1. Only five classes of natural hormones are definitely known: auxins, cytokinins, gibberellins, abscisic acid, and ethylene.

2. Auxins, cytokinins, and gibberellins are hormones that stimulate growth.

3. Abscisic acid and ethylene are hormones that inhibit growth.

4. Plant hormones primarily regulate growth and differentiation.

1. They were particularly interested in gravitropism, growth directed by gravity, and phototropism, growth directed by light.

2. Roots, which grow downward (toward the source of gravitational attraction), are positively gravitropic, and stems, which grow upward, are negatively gravitropic.

3. Stems are usually positively phototropic and grow toward a light source.

4. Bending of the stem doesn't occur at the tip itself, but a few millimeters below it, due to elongation of the stem on the side opposite the light, pushing the tip toward the light (Figure 39.1).

1. Figure 39.2 shows how he made slices halfway through coleoptiles and inserted thin slices of mica, which is impervious to most chemicals.

2. When inserted below the tip on the light side of an illuminated coleoptile, the mica slices had no effect on the bending reaction, but when placed on the dark side, they prevented bending.

3. Boysen-Jensen also showed that a coleoptile will bend normally if its tip is cut off and a block of gelatin is inserted between the tip and the rest of the coleoptile.

4. Arpad Paal extended these observations by demonstrating that cutting out a notch of tissue on one side of a coleoptile will make it bend toward that side in the dark.

1. Went cut off oat coleoptile tips and left them on small blocks of gelatin for a few hours to allow material to diffuse into the blocks.

2. He then placed the blocks on one side of decapitated coleoptiles and found that they would bend, in the dark, away from that side.

3. The coleoptiles bend in proportion to the number of tips that have been placed on a block, providing an assay procedure for measuring the amount of this growth substance from any source (Figure 39.3).

4. Went found that the substance responsible for bending, called auxin, was widespread in nature, but in very low concentrations.

1. Indoleacetic acid (IAA) promotes the elongation of cells in ordinary root and stem growth in the region directly behind a proliferating meristem by increasing the turgor pressure inside these cells while their wall structures are loosened.

2. The acid-growth hypothesis postulates that reducing the pH in the cell wall loosens the wall structure so the wall can expand under increased turgor pressure.

3. The concentration of auxin in a plant is controlled by various enzymes: Some inactivate the hormone, others convert free auxin into storage complexes, and still others release the free hormone from storage.

1. Organic chemists have produced several important artificial compounds related to auxin.

2. A number of the synthetic auxins, such as indolebutyric acid (IBA) and naphthalene acetic acid (NAA), are even more effective than IAA in stimulating root development. Both are used commercially and are readily available.

3. Some synthetic auxins act as herbicides.

1. Light can affect one side of a stem more than the other, but gravity pulls on the top and bottom of a stem equally.

2. Auxin is more concentrated in the lower parts of some cells or in the lower portion of a plant lying horizontally, as if it had been attracted by gravity.

3. Gravity cannot sediment auxin molecules in the viscous fluid of a cell against the forces of diffusion.

1. A statocyte contains several amyloplasts, the plastids that store starch granules, which are dense enough to fall to the bottom of a statocyte and thus determine the downward direction.

2. The time it takes amyloplasts to fall in the cytoplasm is correlated with the time it takes an organ to detect the direction of gravity.

3. Genetic evidence also shows that amyloplasts have a role in some gravitropic responses.

1. The root cap is the gravitational detector for the root, and it appears to be the source of this inhibitor, which, like auxin, must be more concentrated in the lower half of a horizontal root (Figure 39.6).

2. A block on one side of a vertical root makes the root curve away from that site (Figure 39.6).

3. Chemical analysis shows that the inhibitor is likely to be abscisic acid, a plant hormone.

1. Early in this century, Japanese rice crops were threatened by foolish seedling disease (bakanea), an infection by the fungus Gibberella fujikuroi. Infected plants failed to set seed and developed long, straggly stems, so they fell over in the water and died (Figure 39.7).

2. The active agent in this disease was isolated and identified and since then many related compounds have been identified that have the same effects on plant growth. All are called gibberellins or gibberellic acid (GA).

3. Although gibberellins were first found in a fungus, they are widely distributed in plants and have been identified in algae and mosses as well as in angiosperms.

4. GAs are normal hormones that stimulate growth in the shoot system but have little or no effect on roots.

1. Dwarf plants are homozygous for recessive mutations that block gibberellin production.

2. Treating a dwarf corn seedling with a GA induces it to grow as tall as normal corn.

3. This effect, which is also seen in dwarf strains of other plants, indicates that though the mutants cannot produce gibberellins, the targets of the hormone action are perfectly functional.

1. GAs are especially concentrated in developing seeds.

2. Like IAA, GAs promote cell elongation much more than cell division, but in addition to promoting shoot growth, they may also induce flowering, promote seed germination, break dormancy, and stimulate a number of other events.

3. GAs are commonly used commercially, with particular success in increasing yields of seedless grapes and sugar cane.

4. GAs increase the transcription of messenger RNA, but it isn't clear if this is a general increase in all transcription or a specific induction of certain genes.

1. In 1955, Carlos Miller tried to get tobacco pith tissue to grow in tissue cultures containing auxin.

2. He found that the cells would grow, but would not divide.

3. The nuclei of these giant cells continued to divide, but with no cytokinesis, each cell had many nuclei.

4. Miller and his colleagues were able to induce normal cell division by adding extracts from yeast and other materials, thus showing that these extracts contained some kind of growth regulator.

5. From these materials, they eventually isolated a substance they named kinetin, which was later identified as a derivative of adenine.

1. In addition to a number of vitamins and minerals, they found that a factor from coconut milk was essential for cell division.

2. Other sources of this factor were identified later, and in 1964 zeatin was isolated from corn kernels.

3. Zeatin is very similar in structure to kinetin, and all these compounds that promote cell division are known as cytokinins.

1. The path of cell differentiation is controlled by the relative amounts of cytokinin and auxin, as the experiment shown in Figure 39.8 demonstrates.

2. Pith tissue is removed from a tobacco plant and sterilely placed on the surface of nutrient agar medium lacking hormones.

3. The result is a small mass of new, undifferentiated tissue called callus.

4. Low auxin concentrations and intermediate amounts of cytokinin support active cell proliferation, producing massive calluses of new, undifferentiated cells.

5. Keeping the auxin level constant and increasing the cytokinin level inhibits callus formation and makes the shoot system of the plant develop.

6. If the cytokinin level is reduced and the auxin level raised, a root system develops without any shoots.

1. For reasons not yet understood, the application of cytokinins keeps harvested plant products fresh.

2. Lettuce, broccoli, and many other fruits, vegetables, and fungi are treated with cytokinins to prolong their shelf life.

3. Florists use cytokinins to keep cut flowers fresh.

4. The molecular basis of the effects of cytokinins has eluded investigators up to this point.

1. The tall conifers, like pines and firs, have a symmetrical conical shape.

2. Many broad-leaved trees grow with a distinctly pointed top.

3. Apical dominance results when the growth of a stem tends to be dominated by its terminal bud, which suppresses the growth of other buds, especially the axillary buds that form between the stem and leaves (Figure 39.9).

4. This inhibition is due to auxin, which is made in the terminal bud and spreads down through the stem.

5. Auxin inhibits buds near the apex from growing into branches; as the tree grows taller, lower buds escape this inhibition and form branches, the oldest, lowest branches having grown for longer times (Figure 39.10).

1. Increasing the cytokinin concentration overcomes the inhibition caused by auxin.

2. One reason the lower branches of a tree grow longer is that the lower buds are also within reach of cytokinin coming up from the roots.

3. As with so many of the effects of plant hormones, the critical factor is the balance of auxin and cytokinin.

1. Ethylene is synthesized by plant tissues, including seeds, flowers, fruit, leaves, and roots, and it is also produced by human activities like burning natural gas and petroleum products.

2. Ethylene is commonly used to speed the ripening of fruit.

3. When seeds germinate, the stem pushes up through the soil toward the light, its movement facilitated by the formation of a crook, which is controlled by ethylene (Figure 39.11)

4. Excess ethylene can be detrimental to plants.

1. Auxin stimulates ethylene production and ethylene inhibits cell elongation.

2. Ethylene induces a general expansion of cells in all directions rather than in the one preferred direction of stem growth.

3. Ethylene may also be responsible for inhibiting root growth and the growth of axillary buds which has been attributed to auxin.

4. Ethylene is a classical inhibitor of gravitropism; seedlings respond to ethylene with the typical "triple response"–horizontal growth, inhibition of elongation, and swelling.

1. The pathway of ethylene action is being dissected through biochemical and genetic analysis, particularly using mutants of Arabidopsis and tomatoes.

2. Ethylene first binds to a receptor protein associated with cell membranes; since the gas is quite soluble in cell membranes, it is probably able to enter the cytoplasm and bind to other receptors there.

3. Some of these receptors appear to be protein kinases, so the first effect of ethylene is likely to be a kinase cascade (Figure 39.12).

4. The ripening of fruit is an important response to ethylene, and studies in tomatoes have shown how ripening is effected by certain newly induced enzymes (Figure 39.13).

1. The practice of putting unripe fruit in closed containers simply increases the concentration of the gas.

2. A bit of ripe fruit will trigger ripening in nearby fruit on a massive scale because ethylene production is autocatalytic.

3. Ethylene promotes seed germination and flower production.

4. Ethylene also enhances the thickening of tree trunks in order to make more stable ornamental trees.

1. Leaves, flowers, and fruits are separated from plants in the process of abscission, in which the vascular system is sealed off at appropriate points to prevent the loss of water and nutrients and to exclude bacteria, fungi, and other pathogens.

2. An abscission zone, a layer of specialized cells, forms at the base of each leaf or fruit, and its cells die and become hardened by deposits of lignin and suberin, so by the time a leaf or fruit drops, its vascular system has been sealed off.

1. It turns out that ABA has nothing to do with abscission.

2. ABA, which is synthesized mostly in chloroplasts, is a general inhibitor of many processes.

3. The abscission layer forms and hardens under the control of IAA and ethylene.

1. ABA induces tolerance to stress and aids in water conservation during wilting.

2. When a plant loses water and begins to wilt, ABA production is enhanced, and ABA closes the stomata.

3. As soon as enough water returns to the plant, ABA is enzymatically degraded, and the guard cells return to normal.

1. ABA is known to regulate more than 70 genes, both positively and negatively.

2. The entire regulatory pathway is complex and not completely known, but it fits a classic model of genetic regulation by a small ligand.

3. Figure 39.15 summarizes the effects of the various plant hormones and environmental stimuli.

1. Some plants accumulate defensive proteins near the wound, including proteinase inhibitors, that inactivate proteins that insects need for digestion. This often leads to insect malnutrition and death.

2. The signal ligand that induces the wound response is a peptide of 18 amino acids called systemin.

3. Systemin is cleaved off a protein, prosystemin, and carried through the phloem to all tissues near the point of injury.

4. Systemin then induces genes for proteins of the wound response through a pathway that is interesting because of its similarity to animal pathways (Figure 39.16).

5. Precisely what induces the production of systemin in response to a wound is still not clear.

1. The critical factor is not the length of day or night but rather how each type of plant responds to a certain critical photoperiod.

2. A short-day plant is not one that flowers only when the days become short, but one that only flowers if subjected to photoperiods shorter than a certain critical day length.

3. A long-day plant will only flower with photoperiods longer than a critical day length (Figure 39.17).

4. The critical day lengths for some long-day plants are shorter than those for same short-day plants.

5. Typical short-day plants will only flower if they are subjected to increasing dark periods of about 12—14 hours. These plants include strawberries, chrysanthemums, dahlias, goldenrod, sorghum, and violets.

6. Long-day species, such as lettuce, spinach, wheat, potatoes, and larkspur, generally need shorter nights of perhaps 10—12 hours.

7. There are also day-neutral plants, whose flowering is not controlled by the light/dark cycle. These include dandelions, tomatoes, garden beans, snapdragons, cotton, sunflowers, and roses.

1. Sprouting in potatoes is short-day regulated, while in strawberries, flowering is facilitated by short days and the development of runners is stimulated by long days.

2. Dormancy and the breaking of dormancy are also regulated by both photoperiod and temperature.

3. Abscisic acid formation is apparently induced by the short days of autumn, so that even subjecting a plant to artificially short days during the summer can send it into dormancy.

4. Plants have evolved a mechanism that anticipates the changing seasons and predicts that short, cold days will be followed by warmer days when they can grow.

1. In long-day plants, a night-break burst of red light with a wavelength of 660 nm is most effective, suggesting that the light must be absorbed by a pigment with this absorption maximum.

2. The effect will be reversed if a brief exposure to red light (or white light, which contains red) is followed within half an hour by a short exposure to far-red light, around 730 nm.

3. A burst of red light will make a long-day plant flower, while far-red light will inhibit flowering.

4. If a plant is alternately exposed to red and far-red light, the plant will respond to the last burst of light, whichever it is.

1. Phytochrome exists in two forms: Pr absorbs red light, and Pfr absorbs far-red light.

2. Phytochrome's chromophore, the pigment that actually absorbs light, is a tetrapyrrole (Figure 39.18).

3. Phytochrome responds to red or far-red light by shifting from one form (Pr)to the other (Pfr).

4. Pr is metabolically inert, but the active Pfr form has an exposed hydrophobic site.

1. Phytochrome allows plants to orient both leaves and chloroplasts to receive more light of less achieving growth and differentiation of chloroplasts, and synthesizing pigments such as the chlorophyll and carotenoids of leaves and the anthocyanins and flavonoids of flowers and fruits.

2. Phytochrome also regulates more complex processes of growth and morphogenesis.

3. Plants are etiolated when they attempt to grow while totally shielded from light. They commonly have white, straggly stems and few thin, yellow leaves.

4. Etiolation, which is under the control of phytochrome, is eliminated by exposure to red or white light.

1. PhyA, PhyB, and PhyC proteins are well characterized and have different functions, as illustrated by the shade-avoidance reaction of some plants (Figure 39.20).

2. PhyA is necessary for perception of continuous far-red light.

3. PhyB is necessary for perception of continuous red light.

1. Genes turned on by the Pfr form of phytochrome have a specific sequence, a light-responsive element, in their promoter regions.

2. The clearest case of gene regulation by phytochrome is self-regulation of the phyA gene that encodes phytochrome A (Figure 39.22).