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Plant Hormones

Plant Hormones. What kinds of things do plants have to respond to?. Light - phototropism Touch - thigmotropism Gravity – gravitropism Turgor movements Biological clock - circadian rhythms When to open and close stomata Control of flowering - photoperiodism.

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Plant Hormones

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  1. Plant Hormones

  2. What kinds of things do plants have to respond to? • Light - phototropism • Touch - thigmotropism • Gravity – gravitropism • Turgor movements • Biological clock - circadian rhythms • When to open and close stomata • Control of flowering - photoperiodism

  3. Figure 39.2 Review of a general model for signal-transduction pathways

  4. Figure 39.0 A grass seedling growing toward a candle’s light

  5. Figure 39.1 Light-induced greening of dark-sprouted potatoes: a dark-grown potato (left), after a week's exposure to natural sunlight (right)

  6. The Search for the Plant Hormone • Darwin and son his discovered that grass seedlings would not bend towards the light if the tip was removed and covered with an opaque cap. • Boysen and Jensen demonstrated that the signal was a mobile substance. • They placed a gelatin block between the tip and the rest of the plant and demonstrated that the signal diffused through the gelatin. • Went(1926) extracted the chemical messenger and impregnated agar blocks. • The agar blocks were places on various parts of the plant and the plant bent in theopposite direction from which the agar block was placed. • Showed that cells opposite of the elongated causing the stem to bend. • The substance was called auxin and produced growth on the opposite side in which it was concentrated.

  7. Figure 39.4 Early experiments of phototropism

  8. Figure 39.5 The Went experiments

  9. Table 39.1 An Overview of Plant Hormones

  10. Auxins

  11. Plant Hormones Help Coordinate Growth, Development and to Stimuli • Five classes of Hormones: • Auxins • Cytokinins • Gibberillins • Abscisic acid • Ethylene • Hormones are effective in very small concentrations and are the signal is amplified • Can affect gene expression and activity of enzymes.

  12. Auxins • Natural form is called indoleacetic Acid(IAA) • Found mostly in the apical meristem. • Auxin only travels in one direction. • Acid Growth Hypothesis: • Stimulates proton pumps in the region of growth. • Acidic pH breaks down cell walls • Turgor pressure causes cell to elongate • Function: • Stimulates cell division • Differentiation of secondary xylem growth • In developing seeds promotes fruit growth.

  13. Figure 39.6 Polar auxin transport: a chemiosmotic model (Layer 1)

  14. Figure 39.6 Polar auxin transport: a chemiosmotic model (Layer 3)

  15. Figure 39.6 Polar auxin transport: a chemiosmotic model (Layer 2)

  16. Cytokinins • Together with auxin orchestrate root and shoot growth. • Stimulate cell division and differentiation. • Relative amounts of cytokinin and auxin will determine what will occur. • In equal amounts no differentiation occurs. • More cytokinin than auxin will result in root buds. • More auxin than cytokinin will result in shoot buds.

  17. Gibberellins • Causes “bolting” rapid growth of a flower stalk. • Involved in breaking dormancy of apical buds in the spring. • Stimulate growth in both leaves and stems. • In some plants both auxin and gibberellins are cintribute to fruit set.

  18. Bolting “Foolish Seed”

  19. Abscisic Acid Produced in the terminal bud slows growth and inhibits cell division. Primordial leaves develop into scales and protect the apical bud through the winter. Keeps seeds dormant. Can help plants cope with harsh conditions by closing their stomata.

  20. Ethylene • A gas that promotes fruit ripening. • Contributes to aging or “senescence” of parts of the plant. • Promotes degradation of cell walls and decreases chlorophyll content that is associated with fruit ripening. • Involved in leaf abscision.

  21. Tropisms Orient Plants Toward or Away From Stimuli • Phototropism • Cells on the darker sides of the stem elongate faster in response to auxin moving down from the shoot. • Photoreceptor is believed to be a blue light receptor. • Gravitropism • Roots curve downwards. • Two theories • Staholith settling • Protoplast signaling

  22. Thigmotrpism • Climbing plants respond to touch and grow tendrils to grasp onto surfaces. • Stunting height growth in windy environments • Rapid Leaf Movements • Rapid loss of turgor pressure in response to touch. • Specialized motor organs called pulvini in the joints of leaves. • Lose potassium when stimulated causing water loss. • Signals are transmitted either through chemical or electrical impulses called action potentials • Sleep Movements • Daily raising or lowering of leaves

  23. Circadian Rhythms • Internal 24 hour cycle in which most organism keep track of the time of day. • Photoperiodism • Physiological response to day length. • Control of flowering • Short day plants - flower during periods of short day length. • Long day plants - flower during periods of long day length. • Day neutral plants – unaffected by day length. • Researchers have found that night l;ength affects plant flowering. • Leaves detect day length

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