Control of Growth and Responses in Plants 1 Ch. 27 - Plant Responses

1Control of GroControl of Growth and Respowth and Responses in Plantsnses in Plants

Ch. 27 - Plant Responses


Ch. 27 - Plant Responses

TropismsPlant growth toward or away from a unidirectional stimulus is called a tropism Positive is towards stimulusPositive is towards stimulusNegative is away from stimulusNegative is away from stimulusDue to differential growth - one side of organ Due to differential growth - one side of organ elongates faster than the otherelongates faster than the other

Three types of tropisms: - Phototropism - movement in response to light - Gravitropism - movement in response to gravity - Thigmotropism - in response to touch

3Phototropism


Phototropism

Positive phototropism:Studied by Charles & Francis Darwin Occurs because cells on the shady side of the stem elongate

A pigment related to riboflavin thought to act as a photoreceptor when phototropism occurs Plant hormone called Plant hormone called auxinauxin migrates from lighted migrates from lighted side of stem to shady side of stemside of stem to shady side of stemCells on the shady side elongate faster than those Cells on the shady side elongate faster than those on the bright side, causing stem to curve toward the on the bright side, causing stem to curve toward the lightlight


Gravitropism

When a plant is placed on its side, the stem grows upward, opposite of the pull of gravity. This is an example of negative response called gravitropism

Roots, in contrast, show positive gravitropism, as they grow downwards. Roots without root caps don’t respond to gravityRoot cap cells contain sensors called statoliths, which are starch grains located within amyloplasts, a type of plastid.

- Amyloplasts settle to lower part of cell & cause bending of root.

6Gravitropism

7Gravitropism

Negative gravitropism of stems

Positive gravitropism of roots

Sedimentation of statoliths


Gravitropism

Auxin is responsible for:Positive gravitropism of roots, andNegative gravitropism of shoots

How does auxin do this:Amyloplasts come in contact with ER which releases stored calcium ions.

This leads to activation of auxin pumps & auxin enters the cells

Roots & stems respond differently to auxin: - Auxin inhibits growth of root cells, so cells on upper surface elongate so root curves downward

- Auxin stimulates growth of stem cells, so cells on lower surface elongate so stem curves upward

9Gravitropism

Negative gravitropism of stems

Positive gravitropism of roots

Sedimentation of statoliths

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Control of GroControl of Growth and Respowth and Responses in Plantsnses in Plants

Thigmotropism

Unusual growth due to contact with solid objects is called thigmotropismEx: Coiling of tendrils

The plant grows straight until it touches something. Cells in contact with object grow less while those on the opposite side elongate.

Response can be quite rapid; within 10 minutesSometimes it seems to need light which might be a need for ATP for the response.

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Coiling Response

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Nastic Movements

Nastic movements:Do not involve growth andAre not dependent on the stimulus direction

Seismonastic movements result from:Touch, shaking, orThermal stimulation

Due to loss of turgor pressure within a few cells located in a thickening, called a pulvinus, at the base of each leaflet. Touch causes K+ to flow out of cells & then water follows.

•Ex: Mimosa leaves & Venus flytrap

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Seismonastic Movement

Mimosa pudica

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Nastic Movements

Sleep movements:Occur daily in response to light and dark changesEx: Prayer PlantMovement due to changes in turgor pressure of motor cells in a pulvinus located at the base of each leaf.

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Sleep Movement

Prayer plant

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Circadian Rhythms

Biological rhythms with a 24-hour cycleTend to be persistentRhythm is maintained in the absence of Rhythm is maintained in the absence of environmental stimuli environmental stimuli Caused by a Caused by a biological clockbiological clockWithout environmental stimuli, circadian rhythms Without environmental stimuli, circadian rhythms continue but the cycle extends to 25 or 26 hourscontinue but the cycle extends to 25 or 26 hours Believed that the clocks are synchronized by Believed that the clocks are synchronized by external stimuli such as length of daylight external stimuli such as length of daylight compared to length of darkness. This is called the compared to length of darkness. This is called the photoperiod.photoperiod.

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Plant Hormones

Almost all communication on a plant is done by hormonesChemical signals produced in very low concentrations in one part of plants and then active in another part of the plant

Hormones travel within phloem, or from cell to cell, in response to the appropriate stimulus

Each hormone has a specific chemical structure

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Auxins

The most common naturally occurring auxin is indoleacetic acid (IAA).

It is produced in shoot apical meristem and is found in young leaves and in flowers and fruits

Auxins affect many aspects of plant growth & development

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Effects of Auxin

Apical DominanceApically produced auxin prevents the growth of axillary buds (side buds)

When a terminal bud is removed, the nearest lateral buds begin to grow, and the plant branches Pruning the top of a plant generally achieves a Pruning the top of a plant generally achieves a fuller look by removing the apical dominancefuller look by removing the apical dominanceWeak solution of auxin applied to woody cutting Weak solution of auxin applied to woody cutting causes rapid growth of adventitious rootscauses rapid growth of adventitious roots Promotes fruit growthPromotes fruit growth

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Apical Dominance

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Effects of Auxin

Auxin production by seeds also promotes the growth of fruit. As long as auxin is concentrated in leaves or fruits As long as auxin is concentrated in leaves or fruits rather than in the stem, leaves and fruits do not fall rather than in the stem, leaves and fruits do not fall off.off. Trees can be sprayed with auxin to keep mature Trees can be sprayed with auxin to keep mature fruit from falling to groundfruit from falling to ground Auxin is sprayed on tomatoes to induce Auxin is sprayed on tomatoes to induce development of fruit without pollination creating development of fruit without pollination creating seedless tomatoes seedless tomatoes

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Control of GroControl of Growth and Respowth and Responses in Plantsnses in Plants Phototropism Experiments

Darwin & Darwin (1880s) 1. Used coleoptiles (grass shoots)

2. Found that shoots bend if: a. Tips of shoots are present & Normal

Covered with clear cap Opaque base

3. No bending if:a. Tip covered with capb. Tip was removed

4. Concluded tip senses light

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Phototropism Experiments

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Peter Boysen-Jensen (1913)

1. Removed tips of shoots 2. Placed gelatin on stump 3. Replaced tip on top of gelatin:

a. Shoots bent towards light 4. Put piece of impermeable mica between shoot and tip: a. No phototropic response 5. Concluded that some mobile

chemical is responsible for the phototropic response

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Control of GroControl of Growth and Respowth and Responses in Plantsnses in PlantsA. Paal (1918)

1. Removed tips of shoots 2. Put shoots in the dark 3. Replaced tips back on stumps but put them off-center on stumps 4. Tip placed on right side:

a. Shoots bent towards left 5. Tip placed on left side:

a. Shoots bent towards right 6. Suggested tip produces chemical

that moves down shoot & causes cells below it to grow a. Light must alter its amount

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Control of GroControl of Growth and Respowth and Responses in Plantsnses in PlantsFritz Went (1926)

1. Removed tips & placed them on blocks of agar for an hour 2. Put blocks of agar only on cut

ends of stumps 3. If placed in center of stump:

a. Shoots grew straight upward 4. If placed off-center of stump

a. Shoots grew & bent to opposite side

5. Blank agar blocks didn’t grow a. Definitive evidence of a hormone

He named the hormone auxin.

29Demonstrating Phototropism -

Went’s Experiment

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Phototropism

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Control of GroControl of Growth and Respowth and Responses in Plantsnses in PlantsHow Auxins Work

When a stem is exposed to unidirectional light, auxin moves to the shady side.

Auxins bind to plasma membrane receptors which leads to a series of reactions & the generation of at least three specific second messengers:1. Activates a proton, H+, pump

Acidic conditions cause cell wall to loosenCellulose fibrils are weakened

2. Activates Golgi apparatusSends out vesicles laden with cell wall materials

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Control of GroControl of Growth and Respowth and Responses in Plantsnses in PlantsHow Auxins Work

3. Stimulates DNA-binding protein Activates a particular geneLeads to production of growth factors

Cell walls become extensible & then fill with water by osmosis.

Turgor pressure increases due to the entry of water & the cell elongates.

This occurs on side opposite to the light so the stem lengthens on shady side causing a bending toward the light.

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Auxin Mode of Action

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mm

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Gibberellins

Growth promoting hormonesBring about internode elongation of stem cellsGibberellic acid (GA3) = most common Stem elongationStem elongationCan cause dwarf plants to grow hugeCan cause dwarf plants to grow hugeSources of gibberellin:Sources of gibberellin: Young leaves, roots, embryos, seeds & fruitsYoung leaves, roots, embryos, seeds & fruitsCommercial uses:Commercial uses: Break dormancy of buds & seeds, induce flowering, Break dormancy of buds & seeds, induce flowering, increase size of flowers, produce larger seedless increase size of flowers, produce larger seedless grapesgrapes

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Effect of Gibberellins

No treatment

Treated

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Gibberellins

How GA3 acts as a chemical messenger:Embryo produces gibberellinsAmylase, enzyme that breaks down starch, appears in cells just inside seed coat

GA3 is the first messenger Attaches to receptor in plasma membraneAttaches to receptor in plasma membraneSecond messenger, calcium ions, combines with a Second messenger, calcium ions, combines with a DNA-binding proteinDNA-binding protein Believed to activate the gene that codes for Believed to activate the gene that codes for amylase. This acts on starch to release sugars amylase. This acts on starch to release sugars used as source of energy for growing embryoused as source of energy for growing embryo

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Gibberellic Acid:Structure and Mode of Action

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Cytokinins

CytokininsA class of plant hormones that promote cell division (cytokinesis)

First isolated in 1967 from corn = zeatinProduced in dividing tissues or roots & in seeds & fruits Promotes cell divisionPromotes cell division Prevents Prevents senescence senescence (Aging process. Leaves die (Aging process. Leaves die and fall off)and fall off) Initiates leaf growth. Lateral buds will grow when Initiates leaf growth. Lateral buds will grow when cytokinin is applied to them. cytokinin is applied to them.

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Abscisic AcidAbscisic acid (ABA): (aka stress hormone)

Initiates and maintains seed and bud dormancy Brings about closure of stomata Dormancy occurs when a plant readies itself for adverse conditions by stopping growth ABA moves from leaves to vegetative buds in fallABA moves from leaves to vegetative buds in fall Buds are converted to winter buds which get covered by Buds are converted to winter buds which get covered by

thick, hardened scalesthick, hardened scales In spring, reduction in level of ABA & increases in In spring, reduction in level of ABA & increases in

gibberellins break seed and bud dormancy.gibberellins break seed and bud dormancy. Produced by: Any “green tissue” with chloroplastsAny “green tissue” with chloroplasts Monocot endosperm, andMonocot endosperm, and RootsRoots

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Abscisic Acid:Control of Stoma Opening

ABA binding leads to influx of Ca2+ & the opening of K+ channels. Water exits guard cells & stoma closes.

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Ethylene

Ethylene is involved in abscission, the dropping of leaves, fruits & flowers from a plantOnce abscission has begun: Ethylene stimulates certain enzymes like cellulaseEthylene stimulates certain enzymes like cellulaseCauses leaves, fruits, or flowers to dropCauses leaves, fruits, or flowers to drop

Also ripens fruit by increasing activity of enzymes that soften fruit

Uses in agriculture: To hasten ripening of green fruitsTo hasten ripening of green fruits To create pleasing colors before salesTo create pleasing colors before sales

Ethylene is a gas that can induce ripening of nearby fruits

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Functions of Ethylene

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Photoperiodism

Photoperiodism is any physiological response prompted by changes in day or night length

PhotoperiodPhotoperiod1. The relative lengths of day and night2. This changes with the seasons3. Flowering, germination & dormancy all occur at specific times of year4. Thus, photoperiod is the major environmental factor that needs to be measured by plants

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PhotoperiodismThree Types of Plants: 1. Short-Day Plants (Long-night)

a. Flower when days are short (fall, winter)

b. Actually controlled by night length: Night length must be longer than a critical value.

c. Continuity of darkness is what matters. A flash of light will disrupt flowering d. Examples: Chrysanthemums, poinsettias, rice, ragweed

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Photoperiodism and Flowering

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Photoperiodism

2. Long-Day Plants (Short-night) a. Flower when days are long (late spring, summer) b. Night must be shorter than a critical value c. A flash of light during the night can induce flowering during the wrong season d. Examples: spinach, wheat, lettuce, iris, petunia, mustard

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Photoperiodism and Flowering

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Photoperiodism

3. Day-Neutral Plants a. Day length doesn’t matter b. Flower year round

c. Examples: Roses, carnations, dandelions, sunflowers

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Photoperiodism How is photoperiod detected?

1. Involves phytochrome, a light-absorbing pigment that exists in 2 inter-changeable forms:

a. Pr strongly absorbs red light (660-680 nm)

b. Pfr absorbs far-red light (700–730 nm)

2. When Pr absorbs red light it is converted quickly to Pfr

3. When Pfr absorbs far-red light it is converted slowly to Pr

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Phytochrome Conversion Cycle

happens quickly

(happens slowly)

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Photoperiodism

4. Pfr slowly reverts back to Pr in the dark 5. At sunset, far-red light is common

a. So Pfr begins to convert to Pr

b. This marks end of day; start of night.

c. Pr accumulates slowly all night 6. At sunrise, red-light is common

a. So Pr converts to Pfr relatively quickly

b. This marks end of night; start of day.

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Phytochrome Conversion Cycle

happens quickly

(happens slowly)

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Control of Growth and Responses in Plants 1 Ch. 27 - Plant Responses