Albia Dugger • Miami Dade College

Chapter 30Communication

Strategies in Plants

30.1 Prescription: Chocolate

• Cocoa is made from cacao beans, which are the seeds of the Theobroma cacao tree

• Cocoa seeds have a high content of flavonoids such as epicatechin, which functions in plant immunity

• In humans, epicatechin has a protective effect against oxidative tissue damage that occurs after a stroke or heart attack, enhances memory, and kills cancer cells

Cacao Tree

Cacao Fruit

30.2 Introduction to Plant Hormones

• Plant development depends on cell-to-cell communication – mediated by plant hormones

• Plant hormones are extracellular signaling molecules that exerts an effect at very low concentrations

• Hormones affect development and growth of plant parts; defensive responses; circadian rhythms; flowering; fruit and seed formation; aging; and dormancy

Chemical Signaling

• A hormone released by cells in a localized area usually alters the activity of cells in a different area

• A cell’s response depends on the cell and the receptor, and varies with the concentration of the hormone

• Typically, the response involves modification of nuclear or mitochondrial DNA that causes a change in gene expression

• In some cases, cell function is affected with no change in underlying gene expression patterns

Hormone Interactions

• Different hormones can have synergistic or opposing effects; a cell’s response depends on integration of hormonal signals

• Plant hormones interact with one another mainly at the transcriptional level

• Hormone expression may be controlled by negative or positive feedback loops

Plant versus Animal Development

Table 30-1 p507

Table 30-1 p507

Table 30-1 p507

Take-Home Message: What regulates growth and development in plants?

• Plant hormones are signaling molecules that coordinate activities among cells in different parts of the plant body.

• Cells that bear receptors for a hormone—and thus can respond to it—may be in the same tissue as the hormone-releasing cell, or in another region of the plant body.

• Plant hormones are involved in all aspects of growth, development, and function in plants. They often work together, with synergistic or opposing effects on cells.

30.3 Auxin: The Master Growth Hormone

• Auxin (IAA) coordinates the effects of other plant hormones plays a critical role in all aspects of plant development• First division of the zygote• Polarity and tissue pattern in the embryo• Formation of plant parts• Differentiation of vascular tissues• Formation of lateral roots• Responses to environmental stimuli

Auxin and Plant Growth

• Auxins promote or inhibit cell division and elongation, depending on the target tissue

• Auxin increases the activity of transport proteins that pump hydrogen ions from cytoplasm into the cell wall

• Increased acidity softens the wall, and turgor stretches the cell

Experiment: Response to Auxin

time timetime

A A coleoptile stops growing after its auxin-producing tip has been removed.

B A block of agar that absorbs auxin from a cut tip can stimulate a de-tipped coleoptile to resume growth

C If an auxin-containing agar block is placed to one side of a cut tip, the coleoptile will con-tinue to grow, but it will bend as it lengthens.

ANIMATED FIGURE: Auxin's effects

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Polar Transport

• Auxin made in shoot apical must be transported to parts of the body where it is needed

• Auxin from shoots is loaded into phloem, travels to roots, and is unloaded into root cells

• Auxin diffuses into cells or is actively transported through plasma membrane proteins called influx carriers

• Once auxin has entered cytoplasm, it can only leave through active transport proteins called efflux carriers

Figure 30-4a p509

auxin

auxin

auxin

auxin

Importance of Polar Transport

• This mechanism auxin flow is unique among plant hormones, and it is important because it establishes auxin concentration gradients across tissues, organs, and the entire plant

• Auxin coordinates the actions of other hormones, many of which are expressed in different localized patterns

Apical Dominance

• Efflux carriers help balance the growth of a plant’s apical and lateral buds

• Auxin travels through efflux carriers in a growing shoot tip and prevents growth of lateral buds (apical dominance)

• Cell membranes in dormant lateral buds have few efflux carriers; auxin produced by apical meristem is not traveling

• If a shoot’s tip breaks off, strigolactone level declines, lateral buds acquire efflux carriers, and lateral buds begin to grow

Loss of a Shoot Tip Ends Dormancy in Lateral Buds

Take-Home Message: What are the main effects of auxin in plants?

• Auxin is a plant hormone that coordinates other hormones during growth and development at all stages of the plant life cycle.

• A polar distribution system sets up auxin concentration gradients across a plant’s tissues and organs in response to internal and external conditions.

30.4 Cytokinin

• A cytokinin is one of a group of plant hormones derived from the nucleotide adenine

• Cytokinin stimulates cell divisions in shoot apical meristem, and cell differentiation in root apical meristem

• Cytokinin and auxin work together, often antagonistically, and they influence one another’s expression

Cytokinin and Auxin

• Cytokinin opposes auxin’s effect on lateral root formation

• In root apical meristem, cytokinin opposes auxin to maintain the balance of differentiating and undifferentiated cells

• Cytokinin stimulates lateral bud growth by releasing lateral buds from apical dominance

Interaction of Auxin and Cytokinin in Release of Apical Dominance

cytokinin

auxin

auxin auxin

A Auxin flowingthrough a shootkeeps the levelof cytokinin lowin the stem.

B Removing the tipends auxin flow in thestem. As the auxinlevel declines, thecytokinin level rises.

C The cytokinin stim-ulates cell division in apical meristem of lateral buds. The cells begin to produce auxin.

D Auxin gradientsform and direct thedevelopment of thegrowing lateral buds.

Take-Home Message: What are the main effects of cytokinin in plants?

• Cytokinin stimulates cell divisions in shoot apical meristem, and cell differentiation in root apical meristem.

• Cyokinin and auxin act together and often antagonistically. The cytokinin–auxin balance controls cell division and differentiation in shoot and root apical meristem.

30.5 Gibberellin

• A gibberellin is a hormone that promotes growth by inducing cell division and elongation between nodes in stem tissue

• Gibberellin is also involved in slowing the aging of leaves and fruits, breaking dormancy in seeds, germination of seeds, and, in some plants, flowering

Effect ofGibberellins

• Gibberellin works by inhibiting inhibitors –removing the brakes on some cellular processes

Gibberellin and Germination

• Gibberellin and barley seed germination• Barley seed absorbs water• Embryo releases gibberellin• Gibberellin induces transcription of amylase gene • Amylase breaks stored starches into sugars used by

embryo for aerobic respiration

Figure 30-8 p511

gibberellin

amylase

sugars

embryoendospermaleuroneA Absorbed water causes cells of a barleyembryo to release gibberellin, which diffusesthrough the seed into the aleurone layer ofthe endosperm.

B Gibberellin triggers cells of the aleuronelayer to express the gene for amylase. Thisenzyme diffuses into the starch-packedmiddle of the endosperm.

C The amylase hydrolyzes starch into sugarmonomers, which diffuse into the embryoand are used in aerobic respiration. Energyreleased by the reactions of aerobic respirationfuels meristem cell divisions in the embryo.

Take-Home Message: What are the main effects of gibberellin in plants?

• Gibberellin stimulates cell division and elongation in stems, which causes stems to lengthen between nodes.

• Gibberellin affects the expression of genes for nutrient utilization during seed germination.

30.6 Abscisic Acid

• Abscisic acid (ABA) mediates germination, inhibits growth, and is part of protective responses to stress caused by living and nonliving factors in the environment

• ABA also has an important role in embryo maturation, stomata closure, seed and pollen germination, and fruit ripening; and it suppresses lateral root formation

ABA Activity

• ABA synthesis begins in chloroplasts – its concentration is highest in leaves and other photosynthetic parts

• ABA receptors occur on the plasma membrane, in cytoplasm, and in the nucleus – ABA activates transcription factors that govern the expression of thousands of genes

• Example: ABA enhances transcription of genes that encode NADPH oxidase – resulting reactions produce H2O2 and NO

Hydrogen Peroxide and Nitric Oxide

hydrogen peroxide H—O=O—H

nitric oxide N≡O

ABA and Germination

• ABA accumulates in a seed as it forms and prevents the seed from germinating too early by inhibiting expression of genes involved in cell wall expansion and gibberellin synthesis

• A seed cannot germinate until its ABA level declines

• Hydrogen peroxide (from NADPH oxidase) enhances expression of gibberellin synthesis genes

Premature Germination Without ABA

Take-Home Message: What are the main effects of abscisic acid in plants?

• Abscisic acid inhibits germination and growth.

• ABA also stimulates metabolism, stress responses, and embryonic development.

30.7 Ethylene

• Ethylene is a gaseous hormone produced in all parts of a plant from methionine and ATP

• Ethylene helps regulate many metabolic and developmental processes, including germination, growth, abscission, fruit ripening, and stress responses

• Expression of some genes is inhibited by ethylene (negative feedback loop); expression of others is enhanced by ethylene (positive feedback loop)

Ethylene and Fruit Ripening

• Ripening of fleshy fruits such as strawberries occurs after a peak of cellular respiration followed by a burst of ethylene produced in a positive feedback loop

• Chloroplasts are converted to chromoplasts; cell walls break down; starch and organic acids are converted to sugars

• Synthetic ethylene is widely used to artificially ripen fruit

Ethylene Production During Strawberry Ripening

fruit is maturepetals drop fruit forms green fruit enlarges fruit ripens

Days After Flower Opening

Eth

ylen

e p

rod

uc

tio

n

Take-Home Message: What are the main functions of ethylene in plants?

• Ethylene produced in negative feedback loops participates in ongoing metabolic and developmental processes.

• Ethylene produced in positive feedback loops is involved in intermittent processes such as abscission, fruit ripening, and defense responses.

30.8 Tropisms

• Tropisms• Plants adjust the direction and rate of growth in response

to environmental stimuli such as gravity, light, contact, and mechanical stress

• Hormones are typically part of this effect

Gravitropism

• Gravitropism• A growth response to gravity which causes roots to grow

downward and shoots to grow upward

• Statoliths• Amyloplasts containing heavy starch grains that sink to the

bottom of the cell• A change in position results in movement of cell’s auxin

efflux carriers

Gravitropism

Figure 30-13a p514

statoliths

A This micrograph shows heavy, starch-packed statoliths settled on the bottom of gravity-sensing cells in a corn root cap.

Figure 30-13b p514

statoliths

B This micrograph was taken ten minutes after the root in A was rotated 90°. The statoliths are already settling to the new “bottom” of the cells.

Phototropism

• Phototropism• Orientation of certain plant parts toward light• Nonphotosynthetic pigments (phototropins) respond to

blue light, initiating signal cascades• Auxin is redistributed to shady side of plant

• Heliotropism• In some plants, leaves or flowers change position in

response to changing angle of the sun through the day

Figure 30-14 p514

A Sunlight strikes onlyone side of a coleoptile.

B Auxin flow is directed towardthe shaded side, so cells onthat side lengthen more.

Movement of Chloroplasts in Response to Light

• On the interior of a cell, chloroplasts are dragged from one position to another on actin filament tracks of the cytoskeleton

• Chloroplasts move away from high-intensity light, which minimizes damage from excess electrons accumulating in electron transfer chains of the light reactions

• Chloroplasts move toward low-intensity light, maximizing exposure to light for photosynthesis

Movement of Chloroplasts in Response to Light

Thigmotropism

• Thigmotropism• Contact with a solid object changes the direction of plant

growth• Involves several gene products and calcium ions• Results in unequal growth rates on opposite sides of the

shoot

• Mechanical stress (such as wind) inhibits stem lengthening in a similar touch response

Take-Home Message: How do plants respond to environmental cues?

• Via hormones, plants adjust the direction and rate of growth in response to gravity, light, contact, mechanical stress, and other environmental stimuli.

30.9 Sensing Recurring Environmental Changes

• Shifts in biological activity that recur in 24-hour cycles are mediated by cyclic shifts in gene expression

• Seasonal shifts in night length trigger seasonal shifts in development in many plants

Circadian Cycles

• A circadian rhythm is a cycle of biological activity that recurs every 24 hours

• Example: A bean plant holds its leaves horizontally during the day but folds them close to its stem at night

• Circadian rhythms are driven by feedback loops involving transcription factors that regulate their own expression

Rhythmic Leaf Movements of a Bean Plant

Midnight1 A.M. 6 A.M. Noon 3 P.M. 10 P.M.

ANIMATED FIGURE: Rhythmic leaf movements

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Adjusting Circadian Clocks

• Different wavelengths of sunlight provide input to circadian clocks by activating and inactivating photoreceptor pigments such as phytochromes and cryptochromes

• Active phytochromes cause gene transcription for components of rubisco, photosystem II, ATP synthase, and other molecules involved in photosynthesis

• Some gene products are produced during the day and degraded at night; others produced at night are degraded during the day

Circadian Cycles of Gene Expression

Seasonal Changes

• Plants respond to seasonal changes in light availability with seasonally appropriate behaviors such as entering or breaking dormancy

• Photoperiodism is an organism’s response to changes in the length of day relative to night

Photoperiodism and Flowering

• Inputs from phytochrome and cryptochrome converge on the CO gene, which encodes a transcription factor

• The transcription factor induces expression of the FT gene (flowering locus T) in companion cells

• During short-day seasons, CO protein never accumulates to a high enough level to promote flowering in long-day plants

• Short-day plants have the same CO gene, but its product inhibits FT gene expression in these plants

Figure 30-18 p517

A A flash of red light interrupting a long night causes plants to respond asif the night were short. Long-day plants flower; short-day plants do not.

Time (hours)

long-dayplant

short-dayplant

long-dayplant

short-dayplant

critical night length

critical night length

Time (hours)

B A flash of far-red light cancels the effect of a red light flash. Short-dayplants flower; long-day plants do not.

0 4 8 12 16 20 24

0 4 8 12 16 20 24

ANIMATED FIGURE: Flowering response experiments

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Vernalization

• Some biennials and perennials flower in the spring only after exposure to cold winter temperatures (vernalization)

• Plants may perceive temperature via their plasma membrane, which varies in lipid composition and calcium ion permeability

• A “cold” signal influences expression of the FT gene, and of the VRN1 gene, which encodes a transcription factor that promotes flowering when warm temperatures return

Take-Home Message: How do plants respond to recurring environmental change?

• Plants respond to recurring cues from the environment with recurring cycles of activity such as rhythmic leaf movements.

• Photoreceptors that detect daylight provide input into circadian cycles.

• The main environmental cue for flowering is the length of night relative to the length of day, which varies by the season in most places.

• In some species, prolonged exposure to low temperature stimulates flowering in spring.

ANIMATION: Phytochrome conversions

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30.10 Responses to Stress

• Living and nonliving stressors in the environment provoke short-term and long-term defense responses in plants

• Defense responses in plants are mediated by hormones

Abiotic Stressors

• Abscisic acid synthesis is triggered by temperature extremes, lack of water, and other abiotic stressors

• Example: ABA is part of a response that causes a plant’s stomata to close when water is scarce

Biotic Stressors

• Cell surface receptors recognize molecules specific to bacterial pathogens, triggering a burst of ethylene synthesis

• When ethylene is present, it binds to its receptors and locks them in an inactive form that marks them for destruction

• Binding of additional bacteria triggers an ABA-mediated nitric oxide burst that immediately closes stomata

• Beneficial bacteria and fungi avoid triggering defense responses by engaging in a complex cross-talk

Hypersensitive Response

• A pathogen that penetrates a plant’s epidermis triggers a large surge of hydrogen peroxide and nitric oxide that causes cells in the infected region to commit suicide

• This “hypersensitive” response can prevent a pathogen from spreading to other parts of the plant, because it often kills the pathogen along with the infected tissue

Hypersensitive Response

Systemic Acquired Resistance

• Pathogen-triggered systemic acquired resistance increases the plant’s resilience to both biotic and abiotic stress

• An infected tissue releases a signal that triggers cells to produce salicylic acid, which increases transcription of hundreds of genes involved in pathogen resistance

• The chemicals produced differ by species, but all confer general hardiness to the plant

Interspecific Plant Defenses

• Wounding of a leaf by insects triggers production of ABA, hydrogen peroxide, ethylene, and jasmonic acid

• Jasmonic acid increases transcription of genes resulting in release of certain volatile chemicals into the air

• These secondary metabolites are detected by wasps that parasitize insect herbivores

• These chemicals are also detected by neighboring plants, which increase production of ethylene and jasmonic acid

Interspecific Plant Defenses

Take-Home Message:

How do plants respond to stress?

• Abscisic acid is involved in responses to nonliving environmental stresses.

• Detection of plant pathogens can trigger stomatal closure or cell death.

• Systemic acquired resistance triggered by pathogen attacks increases a plant’s ability to withstand biotic and abiotic stresses.