Chap 39

action_spectrum.html: 39_17PhototropicExper.jpg
 EXPERIMENT   Researchers exposed maize (Zea mays) coleoptiles to light to test which wavelengths stimulate the phototropic bending toward light.

 RESULTS   Curvature occurred with wavelengths below 500 nm and was greatest with blue light.

 CONCLUSION   Phototropism is caused by a photoreceptor that is sensitive to blue light.

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Apical dominance.
Intact plant. Auxin from the apical bud inhibits the growth of axillary buds. Cytokinins, transported upward from roots, counter auxin, stimulating the growth of axillary buds. Plant with apical bud removed. Removal of the apical bud from the same plant enabled lateral branches to grow.

auxin.html: 39_07AuxinMovement.jpg
Polar movement of auxin from shoot tip to base Fluorescent antibodies are seen bound to transport proteins at the base of xylem parenchyma cells, where they move the auxin to the apical end of the neighboring cell, resulting in unidirectional transport of the hormone from shoot tip to base.

auxin_response.html: 39_08AuxinResponse.jpg
Cell elongation in response to auxin: the acid growth hypothesis.

de-etiolation.html: 39_01Phototropism.jpg
Light–induced de–etiolation (greening) of dark–grown potatoes.
Before exposure to light. A dark-grown potato has nonexpanded leaves that enable the shoots to penetrate the soil. It has short roots, but little water is lost by the shoots.   After a week’s exposure to natural daylight. The plant now has broad green leaves and long roots; this transformation begins with the reception of light by a pigment, phytochrome.

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Ethylene induces fruit ripening by positive feedback and also mediates plant response to stress. A response to mechanical stress called the triple response allows a growing shoot to avoid obstacles.

gibberellin_grape.html: 39_10GibberellinEffect.jpg
The effect of gibberellin treatment on Thompson seedless grapes. The grape bunch on the left is an untreated control. The bunch on the right is growing from a vine that was sprayed with gibberellin during fruit development.

gibberellin_seed.html: 39_11GAandNutrients.jpg
Gibberellins mobilize nutrients during the germination of grain seeds.

  1. After a seed imbibes water, the embryo releases gibberellin (GA) to the aleurone (outer layer of the seed coat).
  2. The aleurone secretes enzymes such as α-amylase that hydrolyze nutrients in the endosperm.
  3. Nutrients absorbed from the endosperm by the scutellum (cotyledon) are consumed as the embryo grows.

gravitropism.html: 39_25Gravitropism.jpg
Positive gravitropism in roots: the statolith hypothesis.

  1. A horizontal root of maize will bend until its tip becomes vertically oriented.
  2. Soon after the root is placed horizontally, statoliths begin settling to the bottom of root cap cells.
This settling of statoliths (plastids containing dense starch grains) may be the gravity–sensing mechanism that leads to redistribution of auxin and differential rates of elongation by cells on opposite sides of the root.

leaf_abscission.html: 39_16Abscission.jpg
Abscission of a maple leaf.

Abscission is controlled by a change in the balance of ethylene and auxin; a higher proportion of ethylene causes the cells to produce enzymes that digest the cellulose of cell walls.

A layer of cork becomes the leaf scar that protects the plant.

A mixture of pigments such as chlorophylls, carotenoids, and anthocyanins give the dying leaf its fall color in temperate regions.

light_germination.html: 39_18PfPfrGermination.jpg
 EXPERIMENT   Lettuce seeds were exposed to differnt light treatments.

 RESULTS   Germination is greatest in seeds that are last exposed to red light. Germination was inhibited in seeds that were last exposed to far-red light.

 CONCLUSION   Red light stimulates germination, and far-redlight inhibits germination. The final exposure is critical.

parasitoid.html: 39_29HerbivoryDefense.jpg
A maize leaf “recruits” parasitoid wasp as a defensive response to an army–worm caterpillar by releasing a chemical attractant for the wasp when the caterpillar chews on the leaf.

phototropism.html: 39_01Phototropism.jpg
Grass seedling growing toward light.

phytochrome.html: 39_19PhytochromeStructure.jpg
Structure of a phytochrome.

A phytochrome consists of two proteins each with two domains.

One domain contains the pigment chromophore and functions as the photoreceptor.

The other domain has protein kinase activity that triggers cellular responses.

phytochrome_isomers.html: 39_UN804PfPfrConversion.jpg
Phytochrome: a molecular switching mechanism. The chromophore exists as two isomers: in its Pr form, a phytochrome absorbs red light, whereas in its Pfr form, it absorbs far–red light.

phytochrome_switch.html: 39_20PhytochromeSwitch.jpg
Absorption of red light causes Pr to change to the Pfr form. Far–red light reverses this conversion. Shade–avoidance: In the shade of a forest, chlorophyll pigments in the canopy absorb red light and allow far-red light to pass. Trees in the shade shift the phytochrome ratio in favor of Pr and grows taller. Direct sunlight increases the proportion of Pfr, which stimulates branching and inhibits vertical growth.

phytochrome_transduction.html: 39_04PlantSigTransduct.jpg
Light causes the phytochrome receptor to activate cGMP as a second messenger that activates a protein kinase. Phytochrome also increases flow of Ca2+ into the cytoplasm and activate another protein kinase. Both pathways lead to expression of genes for proteins that function in the de-etiolation (greening) response.

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Spines are modified leaves that defend a plant against herbivores.

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The shorter plant on the left was rubbed twice a day. The untouched plant (right) grew much taller.

thigmotropic.html: 39_27Thigmotropism.jpg
The sensitive plant Mimosa pudica responds to touch by folding its leaflets together.

The folding is caused by curving of the pulvinus (motor organ) where cells on one side loses water and becomes flaccid, while cells on the opposite side retain their turgor.

Touch can also affect plant morphology by thigmomorphogenesis.

thorns.html: ../ch53/53_09Mutualism.jpg
Thorns are modified stems that defend a plant against herbivores.

transduction.html: 39_03SignalTransduction.jpg
Review of a general model for signal transduction pathways. A hormone or another signal binding to a specific receptor stimulates the cell to produce relay molecules, such as second messengers, which trigger the cell's various responses to the original signal. In this diagram, the receptor is on the surface of the target cell. In other cases, hormones enter cells and bind to specific receptors inside.

triple_response.html: 39_13EthyleneResponse.jpg
The triple response mediated of stems by ethylene involves