Plant HormonesPlant Hormones

What kinds of things do What kinds of things do plants have to respond plants have to respond to?to?

Light Light - phototropism - phototropism TouchTouch - thigmotropism - thigmotropism GravityGravity – gravitropism – gravitropism TurgorTurgor movements movements Biological clockBiological clock - circadian rhythms - circadian rhythms

When to open and close stomataWhen to open and close stomata

Control of floweringControl of flowering - photoperiodism - photoperiodism

Figure 39.2 Review of a general model for signal-transduction pathways

Figure 39.0 A grass seedling growing toward a candle’s light

Figure 39.1 Light-induced greening of dark-sprouted potatoes: a dark-grown potato (left), after a week's exposure to natural sunlight (right)

The Search for the Plant The Search for the Plant HormoneHormone

Darwin and son his discovered that grass seedlings would not Darwin and son his discovered that grass seedlings would not bend towards the light if the tip was removed and covered with an bend towards the light if the tip was removed and covered with an opaque cap.opaque cap.

Boysen and Jensen demonstrated that the signal was a mobile Boysen and Jensen demonstrated that the signal was a mobile substance.substance. They placed a gelatin block between the tip and the rest of the plant They placed a gelatin block between the tip and the rest of the plant

and demonstrated that the signal diffused through the gelatinand demonstrated that the signal diffused through the gelatin.. Went(1926) extracted the chemical messenger and impregnated Went(1926) extracted the chemical messenger and impregnated

agar blocks.agar blocks. The agar blocks were places on various parts of the plant and the The agar blocks were places on various parts of the plant and the

plant bent in theplant bent in the opposite direction from which the agar block was opposite direction from which the agar block was placed.placed.

Showed that cells opposite of the elongated causing the stem to Showed that cells opposite of the elongated causing the stem to bend.bend.

The substance was called auxin and produced growth on the The substance was called auxin and produced growth on the opposite side in which it was concentrated.opposite side in which it was concentrated.

Figure 39.4 Early experiments of phototropism

Figure 39.5 The Went experiments

Table 39.1 An Overview of Plant Hormones

AuxinsAuxins

Plant Hormones Help Coordinate Plant Hormones Help Coordinate Growth, Development and to Growth, Development and to StimuliStimuli

Five classes of Hormones:Five classes of Hormones: AuxinsAuxins CytokininsCytokinins GibberillinsGibberillins Abscisic acidAbscisic acid EthyleneEthylene

Hormones are effective in very small Hormones are effective in very small concentrations and are the signal is amplifiedconcentrations and are the signal is amplified

Can affect gene expression and activity of Can affect gene expression and activity of enzymes.enzymes.

AuxinsAuxins Natural form is called indoleacetic Acid(IAA)Natural form is called indoleacetic Acid(IAA) Found mostly in the apical meristem.Found mostly in the apical meristem. Auxin only travels in one direction.Auxin only travels in one direction. Acid Growth Hypothesis:Acid Growth Hypothesis:

Stimulates proton pumps in the region of growth.Stimulates proton pumps in the region of growth. Acidic pH breaks down cell wallsAcidic pH breaks down cell walls Turgor pressure causes cell to elongateTurgor pressure causes cell to elongate Function:Function:

Stimulates cell divisionStimulates cell division Differentiation of secondary xylem growthDifferentiation of secondary xylem growth In developing seeds promotes fruit growth.In developing seeds promotes fruit growth.

Figure 39.6 Polar auxin transport: a chemiosmotic model (Layer 1)

Figure 39.6 Polar auxin transport: a chemiosmotic model (Layer 3)

Figure 39.6 Polar auxin transport: a chemiosmotic model (Layer 2)

CytokininsCytokinins Together with auxin orchestrate root and Together with auxin orchestrate root and

shoot growth.shoot growth. Stimulate cell division and differentiation.Stimulate cell division and differentiation. Relative amounts of cytokinin and auxin Relative amounts of cytokinin and auxin

will determine what will occur.will determine what will occur. In equal amounts no differentiation occurs.In equal amounts no differentiation occurs. More cytokinin than auxin will result in root More cytokinin than auxin will result in root

buds.buds. More auxin than cytokinin will result in shoot More auxin than cytokinin will result in shoot

buds. buds.

GibberellinsGibberellins

Causes “bolting” rapid growth of a flower Causes “bolting” rapid growth of a flower stalk.stalk.

Involved in breaking dormancy of apical Involved in breaking dormancy of apical buds in the spring.buds in the spring.

Stimulate growth in both leaves and Stimulate growth in both leaves and stems.stems.

In some plants both auxin and In some plants both auxin and gibberellins are cintribute to fruit set.gibberellins are cintribute to fruit set.

Bolting “Foolish Seed”Bolting “Foolish Seed”

Abscisic Acid Abscisic Acid

Produced in the terminal bud slows growth Produced in the terminal bud slows growth and inhibits cell division.and inhibits cell division.

Primordial leaves develop into scales and Primordial leaves develop into scales and protect the apical bud through the winter.protect the apical bud through the winter.

Keeps seeds dormant.Keeps seeds dormant.

Can help plants cope with harsh conditions Can help plants cope with harsh conditions by closing their stomata.by closing their stomata.

EthyleneEthylene A gas that promotes fruit ripening.A gas that promotes fruit ripening. Contributes to aging or “senescence” of Contributes to aging or “senescence” of

parts of the plant.parts of the plant. Promotes degradation of cell walls and Promotes degradation of cell walls and

decreases chlorophyll content that is decreases chlorophyll content that is associated with fruit ripening.associated with fruit ripening.

Involved in leaf abscision.Involved in leaf abscision.

Tropisms Orient Plants Toward Tropisms Orient Plants Toward or Away From Stimulior Away From Stimuli

PhototropismPhototropism Cells on the darker sides of the stem elongate faster Cells on the darker sides of the stem elongate faster

in response to auxin moving down from the shoot.in response to auxin moving down from the shoot. Photoreceptor is believed to be a blue light receptor.Photoreceptor is believed to be a blue light receptor.

GravitropismGravitropism Roots curve downwards.Roots curve downwards. Two theoriesTwo theories

Staholith settlingStaholith settling Protoplast signalingProtoplast signaling

ThigmotrpismThigmotrpism Climbing plants respond to touch and grow tendrils Climbing plants respond to touch and grow tendrils

to grasp onto surfaces.to grasp onto surfaces. Stunting height growth in windy environmentsStunting height growth in windy environments

Rapid Leaf MovementsRapid Leaf Movements Rapid loss of turgor pressure in response to touch.Rapid loss of turgor pressure in response to touch.

Specialized motor organs called Specialized motor organs called pulvinipulvini in the joints of in the joints of leaves.leaves.

Lose potassium when stimulated causing water loss.Lose potassium when stimulated causing water loss. Signals are transmitted either through chemical or electrical Signals are transmitted either through chemical or electrical

impulses calledimpulses called action potentials action potentials

Sleep MovementsSleep Movements Daily raising or lowering of leavesDaily raising or lowering of leaves

Circadian RhythmsCircadian Rhythms Internal 24 hour cycle in which most organism keep Internal 24 hour cycle in which most organism keep

track of the time of day.track of the time of day.

PhotoperiodismPhotoperiodism Physiological response to day length.Physiological response to day length. Control of floweringControl of flowering

Short day plants - flower during periods of short day Short day plants - flower during periods of short day length.length.

Long day plants - flower during periods of long day length.Long day plants - flower during periods of long day length. Day neutral plants – unaffected by day length.Day neutral plants – unaffected by day length.

Researchers have found that night l;ength affects Researchers have found that night l;ength affects plant flowering.plant flowering.

Leaves detect day lengthLeaves detect day length