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

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

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

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

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.

Figure 39.4 Early experiments of phototropism

Figure 39.5 The Went experiments

Table 39.1 An Overview of Plant Hormones

Auxins

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.

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.

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

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

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

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.

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.

Bolting “Foolish Seed”

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.

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.

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

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

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