Chap 39 Control Systems in Plants. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Table 39.1.

Chap 39

Control Systems in Plants

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Table 39.1

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Table 39.1, continued

• Plant hormones are produced at very low concentrations.– Signal transduction pathways amplify the hormonal

signal many fold and connect it to a cell’s specific responses.

– These include altering the expression of genes, by affecting the activity of existing enzymes, or changing the properties of membranes.

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Elongation of cells along shaded side causes

bending towards the light

Went Experiments

Went Experiments

Auxin causeselongation of

cells

Cryptochromes or light receptors in the tip of the plant may cause it to pump the auxin to the opposite side

Cryptochromes or light receptors in the tip of the plant may cause it to pump the auxin to the opposite side

Charged Auxin can only exit at basal end of cell

Auxin when neutral, can diffuse through top of cell

Unidirectional downwards transport of auxinUnidirectional downwards transport of auxin

• According to the acid growth hypothesis, in a shoot’s region of elongation, auxin stimulates plasma membrane proton pumps, increasing the voltage across the membrane and lowering the pH in the cell wall.– Lowering the pH activates expansin enzymes that

break the cross-links between cellulose microfibrils. – Increasing the voltage enhances ion uptake into

the cell, which causes the osmotic uptake of water– Uptake of water with looser walls elongates the cell.

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Expansins

• Synthetic auxins, such as 2,4-dinitrophenol (2,4-D), are widely used as selective herbicides.– Monocots, such as maize or turfgrass, can rapidly

inactivate these synthetic auxins.– However, dicots cannot, and die from a hormonal

overdose.• Spraying cereal fields or turf with 2,4-D eliminates dicot

(broadleaf) weeds such as dandelions.

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Apical Dominance

the terminal bud releases auxin which

suppresses the lateral axillary buds

Apical Dominance

the terminal bud releases auxin which

suppresses the lateral axillary buds

When the terminal bud is removed the inhibition of the axillary buds are removed and lateral growth starts

When the terminal bud is removed the inhibition of the axillary buds are removed and lateral growth starts

• Cytokinins retard the aging of some plant organs.– They inhibit protein breakdown by stimulating

RNA and protein synthesis, and by mobilizing nutrients from surrounding tissues.

– Leaves removed from a plant and dipped in a cytokinin solution stay green much longer than otherwise.

– Cytokinins also slow deterioration of leaves on intact plants.

– Florists use cytokinin sprays to keep cut flowers fresh.

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• A century ago, farmers in Asia notices that some rice seedlings grew so tall and spindly that they toppled over before they could mature and flower.– In 1926, E. Kurosawa discovered that a fungus in

the genus Gibberella causes this “foolish seedling disease.”

– The fungus induced hyperelongation of rice stems by secreting a chemical, given the name gibberellin.

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Fig. 39.9

Promotes stem elongation in dwarf plants like cabbage

Promotes stem elongation in dwarf plants like cabbage

• Roots and leaves are major sites of gibberellin production.– Gibberellins stimulate growth in both leaves and

stems but have little effect on root growth.– In stems, gibberellins stimulate cell elongation and

cell division.– One hypothesis proposes that gibberellins stimulate

cell wall loosening enzymes that facilitate the penetration of expansin proteins into the cell well.

– Thus, in a growing stem, auxin, by acidifying the cell wall and activating expansins, and gibberellins, by facilitating the penetration of expansins, act in concert to promote elongation.

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• The effects of gibberellins in enhancing stem elongation are evident when certain dwarf varieties of plants are treated with gibberellins.– After treatment with gibberellins, dwarf pea plant

grow to normal height.– However, if applied to

normal plants, there is often no response, perhaps because these plants are already producing the optimal dose of the hormone.

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Fig. 39.10

• In many plants, both auxin and gibberellins must be present for fruit to set.– Spraying of gibberellin during fruit development is

used to make the individual grapes grow larger and to make the internodes of the grape bunch elongate.• This enhances air circulation between the grapes and

makes it harder for yeast and other microorganisms to infect the fruits.

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Fig. 39.11

• Abscisic acid (ABA) was discovered independently in the 1960s by one research group studying bud dormancy and another investigating leaf abscission (the dropping of autumn leaves).– Ironically, ABA is no longer thought to play a

primary role in either bud dormancy or leaf abscission, but it is an important plant hormone with a variety of functions.

– ABA generally slows down growth.– Often ABA antagonizes the actions of the growth

hormones - auxins, cytokinins, and gibberellins.– It is the ratio of ABA to one or more growth

hormones that determines the final physiological outcome.

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• One major affect of ABA on plants is seed dormancy.– The levels of ABA may increase 100-fold during

seed maturation, leading to inhibition of germination and the production of special proteins that help seeds withstand the extreme dehydration that accompanies maturation.

– Seed dormancy has great survival value because it ensures that the seed with germinate only when there are optimal conditions of light, temperature, and moisture.

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• Many types of dormant seeds will germinate when ABA is removed or inactivated.– For example, the seeds of some desert plants break

dormancy only when heavy rains wash ABA out of the seed.

– Other seeds require light or prolonged exposure to cold to trigger the inactivation of ABA.

– A maize mutant that has seeds that germinate while still on the cob lacks a functional transcription factor required for ABA to induce expression of certain genes.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin CummingsFig. 39.12

• ABA is the primary internal signal that enables plants to withstand drought.– When a plant begins to wilt, ABA accumulates in

leaves and causes stomata to close rapidly, reducing transpiration and preventing further water loss.

– ABA causes an increase in the opening of outwardly directed potassium channels in the plasma membrane of guard cells, leading to a massive loss of potassium.

– The accompanying osmotic loss of water leads to a reduction in guard cell turgor and the stomata close.

– In some cases, water shortages in the root system can lead to the transport of ABA from roots to leaves, functioning as an “early warning system.”

• Abscisic acid (ABA)• • Unlike animals, plants cannot flee from potentially harmful conditions like • drought • the approach of winter• They must adapt or die. • The plant hormone abscisic acid (ABA) is the major player in mediating the adaptation of the plant to stress. • Here are a few examples. • 1. Bud dormancy• ABA mediates the conversion of the apical meristem into a dormant bud. The newly developing leaves growing above the meristem become converted into stiff bud scales

that wrap the meristem closely and will protect it from mechanical damage and drying out during the winter. • ABA in the bud also acts to enforce dormancy so if an unseasonably warm spell occurs before winter is over, the buds will not sprout prematurely. Only after a prolonged

period of cold or the lengthening days of spring (photoperiodism) will bud dormancy be lifted. • 2. Seed maturation and dormancy• Seeds are not only important agents of reproduction and dispersal, but they are also essential to the survival of annual and biennial plants. These angiosperms die after

flowering and seed formation is complete. ABA plays a role in seed maturation, at least in some species, and also enforces a period of seed dormancy. As we saw for buds, it is important the seeds not germinate prematurely during unseasonably mild conditions prior to the onset of winter or a dry season. ABA in the seed enforces this dormancy. Not until the seed has been exposed to a prolonged cold spell and/or sufficient water to support germination is dormancy lifted.

• 3. Abscission• ABA also promotes abscission of leaves and fruits (in contrast to auxin, which inhibits abscission). It is, in fact, this action that gave rise to the name abscisic acid. • The dropping of leaves in the autumn is a vital response to the onset of winter when ground water is frozen — and thus cannot support transpiration — and snow load would

threaten to break any branches still in leaf. • Most nondeciduous species in cold climates (e.g., pines) have "needles" for leaves. These are very narrow and have a heavy waterproof cuticle. The shape aids in shedding

snow, and the cuticle cuts down on water loss.• 4. Seedling Growth• ABA inhibits the growth of seedlings. • 5. Apical Dominance• ABA — moving up from the roots to the stem — synergizes with auxin — moving down from the apical meristem to the stem — in suppressing the development of

lateral buds. The result is inhibition of branching or apical dominance. • 6. Closing of stomata• Some 90% of the water taken up by a plant is lost in transpiration. Most of this leaves the plant through the pores — called stomata — in the leaf. Each stoma is flanked by a

pair of guard cells. When the guard cells are turgid, the stoma is open. When turgor is lost, the stoma closes. Discussion of gas exchange in the leaf.• ABA is the hormone that triggers closing of the stomata when soil water is insufficient to keep up with transpiration.

• The mechanism: • ABA binds to G-protein-coupled receptors at the surface of the plasma membrane of the guard cells as well as to other receptors in the cytosol. • Receptor activation produces

– a rise in pH in the cytosol – transfer of Ca2+ from the vacuole and endoplasmic reticulum to the cytosol

• These changes cause ion channels in the plasma membrane to open allowing the release of ions (Cl−, organic [e.g., malate2−], and K+) from the cell. • The loss of these solutes from the cytosol reduces the osmotic pressure of the cell and thus turgor. • The stomata close.

• The loss of leaves each autumn is an adaptation that keeps deciduous trees from desiccating during winter when roots cannot absorb water from the frozen ground.– Before leaves abscise, many essential elements are

salvaged from the dying leaves and stored in stem parenchyma cells.

– These nutrients are recycled back to developing leaves the following spring.

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Abscission layer that forms at the base of a leaf is controlled by a rise in ethylene and a lowering of auxin concentration

Abscission layer that forms at the base of a leaf is controlled by a rise in ethylene and a lowering of auxin concentration

• In 1901, Dimitry Neljubow demonstrated that the gas ethylene was the active factor which caused leaves to drop from trees that were near leaking gas mains.– Plants produce ethylene in response to stresses such

as drought, flooding, mechanical pressure, injury, and infection.

– Ethylene production also occurs during fruit ripening and during programmed cell death.

– Ethylene is also produced in response to high concentrations of externally applied auxins.

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• Ethylene instigates a seedling to perform a growth maneuver called the triple response that enables a seedling to circumvent an obstacle. Thigmotropism – response to mechanical stress

• Ethylene production is induced by mechanical stress on the stem tip.

• In the triple response, stem 1. elongation slows, the stem 2. thickens, and 3. curvature causes the stem to start growing horizontally.

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Fig. 39.13

Adding antisense RNAblocks the transcription

of Ethylene

Ethylene addedat the market to ripen

the tomato

GravitropismGravitropism

Fig. 39.25

Statoliths may redistribute the auxin to the lower part causing differential rates of elongation on either side of the plant

Statoliths may redistribute the auxin to the lower part causing differential rates of elongation on either side of the plant

Root tip

• Plants may tell up from down by the settling of statoliths, specialized plastids containing dense starch grains, to the lower portions of cells.– In one hypothesis, the aggregation of statoliths at

low points in cells of the root cap triggers the redistribution of calcium, which in turn causes lateral transport of auxin within the root.

– The high concentrations of auxin on the lower side of the zone of elongation inhibits cell elongation, slowing growth on that side and curving the root downward.

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The innerside cells of the pulvinus lose turgor pressure causing the Mimosa leaves to close when stimulated

The innerside cells of the pulvinus lose turgor pressure causing the Mimosa leaves to close when stimulated

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Fig. 39.27

If FR light is flashed last the plant will act like it’s a short day

If FR light is flashed last the plant will act like it’s a short day

If Red light is flashed last the plant will act like it’s a long day

If Red light is flashed last the plant will act like it’s a long day

PHYTOCHROME

a photodector homodimer -made of twin protein molecules

each protein has two domains

PHYTOCHROME

a photodector homodimer -made of twin protein molecules

each protein has two domains

Regulates cellular responses

Regulates cellular responses

During the night the amount of Pfr decreases and Pr increases

During the night the amount of Pfr decreases and Pr increases

Strong sunlight favors

the production of Pfr

Strong sunlight favors

the production of Pfr

A strong spike of Pfr in the morning synchronizes the plant’s biological clock

A strong spike of Pfr in the morning synchronizes the plant’s biological clock

Sunlight or

The caterpillar triggers the plant to release an attractant for the parasitoid wasp

The caterpillar triggers the plant to release an attractant for the parasitoid wasp

Hypersentive Response (HR) enhances the production of antimicrobial phytoalexins and pathogenesis related proteins (PR) which ‘seals’ the cell wall to set up a barricade to slow infection and then the cells kill themselves

Hypersentive Response (HR) enhances the production of antimicrobial phytoalexins and pathogenesis related proteins (PR) which ‘seals’ the cell wall to set up a barricade to slow infection and then the cells kill themselves

A signal molecule like salicylic acid will trigger Systemic Acquired Resistance (SAR) in healthy leaves

A signal molecule like salicylic acid will trigger Systemic Acquired Resistance (SAR) in healthy leaves