Plant Hormones — Set 3

Correct Answer: C. Ethylene

• **Ethylene** = Ethylene (C₂H₄) is unique among plant hormones in being a gas at normal temperature and pressure; as a simple two-carbon molecule, it diffuses freely through cell membranes and air spaces in plant tissues, as well as through the surrounding atmosphere to neighbouring plants. • **Neighbour effect** — Because ethylene can diffuse through the air, a ripening fruit in storage releases ethylene that accelerates ripening in surrounding fruits — famously summarised as 'one rotten apple spoils the barrel'; this is exploited commercially by storing fruits in low-ethylene, controlled-atmosphere warehouses. • Ethylene biosynthesis follows the Yang cycle: methionine → SAM → ACC → ethylene; ACC is the immediate precursor and is the compound transported in the phloem before being converted to ethylene in target tissues. • 💡 Option A (Cytokinin) is wrong because cytokinins are complex adenine-derived organic molecules dissolved in plant sap, not gases; Option B (Auxin) is wrong because IAA is an indole compound, a solid organic acid in pure form, not a gas; Option D (Gibberellin) is wrong because gibberellins are large diterpenoid molecules that exist only in solution within plant tissues.

Correct Answer: C. Gibberellin

• **Gibberellin** = GA3 (gibberellic acid) sprays applied to grape clusters promote cell expansion in berry flesh, resulting in larger individual berries; additionally, because gibberellin also elongates the cluster stem (rachis), the berries become more spread out, improving air circulation and reducing fungal disease. • **Thompson Seedless grapes** — The application of GA3 to Thompson Seedless (a widely grown commercial variety) is one of the most economically important uses of plant hormones globally; it increases berry size from about 2g to over 5g per berry, significantly raising market value. • Gibberellin achieves berry enlargement by stimulating cell division early in fruit development and cell expansion later, working through both phases of fruit growth. • 💡 Option A (Cytokinin) is wrong because while it promotes cell division, it is not the hormone used commercially to increase grape berry size; Option B (Abscisic Acid) is wrong because it promotes dormancy and maturation rather than cell expansion in fruits; Option D (Auxin) is wrong because it helps maintain fruit attachment but does not significantly increase individual berry size like gibberellin does.

Correct Answer: C. Abscisic Acid

• **Abscisic Acid** = During drought, ABA is synthesised in wilting leaves and root tips, then rapidly transported to guard cells surrounding stomata; ABA binds to receptors (PYR/PYL proteins) on guard cells and triggers the efflux of K⁺ and Cl⁻ ions, causing guard cells to lose turgor and the stomatal pore to close, dramatically reducing water loss through transpiration. • **Rapid response** — Stomatal closure in response to ABA can occur within minutes of exposure, making it one of the fastest known hormone-mediated responses in plants; this speed is critical for plant survival during sudden drought episodes. • ABA also promotes root growth toward water and activates genes encoding proteins that protect cells from dehydration damage (osmotic stress response). • 💡 Option A (Auxin) is wrong because it regulates cell elongation and tropisms but has no direct role in controlling stomatal aperture in response to water deficit; Option B (Gibberellin) is wrong because it is a growth promoter that does not participate in the drought-stomata signalling pathway; Option D (Cytokinin) is wrong because it promotes stomatal opening (opposite effect) and is associated with nutrient sufficiency, not water-stress closure.

Correct Answer: D. Ethylene

• **Ethylene** = In plants with separate male and female flowers (such as cucumber, cannabis, and certain dioecious species), ethylene application or elevated internal ethylene levels shifts sex expression toward femaleness; it suppresses the development of stamens and promotes pistil and ovary formation, increasing the number of female (pistillate) flowers. • **Cucumber sex determination** — In cucumbers, ethylene promotes female flower production while gibberellins promote male (staminate) flowers; growers exploit this by applying ethephon (an ethylene releaser) to increase female flower counts and thereby boost fruit yield. • This hormonal control of sex determination in plants is a fascinating example of a gas (ethylene) acting as a developmental switch for reproductive organ identity. • 💡 Option A (Auxin) is wrong because it regulates growth and tropic responses, not the sex determination of flowers; Option B (Gibberellin) is wrong because it actually promotes male flower formation in cucurbit plants, the opposite of the desired female shift; Option C (Cytokinin) is wrong because it promotes vegetative cell division and leaf development, with no established role in switching floral sex expression.

Correct Answer: C. Abscisic Acid

• **Abscisic Acid** = ABA is often called the 'anti-gibberellin' because it counteracts nearly every major growth-promoting action of gibberellins: where gibberellin breaks seed dormancy, ABA maintains it; where gibberellin promotes stem elongation, ABA inhibits it; where gibberellin induces alpha-amylase production in germinating seeds, ABA suppresses it. • **Functional antagonism** — This ABA–GA antagonism is most clearly seen in seed biology, where the ratio of the two hormones precisely controls the transition between dormancy (ABA dominant) and active germination (GA dominant); many seed dormancy-breaking treatments (cold stratification, light) work by reducing ABA and elevating GA. • ABA and gibberellins also act antagonistically in stomatal regulation, stem growth, and the expression of various developmental genes. • 💡 Option A (Ethylene) is wrong because while it also inhibits some growth processes, it is not specifically anti-gibberellin; its main functions are in ripening, senescence, and stress responses; Option B (Cytokinin) is wrong because cytokinins are growth promoters that work alongside gibberellins in many contexts rather than opposing them; Option D (Auxin) is wrong because it is itself a growth promoter and interacts with gibberellin cooperatively in many developmental processes.

Correct Answer: D. Cytokinins

• **Cytokinins** = Zeatin (6-(4-hydroxy-3-methylbut-2-enylamino)purine) was the first naturally occurring cytokinin to be isolated, extracted from immature maize (Zea mays) kernels in 1963 by D.S. Letham; its name 'zeatin' is derived directly from 'Zea', the genus name of maize. • **Historical sequence** — The first cytokinin ever identified was kinetin (in 1955, isolated from autoclaved DNA), but kinetin is a synthetic artefact; zeatin's isolation established that naturally occurring cytokinins exist in plants, confirming that cytokinin activity was not merely a laboratory phenomenon. • Zeatin remains one of the most biologically active natural cytokinins and is found in high concentrations in developing seeds, coconut milk, and root tips across many plant species. • 💡 Option A (Gibberellins) is wrong because the key gibberellin discovery is linked to the Bakanae disease of rice, not maize kernels; Option B (Auxins) is wrong because auxin discovery is associated with Darwin's and Went's phototropism experiments using oat coleoptiles; Option C (Ethylene) is wrong because ethylene's hormonal role was established through observations of gas-induced fruit ripening and the triple response in seedlings.

Correct Answer: D. Auxin

• **Auxin** = High concentrations of auxin produced by the developing seeds within a young fruit maintain an active abscission-suppressing signal at the fruit stalk (peduncle); auxin prevents the formation of the abscission layer, ensuring the fruit remains firmly attached while it grows and accumulates nutrients and sugars. • **Auxin decline triggers drop** — As the fruit matures and seed production is complete, auxin levels naturally decline; without sufficient auxin, ABA and ethylene signals dominate, and the abscission zone forms, allowing the fully ripe fruit to detach and fall — which aids seed dispersal. • This mechanism is commercially exploited: synthetic auxins are sprayed on apple and citrus orchards just before harvest to prevent premature fruit drop (pre-harvest drop), extending the harvest window. • 💡 Option A (Ethylene) is wrong because ethylene promotes fruit ripening and abscission — it is responsible for fruit drop, not attachment; Option B (Abscisic Acid) is wrong because ABA promotes the abscission zone formation that causes fruit drop; Option C (Gibberellin) is wrong because while it contributes to fruit growth, it does not specifically maintain the attachment signal at the abscission zone like auxin does.

Correct Answer: D. Gibberellin

• **Gibberellin** = Gibberellins can substitute for the long-day photoperiod requirement in certain long-day plants; when applied exogenously, they trigger the same molecular flowering pathway that is normally activated only when the nights are sufficiently short, allowing the plant to flower even when grown under short-day (long-night) conditions that would normally prevent it. • **Vernalisation substitution** — Gibberellins can also substitute for the cold-temperature (vernalisation) requirement in biennial plants; a GA3 spray causes winter biennials to flower in their first year without experiencing cold, demonstrating that gibberellin integrates both temperature and photoperiod signals in the flowering pathway. • This effect is plant-type specific: gibberellins induce flowering in long-day and cold-requiring plants but have little or no effect on short-day plants, which require a different hormonal signal (possibly related to florigen/FT protein). • 💡 Option A (Ethylene) is wrong because it promotes fruit ripening and senescence, not photoperiod-independent flowering; Option B (Abscisic Acid) is wrong because it inhibits growth and would suppress flowering rather than promote it; Option C (Auxin) is wrong because auxin is not involved in photoperiod perception or substituting for day-length requirements in flowering.

Correct Answer: A. Auxin

• **Auxin** = The most abundant natural auxin, Indole-3-Acetic Acid (IAA), is biosynthesised from the amino acid tryptophan through several enzymatic pathways; the indole ring structure of tryptophan is retained in IAA, making the biosynthetic relationship chemically evident. • **Multiple pathways** — IAA can be synthesised from tryptophan via at least four different routes (the IPA, IAM, TAM, and IAN pathways) and also through tryptophan-independent routes; the availability of multiple pathways ensures plants maintain adequate auxin levels under varying conditions. • IAA is mainly produced in young leaf primordia, apical meristems, and developing seeds — all sites of rapid cell proliferation — and then transported polarly downward through the plant. • 💡 Option B (Ethylene) is wrong because ethylene is biosynthesised from the amino acid methionine (via SAM and ACC), not tryptophan; Option C (Cytokinin) is wrong because cytokinins are adenine (purine) derivatives, not derived from tryptophan or any single amino acid precursor; Option D (Gibberellin) is wrong because gibberellins are diterpenoids synthesised from the mevalonate/MEP pathway starting with acetyl-CoA, not from amino acids.

Correct Answer: A. Plant growth regulators

• **Plant growth regulators** = Phytohormones (from Greek phyton = plant, hormon = to set in motion) are naturally occurring organic compounds synthesised in plants in tiny quantities that regulate physiological and developmental processes — including growth, differentiation, flowering, fruiting, dormancy, and stress responses — often acting at sites remote from where they are produced. • **Five classic categories** — The five classical plant hormones are auxins, gibberellins, cytokinins, abscisic acid, and ethylene; newer categories include brassinosteroids, jasmonates, salicylic acid, and strigolactones, expanding the phytohormone family well beyond the original five. • The distinguishing feature of a phytohormone is that it is effective at extremely low concentrations (often nanomolar range) and acts as a chemical messenger coordinating plant responses to both internal developmental cues and external environmental stimuli. • 💡 Option B (Plant minerals) is wrong because minerals (like N, P, K) are inorganic nutrients essential for plant structure and metabolism, not chemical signal molecules; Option C (Plant fertilizers) is wrong because fertilizers are externally applied nutrient inputs, not endogenous hormonal regulators; Option D (Plant pigments) is wrong because pigments like chlorophyll and carotenoids absorb light for photosynthesis and do not function as growth-regulating chemical messengers.