Five Major Hormones
Each hormone controls distinct aspects of plant growth
Plant hormones (phytohormones) are small organic molecules produced in tiny quantities that coordinate plant growth, development, and responses to the environment. Unlike animal hormones, plant hormones often act at or near their site of synthesis, and the same hormone can have different effects depending on tissue type and concentration.
The five classical plant hormones — auxin, gibberellin, cytokinin, abscisic acid, and ethylene — were all discovered in the early-to-mid 20th century. Each controls distinct processes, though they interact extensively: the ratio of auxin to cytokinin in tissue culture determines whether a plant callus grows roots (high auxin:cytokinin) or shoots (low auxin:cytokinin).
💡 Hormone Interactions — Auxin:Cytokinin Ratio
Plant hormones rarely act alone — their ratios determine developmental outcomes:
High auxin:cytokinin ratio: Root formation. Used in tissue culture to induce rooting of plant callus. Explains why root tips (high auxin) continue producing roots.
High cytokinin:auxin ratio: Shoot formation. Used in tissue culture to induce shoot development. Explains why removing the apical bud (removing auxin source) releases lateral buds (cytokinins from roots now dominate).
ABA vs GA antagonism: ABA promotes seed dormancy; GA breaks dormancy and promotes germination. During seed maturation, ABA rises (induces dormancy). During stratification/imbibition, ABA declines and GA rises (dormancy broken, germination proceeds).
Ethylene + auxin: High auxin can stimulate ethylene production. Ethylene and auxin interact in root growth — at high concentrations, auxin inhibits root elongation partly through ethylene induction.
Aux
Auxin (IAA) — cell elongation and apical dominance
Auxin (indole-3-acetic acid, IAA) is the primary hormone controlling cell elongation, phototropism, gravitropism, and apical dominance. Produced mainly in the shoot apical meristem and young leaves, auxin is transported polarly (strictly from shoot tip to root, basipetal in shoots) by specialized carrier proteins (PIN proteins). This polar auxin transport is unique among plant hormones.
Cell elongation: Auxin activates H⁺-ATPase pumps in the cell membrane → acidification of cell wall → acid growth hypothesis: low pH activates expansins (proteins that loosen cell wall cross-links) → cell wall loosens → cell takes up water by osmosis → cell elongates. This mechanism explains phototropism: more auxin accumulates on the shaded side → shaded side elongates more than lit side → shoot bends toward light.
Apical dominance: High auxin concentration from the apical bud suppresses lateral bud growth. Removing the apical bud (pruning/pinching) removes the auxin source → lateral buds released from inhibition → bushier growth. This is why pinching houseplants makes them fuller.
Memory trick: Auxin = growth hormone of shoots. More auxin = more elongation (at low-moderate concentrations). Apical dominance = tip controls the whole plant. Phototropism = auxin redistributes to shaded side = bends toward light.
Gib
Gibberellin — stem elongation and seed germination
Gibberellins (GAs) are a large family of hormones produced in young leaves, seeds, and roots. Primary effects: stem elongation (stimulating both cell division and elongation in internodes), seed germination, and fruit development.
Stem elongation: Dwarf plant mutants (dwarf peas — Mendel's plants, dwarf maize) typically have defects in gibberellin biosynthesis or signaling. Applying GA to dwarf plants restores normal height. GA stimulates degradation of DELLA proteins (growth repressors), releasing the cell division and elongation machinery.
Seed germination: After a seed imbibes water, GA is released from the embryo → signals the aleurone layer of the endosperm → aleurone cells secrete α-amylase and other hydrolytic enzymes → enzymes digest stored starch and proteins in endosperm → sugars and amino acids released → embryo uses for growth.
Bolting: Long-day plants produce GA in response to long days → rapid stem elongation (bolting) before flowering. Spraying GA on biennial plants can induce bolting and flowering in the first year.
Memory trick: Gibberellin = Grow tall (stem elongation). Dwarf plants lack GA. GA triggers alpha-amylase in germinating seeds. GA = bolt (rapid elongation before flowering).
Cyt
Cytokinin — cell division and delay of aging
Cytokinins are adenine-derived hormones produced primarily in root tips. Primary effects: stimulating cell division (cytokinesis — hence the name), promoting lateral bud growth (antagonizes auxin's apical dominance), and delaying leaf senescence (aging).
Cytokinin:auxin ratio determines shoot vs root development in tissue culture: high cytokinin:auxin → shoot development. Low cytokinin:auxin (high auxin:cytokinin) → root development. This is used in plant tissue culture to propagate plants clonally.
Delay of senescence: Cytokinins maintain green color in leaves by delaying chloroplast breakdown and protein degradation. Florists exploit this — cut flowers treated with cytokinin stay fresh longer. If you place a yellowing leaf on a plate and draw a cytokinin solution on one area, that area stays green while the rest yellows — cytokinin mobilizes nutrients toward itself.
Memory trick: Cytokinin = Cell division. Produced in Roots. Counters apical dominance (promotes lateral buds). Delays senescence (keeps leaves green). Auxin:cytokinin ratio = root:shoot balance.
ABA
Abscisic acid — stress response and dormancy
Abscisic acid (ABA) is the primary stress hormone of plants, produced in roots and leaves in response to drought, cold, and other stresses. Two main effects:
Stomatal closure: During drought, ABA is produced in roots → transported to leaves → guard cells detect ABA → K⁺ channels open → K⁺ flows out of guard cells → water follows by osmosis → guard cells lose turgor → stomata close → water loss reduced. This is the most rapid and critical drought response in plants. ABA-insensitive mutants (abi mutants) cannot close their stomata → wilt rapidly in drought.
Seed dormancy: High ABA in seeds maintains dormancy — prevents germination under unfavorable conditions. ABA levels decline as seeds age or as conditions improve (cold stratification → ABA degraded → dormancy broken). GA and ABA antagonize each other: GA promotes germination, ABA suppresses it.
Memory trick: ABA = Abscisic acid = Adversity hormone. Drought → ABA → stomata CLOSE (guard cells lose turgor). ABA maintains dormancy. ABA opposes GA (dormancy vs germination). 'ABA = Always Be Alert (to stress).'
Eth
Ethylene — fruit ripening and senescence
Ethylene (C₂H₄) is unique among plant hormones — it is a gas at room temperature, allowing it to diffuse through air and affect neighboring plants or fruits. Produced by virtually all plant tissues, especially in response to wounding, stress, ripening fruit, and flooding.
Fruit ripening: Ethylene triggers a cascade of changes during fruit ripening: starch conversion to sugars, cell wall softening (polygalacturonase breaks down pectin), color change (chlorophyll breakdown, anthocyanin synthesis), and aroma production. Ethylene is autocatalytic during ripening — ethylene stimulates more ethylene production → rapid ripening. This explains why one rotten apple spoils the barrel (the ripe/damaged apple produces ethylene gas → triggers ripening in neighbors).
Commercial applications: Fruits are harvested unripe and transported at low O₂/low temperature (suppresses ethylene production) → exposed to ethylene gas before sale to trigger ripening. Bananas are shipped green and gassed with ethylene at the destination.
Senescence and abscission: Ethylene promotes leaf and fruit abscission (separation and drop) by activating enzymes that digest the cell walls of the abscission zone.
Memory trick: Ethylene = a GAS = ripening + aging + abscission. One ripe banana next to unripe ones = ripens them all (ethylene gas spreads). Autocatalytic = ripening accelerates itself. Commercial fruits shipped unripe to avoid ethylene.
🔬 Applied Scenario — Plant Hormones in Agriculture
Plant hormones are exploited in commercial agriculture and horticulture:
A
Auxin as herbicide. Synthetic auxins (2,4-D, 2,4,5-T) at high concentrations cause uncontrolled, disorganized growth in broad-leaved plants (dicots) → rapid cell proliferation → plant death. Grasses (monocots) are relatively resistant. 2,4-D is one of the most widely used herbicides globally — selective for broad-leaved weeds in lawns and grain fields. Agent Orange (a 50:50 mixture of 2,4-D and 2,4,5-T) was used as a defoliant in the Vietnam War.
B
Gibberellin in seedless grape production. Thompson Seedless grapes treated with gibberellin during fruit development produce larger berries with looser clusters — dramatically increasing commercial value. The GA treatment enlarges the berry cells without fertilization (parthenocarpy). This is why supermarket seedless grapes are much larger than wild-type seedless grapes.
C
Ethylene management in postharvest. Controlled atmosphere storage (low O₂, high CO₂, low temperature) suppresses ethylene production and response → fruits can be stored for months without ripening. Apples stored in controlled atmosphere stay firm and fresh for up to 12 months. 1-methylcyclopropene (1-MCP, sold as SmartFresh) irreversibly blocks ethylene receptors → dramatically extends shelf life of cut flowers and fruits.
D
Cytokinin in cut flower industry. Cytokinin treatments (benzylaminopurine/BAP sprays) delay senescence of cut flowers and potted plants by maintaining chlorophyll and preventing protein breakdown. Cytokinin also promotes lateral branching in ornamental plants when applied to axillary buds — important in chrysanthemum and poinsettia production.
📌 Exam Application
1. Auxin (IAA): Cell elongation (acid growth hypothesis), phototropism (accumulates on shaded side), apical dominance (tip suppresses lateral buds), polar transport (shoot tip → root).
2. Gibberellin: Stem elongation (dwarf mutants lack GA), seed germination (triggers alpha-amylase in aleurone layer), bolting.
3. Cytokinin: Cell division, delays senescence, promotes lateral buds. Produced in roots. High cytokinin:auxin = shoots; high auxin:cytokinin = roots.
4. ABA: Drought response (stomata close), seed dormancy. Opposes GA. Stress hormone.
5. Ethylene: Gas. Fruit ripening (autocatalytic), senescence, abscission. One ripe banana ripens neighbors. Shipped unripe commercially.
✓ Quick Self-Test
1. What are the five major plant hormones and one key function of each?
2. How does auxin cause phototropism?
3. What is apical dominance and which hormones are involved?
4. How does ABA cause stomatal closure during drought?
5. What makes ethylene unique among plant hormones?
Answers:
1. Auxin: cell elongation and apical dominance. Gibberellin: stem elongation and seed germination. Cytokinin: cell division and delay of senescence. ABA: stomatal closure and seed dormancy. Ethylene: fruit ripening and leaf abscission.
2. Auxin is produced at the shoot apex and transported toward the base. Light causes auxin to migrate from the lit side to the shaded side of the shoot. The shaded side therefore has more auxin → greater cell elongation → the shoot bends toward the light source. The mechanism of elongation is the acid growth hypothesis: auxin activates H⁺ pumps → cell wall acidifies → expansins loosen wall → cell elongates as water enters by osmosis.
3. Apical dominance is the suppression of lateral bud growth by the apical bud. Auxin produced by the apical meristem is transported downward and inhibits lateral bud development. Cytokinin produced in root tips promotes lateral bud growth. When the apical bud is removed (pruning), the auxin source is eliminated and lateral buds are released from inhibition — the plant becomes bushier.
4. During drought, ABA is synthesized in roots and leaves → transported to guard cells → ABA binds receptors → activates K⁺ efflux channels → K⁺ moves out of guard cells → water follows by osmosis → guard cells lose turgor (deflate) → stomata close → reduces transpirational water loss.
5. Ethylene is a gas (C₂H₄) at room temperature — the only gaseous plant hormone. This allows it to diffuse through air to affect neighboring cells, organs, and even neighboring plants. Ethylene triggers fruit ripening autocatalytically (ethylene stimulates more ethylene production, accelerating ripening), causes leaf and fruit abscission, and promotes senescence.