Chapter 39Plant Responses

Signal transduction pathway

1. reception – signal molecule lands on receptor

2. Transduction – relay molecules called second messengers

3. Response – activation of cellular response

Etiolation – morphological adaptations for growing in darkness› Energy is spent elongating stems

De-etiolation “greening” – shoot reaches sunlight – phytochrome (photoreceptor)› Elongation slows, leaves expand, roots

elongate, shoot produces chlorophyll

Hormones – chemical signals that coordinate the different parts of an organism› In plants, response is governed by

interaction of two or more hormones› First plant hormone discovered was Auxin

(IAA)› Cells have receptors for hormones which

then activate processes in cell such as: Transcription Activate enzymes Membrane transport

Types of HormonesAuxin (IAA)CytokininsGibberellinsBrassinosteroidsAbscisic acidEthylene

Tropism – any growth response that results in curvatures of whole plant organs toward or away from stimuli› Phototropism

Positive phototropism – growth toward light Negative phototropism – growth away from

light

Charles and Francis Darwin’s experiment on oat coleoptiles

Used opaque covers on tip and collar at bottom to test phototropism – light hitting tip caused phototropism

Boysen & Jensen experiments

Cut coleoptile & put it back on with gelatin or mica separating it – a substance moved through the gelatin that caused bending

Frits Went experiment

Extracted auxin from the gelatin In the dark, placed gelatin with auxin

off center and the coleoptile bent away from the side with the gelatin

Conclusions

For monocots, auxin causes phototropism by accumulating on the dark side of the shoot and causing cell elongation

Not the case for dicots

Auxin Produced in shoot tips Causes cell elongation in low conc but

inhibits elongation in higher conc. Stimulates lateral and adventitious root

formation Synthetic auxin in high doses kills

dicots Developing seeds give off auxin which

promotes fruit growth Interplay with cytokinin causes apical

dominance

Apical Dominance

Interplay between auxin & cytokinin How do these two hormones produce theChristmas tree look?

Cytokinin Produced in embryos, roots, fruits –

moves from root upward in xylem Stimulates cell division in conjunction

with auxin Balance b/w 2 causes differentiation

› Apical dominance Cytokinin stimulates lateral buds & growth Auxin inhibits lateral buds & growth

Antiaging effect – cut pieces of leaves stay green by inhibiting senescence (aging)

Gibberellins

Stem elongation Fruit gets larger

› Ex - Thompson seedless grapes – grapes get larger (with auxin) and internodes elongate to make room for bigger grapes

Help seeds break dormancy (stimulate amylase release) & mobilize food

Contributes to flower bolting

Abscisic Acid

Slows growth Promotes seed dormancy – inhibit

germination & help w/ dormancy Drought tolerance

› Causes stomata to close› Warns leaves of H2O shortage

Ethylene Produced in response to stress & high

levels of auxin Triple response to mechanic stress –

allows growing shoot to avoid obstacle Programmed cell death (apoptosis)

› Occurs in xylem vessels, cells in flowers after pollination, leaves in autumn

Leaf abscission Fruit ripening

Brassinosteroids

Produced in seeds, fruit, shoots, leaves, and floral buds

Inhibit root growth; slow leaf abscission; promote xylem differentiation

Photomorphogenesis

The effect of light on plant morphology Light also allows plants to measure the

passage of days and seasons Photoreceptors

› Blue light receptors control: phototropism, stomatal opening,

› Red light receptors (phytochrome) controls: seed germination in lettuce, stimulates branching & inhibits vertical growth, sets circadian rhythms to 24 hrs

Photoperiodism Flowering

› Short day plants require a short enough day and long enough night Tobacco, mums, poinsettias, some soybeans

– late summer or fall› Long day plants require a long enough day

and short enough night Spinach, radish, lettuce, iris, cereal grains –

late spring or early summer› Day neutral flower when they are old

enough Tomatoes, dandelions, rice

› Some plants must have cold treatment before photoperiod will induce it to flower

Other Tropisms Gravitropism – response to gravity

› Roots display positive gravitropism› Shoots exhibit negative gravitropism› Auxin plays rolls in gravitropism› Statoliths – specialized starch plastids

settle to lower portions of cells Thigmotropism – directional growth in

response to touch› Action potentials cause response› Ex – Mimosa pudica and Venus fly trap› Mimosa pudica video › Venus fly trap video

Stresses on plants

Drought

Flooding

Salt

Heat

Cold Problem – lose fluidity of membranes

Problem – loss of turgor, dehydration

Problem – O2 deprivation

Problem – hypertonic environment – roots lose turgur pressureProblem - denaturing proteins