Describe plant hormones (auxin, gibberellin, cytokinin, ABA, ethylene)

The 5 classical hormones — WHAT / WHY / HOW

1. Auxin (IAA — indole-3-acetic acid)

WHAT it does: cell elongation, phototropism/gravitropism, apical dominance, root initiation, fruit growth.
WHY it bends plants (HOW): light knocks auxin to the shaded side of the shoot → that side's cells elongate more → shoot bends toward the light. This is phototropism.
Apical dominance: auxin from the tip suppresses the side (axillary) buds. Cut the tip → auxin drops → side buds grow bushy. (This is why gardeners prune.)

2. Gibberellin (GA)

WHAT: stem elongation (internodes), breaks seed & bud dormancy, triggers germination, bolting/flowering, makes dwarf plants tall.
HOW in germination (classic exam story): water reaches the embryo → embryo releases GA → GA travels to the aleurone layer → aleurone makes α-amylase → amylase digests stored starch in the endosperm into sugars → sugars feed the growing embryo.
Dual coding hook: GA = Growth-tall + Germination.

3. Cytokinin

WHAT: promotes cell division (cytokinesis) , delays leaf senescence (ageing), promotes lateral bud growth (opposes apical dominance).
WHERE made: mainly root tips, moves upward in xylem.
The famous ratio (HOW plant tissue culture works):
- High auxin : cytokinin → roots form.
- High cytokinin : auxin → shoots form.
- Roughly equal → undifferentiated callus.

4. ABA (Abscisic acid)

WHAT: closes stomata in drought, maintains dormancy of seeds/buds, general growth inhibitor — the antagonist of GA and auxin.
HOW stomata close: water stress → ABA rises → guard cells lose $\text{K}^+$ and water → they go flaccid → stomata close → less transpiration → water saved.

5. Ethylene

WHAT: fruit ripening, abscission (dropping of leaves/fruit/flowers), senescence, triple response to stress.
HOW ripening self-spreads: ethylene is a gas → one ripe fruit releases it → neighbouring fruits ripen → "one bad apple spoils the barrel."

Figure — Describe plant hormones (auxin, gibberellin, cytokinin, ABA, ethylene)

GROW vs STOP — the 80/20 table (learn this and you have 80% of the marks)

Hormone	Main jobs	Grow or Stop	Made / moves
Auxin	elongation, tropisms, apical dominance, roots	GROW	shoot tip → down (polar)
Gibberellin	stem elongation, germination, breaks dormancy	GROW	young tissues/seeds
Cytokinin	cell division, delays ageing, lateral buds	GROW	root tip → up (xylem)
ABA	closes stomata, keeps dormancy	STOP	stressed tissues
Ethylene	ripening, leaf/fruit drop	STOP/age	ripening fruit (gas)

Common mistakes (Steel-man → Fix)

Flashcards

Which hormone is produced at the shoot tip and causes cell elongation?

Auxin (IAA)

By what mechanism does auxin loosen the cell wall for elongation?

The acid-growth theory — it activates H⁺ pumps that lower wall pH, activating expansins that loosen cellulose

Why does a shoot bend toward light?

Light shifts auxin to the shaded side; that side elongates more, bending the shoot toward the light

What is apical dominance and how is it caused?

Auxin from the shoot tip suppresses growth of lateral (axillary) buds

Which hormone breaks seed dormancy and triggers germination?

Gibberellin (GA)

In germinating seeds, GA makes the aleurone layer produce which enzyme, and what does it do?

α-amylase, which digests stored starch in the endosperm into sugars for the embryo

Which hormone rescues dwarf plants into tall ones?

Gibberellin

Which hormone promotes cell division and delays leaf ageing (senescence)?

Cytokinin

Where is cytokinin made and how does it travel?

In root tips; upward in the xylem

State the tissue-culture ratio rule for auxin:cytokinin.

High auxin:cytokinin → roots; high cytokinin:auxin → shoots; equal → callus

Which hormone closes stomata during drought and how?

ABA — guard cells lose K⁺ and water, become flaccid, stomata close

Which two hormones are antagonists over seed dormancy?

GA (breaks dormancy) vs ABA (maintains dormancy)

Which is the only gaseous plant hormone?

Ethylene

Give two roles of ethylene.

Fruit ripening and abscission (leaf/fruit/flower drop)

Why does one ripe fruit ripen its neighbours?

Ethylene is a gas that diffuses and triggers ripening in nearby fruit

Which three hormones mainly promote growth and which two inhibit/age?

Grow: auxin, gibberellin, cytokinin; Stop/age: ABA, ethylene

Recall Feynman: explain it to a 12-year-old

Plants can't move, so they "talk" to their own body parts using tiny chemical messages called hormones. Three of them are the "grow" gang: auxin makes cells stretch (and makes the plant lean toward light), gibberellin makes stems long and wakes up sleeping seeds, and cytokinin makes cells multiply and keeps leaves young. Two are the "chill out / survive" gang: ABA is the panic button — when the plant is thirsty it shuts the little pores (stomata) so water doesn't escape, and it keeps seeds asleep until it's safe. Ethylene is a smelly gas that ripens fruit and tells old leaves to fall off — which is why one ripe banana makes the others ripen too.

Connections

Phototropism and Gravitropism — auxin redistribution in action
Stomata and Transpiration — where ABA does its water-saving job
Seed Germination — GA vs ABA tug-of-war
Plant Tissue Culture — auxin:cytokinin ratio
Osmosis and Turgor — the pressure that auxin's loose walls exploit
Fruit Ripening and Food Storage — ethylene in agriculture

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, plants na bhaag sakte hain na sun sakte hain, toh apne body parts ko messages bhejne ke liye woh chemicals use karte hain jinhe plant hormones kehte hain. Inko do teams me baant lo: ek "GROW" team (Auxin, Gibberellin, Cytokinin) jo growth badhati hai, aur ek "STOP/survive" team (ABA, Ethylene) jo growth rokti hai ya ageing karvati hai. Zyada tar exam answer isi tug-of-war ke baare me hota hai — kaha kaun jeetta hai.

Auxin shoot tip pe banta hai aur neeche jaata hai (polar transport). Yeh cells ko lamba karta hai. Jab light ek side se aati hai, auxin shaded side pe jama ho jaata hai, wahan cells zyada stretch karte hain, isliye plant light ki taraf jhuk jaata hai — yeh phototropism hai. Auxin tip se side buds ko dabata bhi hai (apical dominance), isliye tip kaato toh plant bushy ho jaata hai. Gibberellin stem ko lamba karta hai aur seed ko germinate karvata hai (aleurone layer se amylase banwakar starch ko sugar me todta hai). Cytokinin cell division karvata hai aur leaves ko jaldi budhi hone se rokta hai — yeh root tip me banta hai aur xylem se upar aata hai.

Ab STOP team: ABA stress hormone hai — jab paani kam ho, ABA badhta hai, guard cells se K⁺ aur paani nikal jaata hai, stomata band ho jaate hain, aur paani bachta hai. ABA seed ko dormant (sleeping) rakhta hai — isiliye ABA vs GA antagonist hain. Ethylene ekmaatra gas hormone hai jo fruit ripening aur leaf/fruit girna (abscission) karvata hai. Isiliye ek pakka hua fruit doosron ko bhi pakaa deta hai — "one bad apple spoils the barrel".

Yaad rakhne ke liye trick: "A Girl Called ABE" → Auxin, Gibberellin, Cytokinin, ABA, Ethylene. Pehle teen GROW, aakhri do STOP. Bas itna table aur ratio (auxin:cytokinin → roots vs shoots) rata lo, 80% marks pakke.