Chapter Outline

26.1 Plant Hormones
1.     One defining characteristic of life is an ability to respond to stimuli.
2.     Adaptive organisms respond to environmental stimuli because it leads to longevity and survival of the species.
3.     Plant cells utilize signal transduction when they respond to stimuli.
4.     Signal transduction involves:
a.     Receptors
i.      Plant cells have proteins that are activated by a specific signal.
b.     Transduction pathway
i.      There are a series of relay proteins or enzymes that transform the signal to a signal that is understood by the cell’s machinery.
ii.     The stimulated receptor can either communicate immediately with the transduction pathway, or use a second messenger to initiate a response.
c.     Cellular response
i.      The response is the result of the transduction pathway.
ii.     The response brings about the observed microscopic response, i.e., closing stomata.

• For plants to respond to stimuli, activities of plant cells and structures have to be coordinated.
• Almost all plant communication is done by hormones.
• Hormones are chemical messengers, produced in very low concentrations that are active in another part of the organism.
• Hormones are synthesized in one part of a plant; they travel in the phloem after a plant receives an appropriate stimulus.

A.    Auxins
1.     Apically produced auxin prevents the growth of axillary buds, a phenomenon called apical dominance.
a.     When a terminal bud is removed, the nearest buds begin to grow and the plant branches.
b.     Application of a weak solution of auxin causes roots to develop from the ends of cuttings.
c.     Auxin production by seeds promotes growth of fruit.
d.     As long as auxin is concentrated in leaves and fruits rather than stem, they do not fall off.
2.     Auxin-controlled cell elongation is involved in gravitropism and phototropism.
a.     When gravity is perceived, auxin moves to the lower surface of roots and stems.
b.     Charles Darwin and his son Francis discovered with oat seedlings, phototropism would not occur if the tip of a seedling is cut off or covered by a cap; they concluded the cause of curvature moved from the coleoptile (the protective sheath for the leaves of a seedling) tip to the rest of the shoot.
3.     Frits W. Went (1926) experimented with coleoptiles.
a.     He cut off tips and placed them on agar.
b.     When an agar block was placed to one side, the coleoptile would curve away from that side regardless of the light.
c.     He deduced a chemical caused the curved growth, and he named it auxin after the Greek word for “promoting growth.”
4.     How Auxins Cause Stems to Bend
a.     In a plant exposed to unidirectional light, auxin moves from the bright side to the shady side of a stem.
b.     Auxin binds to receptors and activates the ATP‑driven proton (H+) pump.
c.     As hydrogen ions are pumped out of the cell, the cell wall becomes acidic, breaking hydrogen bonds.
d.     Cellulose fibrils are weakened and activated enzymes further degrade the cell wall.
e.     The electrochemical gradient established causes uptake of solutes and water follows by osmosis.
f.      The result is elongation of the stem on the shady side so that it bends toward the light.
B.     Gibberellins
1.     Gibberellins are a group of some 70 plant hormones that chemically differ only slightly.
2.     GA3 is the most common of the natural gibberellins.
3.     Gibberellins are growth promoters that elongate cells.
4.     Gibberellins were discovered in 1926 by Ewiti Kurosawa, a Japanese scientist investigating a fungal disease of rice plants called “foolish seedling disease.”
a.     His fungus-infected plants produced an excess chemical gibberellin, named after the fungus.
b.     By 1956, gibberellic acid was finally isolated from a flowering plant rather than fungus.

• The dormancy of seeds and buds can be broken by applying gibberellins.
• After the embryo produces gibberellins, amylase appears. So it seems that gibberellins lead to activation of the gene that code for amylase.

C.     Cytokinins
1.     Cytokinins are a class of plant hormones that promote cell division.
2.     Cytokinins are derivatives of the purine base adenine.
3.     A natural cytokinin zeatin is found in corn kernels; kinetin is a synthetic cytokinin.
4.     Researchers discovered cytokinins in work on growing plant tissues in culture.
5.     Oligosaccharins, chemical fragments released from the cell wall, also direct differentiation.
6.     Researchers hypothesize that auxin and cytokinins are part of a reception-transduction response pathway that activates enzymes that release these fragments from the cell wall.
7.     Aging processes are senescence; large molecules break down and are transported elsewhere in the plant.
a.     Cytokinins prevent senescence of leaves; they also initiate development of leaf growth.
b.     Cytokinins initiate growth of lateral buds despite apical dominance.
D.    Abscisic Acid
1.     Abscisic acid (ABA) is produced by any tissue that contains chloroplasts, monocot stems, and roots.
2.     ABA is sometimes called the “stress hormone”; it maintains seed and bud dormancy and causes closure of stomata.
3.     It was once thought that ABA functioned in abscission, but now the hormone is no longer believed to function naturally in this process.
4.     Dormancy occurs when a plant organ readies itself for adverse conditions by stopping growth.
a.     ABA moves from leaves to vegetative buds in the fall; thereafter these buds are converted to winter buds which are covered by thick, hardened scales.
b.     Reduction in ABA and increase in gibberellins break seed and bud dormancy; seeds germinate and buds send forth leaves.
5.     Abscisic acid brings about the closing of stomata when a plant is under water stress.
a.     By some unknown mechanism, ABA causes K+ ions to leave guard cells.
b.     As a result, guard cells lose water and the stomata close.
E      Ethylene
1.     Ethylene is a gas involved in abscission and ripening of fruits. 
2.     Auxin and perhaps gibberellin initiates abscission, but once abscission has begun, ethylene stimulates production of enzymes such as cellulase that cause leaf, fruit, or flower drop.
3.     It was an early practice to prepare citrus fruit for market by storage in a room with a kerosene stove.
4.     Later work revealed incomplete combustion of kerosene produced ethylene which ripens fruit.
5.     Ethylene stimulates production of cellulase, an enzyme that hydrolyzes cellulose in plant cell walls.
6.     A barrel of ripening apples can induce ripening of a bunch of bananas some distance away.
7.     Ethylene releases from the site of a physical wound; therefore one rotten apple spoils the whole bunch.
8.     The presence of ethylene in air inhibits the growth of plants in general.
26.2   Plant Responses
A.    Tropisms
1.     A tropism is plant growth toward or away from a unidirectional stimulus.
a.     The stimulus comes from only one direction instead of many.
b.     Growth toward a stimulus is a positive tropism; growth away from a stimulus is a negative tropism.
c.     By differential growth, one side elongates faster; the result is a curving toward or away from a stimulus.
2.     Three well-known tropisms are named for the stimulus that causes the response.
a.     Gravitropism is response to Earth’s gravity; roots demonstrate positive gravitropism
and stems demonstrate negative gravitropism.
b.     Phototropism is growth of plants in response to light; stems show positive phototropism.
c.     Thigmotropism is unequal growth due to touch (e.g., coiling of tendrils around a pole).
3.     Response to a stimulus first involves reception of the stimulus.
4.     The next step is transduction of the stimulus into a form meaningful to the organism.
5.     Finally, there is a response by the organism.
B.     Gravitropism
1.     An upright plant placed on its side displays negative gravitropism; it grows upward opposite gravity.
2.     Charles Darwin and his son were the first to show that roots display positive gravitropism.
a.     If the root cap is removed, roots no longer respond to gravity.
b.     Later researchers showed root cap cells contain statoliths, starch grains within amyloplasts.
c.     Due to gravity, amyloplasts settle to the lowest part of the cell.
3.     The connection between amyloplasts is uncertain.
a.   Amyloplasts could cause stems and roots to respond differently to auxin.
b.    The upper surface of a root elongates so that the root curves downward and the lower surface of a stem elongates, so that the stem curves upwards.
C.     Phototropism
1.     Phototropism occurs because cells on shady side of stems elongate due to the presence of auxin.
2.     Negative tropism occurs when a plant curves away from light.
3.     It is now known that phototropism occurs because plants respond to blue light.
a.     When the blue light is formed, the phototropin undergoes a conformation change.
b.     The change results in a transfer of a phosphate group from ATP, to a protein portion of the photoreceptor.
c.     The photoreceptor then triggers a transduction pathway that leads to the entry of auxin into the cell.
D.    Thigmotropism
1.     Unequal growth due to contact with solid objects is thigmotropism.
2.     The coiling of morning glory or pea tendrils around posts, etc., is a common example.
3.     Cells in contact with an object grow less while those on the opposite side elongate.
4.     This process is quite rapid; tendrils have been observed to encircle an object in ten minutes.
5.     A couple of minutes of touching can bring about a response that lasts for several days.
6.     The response can be delayed; tendrils touched in the dark will respond when illuminated.
a.     ATP rather than light can cause the response; the need for light is simply a need for ATP.
b.     The hormones auxin and ethylene are involved; they induce curvature of tendrils in the absence of touch.
7.     Thigmomorphogenesis is a touch response involving the whole plant.
a.     An entire plant responds to the presence of wind or rain.
b.     A plant growing in a windy location has a shorter, thicker trunk.
c.     Simple rubbing of a plant inhibits cellular elongation and produces a shorter, sturdier plant.
E.     Arabidopsis Is a Model Organism (Nature of Science reading)
1.     To study the actions of genes, including those that control growth and development, scientists have chosen the weed Arabidopsis thaliana as a study plant because:
a.     The plant is small so many hundreds of plants can grow in a small amount of space.
b.     The plant has a short generation time (5–6 weeks), and can produce about 10,000 seeds.
c.     The plant typically self-pollinates, but can cross pollinate, which is helpful for gene mapping and production of strains with multiple mutations.
d.     The number of DNA base pairs is relatively small.
2.     Since the Arabidopsis genome has been sequenced, genes can be cloned and used as probes for the isolation of the homologous genes from plants of economic value.
3.     Mutant plants are created to discover what each of its genes do.
a.     For example, one of the mutant genes that alters the development of flowers has been cloned and reintroduced into tobacco plants, where it caused sepals and stamens to appear where petals would ordinarily be.
4.     Knowledge about the development of the flowers in Arabidopsis can have far-ranging applications, including leading to more productive crops.
5.     In addition, the study of the Arabidopsis genome may promote plant molecular genetics, in general.
F.     Nastic Movements
1.     Turgor movements are dependent on turgor pressure changes in plant cells.
2.     In contrast to tropisms, turgor movements do not involve growth and are not related to the source of the stimulus.
3.     Turgor movements result from touch, shaking, or thermal stimulation.
4.     When a Mimosa pudica leaf is touched, the leaflets fold because the petiole droops.
a.     This response takes only a second or two and is due to a loss of turgor pressure within cells.
b.     A pulvinus is a thickening at the base of such leaflets where turgor pressure can rapidly drop.
c.     Potassium ions move out of the cell and water follows by osmosis.
d.     A single stimulus, such as a hot needle, can cause all of the leaves to respond; this requires a nerve impulse-like stimulus for communication.
5.     Venus flytrap
a.     This plant has three sensitive hairs at the base of the trap.
b.     When touched by an insect, an impulse-type stimulus triggers the trap to close.
c.     Two ideas as to what causes the electrical charge are:
i.      The cells located near the outer region of the lobes rapidly secrete hydrogen ions (H+) into their cell walls, loosening them, and allowing the walls to swell rapidly by osmosis; or
ii.     The cells in the inner portion of the lobes and the midrib rapidly lose ions, leading to a loss of water by osmosis and collapse of these cells.
ii.     Whatever the mechanism, turgor movements are involved.
6.     Sleep Movements
a.     Circadian rhythms are sleep movements that occur regularly in a 24-hours cycle.
b.     A common example of circadian rhythms is in the prayer plant (Maranta leuconeura).
i.      At night, the leaves of the prayer plant fold upward into a shape resembling hands of prayer.
ii.     This movement is also due to changes in the turgor pressure of motor cells in the pulvinus located at the base of each leaf.
c.     Other example of circadian rhythms are:
i.      Morning glories (Ipomoea leptophylla) open its flowers during the early part of the day and close them at night.
ii.     In most plants the stomata open in the morning and close at night.
iii.    In some plants nectar is secreted at the same time of the day or night.
d.     To qualify as a circadian rhythm, the activity must:
i.      Occur every 24-hours.
ii.     Take place in the absence of external stimuli.
iii.    Be able to be reset if external cues are provided.
5.     A biological clock is an internal mechanism maintaining circadian rhythm in the absence of stimuli.
a.     Biological clocks are synchronized by external stimuli to 24-hour rhythms.
b.     Photoperiod is more reliable an indicator of seasonal changes than temperature change.
c.     The biological clock may involve the transcription of a number of “clock genes.”
i.      One model proposes that the information-transfer system from DNA to RNA to enzyme to metabolite, with all its feedback controls, is intrinsically cyclical and could be the basis for biological clocks.
ii.     These genes control sleep movements, opening and closing of stomata, the discharge of floral fragrances, and metabolic activities associated with photosynthesis.
d.     The biological clock also influences seasonal cycles that depend on day-night length.
G.    Photoperiodism
1.     Many physiological changes in plants (e.g., seed germination, the breaking of bud dormancy, and the onset of senescence) are related to a seasonal change in day length.
2.     Photoperiodism is a physiological response prompted by changes in the length of a day or night in a 24-hour daily cycle.
3.     Phytochrome is a blue-green leaf pigment that is present in the cytoplasm of plant cells.
a.     Pr (phytochrome red) absorbs red light (wavelength of 660 nm); it is converted to Pfr.
b.     Pfr is phytochrome far-red and absorbs far-red light (wavelength of 730 nm); it is converted to Pr.
c.     During a 24-hour period, there is a shift in ratio of these two pigments.
i.      Direct sunlight contains more red than far-red light; Pfr is present in plant leaves during the day.
ii.     Shade and sunsets have more far-red than red light; Pfr is converted to Pr as night approaches.
iii.    There is a slow metabolic replacement of Pfr by Pr during night.
4.     Functions of Phytochrome
a.     The Pr à Pfr conversion cycle controls other growth functions in plants.
b.     In addition to being involved in flowering, Pfr promotes seed germination and inhibits stem elongation.
c.     Following germination, the presence of Pr dominates; the stem elongates and grows toward sunlight while the leaves remain small.
d.     Once a plant is exposed to sunlight and Pr is converted to Pfr, the plant begins to grow normally—leaves expand and the stem branches.
e.     Flowering plants can be divided into three groups, based their flowering status:
i.      Short-day Plants
a)     These plants flower when day length is shorter than a critical length.
b)    Examples include cocklebur, goldenrod, poinsettia, and chrysanthemum.
c)     In effect, they require a period of darkness that is longer than a critical length to flower.
ii.     Long-day Plants
a)     These plants flower when the day length is longer than a critical length.
b)    Examples include wheat, barley, rose, iris, clover, and spinach.
c)     In effect, they require a period of darkness that is shorter than a critical length to flower.
iii.    Day-neutral Plants
a)     These are plants for which flowering is not dependent on day length.
b)    Examples include tomato and cucumber.
f.      A long-day and a short-day plant can have the same critical length.
i.      These plants differ in specific sequence of day lengths as a season progresses.
ii.     Spinach is a long-day plant that flowers in summer when day length increases to 14 hours.
iii.    Ragweed is a short-day plant that flowers in fall when day length shortens to 14 hours or less.
g.     Scientists experimented with artificial lengths of dark and light periods.
i.      Cocklebur, a short-day plant, flowers as long as the dark period lasts over 8.5 hours.
ii.     If the dark period is interrupted by a flash, it does not flower; but darkness amidst a day cycle has no effect.
iii.    Long-day plants require a dark period shorter than a critical length regardless of the length of the light period.
iv.    Therefore, length of the dark period controls flowering, not length of the light period.
H.    Responses to the Biotic Environment
1.     Plants have many defense mechanisms to protect themselves from herbivore and parasite attacks.
2.     Plants have physical defenses such as cuticle-covered epidermis and bark.
3.     Despite the physical defenses, herbivores can still attack the plant, so plants need several other types of defenses.
4.     Plants produce secondary metabolites as a defense mechanism.
a.     Tannins, in or on the leaf epidermis, are defensive compounds that interfere with the outer proteins of bacteria and fungi.
b.     Tannins also deter predators because of the astringent effect on the mouth and interference with digestion.
c.     Alkaloids (e.g., morphine, nicotine, caffeine) are also secondary metabolites.
i.      Coffee plant seedlings have a high concentration of caffeine, which can kill insects and fungi by blocking DNA and RNA synthesis.
d.     Cyanogenic glycosides break down to cyanide and inhibit cellular respiration.
i.      Foxglove (Digitalis purpurea) produces deadly cardiac and steroid glycosides, which cause nausea, hallucinations, and death in animals that ingest them.
5.     Plants also respond when they have been wounded or attacked.
a.     After a leaf has been chewed or injured, the plant produces proteinase inhibitors, a chemical that destroys the digestive enzymes of a predator feeding on them.
b.     Systemin is a small peptide that serves as the defense hormone, bringing about the effect of proteinase inhibitors.
6.     Sometimes plants produce a specific gene product that binds to either a viral, bacterial, or fungal gene product made within the cell, so the plant can “recognize” a particular pathogen.
a.     Following gene production, a transduction pathway ensues, and the final result is a hypersensitive response (HR) that seals off the infected area and will also initiate a wound response.
7.     Some plant defense mechanisms are indirect defense mechanisms.
a.     Since female butterflies are less likely to lay their eggs on plants that already have butterfly eggs, the leaves of some passion flowers (genus Passiflora) display physical structures resembling the eggs, so the butterflies do not lay eggs on those plants.
b.     Lima beans produce volatiles that attract carnivore mites only when they are being damaged by a spider mite.
c.     Corn and cotton plants release volatiles that attract wasps, which then inject their eggs into caterpillars eating their leaves.
8.     Some plants have mutualistic relationships with other animals to protect them from herbivores.
a.     A species of acacia ant, Pseudomyrmex ferruginea, has swollen thorns with a hollow interior and ants will live and feed (without harming) off the acacia. In turn, the ants protect the plant by attacking and stinging herbivores.