1. Chapter 39
Plant Responses

2. Signal transduction pathway
1. reception – signal molecule lands on receptor
Transduction – relay molecules called second messengers
Response – activation of cellular response

3. 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

4. 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

5. Types of Hormones Auxin (IAA) Cytokinins Gibberellins Brassinosteroids
Abscisic acid
Ethylene

6. 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

7. 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

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

9. 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

10. Conclusions
For monocots, auxin causes phototropism by accumulating on the dark side of the shoot and causing cell elongation
Not the case for dicots

11. 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

12. 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)

13. Apical Dominance Interplay between auxin & cytokinin How do these two
hormones produce the
Christmas tree look?

14. 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

15. Abscisic Acid Slows growth
Promotes seed dormancy – inhibit germination & help w/ dormancy
Drought tolerance
Causes stomata to close
Warns leaves of H2O shortage

16. 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

17. Brassinosteroids
Produced in seeds, fruit, shoots, leaves, and floral buds
Inhibit root growth; slow leaf abscission; promote xylem differentiation

18. 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

19. 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
Florigen –flowering hormone

20. 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

21. Stresses on plants Drought Flooding Salt Heat Cold
Problem – loss of turgor, dehydration
Problem – O2 deprivation
Problem – hypertonic environment – roots lose turgor pressure
Problem - denaturing proteins
Problem – lose fluidity of membranes

22. Plants defend themselves against herbivores and pathogens
Herbivores – animals Pathogens – bacteria and fungi.
Physical Defenses (Thorns, chemicals)
Example – jasmonic acid and spider mites (attracts predators)
Symbiotic Responses (other insects/animals)

23. Pathogens (Fungus, bacteria, etc)
R-genes (resistance genes) counteract avr genes (avirulant)
Elicitors –braod plant response
Oligosaccarins release with cell damage a stimulate phytoalexins (antimicrobial)
PR proteins – pathogenesis related proteins also antimicrobial attacking bacterial cell walls.

24. stimulating production of phytoalexins and PR proteins.
HR hypersensitive response - enhanced production of phytoalexins and PR proteins, and the “sealing” response that contains the infection is more effective (local).
Systemic Acquired Resistance- nonspecific, providing protection against a diversity of pathogens for days (whole plant).
stimulating production of phytoalexins and PR proteins.
one of the hormones responsible for activating SAR is salicylic acid.
A modified form of this compound, acetylsalicylic acid, is the active ingredient in aspirin.
chewing the bark of a willow tree