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

4.9.7 · Biology › Plant Biology

Intuition Bada picture

Plants na bhaag sakte hain, na chhup sakte hain, na soch sakte hain — phir bhi unhe light, gravity, drought, wounds aur changing seasons ka jawab dena hota hai. Unka poora "nervous system" chemistry hai: chote-chote signalling molecules jo ek jagah bante hain, doosri jagah jaate hain, aur growth ko ON ya OFF karte hain. Inhe plant hormones (phytohormones) kehte hain. WHY ye exist karte hain: ek rooted organism ko ek slow, chemical tarika chahiye jisse woh cells ko coordinate kar sake jo millimetres se metres door hain. Master theme: growth ek tug-of-war hai un hormones ke beech jo kehte hain "GROW / GO" (auxin, gibberellin, cytokinin) aur jinke kehna hai "STOP / SURVIVE" (ABA, ethylene). Almost har exam answer actually is baare mein hai ki kaunsa hormone kahan jeetta hai.

Plant hormone KYA hota hai?

5 classical hormones — KYA / KYU / KAISE

1. Auxin (IAA — indole-3-acetic acid)

YE KYA KARTA HAI: cell elongation, phototropism/gravitropism, apical dominance, root initiation, fruit growth.
YE PLANTS KO BEND KYU KARTA HAI (KAISE): light auxin ko shoot ki shaded side par dhakelta hai → us side ke cells zyada elongate hote hain → shoot light ki taraf bend ho jaata hai. Ise phototropism kehte hain.
Apical dominance: tip ka auxin side (axillary) buds ko suppress karta hai. Tip kaat do → auxin girega → side buds bushy grow honge. (Isliye gardeners prune karte hain.)

2. Gibberellin (GA)

KYA: stem elongation (internodes), seed & bud dormancy todi, germination trigger karta hai, bolting/flowering, dwarf plants ko tall banata hai.
Germination mein KAISE (classic exam story): paani embryo tak pahunchta hai → embryo GA release karta hai → GA aleurone layer tak jaata hai → aleurone α-amylase banata hai → amylase endosperm mein stored starch ko sugars mein digest karta hai → sugars growing embryo ko feed karte hain.
Dual coding hook: GA = Growth-tall + Germination.

3. Cytokinin

KYA: cell division (cytokinesis) promote karta hai, leaf senescence (ageing) delay karta hai, lateral bud growth promote karta hai (apical dominance ka oppose karta hai).
KAHAN BANTA HAI: mainly root tips mein, xylem mein upar move karta hai.
Famous ratio (KAISE plant tissue culture kaam karta hai):
- High auxin : cytokinin → roots bante hain.
- High cytokinin : auxin → shoots bante hain.
- Roughly equal → undifferentiated callus.

4. ABA (Abscisic acid)

KYA: drought mein stomata band karta hai, seeds/buds ki dormancy maintain karta hai, general growth inhibitor hai — GA aur auxin ka antagonist.
KAISE stomata band hote hain: water stress → ABA badhta hai → guard cells $K^{+}$ aur paani kho dete hain → wo flaccid ho jaate hain → stomata band ho jaate hain → kam transpiration → paani bachta hai.

5. Ethylene

KYA: fruit ripening, abscission (leaves/fruit/flowers ka girna), senescence, stress ka triple response.
KAISE ripening self-spread hoti hai: ethylene ek gas hai → ek ripe fruit ise release karta hai → paas ke fruits ripen hote hain → "ek kharab seb poori tokri kharaab kar deta hai."

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

GROW vs STOP — 80/20 table (yeh seekh lo aur 80% marks pakke)

Hormone	Main kaam	Grow ya Stop	Banta hai / move karta hai
Auxin	elongation, tropisms, apical dominance, roots	GROW	shoot tip → neeche (polar)
Gibberellin	stem elongation, germination, dormancy todna	GROW	young tissues/seeds
Cytokinin	cell division, ageing delay, lateral buds	GROW	root tip → upar (xylem)
ABA	stomata band karna, dormancy maintain karna	STOP	stressed tissues
Ethylene	ripening, leaf/fruit drop	STOP/age	ripening fruit (gas)

Worked examples

Common mistakes (Steel-man → Fix)

Flashcards

Shoot tip par kaun sa hormone banta hai aur cell elongation karta hai?

Auxin (IAA)

Auxin cell wall ko elongation ke liye kaise loosen karta hai?

Acid-growth theory — yeh H⁺ pumps activate karta hai jo wall pH kam karte hain, expansins activate hote hain jo cellulose loosen karte hain

Shoot light ki taraf kyu bend karta hai?

Light auxin ko shaded side par shift karti hai; woh side zyada elongate hoti hai, shoot light ki taraf bend ho jaata hai

Apical dominance kya hai aur yeh kaise hota hai?

Shoot tip ka auxin lateral (axillary) buds ki growth suppress karta hai

Kaun sa hormone seed dormancy todta hai aur germination trigger karta hai?

Gibberellin (GA)

Germinating seeds mein, GA aleurone layer se kaun sa enzyme banwata hai, aur woh kya karta hai?

α-amylase, jo endosperm mein stored starch ko embryo ke liye sugars mein digest karta hai

Kaun sa hormone dwarf plants ko tall banata hai?

Gibberellin

Kaun sa hormone cell division promote karta hai aur leaf ageing (senescence) delay karta hai?

Cytokinin

Cytokinin kahan banta hai aur kaise travel karta hai?

Root tips mein; xylem mein upar

Tissue culture ke liye auxin:cytokinin ka ratio rule batao.

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

Drought mein kaun sa hormone stomata band karta hai aur kaise?

ABA — guard cells K⁺ aur paani kho dete hain, flaccid ho jaate hain, stomata band ho jaate hain

Seed dormancy par kaun se do hormones antagonists hain?

GA (dormancy todta hai) vs ABA (dormancy maintain karta hai)

Eklauta gaseous plant hormone kaun sa hai?

Ethylene

Ethylene ke do kaam batao.

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

Ek ripe fruit apne padosi fruits ko kyu ripen karta hai?

Ethylene ek gas hai jo diffuse hoti hai aur paas ke fruit mein ripening trigger karti hai

Kaun se teen hormones mainly growth promote karte hain aur kaun se do inhibit/age karte hain?

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

Recall Feynman: 12-saal ke bachche ko explain karo

Plants move nahi kar sakte, isliye woh apne body parts se hormones naam ke tiny chemical messages ke zariye "baat" karte hain. Inme se teen "grow" gang ke hain: auxin cells ko stretch karta hai (aur plant ko light ki taraf jhukkata hai), gibberellin stems ko lamba karta hai aur sote hue seeds ko jagaata hai, aur cytokinin cells ko multiply karta hai aur leaves ko young rakhta hai. Do "chill out / survive" gang ke hain: ABA panic button hai — jab plant pyaasa hota hai toh woh chote pores (stomata) band kar deta hai taaki paani na nikle, aur seeds ko tab tak sone deta hai jab tak safe na ho. Ethylene ek smelly gas hai jo fruit ripen karta hai aur purani leaves ko girne ka signal deta hai — isliye ek ripe banana baaki sabko bhi ripen kar deta hai.

Connections

Phototropism and Gravitropism — auxin redistribution in action
Stomata and Transpiration — jahan ABA apna water-saving kaam karta hai
Seed Germination — GA vs ABA tug-of-war
Plant Tissue Culture — auxin:cytokinin ratio
Osmosis and Turgor — woh pressure jise auxin ki loose walls exploit karti hain
Fruit Ripening and Food Storage — ethylene in agriculture