Plant Hormones and Ethylene Effects Quiz
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Questions and Answers

Which of the following is NOT an effect of ethylene on plant development?

  • Promotion of root and root hair formation
  • Promotion of chloroplast development (correct)
  • Induction of flowering in the pineapple family
  • Induction of lateral cell expansion

    • What is the rate-limiting enzyme in ethylene biosynthesis?

  • Methionine synthase
  • ACC synthase (correct)
  • Ethylene reductase
  • SAM synthase

    • Which of the following is NOT a characteristic of the Yang cycle?

  • Breakdown of ethylene to carbon dioxide and water (correct)
  • Salvaging and recycling of sulfur
  • Involvement in the biosynthesis of ethylene
  • Production of 1-aminocyclopropane-1-carboxylic acid (ACC)

    • How does ethylene promote the ripening of some fruits?

    <p>By inducing cell wall degradation (A)</p> Signup and view all the answers

    Which of the following is NOT a typical site for ethylene biosynthesis in plants?

    <p>Meristems (D)</p> Signup and view all the answers

    Which of the following plant species is NOT known to exhibit a response to ethylene?

    <p>Pinus sylvestris (D)</p> Signup and view all the answers

    What is the primary role of cytokinins in plant development?

    <p>Promoting cell division (B)</p> Signup and view all the answers

    Which of the following scenarios would likely result in the development of roots?

    <p>High auxin and low kinetin concentrations (B)</p> Signup and view all the answers

    What is the commercial application of the synthetic chemical ethephon?

    <p>Promoting fruit ripening (D)</p> Signup and view all the answers

    What is the primary role of auxin in plant development?

    <p>Promoting cell elongation (A)</p> Signup and view all the answers

    What is the role of proton pumps in auxin transport?

    <p>Proton pumps help create an electrochemical gradient that facilitates passive auxin transport. (A)</p> Signup and view all the answers

    What is the primary function of the protein expansins in cell elongation?

    <p>Expansins break down cellulose fibers, allowing cell wall expansion. (A)</p> Signup and view all the answers

    In which direction does auxin move in the stem parenchyma cells?

    <p>Auxin mainly moves toward the root tips, facilitated by active transport through the plasma membrane. (D)</p> Signup and view all the answers

    Which of the following statements accurately describes auxin transport in phloem sieve tubes?

    <p>Auxin is primarily transported through phloem sieve tubes when moving towards the root tips. (D)</p> Signup and view all the answers

    Which of the following is NOT a mechanism by which auxin stimulates cell elongation?

    <p>Auxin directly activates the synthesis of new cellulose fibers. (A)</p> Signup and view all the answers

    How does auxin influence the tropic response in plants?

    <p>Auxin accumulates on the shaded side of the plant, leading to uneven cell elongation and bending towards light. (D)</p> Signup and view all the answers

    What is the role of auxin in wound tissue repair in plants?

    <p>Auxin promotes the activity of cambium cells, leading to the production of new xylem and phloem. (D)</p> Signup and view all the answers

    What is the relationship between auxin and the chemiosmotic model of polar auxin transport?

    <p>The chemiosmotic model describes the mechanism by which auxin crosses the plasma membrane, driven by the proton gradient. (B)</p> Signup and view all the answers

    Which of the following statements accurately describes auxin's role in secondary growth?

    <p>Auxin promotes the division of cambium cells, leading to the formation of secondary xylem and phloem. (B)</p> Signup and view all the answers

    How does auxin influence the formation of new vascular tissue around a wound?

    <p>Auxin promotes the division and differentiation of cambium cells, leading to the formation of new vascular tissue. (B)</p> Signup and view all the answers

    Which of the following plant hormones is involved in regulating stomatal closure in response to water stress?

    <p>Abscisic acid (ABA) (A)</p> Signup and view all the answers

    Which of the following plant hormones is synthesized in response to wound signals and activates signal transduction pathways leading to the production of protease inhibitors?

    <p>Jasmonates (B)</p> Signup and view all the answers

    Which of the following hormones is deactivated by epimerization of the α-hydroxyl groups on the A ring, followed by esterification with fatty acids or glucosylation?

    <p>Brassinosteroids (C)</p> Signup and view all the answers

    Which of the following pairs of hormones are both involved in mediating plant defense responses against pathogens?

    <p>Salicylic acid and Jasmonates (A)</p> Signup and view all the answers

    Which of the following plant hormones is NOT primarily synthesized in response to stress conditions?

    <p>Brassinosteroids (D)</p> Signup and view all the answers

    Which of the following plant hormones is a triterpene derivative with a chemical structure similar to animal steroid hormones?

    <p>Brassinosteroids (D)</p> Signup and view all the answers

    Which of the following plant hormones is known to promote shoot elongation and inhibit root growth?

    <p>Brassinosteroids (D)</p> Signup and view all the answers

    Which of the following hormones is a small peptide produced in wound tissue and acts as a signal molecule in plant defense responses?

    <p>Systemin (D)</p> Signup and view all the answers

    Which plant hormone is responsible for inhibiting cell growth and is associated with the plant's response to stressful conditions?

    <p>Abscisic acid (D)</p> Signup and view all the answers

    The discovery of auxin involved a series of experiments conducted by several scientists. Which of the following scientists demonstrated that the tip of a coleoptile was responsible for the phototropic response?

    <p>Peter Boysen-Jensen (B)</p> Signup and view all the answers

    Plant hormones work by influencing a variety of cellular activities, including gene expression. How do plant hormones impact gene expression?

    <p>They trigger the production of proteins called transcription factors that regulate gene expression. (B)</p> Signup and view all the answers

    What is the primary function of growth promoting hormones, such as auxin, gibberellins, and cytokinins, in plant development?

    <p>They promote the formation of new roots and shoots. (A)</p> Signup and view all the answers

    What are some of the mechanisms by which plants maintain a stable pool of hormones?

    <p>All of the above. (D)</p> Signup and view all the answers

    In the context of plant hormones, why is it significant that they work in small concentrations?

    <p>Small concentrations allow for precise control of developmental processes. (B)</p> Signup and view all the answers

    Why are plant hormones often classified into two broad categories based on their effects on growth?

    <p>To highlight the opposing and complementary roles of hormones in plant development. (D)</p> Signup and view all the answers

    What is the significance of the discovery of auxin?

    <p>It was the first hormone identified in plants, paving the way for research in plant physiology. (C)</p> Signup and view all the answers

    Which of the following is NOT a property of the hormone gibberellin?

    <p>It is found in high concentrations within roots and shoots. (B)</p> Signup and view all the answers

    Based on the text provided, what is a likely consequence of applying auxin paste to a developing ovary?

    <p>Promotion of fruit development without viable seeds. (A)</p> Signup and view all the answers

    What is the name of the disease that involves abnormally elongated rice seedlings due to the fungal secretion of gibberellins?

    <p>Foolish seedling disease (A)</p> Signup and view all the answers

    Which of the following pairs of hormones have opposing effects on lateral bud development?

    <p>Auxin and cytokinins (D)</p> Signup and view all the answers

    Which of the following scenarios would be most likely to lead to an increase in ethylene production?

    <p>High concentrations of auxin in a plant. (C)</p> Signup and view all the answers

    What is the role of auxin in fruit development?

    <p>Auxin produced by the developing seeds stimulates fruit development. (D)</p> Signup and view all the answers

    What is the primary function of cytokinins in relation to auxin?

    <p>To antagonize auxin's effects on lateral bud development. (A)</p> Signup and view all the answers

    Which of the following is NOT a characteristic of 'foolish seedling' disease in rice plants?

    <p>The disease is caused by a fungal secretion of cytokinins. (A)</p> Signup and view all the answers

    What role does auxin play in the process of leaf abscission?

    <p>Loss of auxin initiates leaf abscission. (C)</p> Signup and view all the answers

    Which of the following statements regarding gibberellins is TRUE?

    <p>Gibberellins can induce stem elongation in certain plant types. (D)</p> Signup and view all the answers

    In which plant organ is abscisic acid (ABA) NOT primarily synthesized?

    <p>Flowers (C)</p> Signup and view all the answers

    Which of the following is NOT a known effect of ethylene on plant growth and development?

    <p>Inhibiting lateral bud growth (C)</p> Signup and view all the answers

    Which of the following statements accurately describes the relationship between gibberellins and ABA in seed dormancy?

    <p>Gibberellins and ABA antagonistically interact, with ABA promoting dormancy and gibberellins promoting germination. (B)</p> Signup and view all the answers

    How does ABA primarily promote stomatal closure during drought stress?

    <p>By activating K+ ion transport out of guard cells. (D)</p> Signup and view all the answers

    What is the primary precursor for the synthesis of abscisic acid (ABA) in plants?

    <p>Violaxanthin (D)</p> Signup and view all the answers

    Which of the following scenarios would most likely lead to an increase in ABA levels within a plant?

    <p>Exposure to drought stress conditions. (D)</p> Signup and view all the answers

    Which of the following best describes the role of ABA in seed dormancy?

    <p>ABA maintains a state of suppressed metabolic activity and inhibits germination. (D)</p> Signup and view all the answers

    Which of the following is a common commercial application of ABA derivatives (dormins)?

    <p>Maintaining dormant conditions in plant materials for shipping purposes. (B)</p> Signup and view all the answers

    What is the primary role of ABA in promoting stomatal closure?

    <p>ABA activates K+ ion transport out of guard cells, reducing their turgor pressure. (C)</p> Signup and view all the answers

    Which of the following processes is NOT directly influenced by ABA?

    <p>Fruit ripening (D)</p> Signup and view all the answers

    Study Notes

    Plant Growth Regulators

    • Plant hormones are chemicals produced by plants that alter growth patterns and/or maintenance of the plant.
    • These hormones are found in many cells and tissues, but are concentrated in meristems and buds (dormant shoot meristems).
    • They control cell activities by sending chemical signals, activating genes that code for specific enzymes.
    • Hormones can either inhibit or promote cellular activities.
    • Common plant hormones regulate cell division, elongation, and differentiation.

    Multiple Effects

    • Most plant hormones have multiple effects.
    • They often work together, showing overlapping effects.
    • They also work with environmental factors.
    • The effectiveness of individual hormones depends on maintaining a closely regulated pool size.
    • This balance is achieved through biosynthesis, storage as inactive conjugates, and catabolic degradation.

    Six Recognized Groups

    • Presently, there are six recognized groups of plant hormones: Auxin, Gibberellins, Cytokinins, Abscisic acid, Ethylene, and Brassinosteroids.

    Classification by Function

    • Growth-promoting hormones/promoters: Increase plant growth (e.g., Auxins, Gibberellins, Cytokinins).
    • Growth-inhibiting hormones/retardants: Inhibit plant growth (e.g., Abscisic acid, Ethylene).

    Hormone Action

    • Apical meristem: Cell division (auxin, cytokinins, gibberellins); elongation (auxin, gibberellins); dormancy (abscisic acid).
    • Flower: Initiation (unknown); differentiation (auxin, gibberellins).
    • Fruit: Growth (auxin, cytokinins); ripening (ethylene).
    • Leaf: Growth (gibberellins); senescence (ethylene); inhibition of senescence (cytokinins).
    • Lateral bud: Repression (auxin); release from apical dominance (cytokinins).
    • Stem: Elongation (auxin, gibberellins).
    • Roots: Initiation (auxin promotes, gibberellins inhibit); growth (auxin inhibits).

    Auxin

    • Auxin is the first hormone discovered in plants.

    • Early experiments in auxin research were conducted by researchers like Darwin (1880), Peter Boysen-Jensen (1913), Paál (1919), and Frits Went (1926).

    • Coleoptile is the pointed, protective sheath covering the shoot of monocots.

    • Auxin moves down the stem parenchyma to roots.

    • Auxin enters cells as IAAH (passively) or IAA (active cotransport).

    • Auxin transport proteins in the basal end of the cell are needed to carry auxin through the plasma membrane.

    • Auxin destined for root tissue moves through phloem sieve tubes.

    • Auxin promotes elongation and cell enlargement by increasing H⁺ concentration in cell walls.

    • Auxin triggers expansins (proteins disrupting hydrogen bonds) and breaks cross-linkages in cellulose.

    • This allows wall expansion when cells take in more water.

    • Involved in tropic responses: Auxin moves away from light.

    • Involved in secondary growth: Stimulates cambium cells to divide and secondary xylem to differentiate.

    • Involved in wound-tissue repair in vascular bundles.

    • Influences apical dominance.

    • Apical buds tend to inhibit the activation of lower buds/stems.

    • Auxin's effect lessens with distance from the shoot tip. This regulation is counteracted by cytokinins.

    Gibberellins

    • Ewiti Kurosawa discovered gibberellins.
    • The fungus Gibberella fujikuroi secreted a chemical that led to the discovery of gibberellins.
    • Produced in roots, shoot tips, and younger leaves, with seeds having the highest concentration.
    • An extensive family of molecules (more than 125 known).
    • Diterpenes based on the 20-C ent-gibberellane structure.
    • Induce dramatic internode elongation in dwarf and rosette species.
    • Related biosynthetically to carotenes and other isoprene derivatives.
    • Work with auxins to promote rapid elongation and division of stem tissue.
    • Determine microtubule alignment in the preprophase band that determines the plane of cellulose expansion.
    • Influence or cause bolting (biennial plants producing flowers in their first growing season) and reverse genetic dwarfism
    • Used commercially for fruit production (grapes), malting (barley), and increasing sugarcane yields.
    • Used in plant breeding, for reducing time to seed production by inducing cone formation, promoting male flowers in cucurbits and for stimulating bolting (biennial rosette crops). Gibberellin biosynthesis inhibitors (like ancymidol or paclobutrazol) are used to prevent elongation growth in some plants.

    Cytokinins

    • A group of phenylurea derivatives of adenine.
    • Chemically isolated in 1913.
    • Initially studied using coconut endosperm.
    • A potent growth promoter.
    • Used in tissue culture and embryo development studies.
    • Kinetin is the first identified cytokinin molecule.
    • Synthesized by condensing an isopentenyl group with the amino group of adenosine monophosphate (AMP)
    • Form conjugates with sugars.
    • Metabolized by oxidation.
    • Found in actively dividing tissues (seeds, fruits, leaves, root tips, and wound tissue sites). Root tips are likely the main production site. They are transported via xylem throughout the plant.
    • Localized cytokinins released buds from dormancy.
    • Regulate cell division in shoots and roots.
    • Regulate specific components of the cell cycle.
    • Levels peak at the end of S phase, mitosis, and G1 phase in synchronized tobacco cells.
    • Auxin:cytokinin ratios regulate morphogenesis in cultured tissues. This is crucial in tissue culture, influencing bud, root, and callus development.
    • They modify apical dominance and promote lateral bud growth, induce bud formation in moss
    • Overproduction has been implicated in genetic tumors.

    Ethylene

    • A gaseous hydrocarbon.
    • Affects root and shoot development.
    • Synthesized from the amino acid methionine (through SAM—S-adenosylmethionine) and 1-aminocyclopropane-1-carboxylic acid (ACC).
    • The sulphur atoms are salvaged and recycled by the Yang cycle.
    • Ethylene biosynthesis controlled by transcriptional regulation (rate-limiting enzyme, ACC synthase).
    • Readily released into the atmosphere but can be inactivated by oxidation.
    • Found in large amounts in tissues undergoing senescence or ripening.
    • Formed in all plant organs (roots, stems, leaves, bulbs, tubers, fruits, seeds).
    • The rate and stage of the organ's development.
    • Affect fruit ripening, lateral cell expansion, seed/bud dormancy in various species (cereals, potatoes, tubers), and elongation growth in submerged aquatic species (certain dicots, ferns, and cereal deepwater rice).
    • It also induces root and root hair formation, affects flowering in certain families (pineapple and mango), might change the sex of flowers (promotion of female flower formation in cucumber), and enhances leaf senescence.
    • Ethylene is commercially important in fruit ripening, degreening citrus fruit, and synchronizing flowering and fruit set in pineapple.
    • Ethephon (2-chloroethylphosphonic acid) or Ethrel, is used as a commercial ethylene-releasing agent.

    Abscisic Acid (ABA)

    • Plays major roles in seed and bud dormancy and responses to water stress.
    • A 15-carbon terpenoid compound derived from terminal portions of carotenoids.
    • Synthesized by cleavage from a 40-carbon xanthophyll (violaxanthin).
    • Degraded by oxidation to phaseic acid, further reduced to dihydrophaseic acid.
    • Found in every major organ/tissue, from the root cap to the apical bud.
    • Synthesized in almost all cells containing chloroplasts or amyloplasts.
    • Promotes seed dormancy, with high levels present in mature seeds. Seeds germinate when ABA is degraded.
    • Plays a role in promoting winter bud scale formation on woody plants.
    • Derivatives (dormins) used in nurseries to keep materials in dormant conditions.
    • Promotes stomata closure during leaf water deficits by activating K⁺ ion transport out of guard cells, through signal transduction pathways involving calcium as a secondary messenger. The ABA originates in roots, detects low water levels, and travels to leaves to induce stomata closure.

    Brassinosteroids

    • Triterpene derivatives with a chemical structure similar to animal steroid hormones.
    • Brassinolide is the most biologically active brassinosteroid.
    • Synthesized from the sterol campesterol.
    • Deactivated by epimerization and subsequent esterification/glucosylation.
    • Produced in pollen, immature seeds, shoots, and leaves.
    • Promotes shoot elongation, ethylene production, and inhibits root growth/development.

    Other plant hormones

    • Salicylic acid (phenolic): activates defense genes for resistance against pathogens (hypersensitive response).
    • Oligosaccharins (short-chain sugars): plays a role in defense against pathogens; helps in regulating growth, differentiation, and flower development; acts as signaling molecules.
    • Systemin: small peptide in wound tissue, stimulated defense activities as signal molecules.
    • Jasmonates: fatty acid derivatives; play roles in seed germination, root growth, and protein storage.

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    Test your knowledge on plant hormones, particularly focusing on ethylene and its role in plant development. This quiz covers various aspects of ethylene biosynthesis, its effects on fruit ripening, and the function of other hormones such as cytokinins and auxins. Challenge yourself with questions about the physiological roles and commercial applications of these hormones.

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