Plant Hormones — Set 5

Correct Answer: A. Auxin

• **Auxin** = Auxin drives cell expansion in stems through the 'acid growth theory': it activates H⁺-ATPase pumps that acidify the cell wall, activating expansins — proteins that loosen cellulose microfibrils — allowing the cell to absorb water and enlarge. • **Directional growth** — Because auxin concentration is unequal on shaded vs. lit sides of a stem, the shaded side expands faster, bending the stem toward light (phototropism). • Auxin works only in elongating (not mature) cells; once the cell wall becomes permanently thickened, auxin can no longer induce expansion. • 💡 Option B (Cytokinin) is wrong because cytokinin promotes cell division (cytokinesis) rather than cell elongation; Option C (Gibberellin) is wrong because gibberellin elongates internodes by a different pathway involving DELLA protein degradation, which is most prominent in nodes, not general stem cell expansion; Option D (Abscisic Acid) is wrong because ABA inhibits cell growth and is the primary growth suppressor under stress.

Correct Answer: B. Cytokinin

• **Cytokinin** = Cytokinins promote stomatal opening by stimulating K⁺ ion uptake into guard cells, increasing their osmotic pressure, drawing in water, and causing them to swell and open the pore. • **ABA antagonist** — ABA is the hormone that forcibly closes stomata during drought; cytokinin directly opposes this by reversing the K⁺ flux and maintaining aperture under non-stress conditions. • Stomatal opening regulated by cytokinin is particularly important during high-light, high-photosynthesis periods when maximum CO₂ entry is needed. • 💡 Option A (Abscisic Acid) is wrong because ABA is the primary stomatal-closing hormone — it triggers guard cell K⁺ efflux and calcium signalling to shut stomata; Option C (Auxin) is wrong because auxin has an indirect and dose-dependent effect on stomata but is not the primary opening signal; Option D (Gibberellin) is wrong because gibberellin is primarily associated with stem elongation and seed germination, not stomatal regulation.

Correct Answer: B. Gibberellin

• **Gibberellin** = During malting, barley grains are steeped in water and allowed to partially germinate; gibberellins (naturally produced or externally applied) activate the aleurone layer to secrete α-amylase and other hydrolytic enzymes that convert stored starch into fermentable sugars (maltose). • **Industrial shortcut** — Commercial breweries apply exogenous gibberellic acid to barley to speed up and standardise the malting process, reducing the time and space needed for germination. • The same enzyme-activating property of gibberellins is why the hormone is used to break dormancy in seeds across agriculture. • 💡 Option A (Ethylene) is wrong because ethylene triggers fruit ripening and senescence but does not activate starch-digesting enzymes in aleurone layers; Option C (Cytokinin) is wrong because cytokinin promotes cell division and delays senescence but does not stimulate aleurone enzyme secretion; Option D (Auxin) is wrong because auxin is involved in cell elongation and rooting, not in the starch hydrolysis needed for malting.

Correct Answer: A. Cytokinin

• **Cytokinin** = Cytokinin overcomes 'apical dominance' — the suppression of lateral buds by high auxin concentrations from the shoot tip — by promoting cell division in axillary buds and allowing them to grow into branches. • **Auxin-cytokinin balance** — When a shoot tip is removed (pruning), auxin levels fall and cytokinin from roots becomes dominant, releasing lateral buds from suppression; this is why pruning makes plants bushier. • Cytokinins are used commercially on ornamental plants and fruit trees to encourage branching and fuller canopy development. • 💡 Option B (Gibberellin) is wrong because gibberellin promotes main-stem elongation and seed germination rather than lateral bud release; Option C (Auxin) is wrong because auxin from the apical tip is precisely what suppresses lateral buds — high auxin inhibits rather than promotes their growth; Option D (Abscisic Acid) is wrong because ABA enforces dormancy in buds rather than activating their growth.

Correct Answer: B. Ethylene

• **Ethylene** = Ethylene promotes abscission of flowers and small fruitlets by activating cell wall-degrading enzymes (cellulase and polygalacturonase) in the abscission zone, causing excess flowers and tiny fruits to drop naturally — a process called chemical thinning. • **Yield quality benefit** — Removing surplus fruit early redirects the tree's photosynthate to fewer, larger fruits, improving size, colour, and sugar content at harvest. • Ethephon (Ethrel) is the commercial ethylene-releasing compound sprayed in orchards for this purpose on apples, peaches, and cotton bolls. • 💡 Option A (Auxin) is wrong because auxin suppresses abscission and is used to prevent fruit drop, not cause it; Option C (Gibberellin) is wrong because gibberellin is often used to increase fruit size (e.g., seedless grapes) or delay ripening, not thin fruit clusters; Option D (Abscisic Acid) is wrong because while ABA can promote leaf senescence, it is not the agent used commercially for fruit thinning in orchards.

Correct Answer: D. Cytokinin

• **Cytokinin** = Cytokinins are N⁶-substituted adenine derivatives — their core structure is the purine base adenine with a side chain (isopentenyl, zeatin, or other groups) attached to the nitrogen at position 6, making them structurally related to the adenine found in DNA, RNA, and ATP. • **First cytokinin** — Kinetin (6-furfurylaminopurine), the first cytokinin discovered by Miller and Skoog in 1955, was isolated from autoclaved herring sperm DNA — an adenine-containing material. • The adenine backbone means cytokinins can interact with certain tRNA anticodon regions, which is one mechanism by which they modulate protein synthesis in dividing cells. • 💡 Option A (Abscisic Acid) is wrong because ABA is a terpenoid (sesquiterpenoid) derived from carotenoids, not from adenine; Option B (Gibberellin) is wrong because gibberellins are diterpenoid acids derived from the terpenoid pathway via geranylgeranyl pyrophosphate; Option C (Auxin) is wrong because auxin (IAA) is an indole compound derived from the amino acid tryptophan.

Correct Answer: B. Ethylene

• **Ethylene** = Ethylene accelerates senescence by activating genes that encode chlorophyll-degrading enzymes (chlorophyllases), proteases, and RNases — causing leaves and flowers to yellow, wilt, and die in a coordinated programmed manner. • **Triple response** — In addition to senescence, ethylene in seedlings causes stem thickening, reduced elongation, and horizontal growth — the 'triple response' used as a classic bioassay for ethylene activity. • Ethylene is a gas produced by all plant tissues but rises sharply during ripening, wounding, and aging; enclosing flowers in a refrigerated ethylene-free atmosphere dramatically extends their shelf life. • 💡 Option A (Cytokinin) is wrong because cytokinin actively delays senescence — it maintains chloroplast integrity and protein synthesis, which is why spraying cytokinin on cut leaves keeps them green; Option C (Auxin) is wrong because auxin promotes growth and prevents abscission of young tissue rather than triggering breakdown; Option D (Gibberellin) is wrong because gibberellin is a growth-promoting hormone associated with elongation and germination, not the aging and breakdown of tissue.

Correct Answer: A. Auxin

• **Auxin** = The root cap contains specialised statocytes filled with dense starch granules called statoliths; when a root is reoriented, statoliths settle toward gravity, triggering asymmetric auxin redistribution — more auxin accumulates on the lower side, inhibiting that side's cell elongation and curving the root downward (positive gravitropism). • **Shoot vs. root response** — High auxin concentrations stimulate shoot cells but inhibit root cells; this difference in sensitivity means the same gravity-induced auxin redistribution makes roots grow downward while shoots grow upward. • Surgical removal of the root cap abolishes gravitropic response, confirming it as the gravity-sensing organ. • 💡 Option B (Abscisic Acid) is wrong because ABA is the stress hormone synthesised in leaf and root plastids to close stomata, not the gravitropic signal in the root cap; Option C (Cytokinin) is wrong because cytokinin is produced in root meristems to promote cell division, not to perceive gravity; Option D (Gibberellin) is wrong because gibberellin promotes elongation uniformly rather than directing gravity-sensing asymmetric growth.

Correct Answer: A. Ethylene

• **Ethylene** = Ethylene weakens the abscission zone — a specialised band of cells at the junction of the fruit stalk — by upregulating cellulase and pectinase enzymes that dissolve the middle lamella, making the fruit detach easily with minimal force when shaken mechanically. • **Harvest technology** — Ethephon spray is applied to orange and lemon orchards 1–2 weeks before mechanical harvest; it releases ethylene in plant tissues, synchronising abscission across the whole orchard. • This practice reduces bruising and labour cost compared to hand-picking while ensuring a clean break without tearing the peel. • 💡 Option B (Gibberellin) is wrong because gibberellin is sometimes applied to delay ripening and maintain firmness in citrus, which is the opposite of promoting detachment; Option C (Abscisic Acid) is wrong because ABA closes stomata and induces dormancy — it does not specifically activate the abscission zone enzymes for fruit drop; Option D (Auxin) is wrong because auxin produced in the fruit actively prevents abscission zone activation, so high auxin keeps the fruit attached to the tree.

Correct Answer: C. Auxin

• **Auxin** = Auxin produced in young leaves and shoot tips flows basipetally through procambial strands, acting as the positional signal that directs vascular cambium cells to differentiate into xylem (at high auxin concentrations) or, in combination with cytokinin, into phloem — making it the master regulator of vascular tissue patterning. • **Concentration gradient** — Experiments by Tsvi Sachs showed that supplying auxin to a wounded area is sufficient to induce canalization — a self-reinforcing channel of vascular differentiation — confirming auxin as the vascular-inducing signal. • This role is critical for wound-healing responses: auxin from leaves re-routes vascular tissue around physical damage, restoring connectivity. • 💡 Option A (Gibberellin) is wrong because gibberellin promotes secondary cell wall thickening in xylem to some degree but is not the primary differentiating signal that establishes where xylem and phloem form; Option B (Abscisic Acid) is wrong because ABA is a growth inhibitor and stress-response hormone, not involved in vascular tissue patterning; Option D (Cytokinin) is wrong because while cytokinin promotes phloem differentiation in combination with auxin, auxin alone is both necessary and sufficient to initiate vascular differentiation, making it the primary hormone.