Control Systems In Plants

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I.  Research on how plants grow toward light led to the discovery of plant hormones.

A.       Hormones are chemical signals that coordinate the parts of the organism.

B.       Only minute concentrations are required to induce substantial changes in the organism.

C.       Positive phototropism is the growth of a shoot toward light.

D.       Charles and Francis Darwin observed that a grass seedling could bend toward light only if the tip of the coleoptile was present.

1.        If the tip were removed, the coleoptile would not curve.

2.        The actual growth response occurred some distance below the tip.

3.        They proposed the hypothesis that some signal was transmitted downward from the tip to the elongating region of the coleoptile.

E.        Peter Boysen-Jenson demonstrated that the signal was a mobile substance by separating the tip from the remainder of the coleoptile by a block of gelatin.

1.        Cellular contact was block but chemicals could pass.

2.        The seedlings behaved normally.

F.        F.W. Went extracted the chemical messenger by removing the coleoptile tip and placing it on a block of agar.

1.        He concluded the block contained a chemical produced in the coleoptile tip and it stimulated growth as it passed down the coleoptile.

2.        Named the hormone auxin.

II.  Plant hormones help coordinate growth, development, and responses to environmental stimuli.

A.       Five major classes of plant hormones.

B.       Control plant growth and development by affecting the division, elongation, and differentiation of cells.

C.       Some also mediate shorter-term physiological responses.

D.       Each has a multiplicity of effects depending on the site of action, the developmental stage of the plant, and the concentration of the hormone compared to others.

E.        Transport from cell to cell involves the passage across cell walls.

F.        Found in very small concentrations.

G.       May act by altering the expression of genes, by affecting the activity of existing enzymes, or by changing properties of membranes.

H.       Reaction usually depends not on the absolute amount of the hormones but its relative concentration compared to others.

I.          Hormonal balance may control the growth and development of the plant.

J.        Auxin

1.        Describes any chemical substance that promotes elongation of the coleoptiles.

2.        The natural auxin is called indoleacetic acid of IAA.

3.        The apical meristem is a major site of auxin synthesis.

4.        The auxin moves down the shoot apex to the region of cell elongation and stimulates growth of cell.

5.        At high concentrations, it may inhibit cell elongation.

6.        This is probably due to a high level of auxin inducing the synthesis of another hormone, ethylene, which generally acts as an inhibitor of plant growth.

7.        Transported directly through parenchyma tissue from one cell to the next.

8.        Moves only from shoot tip to base and is called polar transport although it has nothing to do with gravity.

9.        Polar auxin transport requires energy using proton pumps driven by ATP.

10.     Auxin exits each cell by a specific carrier protein that is restricted to the basal end of the cell.

11.     Proton pumps located at the plasma membrane also play a response by lowering the pH of the wall which makes it more plastic and is free to take up additional water by osmosis and this allows for elongation.

12.     Also stimulates longer-term growth responses.

13.     Affects secondary growth by inducing cell division in the vascular cambium and by influencing the differentiation of secondary xylem.

14.     Promotes the formation of adventitious roots at the cut base of the stem.

15.     Developing seeds also synthesize auxin, which promotes the growth of fruit in many plants.

16.     Used as a herbicide (2,4-D)

K.       Cytokinins

1.        Discovered by accident by Johannes van Overbeek using coconut milk on developing embryos.

2.        Named because they stimulate cytokinesis or cell division.

3.        Produced in actively growth tissues in roots embryos and fruits.

4.        Reach target tissues by moving up the plant by the xylem sap.

5.        Stimulate cell division and influence the pathway of differentiation.

6.        The ration of Cytokinin to auxin controls the differentiation of cells.

7.        If more Cytokinin than auxin, shoot buds appear.

8.        If more auxin that Cytokinin, roots form.

9.        Auxin transported from the shoot restrains axillary buds from growing causing the shoot to lengthen.

10.     Cytokinin entering the shoot system from the roots counter the action by signaling the axillary buds to grow.

11.     As roots become more extensive, the increased level of cytokinins would signal the shoot system to form more branches.

12.     Used to retard aging of some plant organs by inhibiting protein breakdowns, by stimulating RNA and protein synthesis and by mobilizing nutrients from surrounding tissues.

L.        Gibberellins

1.        E. Kurosawa discovered that the disease of rice was caused by a fungus and then later determined that the hyperelongation of rice stems came from a chemical called gibberellin.

2.        Roots and young leaves are major sites of production.

3.        Stimulate growth in both the leaves and stem but have little effect on root growth.

4.        Stimulate cell elongation and cell division.

5.        Gibberellins and auxin must be acting simultaneously.

6.        Also used in fruit growth.

7.        In some plants both gibberellins and auxins must be present.

8.        Many seed have a high concentration of gibberellins from the embryo.

9.        After water is imbibed, the release of gibberellins from the embryo signals the seeds to break dormancy and germinate.

10.     Also function to break dormancy in the resumption of growth by apical buds in spring.

M.       Abscisic Acid

1.        Produced in the terminal bud.

2.        Slows growth and directs leaf primordial to develop into the scales that will protect the dormant buds during winter.

3.        Inhibits cell division of the vascular cambium.

4.        Suspends both primary and secondary growth.

5.        Also used as a stress hormone to help the plant cope with adverse conditions.

N.       Ethylene

1.        Mistakenly discovered by ripening fruit in the presence of kerosene heaters.

2.        Kerosene produces ethylene which is a gaseous by-product.

3.        It is a gas that moves through the plants in the air spaces between cells.

4.        Can also move in the cytosol traveling from cell to cell through the symplast and in the phloem.

5.        Inhibits cell elongation.

6.        Associated with a variety of aging processes in plants.

7.        Aging or senescence is a progression of irreversible change that eventually leads to death.

8.        May occur at cell level, organ level or whole plant levels.

9.        Includes the degradation of cell walls which softens the fruit and decreases the chlorophyll content causing the fruiting to ripen.

10.     As it triggers the aging process, the aging cells then release more ethylene.

11.     The signal to ripen even spreads from fruit to fruit.

12.     Circulating air prevents ethylene from accumulating and carbon dioxide inhibits the action of whatever has not been flushed away.

13.     The loss of leaves is controlled by the ethylene and auxin levels.

14.     Abscission is the loss of leaves at the base of the petiole.

15.     Parenchyma cells of this layer are very thin. And enzymes hydrolyze the polysaccharides which causes the leaves to fall.

16.     An aging leaf produces less and less auxin.

III.  Tropisms orient the growth of plant organs toward or away from stimuli

A.       Tropisms are growth responses that result in curvatures of whole plant organs toward or away from stimuli.

B.       The mechanism is differential rate of elongation of cells opposite sides of the organ.

C.       Three stimuli are used to induce and are gravity, light, and touch.

D.       Phototropism

1.        Results from auxin stimulating cell elongation on the darker side of the stem or some other chemical messenger inhibiting elongation on the lighter side.

2.        Shot tip is the site of photoreception that triggers the growth response and are made of pigment molecules that are most sensitive to blue light.  These are probably yell and related to riboflavin.

E.        Gravitropism

1.        Roots curve downward in response to gravity.

2.        Roots display positive gravitropism and shoots display negative gravitropism.

3.        Uses statoliths which are specialized plastids containing dense starch grains and moving them to the low points of the cells.

4.        In roots they are located in certain cells of the root cap.

F.        Thigmotropism

1.        Grasping organs usually grow straight until they touch something and the contact stimulates a coiling response caused by differential growth of cells on opposite sides of the tendril.

2.        The developmental response to mechanical perturbation is called thigmomorphogenesis and usually results from an increased production of ethylene in response to chronic mechanical stimulation like wind.

IV.  Turgor movements are relatively rapid, reversible plant responses

A.       Rapid leaf movements

1.        Results from a rapid loss of turgor by cells within pulvini which are specialized motor organs located at the joints of the leaf.

2.        The motor cells suddenly become flaccid after stimulation because the lose potassium which causes water to leave the cells by osmosis.

3.        Helps the plant conserve water.

4.        From the point of stimulation, the message that produces this response travels wavelike through the plant at a speed of about a centimeter per second.

5.        Chemical messengers probably have a role in transmission but an electrical impulse can also be detected and is called action potentials.

B.       Sleep Movements

1.        Many plants lower their leaves in the evening and raise them to a horizontal position in the morning.

2.        Powered by daily changes in the turgor pressure of motor cells in pulvini.

3.        When leaves are horizontal the cells on side of the pulvinus are turgid and those on the other side are flaccid.

4.        Paralleling the opposing changes in volume is massive migration of potassium from one side to another.

V.  Photoperiodism synchronizes many plant responses to changes of season.

A.       A physical response to day length such as flowering is photoperiodism

B.       Photoperiodic control of flowering

1.        Studied by Garner and Allard

2.        Found that if the plants were kept in light-tight boxes so that lamps could manipulate the duration of light and dark, flowering would occur at other times.

3.        Short-day plants require a light period shorter than a critical length to flower.

4.        These include mums, poinsettias, and soybeans.

5.        Long-day plants tend to flower in late spring or early summer when the light period is longer.

6.        Day-neutral plants flower when they reach the correct stage of maturity regardless of day length at the time.

7.        Researchers discovered in the 1940’s that is was the length of night that controlled these plants and not the length of day.

8.        Some have a pretreatment to cold required before they will flower.  This is called vernalization.

9.        Buds produce flowers but leaves detect the photoperiod.