Plant Responses to the Environment

Chapter 45

Plant Hormones

• Hormones – chemicals secreted by cells, transported to other cells where they exert effect

• Released in response to an environmental stimuli

• Promote growth, development, aging

Six Plant Hormones

• Auxins• Gibberellins• Cytokinins• Ethylene• Abscisic acid• Florigens

Auxins

• Promote or inhibit elongation in target cells– Shoot – high conc. causes elongation– Root – low levels stimulate elongation, high conc.

inhibit

• Synthetic auxin (2,4 D) is used to kill dicots• Commercially used to promote root

formation in plant cuttings, stimulate fruit development, delay fruit fall

Gibberellins

• Primarily in plant shoots• Promote stem elongation by increasing

cell elongation and division• Stimulate bud sprouting, flowering, fruit

production and development, seed germination

• Produced in the shoot apical meristem, young leaves, plant embryos

Cytokinins

• Promote cell division• Synthesized in root apical meristem• Inhibit formation of root branches,

cause nutrients to be transported to leaves, stimulating chlorophyll production and delaying aging

• Commercially - sprayed on cut flowers to keep them fresh

Ethylene

• Gas• Produced in plant tissues, released in

response to a range of environmental stimuli• Stress hormone – produced in response to

wounding, flooding, drought, extreme temp.• Stimulates weak celled abscission layers -

leaves, petals, fruit drop off at appropriate times

• Commercially – used to ripen fruit

Abscisic Acid

• Synthesized in tissues throughout the plant

• Helps plants to withstand unfavorable environmental conditions

• Causes stomata to close when water is scarce

• Promotes root growth, inhibits stem growth in dry conditions.

• Helps maintain dormancy

Florigens

• Synthesized in leaves

• Control flowering in response to environment

• Discovered in 2007

Hormones regulate plant life cycles

• Hormones are produced in response to an environmental stimulus

• These hormones may influence the activity of genes (activate, repress)

Maintaining Dormancy of Seeds

• In temperate zones, seeds remain dormant until spring• Cold weather reduces abscisic acid,

preparing plant for spring germination

• In desert plants, some seeds have high levels of abscisic acid in their coats. It may be washed away by rain

• Grasslands, chaparral, forest – require fire for germination

Gibberellin Stimulates Germination

• -Abscisic acid and +Gibberellin = germination

• Gibberellin is produced by the embryo• Enzymes break down starch for energy

Auxin controls Orientation

• Light and gravity help the seedling figure which way is up

• Auxin controls phototrophism – growth towards light, in shoots

• Gravitropism – growth to/away from gravity, in shoots and roots

• Gravirtopism and phototrophism work together to cause shoot to grow upward

Auxin mediates Gravitropism

• Vertical stem – auxin distributed evenly

• Horizontal stem – position detected and auxin distributed to lower side of stem. • Lower cells elongate, bending stem upwards

(-gravitropism)• When stem is vertical auxin is evenly

distributed

Auxin from the shoot tip travels down and collects in the root tip

Auxin, produced in the shoot tip, is distributed evenly across the shoot and root as it travels downward

root

seed

shoot

(a) The shoot and root are orientedvertically

Gravitropism

Auxin is transported to the lower side of theshoot, where it stimulates cell elongationand causes the stem to bend upward

Auxin is transported to the lower side of theroot, where it inhibits cell elongation andcauses the root to bend downward

(b) The shoot and root are orientedhorizontally

Gravitropism

Negative Gravitropism

Positive Gravitropism

Auxin mediates Phototropism

• Auxin accumulates in the side of the shoot that is away from the light.• Cells elongate and

bend towards the light

Animation: Hormone Characteristics

Animation: Hormone Transport and Activity

Auxin mediates Root Elongation

• Toward gravity• If root is horizontal, they sense gravity

and cause auxin transport to lower side• Lower side cells elongate, causing root

to grow towards gravity

• How do they sense gravity? Statoliths – starch filled plastids settle into the lower part of the cell

Statoliths May Be Gravity Detectors

root

statoliths

nucleus

cell inroot cap

Plant responds to Environment

• When shoot or root push against soil, ethylene is given off.

• Elongation slows and cells become thicker and stronger

• More able to force their way through soil

• Dicots – ethylene causes formation of hook in forming shoot

Thimotropism

• Directional movement or growth in response to touch

• Cell elongation on contact side is inhibited, tendrils grow

• Etheylene may be produced by cells touching the object

Shoot and Root Branching

• Controlled by Auxin and Cytokinin

• Growth of shoot must be balanced by root growth• Water, mineral, anchorage of plant

• Stems – auxin inhibits later bud growth to form branches, cytokinin promotes this growth

• Root – cytokinin stimulates root branching, auxin promotes this growth

Apical Dominance

• Pinching back the tip of a plant causes bushy growth because apical meristems release auxin which suppresses bud development into branches.

Lateral bud sprouting

• Auxin is transported from the stem to the root, decreasing in concentration.

• Cytokinin is transported from the root to the stem, decreasing in concentration.

• Lateral buds closes to shoot receive enough auxin to inhibit growth, very little cytokinin = remain dormant

• Lower lateral buds receive less auxin and more cytokinin = stimulate to grow into branches

Root branch formation

• Auxin, transported down from the stem stimulates branch roots to form

• Cytokinin produced in the root apical meristem inhibits root branching and is transported from the root towards the shoot

• Roots closer to the shoot develop branch roots

• Gradient of hormones keeps size of root and shoot in balance

Lateral buds are inhibited by high auxin levels

Branch roots develop (optimal ratio of cytokinin to auxin)

Branch roots are inhibited by high cytokinin levels

Lateral buds develop into branches (optimal ratio of auxin to cytokinin)

auxin

cytokinin

high

high

shoot tip

root tip

Gradient of Auxin and Cytokinin

Response to Light and Dark

• Timing of flowering and seed production is crucial

• Environmental clues like water and temperature are unpredictable

• Day length is very reliable• Shortening vs. lengthening days

It’s about the amount of darkness

• Day-neutral plants – flower independently of day length • Roses, tomatoes, cucumbers, corn

• Long-day plants – flower when uninterrupted dark is shorter than species-specific duration• Iris, lettuce, spinach, hollyhocks

• Short-day plants – flower when uninterrupted dark is longer than species-specific duration• Cockleburs, chrysanthemums, asters, potato, goldenrod

• It is really more about the amount of darkness than light

long night short night

night day

interrupted night

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The Effects of Darkness on Flowering

Phytochrome

• Plants measure darkness using a biological clock that isn’t well understood

• Each time the phytochrome molecule is exposed to light, the clock resets to 0

• If a plant needs 8 hours of dark to flower but is interrupted with a flash of light at 4 hours, the clock will reset

• Light changes the shape of the phytochrome molecule

conversion in light

absorbsred light

Pr

(inactive)Pfr

(active)Pfr stimulates or inhibits a

response

conversion in dark

absorbsfar-red light

both forms are present in daylight

The Light-Sensitive Phytochrome Pigment

Author Animation: Phytochrome

Author Animation: Morning Glory

Animation: Seedling Elongation

Florigen Stimulates Flowering

• Leaves produce florigen in response to the biological clock

• Transported on phloem to the apical meristem where it activates genes that are responsible for flowering

Coordination of Seeds and Fruit

• Auxin and gibberellin promote growth of ovary

• Apply to fruit and they grow larger and looser

Ethylene

• Unripe fruit is green, bitter• Ripened fruit turns color &

attracts animals

• Ethylene gas stimulates ripening, as they ripen fruits give off ethylene gas to stimulate ripening of adjacent fruit

• Bananas & tomatoes are picked & shipped green

Senscence

• Genetically programmed series of events that prepare the plant for winter

• Ethylene production increases

• Auxin and cytokinin production decreases

• Starches and chlorophyll are broken down and stored in the stem and roots

The Abscission Layer

• A layer located where the fruit or leaf join the stem

• Ethylene promotes breakdown of this layer

• Leaves/fruit drop at correct time

• Can also be triggered by stress

Plant Communication

• Plants summon insect bodyguards• When attacked by caterpillars, corn releases

chemicals, stimulated by volicitin (in caterpillar saliva)

• Parasitic wasps are attracted to chemical, lay eggs in caterpillar

• Lima beans, attacked by spider mites release chemical that attracts carnivorous mite that preys on spider mite

A caterpillar chews on a corn leaf, leaving traces of saliva that contains volicitin

1

Volicitin and leaf damage cause the plant to synthesize and release volatile chemicals

2

The wasps lay their eggs on the caterpillar, which will provide food for their larvae

4

The released chemicals attract female parasitic wasps

3

A Chemical Cry for Help

Animation: Chemical Messengers

Plant Defense

• Some plants, when damaged by insects, produce a signaling molecule that moves through the plant.

• The plant then makes a distasteful chemical

• Radishes and caterpillars

Warning the Neighbors

• Healthy plants sense chemicals released by neighbors that have been wounded by insects.

• Salicylic acid methyl salicylate (volitile)

• Neighbors boost their defenses

Mimosa

• Thigmotropism - sensitive to touch

• Stimulated by electric signals conducted through motor cells at the base of each leaf

Carnivorous Plants

• Sundew - movement of trapped insects triggers thigmotropism in the hairs, secrete sticky goop, smothering the insect

• Bladderwort – trapdoor is sprung by insect, opens inward suddenly, sucking insect into the bladder where it is digested.

A Sundew and Its Insect Prey

The Bladderwort Snares Tiny Aquatic Organisms

Author Animation: Venus Fly Trap