.
3. (c) Tip covered by an opaque cap: no bending.
4. (d) Tip under a transparent cap, lower part shaded: bends towards the light.

2.2 Gibberellin

• ★ Exam imp The bakanae (foolish seedling) disease of rice seedlings was caused by the fungal pathogen Gibberella fujikuroi.
• E. Kurosawa (1926) found the disease symptoms in rice seedlings treated with sterile filtrates of the fungus.
• The active substances were later identified as gibberellic acid.

2.3 Cytokinin

• F. Skoog and his co-workers worked with internodal segments of tobacco stems.
• Callus: a mass of undifferentiated cells.
• ★ Exam imp The callus proliferated only if, in addition to auxins, the medium was supplemented with one of: extracts of vascular tissues, yeast extract, coconut milk or DNA.
• Miller et al. (1955) later identified and crystallised the cytokinesis-promoting active substance and named it kinetin.

2.4 Abscisic acid

• During the mid-1960s, three independent researches purified and characterised three kinds of inhibitors.
• ★ Exam imp These were inhibitor-B, abscission II and dormin; all three were later proved chemically identical and named abscisic acid (ABA).

2.5 Ethylene

• H.H. Cousins (1910) confirmed the release of a volatile substance from ripened oranges.
• This substance hastened the ripening of stored, unripened bananas.
• It was later identified as ethylene, a gaseous PGR.

3. Auxins

• The name comes from the Greek auxein, meaning to grow.
• ★ Exam imp Auxin was first isolated from human urine.
• The term auxin applies to IAA and to other natural and synthetic compounds with certain growth-regulating properties.
• Auxins are generally produced by the growing apices of stems and roots, and migrate to the regions of their action.

• All four of these auxins have been used extensively in agricultural and horticultural practices.

3.1 Physiological effects and uses of auxins

• Rooting: initiate rooting in stem cuttings, widely used for plant propagation.
• Flowering: promote flowering, e.g. in pineapples.
• Leaf and fruit drop: prevent fruit and leaf drop at early stages, but promote abscission of older, mature leaves and fruits.
• Apical dominance: the growing apical bud inhibits the growth of lateral buds.
• Parthenocarpy: induce the formation of fruits without fertilisation, e.g. in tomatoes.
• Herbicides: widely used as weed killers; 2,4-D kills dicotyledonous weeds but does not affect mature monocotyledonous plants.
• Weed-free lawns: 2,4-D is used by gardeners to prepare lawns free of weeds.
• Xylem and cell division: auxin controls xylem differentiation and helps in cell division.

3.2 Apical dominance

• ★ Exam imp Removal of shoot tips (decapitation) usually results in the growth of lateral buds.
• Decapitation is widely applied in tea plantations and hedge-making.
• Removing the shoot tip lets the lateral buds grow into branches, so the plant becomes bushy with more leaves and a dense hedge.

4. Gibberellins

• Gibberellins are another kind of promotory PGR.
• ★ Exam imp More than 100 gibberellins are reported from widely different organisms such as fungi and higher plants.
• They are denoted as $\mathrm{G}{\mathrm{A}}_1$, $\mathrm{G}{\mathrm{A}}_2$, $\mathrm{G}{\mathrm{A}}_3$ and so on.
• Gibberellic acid ($\mathrm{G}{\mathrm{A}}_3$) was one of the first gibberellins to be discovered, and remains the most intensively studied form.
• ★ Exam imp All GAs are acidic. They produce a wide range of physiological responses.

Physiological effects and uses of gibberellins

• Grapes: their ability to increase the length of the axis is used to increase the length of grape stalks.
• Apples: they cause fruits like apple to elongate and improve their shape.
• Delay of senescence: fruits can be left on the tree longer, which extends the market period.
• Malting: $\mathrm{G}{\mathrm{A}}_3$ is used to speed up the malting process in the brewing industry.
• Sugarcane: it stores carbohydrate as sugar in its stems; spraying with gibberellins increases stem length and the yield by as much as 20 tonnes per acre.
• Conifers: spraying juvenile conifers with GAs hastens the maturity period, leading to early seed production.
• Bolting: gibberellins promote bolting in beet, cabbages and many plants with rosette habit.

5. Cytokinins

• Cytokinins have specific effects on cytokinesis (division of the cytoplasm).
• ★ Exam imp They were discovered as kinetin, a modified form of adenine (a purine), from autoclaved herring sperm DNA.
• ★ Exam imp Kinetin does not occur naturally in plants.
• The search for natural substances with cytokinin-like activity led to the isolation of zeatin from corn kernels and coconut milk.
• Since then, several naturally occurring cytokinins and some synthetic compounds with cell-division-promoting activity have been identified.
• Natural cytokinins are synthesised in regions of rapid cell division: root apices, developing shoot buds and young fruits.

Physiological effects of cytokinins

• Help to produce new leaves and chloroplasts in leaves.
• Promote lateral shoot growth and adventitious shoot formation.
• Help to overcome apical dominance.
• Promote nutrient mobilisation, which helps to delay leaf senescence.

6. Ethylene

• Ethylene is a simple gaseous PGR.
• ★ Exam imp It is synthesised in large amounts by tissues undergoing senescence and by ripening fruits.

Physiological effects of ethylene

• Seedlings: horizontal growth of seedlings, swelling of the axis and apical hook formation in dicot seedlings.
• Senescence and abscission: promotes both, especially of leaves and flowers.
• Fruit ripening: highly effective; it enhances the respiration rate during ripening.
• Dormancy and germination: breaks seed and bud dormancy, initiates germination in peanut seeds and sprouting of potato tubers.
• Deep-water rice: promotes rapid internode and petiole elongation, which keeps the leaves and upper shoot above water.
• Roots: promotes root growth and root hair formation, increasing the absorption surface.
• Flowering: used to initiate flowering and synchronise fruit-set in pineapples; induces flowering in mango.

• ★ Exam imp Because ethylene regulates so many processes, it is one of the most widely used PGRs in agriculture.

6.1 Ethephon

7. Abscisic Acid (ABA)

• ABA was discovered for its role in regulating abscission and dormancy; it also has other wide-ranging effects.
• ★ Exam imp It acts as a general plant growth inhibitor and an inhibitor of plant metabolism.
• ABA inhibits seed germination.

• ABA plays an important role in seed development, maturation and dormancy.
• By inducing dormancy, ABA helps seeds withstand desiccation (drying) and other factors unfavourable for growth.
• ★ Exam imp In most situations, ABA acts as an antagonist to GAs.

8. PGRs Working Together

• For every phase of growth, differentiation and development, one PGR or another has some role to play.
• ★ Exam imp The roles may be complementary or antagonistic, and individualistic or synergistic.
• PGRs are synthesised in various parts of the plant and control different differentiation and developmental events.
• Any PGR has diverse physiological effects, and diverse PGRs also show similar effects.
• Several events in a plant's life involve more than one PGR: dormancy in seeds and buds, abscission, senescence and apical dominance.
• PGRs are only one kind of intrinsic control; genomic control and extrinsic factors also play an important role.
• Many extrinsic factors, such as temperature and light, control growth and development via PGRs.
• Such events include vernalisation, flowering, dormancy, seed germination and plant movements.
• Plant growth and development are also affected by light, temperature, nutrition, oxygen status and gravity.

9. Exam Essentials

Pairs to Match

Examples to Remember

Numbers to Remember

• Five major groups of PGRs.
• More than 100 gibberellins reported; $\mathrm{G}{\mathrm{A}}_3$ was one of the first discovered.
• 20 tonnes per acre: increase in sugarcane yield after spraying gibberellins.
• 1910: H.H. Cousins, ethylene from ripened oranges.
• 1926: E. Kurosawa, bakanae disease of rice.
• 1955: Miller et al., kinetin identified and crystallised.
• Mid-1960s: inhibitor-B, abscission II and dormin, later named ABA.

10. Quick Revision

• PGRs are small, simple molecules: indole compounds, adenine derivatives, carotenoid derivatives, terpenes and gases.
• Auxins, gibberellins and cytokinins are growth promoters; ABA is an inhibitor; ethylene is largely an inhibitor.
• All five groups were discovered accidentally; the Darwins showed that the coleoptile tip causes phototropic bending.
• Went isolated auxin from oat coleoptile tips; Kurosawa (1926) produced bakanae symptoms with sterile filtrates of Gibberella fujikuroi.
• Skoog's tobacco callus needed auxin plus a supplement; Miller et al. (1955) crystallised kinetin.
• Inhibitor-B, abscission II and dormin are ABA; Cousins (1910) found ethylene from ripened oranges.
• Natural auxins: IAA, IBA; synthetic: NAA, 2,4-D. Auxins root cuttings and cause apical dominance and parthenocarpy.
• Decapitation removes apical dominance; it is used in tea plantations and hedge-making.
• Over 100 GAs, all acidic; they lengthen grape stalks and sugarcane, shape apples, speed malting and cause bolting.
• Kinetin (from herring sperm DNA) is not natural; zeatin is. Cytokinins overcome apical dominance and delay leaf senescence.
• Ethylene ripens fruits (respiratory climactic), promotes senescence and abscission, and breaks dormancy.
• Ethephon releases ethylene slowly: ripening in tomato and apple, thinning, and female flowers in cucumber.
• ABA inhibits germination, closes stomata, induces seed dormancy and opposes GAs; it is the stress hormone.
• PGRs act in complementary, antagonistic, individualistic or synergistic ways; light and temperature often act through them.

11. Solved Examples

Common Mistakes to Avoid