Plant Hormones — Set 4

Correct Answer: C. Auxin

• **Auxin** = Auxin stimulates the walls of the ovary to expand and develop into a fruit after pollination — making it the direct driver of fruit and flower growth. • **Phototropism link** — Auxin is synthesised in shoot tips and redistributes toward the shaded side, explaining both bending toward light and tissue enlargement. • Commercially, synthetic auxins like NAA are sprayed on orchards to increase fruit size and prevent premature drop. • 💡 Option A (Abscisic Acid) is wrong because ABA inhibits growth and promotes dormancy, not fruit development; Option B (Gibberellin) is wrong because gibberellins mainly elongate internodes and break seed dormancy, not drive ovary-to-fruit conversion; Option D (Ethylene) is wrong because ethylene ripens already-formed fruits rather than initiating their growth.

Correct Answer: A. Gibberellin

• **Gibberellin** = Spraying gibberellins on sugarcane dramatically elongates the internodal sections of the stem, which directly increases plant height and total biomass. • **Sugar yield link** — Taller internodes store more sucrose-rich parenchyma cells, so gibberellin application raises sugar yield per plant by 15–20 % in commercial farms. • Gibberellins were first discovered from the fungus Gibberella fujikuroi, which caused 'bakanae' (foolish seedling) disease — abnormally tall, thin rice plants. • 💡 Option B (Auxin) is wrong because auxin promotes lateral cell expansion in stems rather than internode elongation; Option C (Cytokinin) is wrong because cytokinin promotes cell division and delays leaf senescence but does not specifically elongate internodes; Option D (Ethylene) is wrong because ethylene is a gaseous hormone associated with ripening and senescence, not stem elongation.

Correct Answer: A. Gibberellin

• **Gibberellin** = Gibberellins signal the embryo that conditions are right for growth; they trigger the aleurone layer of seeds to produce α-amylase, which breaks down starch reserves to fuel germination. • **ABA antagonism** — Dormancy is enforced by Abscisic Acid; gibberellin directly counteracts ABA, shifting the balance from dormancy to active germination. • In malting (barley processing for beer), gibberellin is applied externally to speed up uniform germination of the grain. • 💡 Option B (Cytokinin) is wrong because cytokinin promotes cell division and shoot growth but is not the primary dormancy-breaking signal; Option C (Abscisic Acid) is wrong because ABA is precisely the hormone that enforces seed dormancy, not breaks it; Option D (Ethylene) is wrong because ethylene promotes fruit ripening and senescence rather than initiating seed germination.

Correct Answer: C. Cytokinin

• **Cytokinin** = Cytokinins promote the differentiation of proplastids into fully functional chloroplasts by activating genes involved in chlorophyll synthesis and thylakoid membrane formation. • **Anti-senescence role** — By stimulating chloroplast production, cytokinins delay leaf yellowing; a detached leaf stays green much longer when soaked in cytokinin solution. • Cytokinins are synthesised primarily in root tips and transported upward in xylem sap, acting as a root-to-shoot signal about nutrient availability. • 💡 Option A (Gibberellin) is wrong because gibberellins promote stem elongation and seed germination, not chloroplast biogenesis; Option B (Auxin) is wrong because auxin mainly regulates cell elongation and apical dominance; Option D (Abscisic Acid) is wrong because ABA accelerates leaf senescence and chloroplast degradation — the opposite effect.

Correct Answer: B. Abscisic Acid

• **Abscisic Acid (ABA)** = ABA levels spike rapidly when a plant senses drought, salinity, or extreme cold — earning it the title 'stress hormone'; its most immediate action is to close stomata by triggering K⁺ efflux from guard cells, reducing water loss. • **Seed dormancy enforcer** — ABA also keeps seeds dormant during unfavourable seasons, preventing premature germination. • ABA is synthesised from carotenoid precursors in chloroplasts and moves systemically through the phloem to coordinate whole-plant stress responses. • 💡 Option A (Auxin) is wrong because auxin governs phototropism and cell elongation, not stress responses; Option C (Gibberellin) is wrong because gibberellin counteracts dormancy and promotes growth — the opposite of a stress response; Option D (Ethylene) is wrong because while ethylene is produced under some stress conditions, it is specifically known as the ripening and senescence hormone, not the primary stress hormone.

Correct Answer: A. Ethylene

• **Ethylene** = Ethylene is a gaseous hormone that triggers the climacteric burst — a sharp rise in cellular respiration that accelerates starch-to-sugar conversion, softening of cell walls, and colour change in fruits like bananas, mangoes, and apples. • **Autocatalytic ripening** — Ethylene production is self-amplifying in climacteric fruits: once ripening begins, ethylene stimulates more ethylene synthesis, causing rapid, synchronized ripening of nearby fruit. • This is why placing a ripe banana next to unripe fruit speeds up the latter's ripening — the ethylene gas diffuses across. • 💡 Option B (Gibberellin) is wrong because gibberellin delays senescence and ripening in some fruits; Option C (Cytokinin) is wrong because cytokinin promotes cell division and delays senescence rather than triggering the climacteric ripening burst; Option D (Auxin) is wrong because high auxin levels actually suppress abscission and delay ripening at early developmental stages.

Correct Answer: D. Auxin

• **Auxin** = When a tendril touches a solid support, auxin redistributes away from the contact side; the uncontacted side then grows faster, causing the tendril to curve toward and wrap around the support — this is thigmotropism. • **Differential growth principle** — The same mechanism underlies phototropism: unequal auxin distribution on two sides of an organ creates unequal elongation, bending the organ toward or away from a stimulus. • Tendrils of plants like peas and grapevines can complete a full coil in as little as 20 minutes after contact, driven by this rapid auxin shift. • 💡 Option A (Ethylene) is wrong because ethylene promotes senescence and abscission rather than directional growth responses; Option B (Gibberellin) is wrong because gibberellin promotes uniform elongation of internodes, not asymmetric bending; Option C (Cytokinin) is wrong because cytokinin promotes cell division and chloroplast development but is not involved in tropistic bending movements.

Correct Answer: C. High Auxin and Low Cytokinin

• **High Auxin and Low Cytokinin** = In tissue culture, the auxin-to-cytokinin ratio determines organ fate: a high auxin/low cytokinin ratio pushes the undifferentiated callus to differentiate into roots by activating root-specific developmental genes. • **Skoog and Miller Rule** — Researchers F. Skoog and C. Miller established in 1957 that flipping the ratio (low auxin, high cytokinin) induces shoot formation instead — a foundational principle of plant biotechnology. • Equal ratios maintain callus growth without differentiation; this balance is used when large quantities of undifferentiated cells are needed for secondary metabolite production. • 💡 Option A (Only Gibberellin) is wrong because gibberellin alone does not control the root-vs-shoot differentiation decision in callus; Option B (Low Auxin and High Cytokinin) is wrong because this combination triggers shoot and bud formation, not roots; Option D (Equal Auxin and Cytokinin) is wrong because balanced ratios keep cells in an undifferentiated callus state rather than directing them toward roots.

Correct Answer: A. Abscisic Acid

• **Abscisic Acid (ABA)** = ABA is biosynthesised inside plastids (mainly chloroplasts and amyloplasts) from carotenoid precursors — specifically xanthoxin derived from the cleavage of violaxanthin — in the mesophyll cells of leaves and in root cap cells. • **Drought-triggered synthesis** — When leaf water potential drops, carotenoid cleavage accelerates rapidly, causing ABA levels to rise within minutes and stomata to close. • ABA can also be synthesised in seeds during late embryogenesis to establish dormancy before dispersal. • 💡 Option B (Auxin) is wrong because auxin (IAA) is synthesised mainly in young shoot tips and leaf primordia via tryptophan metabolism, not in plastids; Option C (Cytokinin) is wrong because cytokinins are synthesised in root meristems through modification of adenine; Option D (Gibberellin) is wrong because gibberellins are synthesised in the cytoplasm and plastids of young shoots and seeds via the terpenoid pathway, not specifically in root/leaf plastids.

Correct Answer: A. Auxin

• **Auxin** = Auxin produced in developing seeds and young leaves suppresses the formation of the abscission zone — a layer of separation cells at the base of petioles and fruit stalks — thereby preventing premature shedding. • **ABA/Ethylene countered** — As fruit matures and auxin levels drop, ethylene and ABA promote abscission zone activation, allowing ripe fruits to fall naturally. • Fruit growers spray synthetic auxins (such as 2,4-D) on young fruits to prevent pre-harvest drop, protecting yield. • 💡 Option B (Abscisic Acid) is wrong because ABA actively promotes abscission by activating the separation layer — the exact opposite of preventing shedding; Option C (Gibberellin) is wrong because gibberellin delays senescence in some contexts but is not the primary abscission-zone suppressor; Option D (Ethylene) is wrong because ethylene is the key trigger that activates the abscission zone and causes shedding.