Plants

1. Plants

2. Basic Structures

3. Apical meristems enable the plant to grow in length. -located in tips of roots and in the buds.

4. Plants, being rooted to the groundMust respond to whatever environmental change comes their way

5. Plants have cellular receptors • That they use to detect important changes in their environment • For a stimulus to elicit a response • Certain cells must have an appropriate receptor

6. CYTOPLASM CELL WALL 3 Response   1 Reception 2 Transduction Activation of cellular responses Relay molecules Receptor Hormone or environmental stimulus Plasma membrane Figure 39.3 • The potato’s response to light • Is an example of cell-signal processing

7. Plant hormones help coordinate growth, development, and responses to stimuli • Hormones • Are chemical signals that coordinate the different parts of an organism

8. The Discovery of Plant Hormones • Any growth response • That results in curvatures of whole plant organs toward or away from a stimulus is called a tropism • Is often caused by hormones

9. EXPERIMENT In 1880, Charles Darwin and his son Francis designed an experiment to determine what part of the coleoptile senses light. In 1913, Peter Boysen-Jensen conducted an experiment to determine how the signal for phototropism is transmitted. Boysen-Jensen (1913) Control Darwin and Darwin (1880) Shaded side of coleoptile Light RESULTS Light Light Base covered by opaqueshield Tip separated by gelatinblock Tip separated by mica Illuminated side of coleoptile Tip removed Tip covered by opaque cap Tip covered by trans-parentcap CONCLUSION In the Darwins’ experiment, a phototropic response occurred only when light could reach the tip of coleoptile. Therefore, they concluded that only the tip senses light. Boysen-Jensen observed that a phototropic response occurred if the tip was separated by a permeable barrier (gelatin)but not if separated by an impermeable solid barrier (a mineral called mica). These results suggested that the signal is a light-activated mobile chemical. • Charles Darwin and his son Francis • Conducted some of the earliest experiments on phototropism, a plant’s response to light, in the late 19th century

10. EXPERIMENT In 1926, Frits Went’s experiment identified how a growth-promoting chemical causes a coleoptile to grow toward light. He placed coleoptiles in the dark and removed their tips, putting some tips on agar blocks that he predicted would absorb the chemical. On a control coleoptile, he placed a block that lacked the chemical. On others,he placed blocks containing the chemical, either centered on top of the coleoptile to distribute the chemical evenly or offset to increase the concentration on one side. RESULTS The coleoptile grew straight if the chemical was distributed evenly. If the chemical was distributed unevenly, the coleoptile curved away from the side with the block, as if growing toward light, even though it was grown in the dark. Excised tip placed on agar block Growth-promotingchemical diffusesinto agar block Agar blockwith chemicalstimulates growth Control(agar blocklackingchemical)has noeffect Offset blockscause curvature Control CONCLUSION Went concluded that a coleoptile curved toward light because its dark side had a higher concentration of the growth-promoting chemical, which he named auxin. • In 1926, Frits Went • Extracted the chemical messenger for phototropism, auxin, by removing the coleoptile tip & placed it on a block of agar. This will allow the chemical to travel through.

11. A Survey of Plant Hormones

12. In general, hormones control plant growth and development • By affecting the division, elongation, and differentiation of cells • Plant hormones are produced in very low concentrations • But a minute amount can have a profound effect on the growth and development of a plant organ

13. Auxin • Is used for any chemical substance that promotes cell elongation in different target tissues • Auxin transporters • Move the hormone out of the basal end of one cell, and into the apical end of neighboring cells • Auxin • Is involved in the formation and branching of roots

14. 3 Wedge-shaped expansins, activated by low pH, separate cellulose microfibrils from cross-linking polysaccharides. The exposed cross-linking polysaccharides are now more accessible to cell wall enzymes. Cell wallenzymes Expansin Cross-linkingcell wallpolysaccharides 4 The enzymatic cleavingof the cross-linking polysaccharides allowsthe microfibrils to slide.The extensibility of thecell wall is increased. Turgorcauses the cell to expand. CELL WALL Microfibril H2O Cell wall Plasma membrane H+ H+ 2 The cell wallbecomes moreacidic. H+ H+ H+ H+ H+ H+ 1 Auxinincreases theactivity ofproton pumps. Cytoplasm Nucleus Vacuole ATP Plasma membrane H+ 5 With the cellulose loosened, the cell can elongate. Cytoplasm • Cell elongation in response to auxin Figure 39.8

15. Other Effects of Auxin • Auxin affects secondary growth • By inducing cell division in the vascular cambium and influencing differentiation of secondary xylem • Developing seeds synthesize auxin • tomatoes grown in greenhouse conditions sprayed with auxin induce fruit development without a need for pollination • This allows for seedless tomatoes

16. Cytokinins • Cytokinins • Stimulate cell division • Are produced in actively growing tissues such as roots, embryos, and fruits • Work together with auxin

17. Axillary buds Control of Apical Dominance • Cytokinins, auxin, and other factors interact in the control of apical dominance • The ability of a terminal bud to suppress development of axillary buds Figure 39.9a

18. “Stump” afterremoval ofapical bud Lateral branches • If the terminal (apical) bud is removed • Plants become bushier Figure 39.9b

19. Anti-Aging Effects • Cytokinins retard the aging of some plant organs • By inhibiting protein breakdown, stimulating RNA and protein synthesis, and mobilizing nutrients from surrounding tissues

20. Gibberellins • Gibberellins have a variety of effects • Such as stem elongation, fruit growth, and seed germination • Gibberellins stimulate growth of both leaves and stems • In stems • Gibberellins stimulate cell elongation and cell division The release of gibberellins from the embryo of a seed signals the seed to break dormancy and germinate.

21. Fruit Growth • In many plants • Both auxin and gibberellins must be present for fruit to set • Gibberellins are used commercially • In the spraying of Thompson seedless grapes making them grow larger.

22. Abscisic Acid • Two of the many effects of abscisic acid (ABA) are • Seed dormancy • Seed dormancy has great survival value • Because it ensures that the seed will germinate only when there are optimal conditions • Prepares it for winter • Drought tolerance • ABA is the primary internal signal • That enables plants to withstand drought

23. Ethylene • Plants produce ethylene • In response to stresses such as drought, flooding, mechanical pressure, injury, and infection

24. Germinating pea seedlings were placed in the dark and exposed to varying ethylene concentrations. Their growthwas compared with a control seedling not treated with ethylene. EXPERIMENT All the treated seedlings exhibited the tripleresponse. Response was greater with increased concentration. RESULTS 0.10 0.00 0.40 0.20 0.80 Ethylene concentration (parts per million) Ethylene induces the triple response in pea seedlings,with increased ethylene concentration causing increased response. CONCLUSION The Triple Response to Mechanical Stress • Ethylene induces the triple response • Which allows a growing shoot to avoid obstacles 1. Slowing of stem elongation 2. Thickening of the stem 3. Curvature causing stem to grow horizontally. Figure 39.13

25. Apoptosis: Programmed Cell Death • A burst of ethylene • Is associated with the programmed destruction of cells, organs, or whole plants Fruit Ripening • A burst of ethylene production in the fruit • Triggers the ripening process

26. 0.5 mm Abscission layer Protective layer Stem Petiole Leaf Abscission • A change in the balance of auxin and ethylene controls leaf abscission • The process that occurs in autumn when a leaf falls Figure 39.16