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 the opposite 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 Rapid Leaf Movements Sleep Movements
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 Photoperiodism
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