Plant Response

Plant reactions

• Stimuli & a Stationary Life– Animals respond to stimuli

by changing behavior• Move toward positive

stimuli• Move away from negative

stimuli

– Plants respond to stimuli by adjusting growth & development

Signal Transduction Pathways in Plants• Plants have cellular receptors that

detect changes in their environment– For a stimulus to elicit a response,

certain cells must have an appropriate receptor

• Signal triggers receptor• Receptor triggers internal cellular

messengers which transfer and amplify signals from receptors to proteins that cause responses

• Cellular response– may involve increased activity of

enzymes by:• Stimulating transcription of mRNA for

an enzyme (Transcriptional regulation) • Activating an existing enzyme (Post-

translational modification)

Transcriptional Regulation

• Specific transcription factors bind directly to specific regions of DNA and control transcription of genes

• Positive transcription factors – increase the transcription of specific genes

• Negative transcription factors – decrease the transcription of specific genes

Post-Translational Modification of Proteins

• Involves modification of existing proteins in the signal response– Ex. phosphorylation of enzymes for activation

Signal Transduction Pathway Example

Both pathways lead to expression of genes for proteins that functions in greening response of plants

Greening Response in Potato Plants

Grown in dark 1 week exposure to light

Discovery of Plant Hormones• In the late 1800s, Charles Darwin and his son Francis conducted

experiments on phototropism, a plant’s response to light– Curvatures of whole plant organs toward or away from stimuli is called

a tropism– Grass seedling could bend toward light only if the tip of the coleoptile

was present– They postulated that a signal was transmitted from the tip to the

elongating region

Discovery of Plant Hormones

• In 1913, Peter Boysen-Jensen demonstrated that the signal was a mobile chemical substance

Discovery of Plant Hormones

• In 1926, Frits Went extracted the chemical messenger for phototropism, auxin, by modifying earlier experiments

Plant hormones

• Chemical signals that coordinate different parts of an organism– Only minute amounts are required– Produced in one part of the body and is

transported to another part– Binds to specific receptor– Triggers response in target cells & tissues

Plant hormones

• Auxins• Cytokinins• Gibberellins• Brassinosteroids• Abscisic acid• ethylene

Table 39-1

Auxin• Indoleacetic acid (IAA)– Stimulates cell elongation– Involved in lateral root

formation and branching– Enhances apical dominance– An overdose of synthetic auxins

can kill eudicots (used as herbicide)

– Classical explanation of phototropism• Asymmetrical distribution of

auxin• Cells on darker side elongate

faster than cells on brighter side

Cytokinins • Produced in actively growing tissues:

roots, fruits & embryos• Effects– Control of cell division &

differentiation– Enhances apical dominance• Terminal bud suppresses

development of axillary buds• If terminal bud is removed,

plants become bushier– Retard aging of some plant organs

by inhibiting protein breakdown and stimulating RNA and protein synthesis

– auxin & cytokinins interact to control cell division and differentiation

Gibberellins• Over 100 different

gibberellins identified• Effects– Stem elongation

• Stimulate cell elongation and cell division

– Fruit growth • Auxin and gibberellins must be

present for fruit to set– Seed germination

• After seed is imbibed, release of gibberellins from embryo signals seeds to germinate

Fig. 39-11

Gibberellins (GA)send signal toaleurone.

Aleurone secretes α-amylase and other enzymes.

Sugars and other nutrients are consumed.

AleuroneEndosperm

Water

Scutellum (cotyledon)

Radicle

12 3

GA

GA

-amylaseSugar

Brassinosteroids

• Similar to sex hormones of animals• Effects – Similar to auxins– Cell elongation & division in shoots & seedlings

Abscisic acid (ABA)• Effects– Slows growth– High concentration of ABA promotes

seed dormancy• Germination occurs only after ABA is inactivated or

leached out • Ensures that seeds will germinate only under optimal

conditions

– Drought tolerance• Rapid stomata closure to allow plant to withstand drought

Ethylene• Ethylene is a gas released by plant cells

in response to stresses such as drought, flooding, injury, infection

• Multiple effects–Response to mechanical stress• Such as a seedling growing around

a stone (an obstacle) –Apoptosis• Ex. shedding leaves in autumn

Ethylene Effects: Fruit Ripening

• Hard, tart fruit protects developing seeds from herbivores

• Ripe, sweet, soft fruit attracts animals to disperse seed– Burst of ethylene triggers ripening process• Breakdown of cell wall = softening• Conversion of starch to sugar = sweetening

– Positive feedback system• Ethylene triggers ripening• Ripening stimulates more ethylene production

Applications• “One bad apple DOES spoil the whole bunch”– Ripening apple releases ethylene to speed ripening of fruit

nearby

• Ripen green bananas by bagging them with an apple• Climate control storage of apples– Air is circulated to prevent ethylene buildup– Stored in high amounts of CO2 which inhibits the release of

ethylene

Responses to light • Photomorphogenesis– Effect of light on plant growth

• Plants can detect – Presence of light– Intensity of light– Direction of light– Wavelength (color)

• Blue-light receptors• Phytochromes (red-light receptors)• An action spectrum depicts responses of a plant process

to different wavelengths

Blue-Light Photoreceptors• Various blue-light photoreceptors control hypocotyl

elongation, stomatal opening, and phototropism

Phytochromes as Photoreceptors

• Phytochromes are pigments that regulate many of a plant’s responses to light throughout its life

• These responses include seed germination and shade avoidance

• Photoreceptor activity– In each subunit, one domain, which

functions as a photoreceptor, is covalently bonded to a nonprotein pigment, chromophore

• Kinase activity– The other domain has protein kinase

activity. The two domains interact linking light reception to cellular responses triggered by the kinase

Phytochromes as Photoreceptors

• The chromophore of a phytochrome is photoreversible, reverting back and forth between two isomeric forms, depending on the color of light– Pr absorbs red (r) light maximally

– Pfr absorbs far-red (fr) light

• The conversion triggers many developmental responses such as germination

Phytochrome photoreceptors• Molecular switch reaction to red light – Conversion of Pr Pfr in sunlight stimulates germination,

flowering, branching…– Conversion of Pfr Pr in dark inhibits response &

stimulates other responses: growth in height• “Shade avoidance” response

Circadian Rhythms• Internal 24-hour cycles

Morning Glory

The Effect of Light on the Biological Clock

• The clock may depend on synthesis of a protein regulated through feedback control

• Phytochrome conversion marks sunrise and sunset, providing the biological clock with environmental cues

Photoperiodism and Responses to Seasons

• Photoperiod, the relative lengths of night and day, is the environmental stimulus plants use most often to detect the time of year

• Photoperiodism is a physiological response to photoperiod

Photoperiodism and Control of Flowering• Flowering in many species require a certain

photoperiod• Plants that flower when a light period is shorter

than a critical length are called short-day plants– Chrysanthemums, soybeans

• Plants that flower when a light period is longer than a certain number of hours are called long-day plants– Spinach, lettuce, iris

• Flowering in day-neutral plants is controlled by plant maturity, not photoperiod– Tomatoes, rice, dandelions

Flowering Response• Controlled by night length

– “critical period”– Short-day plants (long-

night) flower when night exceeds a minimum number of hours of darkness

– Long-day plants (short-night) flower only if the night is shorter than a critical dark period

– Flash of light can interrupt the nighttime portion of the photoperiod

• Red light can interrupt the nighttime portion of the photoperiod

• Action spectra and photoreversibility experiments show that phytochrome is the pigment that detects red light– If a flash of R light during

dark period is followed by a flash of FR light, the plant detects no interruption of the night length

Flowering Response

• Some plants flower after only a single exposure to the required photoperiod

• Other plants need several successive days of the required photoperiod

• Still others need an environmental stimulus in addition to the required photoperiod– For example, vernalization is a pretreatment with

cold to induce flowering

Flowering Response

Is there a flowering hormone?

• A flowering signal, not yet chemically identified is called florigen

Responses to stimuli: gravity• How does a sprouting

shoot “know” to grow towards the surface from underground?– Environmental cues

• Roots = positive gravitropism

• Shoots = negative gravitropism

• Settling of statoliths (dense starch grains in plastids) may detect gravity

Responses to stimuli: touch• Thigmotropism – Growth in response to

touch– Caused by changes in

osmotic pressure = rapid loss of K+ = rapid loss of H2O = loss of turgor in cells

• Example – Mimosa closes leaves in

response to touch

Responses to Stimuli: Touch

• Thigmomorphogenesis– Changes in form resulting

from mechanical disturbance– Ex. Rubbing stems of young

plants a couple of times daily results in plants that are shorter than controls

Responses to Stimuli• Environmental Stresses– have a potentially adverse effect on survival, growth, and

reproduction– Stresses can be abiotic or biotic

• Abiotic stresses include drought, flooding, salt stress, heat stress, and cold stress

• Drought– During drought, plants reduce transpiration by closing stomata,

slowing leaf growth, and reducing exposed surface area– Growth of shallow roots is inhibited, while deeper roots continue to

grow• Flooding

– Enzymatic destruction of root cortex cells creates air tubes that help plants survive oxygen deprivation during flooding

Responses to Stimuli• Salt Stress

– Salt can lower the water potential of the soil solution and reduce water uptake

– Plants respond to salt stress by producing solutes tolerated at high concentrations• water potential of cells becomes more negative than that of the soil

solution

• Heat Stress– Excessive heat can denature a plant’s enzymes

• Heat-shock proteins help protect other proteins from heat stress

• Cold Stress– Cold temperatures decrease membrane fluidity

• Plants can alter lipid composition of membranes

– Freezing causes ice to form in a plant’s cell walls and intercellular spaces

Plant defenses• Defenses against

herbivores– Thorns– Chemicals – Recruitment of predatory

animal to defend against specific herbivores

– Volatile chemicals to warn other plants of same species• Methyljasmonic acid can

activate expression of genes involved in plant defenses

Plant Defenses

• Defenses against pathogens– First line of defense• Epidermis and periderm

– Second line of defense• Chemical attack that kills pathogen• Enhanced by ability to recognize pathogens

• involves recognition of pathogen-derived molecules by protein products of specific plant disease resistance (R) genes– An R protein recognizes a

corresponding molecule made by the pathogen’s Avr (avirulence) gene

– R proteins activate plant defenses by triggering signal transduction pathways• hypersensitive response and

systemic acquired resistance

Gene-for-gene Recognition

Signal Transduction Pathways

• The hypersensitive response– Causes cell and tissue death near the infection site– Induces production of phytoalexins and PR (pathogenesis-

related) proteins which attack the pathogen– Stimulates changes in the cell wall that confine the pathogen

• Systemic acquired resistance causes systemic expression of defense genes and is a long-lasting response– Salicylic acid is synthesized around the infection site and is likely

the signal that triggers systemic acquired resistance

Plant defenses: Signal Transduction Pathways 1. Specific resistance is based on

pathogen-receptor binding2. Signal transduction pathway is

triggered3. Antimicrobial molecules that seal

off infected areas are produced4. Infected cells release

methylsalicylic acid before they die

5. The signaling molecule is distributed to rest of the plant

6. In cells away from infection site, methylsalicylic acid is converted to salicylic acid which initiates a signal transduction pathway

7. Molecules that help protect the cell against a diversity of pathogens for several days are produced