Plant Responses to Internal and External Signals Control systems…

• Keep track of

the time of day

• Notice seasons • Sense gravity • Sense direction

of sunlight

Signal Transduction Pathways Link Signal Reception to Response Reception

• Cell signals are detected by

receptors that undergo changes in shape as a response to a specific stimulus Transduction

• Multistep pathway that amplifies

the response using relay proteins and second messengers Response

• Activation of cellular responses – Activating existing enzyme molecules – Increasing or decreasing mRNA

production

Signal Transduction Pathways Link Signal Reception to Response

Etiolation

• Morphological

adaptations for growing in darkness De-Etiolation

• Greening • Profound changes

that occur due to exposure to light

Plant Hormones Help Coordinate Growth, Development, and Responses to Stimuli Hormone

• compound produced by one part of an

organism that is transported and triggers a response in another part

• Animals transport in circulatory system • Plants are different b/c… – No circulatory system – Only act locally – Present in high concentrations – Referred to as plant growth regulators

Discovery of Plant Hormones Tropism

• any growth response

that results in a plant’s organs curving toward or away from a stimulus

• Phototropism

What part of a grass coleoptile senses light, and how is the signal transmitted?

• Only the tip senses

light

Discovery of Plant Hormones Does asymmetrical distribution of a growth promoting chemical cause a coleoptile to grown toward the light?

• A coleoptile grows

toward light because its dark side has a higher concentration of auxin (growth promoting hormone)

Survey of Plant Hormones

Survey of Plant Hormones

Apical dominance: with apical bud (left), apical bud removed (right) Controlled by cytokinins, auxin, and strigolactones

Effects of Gibberellins

Effects of Abscisic Acid

Effects of Brassinosteroids [Photo: The level of brassinosteroids regulates both the size and senescence of tobacco. With low levels, tobacco is dwarfed (some as small as 10 inches tall; see plant in front) and the leaves do not senesce, while at normal levels of brassinosteroids, tobacco stands almost 6 feet tall and the leaves turn yellow as they age (plant in back).

Effects of Strigolactones

Figure 1: Rice plants showing the impact of strigolactone on branching—normal plant (left); mutant plant which produces low levels of strigolactone (center); mutant plant treated with strigolactone (right).

Ethylene Induced Triple Response

Abscission caused by a change in the ratio of ethylene to auxin

Responses to Light are Critical for Plant Success

Photomorphogenesis

• Effects of light on plant morphology

Plants detect…

• Presence of light • Direction of light • Intensity of light • Wavelength of light

Two major classes of light receptors

• Blue light photoreceptors • phytochromes

Blue-Light Photoreceptors

Blue light triggers…

• Phototropism • Opening of stomata • Slowing of hypocotyl elongation

Phytochromes

Protein containing a chromophore responsible for a plant’s response to the photoperiod

• Alternate between 2 forms (P

r and P fr )

• P

r

• P

fr

• P

fr only converts to P fr in presence of light triggers many plant responses degrades back to P r at night

Biological Clocks & Circadian Rhythms

Circadian rhythms

• Cycle of about 24 hours • Does not require environmental cues • Internally set • Daily signals from the environment can set

the circadian clock to about 24 hrs.

Photoperiodism and Response to the Seasons Physiological response to the photoperiod Critical night length

• day length does not trigger flowering • If daytime is interrupted, no effect on flowering • If night period is interrupted by a short period

of light, plants do not flower

– Could be effected by a single exposure or may

require several exposures Vernalization

• Use of pretreatment with cold to induce

flowering

Photoperiodic control of flowering

Reversible effects of red and far-red light on photoperiodic response

Photoperiodism

Short day plants

• night is longer than a critical length • Flower in late summer, fall, winter • Mums, poinsettias, soybeans

Long day plants

• night is shorter than a critical length • Flower in late spring, early summer • Radishes, lettuce, irises

Day Neutral plants

• unaffected by photoperiod • triggered by maturity • Tomatoes, rice, dandelions

Evidence for a flowering hormone(s)

Flowering locus T (FT) gene\

•

activated in leaf cells and the FT protein travels through the symplastic route to the apical meristems and induces flowering

Tropisms

• Growth responses that result in

curvatures of the whole plant toward/away from a stimulus Phototropism

• stimulus is light

Tropisms

Gravitropism

•

stimulus is gravity detected by statholiths

•

Dense cytoplasmic components that settle at the bottom of the cell due to gravity

•

Positive – downward (roots)

•

Negative – upward (shoots)

Tropisms

Thigmomorphogenesis

• Refers to changes in form that result from

mechanical perturbation Thigmotropism

• stimulus is touch

Turgor Movements

Turgor Movements

• Reversible movements

caused by changes in turgor pressure Rapid Leaf movements

• reduce water loss or

protect from herbivores

• touch causes leaf to

collapse (causes a rapid loss of turgor pressure by cells causing them to become flaccid)

• motor cells lose K

Mimosa clip +

• 10 minute restoration

Turgor Movements

Sleep movements

• lowering of leaves to

vertical position in evening

• raising leaves to a

horizontal position in the morning

• one side of plant is

turgid while the other is flaccid

– Daily changes in

turgor pressure

Environmental Stress

• An environmental condition that can have an

adverse effect on a plant’s growth, reproduction, & survival Abiotic

• Water deficit, flooding, salt stress, heat,

cold Biotic

• Herbivores, pathogens

Drought

• Control systems in both leaves and roots

Leaves/Shoots

• Guard cells lose turgor and close • Mesophyll releases abscisic acid • Young leaf growth is inhibited • Wilting reduces surface area

Roots

• Shallow root growth inhibited; deeper

roots continue to grow

Flooding (Oxygen Deprivation)

• Waterlogged soil

lacks air spaces to hold oxygen

• May form air tubes

from roots to the surface

• Submerged roots

may be continuous with aerial roots

Salt Stress

• Lowers water potential of soil causing a water

deficit even if enough water is present

• Produce compatible solutes in response to

moderately saline soils

– Keeps water potential of cells more negative

than the soil solution w/o admitting toxic quantities of salt

Heat Stress

• Transpiration reduces temperature and

keeps enzymes from denaturing

• Produce shock proteins – a back up plan to transpiration

Cold Stress

• Subfreezing temps cause ice crystals to

form in protoplast

– death • Lipids become locked and causes a loss

of fluidity in membranes

• Alter lipid composition by increasing

saturated fatty acids

Herbivores

Chemical

• Distasteful • Toxic

Structural

• Thorns, etc.

Recruit predatory animals

• Plants attract wasps that

lay eggs in caterpillars Canavanine

• Replaces arginine • Proteins cannot be

made…insect dies

Defense Against Pathogens

Virulent pathogens

• Plants have virtually no defense

Avirulent pathogens

• Mildly harm but do not kill

Gene-for-gene recognition

• Resistance to a disease depends on a

precise match up between an allele in a plant and an allele in the pathogen

Defense Against Pathogens

Hypersensitive response

• chemical signaling system to resist

infection

• Phytoalexins – Compounds with fungicidal and bactericidal

properties Systemic acquired resistance

• protects unaffected tissues from a

pathogen spreading

Defense responses against an avirulent pathogen