Chapter 39

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

13. 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)

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

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 Problem – loss of turgor, dehydration • Drought • Flooding • Salt • Heat • Cold Problem – O2 deprivation Problem – hypertonic environment – roots lose turgur pressure Problem - denaturing proteins Problem – lose fluidity of membranes