Plant Responses

Chapter 39

Signal Transduction

Plants have cellular receptors that detect important changes in their environment

Etiolation Greening Response

Grown in Dark 1 week exposure to Light

Signal Transduction Pathway Model signal triggers receptor receptor triggers internal cellular

messengers & then cellular response

receptor signal pathway(2° messengers)

response

What kinds of molecules are the receptors?

Signal Transduction Pathway

Plant Hormones

Chemical signals that coordinate different parts of an organismonly minute amounts are required produced by 1 part of bodytransported to another partbinds to specific receptortriggers response in target cells &

tissues

Plant Hormones

auxins cytokinins gibberellins brassinosteroids abscisic acid ethylene

The Discovery of Plant Hormones

Tropisms : Phototropism

Signal is a a mobile chemical

The Went Experiments

Went named the chemical “auxin”

Later purified as indolacetic acid(IAA)

How Auxins Cause Cell Elongation

Increase in H+ activates expansins Weakened cell wall allows more water to

enter

H+ pump

Auxins

Herbicides2-4-D Affects secondary growth Synthetic auxins

sprayed on green house plants induce fruit development

Cytokinins

Stimulate Cytokinesis Are produced in actively growing

tissues

Gibberellins Stimulate cell elongation and division Bolting Fruit Growth Germination Effects

stem elongation fruit growth seed germination

Figure 39.11 The effect of gibberellin treatment on seedless grapes

Abscisic Acid (ABA)

Slows growth Maintains seed dormancy Drought tolerance

Ethylene A magic gas?

Response to stress Leaf abscission Fruit ripening Slowing,

thickening, curving

Ethylene induces the triple response in pea seedlingsSlowing, thickening,curvingETHYLENE

A magic gas?

Response to stress

Leaf abscission Fruit ripening

Abscission of a maple leaf

Apoptosis Caused by

ethylene/auxin shift

Responses to 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) may detect gravity

Molecular Switch Mechanism of Phytochromes

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

Light induced

Phytochrome

ChromophorePhotorecptor

Kinase activity

Response:Vertical growth

Phytochrome

Circadian Rhythms Internal (endogenous) 24-hour cycles

Morning glory

4 O’clock

Noon Midnight

Figure 39.21 Sleep movements of a bean plant

Figure 39.x1 Biological clocks

Figure 39.22 Photoperiodic control of flowering

Figure 39.23 Reversible effects of red and far-red light on photoperiodic response

Is there a flowering hormone? Plant on left is

induced to flower & then grafted onto plant on rightplant on right is

triggered to flower

What can you conclude?

Thigmomorphogenesis Change in form

resulting from mechanical perturbation

Altering gene expression by touch in Arabidopsis

Figure 39.27 Rapid turgor movements by the sensitive plant (Mimosa pudica)

Plant Defenses Defense against herbivores Defense against pathogens Systematic acquired resistance

Figure 39.29 A corn leaf recruits a parasitoid wasp as a defensive response to an herbivore, an army-worm caterpillar

Figure 39.31 Defense responses against an avirulent pathogen