BM

Plant Hormones Detailed Notes

Plant Hormones

Hormones Regulate Growth Internally

  • As a plant grows, it differentiates, develops, and responds to the environment.

  • Major internal regulators of growth and development are chemical.

  • Hormones: Chemical signals that regulate metabolism, growth, and morphogenesis.

    • Occur in plants and animals.

    • Phytohormones = Plant hormones.

Phytohormones

  • Development of plant tissues and organs includes:

    • Cell division.

    • Cell enlargement.

    • Cell differentiation - Results from selective expression of certain genes and silencing others.

  • Hormones coordinate these cellular processes during development.

    • Chemical messengers between cells.

  • Hormones either repress or stimulate specific genes in the nucleus.

  • Many observable hormone responses result from differential gene expression.

  • Plant hormones are complex.

    • Synthesized at multiple locations throughout the plant, not by a singular tissue or gland.

    • Active in very small quantities.

    • Different quantities are needed by different tissues (sensitivity).

    • Plant response depends on how target tissue “reads” the hormone.

      • The same hormone can cause distinct responses in different tissues or at different times.

Phytohormones - 5 Major Classes

  • Auxins

  • Cytokinins

  • Ethylene

  • Abscisic Acid

  • Gibberellins

Others Include:

  • Brassinosteroids

  • Salicylic acid

  • Jasmonic acid

  • Systemin

  • Florigen

Auxins

Effects

  • Wounded tissue regeneration.

  • Establishing polarity during embryogenesis.

  • Apical dominance.

  • Adventitious root growth on cuttings.

  • Ethylene synthesis.

  • Flowering inhibition/promotion.

  • Stimulation of fruit development.

  • Tropic responses

Auxins Include:

  • IAA (indole-3-acetic acid)

  • NAA (1-Naphthaleneacetic acid)

  • 2,4-D (2,4-Dichlorophenoxyacetic acid)

Auxins - Synthesis

  • Primary Auxin Synthesis Sites:

    • Shoot apical meristems.

    • Young leaves.

    • Developing fruits and seeds

  • Auxin movement is both Basipetal and Acropetal

    • Basipetal: proceeding toward the base (away from apex)

    • Acropetal: proceeding toward the apex

Auxins - Transport

  • Major transport of Auxins:

    • Epidermal, cortical, and vascular parenchyma cells

    • Phloem sieve tubes

    • Auxin distribution

    • Key

      • PIN1

      • PIN4

      • PIN7

      • ABCB1

      • ABCB19

    • Auxin concentration gradient (low-high)

    • SAM - Future shoot apical meristem

    • s - Suspensor cells

    • a - Apical cell

    • c - Cotyledon

    • h - Hypophysis

Auxins & Tropisms

  • Tropism: Movement in response to an external stimulus.

  • Tropic Responses in Plants:

    • Phototropism

    • Gravitropism

    • Hydrotropism

    • Thigmotropism

  • Stimulus reaction can be positive or negative

  • Auxin is a key hormone in plant tropisms

Auxins…and Charles Darwin

  • The Power of Movement in Plants (Charles and Francis Darwin)

    • Question: Which part of seedling reacts to the light stimulus and causes the response of bending toward light (phototropism)?

    • Method: Test which part of the plant responds to the light stimulus

    • Results: No response if the seedling tip was removed or covered by dark glass

Auxins - Phototropism

  • What must happen for plant to bend? Where is growth occurring?

    • Bending is caused by uneven cell elongation.

    • The accumulation of auxin hormone on the far side of the shoot stimulates cell elongation.

Auxins - Gravitropism

  • (A) Vertical orientation

    • IAA is synthesized in the shoot and transported to the root in the stele.

    • When the root is vertical, the statoliths in the cap settle to the basal ends of the cells. Auxin transported acropetally in the root via the stele is distributed equally on all sides of the root cap. The IAA is then transported basipetally within the cortex to the elongation zone, where it regulates cell elongation.

  • (B) Horizontal orientation

    • In a horizontal root, the statoliths settle to the side of the cap cells, triggering polar transport of IAA to the lower side of the cap.

    • The majority of the auxin in the cap is then transported basipetally in the cortex on the lower side of the root.

    • The high concentration of auxin on the lower side of the root inhibits growth.

    • The decreased auxin concentration on the upper side stimulates the upper side to grow. As a result, the root bends down.

Auxins - Differentiation of Vascular Tissue

  • Auxins

    • Initiates the formation of veins in developing leaves.

    • Join vascular traces from developing leaves to stem.

    • Vascular tissue regeneration (from wound).

  • Wound severs vascular tissue

    • Auxin regenerates lost vascular tissue.

    • Removing upper leaves/buds delays regeneration

      • Caused by delayed auxin production

      • Evidence of basipetal movement of auxin from above.

    • IAA applied to the upper internode after wounding

      • Regeneration of vascular tissue is re-initiated.

Auxins - Apical Dominance

  • Apical dominance: Apical bud inhibiting lateral bud growth

  • Auxin production in apical buds inhibits the development of lateral buds.

    • Remove apical meristem à lateral buds develop

    • Apply synthetic auxin and lateral bud growth is inhibited.

Auxin - Fruit Development

  • Usually, if the flower is not pollinated and fertilized, no fruit develops.

  • Flower stigmas treated with auxin can lead to parthenocarpic (virgin) fruit.

    • Ex: Seedless tomatoes, cucumbers, eggplants, watermelons, etc.

    • Ovules are still present but not fertilized

  • Seeds are auxin source promoting fruit development.

  • If seeds are removed from developing strawberries, the receptacle stops developing.

Synthetic Auxins - Herbicides

  • Synthetic Auxins as Herbicides:

    • Extensive agricultural use

    • Some block electron flow in photosynthesis.

      • Others are difficult for plants to break down and cause lethal bioaccumulation of auxins.

  • Examples:

    • 2,4-D (2,4-dichlorophenoxyacetic acid)

    • Paraquat

    • Dichlorophenyldimethylurea

Knowledge Check - Auxin

  • How does Auxin affect plants?

  • Where is it synthesized?

  • How is it transported?

  • Generally explain phototropism and gravitropism.

  • Explain apical dominance.

Cytokinins

Effects

  • Major Effects:

    • Promotes cell division

      • Found in actively dividing tissues

      • (ex: seeds, fruit, leaves, root tips)

    • Promotes shoot formation

    • Delays leaf senescence

    • Releases lateral buds from auxin apical dominance

    • Increase root development in arid conditions

Cytokinins Include:

  • Zeatin

  • BAP (6-Benzylamino purine)

  • Kinetin

Cytokinins - Synthesis

  • Primary Synthesis Sites:

    • Root tips

  • Research is ongoing…

Cytokinins - Transport

  • Major Transport

    • Within xylem from roots toward shoots

  • Target Locations

    • Actively dividing tissues

      • Seeds, fruits, leaves, root tips

    • Bleeding sap

      • From cracks, splits, pruning cuts, and other wounds

Cytokinin: Auxin Ratio

  • An equal combination of cytokinin + auxin is required for cell division.

    • Maintains meristematic tissue throughout the plant lifespan

  • When applied to callus tissue (culture of undifferentiated plant cells):

    • \uparrow cytokinin : \uparrow auxin à roots form

    • \uparrow cytokinin : \downarrow auxin à buds form

    • 1 cytokinin : 1 auxin à undifferentiated cell production

  • Plants must regulate growth via hormones:

    • Without access to sufficient nutrients, additional growth is not beneficial.

    • Plants mediate hormone synthesis to prevent additional growth in unfavorable conditions.

    • Arabidopsis thaliana mutants lacking auxin and cytokinin have not been found

Cytokinins - Leaf Senescence

  • Senescence: Biological aging; often leads to shedding tissues and dieback in plants

  • Cytokinins Delay Leaf Senescence

    • Leaves turn yellow once removed from plant

      • Due to loss of chlorophyll

    • When cytokinins are applied to excised leaves, the leaves remain green

Knowledge Check - Cytokinins

  • How does cytokinin affect plants?

  • Where is it synthesized?

  • How is it transported?

  • Generally explain cytokinin : auxin ratios.

  • Explain its role in leaf senescence.

Ethylene

Effects

  • Major Effects:

    • Fruit ripening

    • Leaf and flower senescence

    • Leaf and fruit abscission

Ethylenes Include:

  • Ethylene gas (C2H4)

Ethylene - Synthesis

  • Primary Synthesis Sites:

    • Most tissues in response to stress

    • Primarily tissues undergoing senescence or ripening

Ethylene - Transport

  • Major Transport of Ethylene Gas

    • Diffusion from the synthesis site

  • Target Locations

    • Actively dividing tissues

      • Seeds, fruits, leaves, root tips

    • Bleeding sap

      • From cracks, splits, pruning cuts, and other wounds

Ethylene - Fruit Ripening

  • Climacteric fruits: have a large increase in cellular respiration (CO2 production) while ripening (ripening has accelerated stage)

    • Increased ethylene synthesis precedes and accelerates ripening.

    • Ex: Bananas, tomatoes, peaches, avocados, apples, pears

  • Nonclimacteric fruits: have a gradual decrease in cellular respiration while ripening (ripening is gradual)

    • Ex: citrus, grapes, cherries, strawberries

  • What are the agricultural implications?

Ethylene - Promotes Abscission

  • In leaves, ethylene triggers enzymes dissolving cell wall for abscission

    • Agricultural use: applied to plants to loosen fruits to ease harvest

  • Auxin reduces sensitivity to ethylene in the abscission zone

    • Agricultural use: auxin treatment prevents preharvest drop of citrus fruits

Ethylene - Regulating Flower Sex Expression

  • Ethylene determines flower sex in some monoecious plants

    • Ex: Cucurbitaceae (cucumber, squash)

    • Flower primordia can develop into either male or female flowers

    • Ethylene applications cause immature stamens to undergo programmed cell death

    • Results in female flower formation

  • Agricultural applications?

Knowledge Check - Ethylene

  • How does ethylene affect plants?

  • Where is it synthesized?

  • How is it transported?

  • Generally, explain its role in fruit ripening.

Abscisic Acid

Effects

  • Major Effects:

    • Plant growth inhibitor

    • Stomatal closure

    • Storage-protein synthesis in seeds

    • Promotes seed dormancy

    • Embryogenesis

Abscisic Acids Include:

  • ABA (abscisic acid)

    • Misnomer because Abscisic Acid has no direct role in abscission

Abscisic Acid - Synthesis

  • Primary Synthesis Sites:

    • Mature leaves and roots (within chloroplasts and other plastids)

  • Synthesis occurs especially in response to water stress

    • Plants restrain growth under stressful conditions

Abscisic Acid - Inhibits Seed Germination

  • Abscisic acid levels increase during early seed development

    • Stimulates production of seed storage proteins

    • Promotes seed dormancy (Prevents premature germination)

      • Breaking of dormancy correlated with declining abscisic acid in/on seed

    • Mutant embryos that don’t produce abscisic acid à vivipary (plantlets grow from parent plant)

Abscisic Acid - Drought Response

  • Plant roots respond to dry conditions using abscisic acid

    • Roots increase ABA synthesis

    • Releases ABA into the xylem à leaves

    • Stomata respond to increased ABA and close

    • Mutant plants without ABA synthesis can only be grown in humid conditions

Knowledge Check - Abscisic Acid (ABA)

  • How does ABA affect plants?

  • Where is it synthesized?

  • How is it transported?

  • Generally, explain phototropism and gravitropism.

  • Explain apical dominance.

Gibberellins

Effects

  • Major Effects:

    • Hyperelongation of shoots

    • Breaking seed dormancy

    • Stimulates flowering in long-day plants and biennials

Gibberellins Include:

  • Gibberellic acid (GA3)

Gibberellins - Synthesis

  • Primary Synthesis Sites:

    • Young shoots

    • Developing seeds

  • Grass seeds rehydrate, activating embryo growth and metabolism.

  • The young shoot secretes gibberellic acid from the cotyledon into the outermost endosperm, the aleurone.

  • In response, the aleurone secretes digestive enzymes into the central endosperm. The enzymes break down stored starch into sugars. Sugars are used for seedling growth.

Gibberellins - Transport

  • Major Transport

    • Xylem and Phloem (probably)

    • More research is needed…

Gibberellin - Hyperelongation of Shoots

  • Gibberellin treatment to dwarf mutants

    • Dwarfs unable to synthesize gibberellin and cannot elongate normally

    • With treatment, dwarfs indistinguishable from non-mutant plants

  • Agricultural applications:

    • Removing gibberellin genes produces short, stout varieties

      • More energy into fruits and less to shoot growth

      • More resistant to wind and rain damage than wild-type

Gibberellins - Breaking Seed Dormancy

  • Many seeds require a dormant period before germination

    • Dormant seeds require a specific trigger to germinate.

    • Gibberellins applied to dormant seeds artificially breaks dormancy

      • Enhance cell elongation causing the root to penetrate the seed coat

    • Many grass seeds naturally produce gibberellins once moistened

      • Stimulates elongation of embryo and digestion of stored starch in seeds

Knowledge Check - Gibberellins

  • How do gibberellins affect plants?

  • Where are they synthesized?

  • How are they transported?

Learning Outcomes

  • Identify the 5 major phytohormones, their major effects in plants, modes of transport (if known), and regions of synthesis (if known)

  • Explain apical dominance

  • Distinguish climacteric from non-climacteric fruits

  • Explain basipetal and acropetal

  • Do phytohormones work independently or collaboratively? Do they promote or inhibit plant growth and response to a stimulus?

  • Define tropism

    • Identify the different types of tropisms

    • Which hormone discussed is associated with tropic responses?

  • What is apical dominance? Senescence?

  • Describe some of the applied agricultural uses of plant hormones