39.0 Introduction

1. Plants Respond to Their Environment
1. Plants Do Not Move Like Animals Do
2. Plants Undergo Continuous Development fig 39.1
1. Genetic blueprint controls various events
2. Events greatly influenced by external factors
3. Differentiation of specific tissues controlled by hormones
4. Hormones mediated by genes and environmental factors

1. Differentiation in Plants: Experimental Evidence
1. Totipotency of Single Cells
1. Plant differentiation is fully reversible: Dedifferentiation
2. Gene expression reactivated in cells retaining protoplast and nucleus at maturity
3. Reactivation may lead to alternative differentiation or complete plant
4. Haberlandt proposed that all living plant cells are totipotent
1. Possess full genetic potential of the organism
2. Hypothesis not confirmed until cells could be grown in culture

2. Cell Culture
1. Relatively easy to isolate individual cells
2. Repeated division could not be stimulated
3. Solution utilized filter paper floating on established cell culture
1. Single cell in culture media placed on filter paper
2. Isolated from other cells, but still influenced by them
3. Isolated cell obtained various growth promoting substances
4. Established mass of undifferentiated cells called callus

4. Most plants require the addition of coconut milk to culture medium

3. Tissue Culture
1. Steward supplied differentiated cells with substances from dividing cells
2. Small bits of carrot secondary phloem tissue isolated and placed in flask fig 39.2
1. Growth media contained sucrose, minerals and vitamins
2. New cell clumps differentiated roots fig 39.3a
3. Developed shoots when placed on agar fig 39.3b
4. Grew into whole plants, confirmed Haberlandt's hypothesis fig 39.3c

3. Demonstrated differentiated phloem contained all genetic potential for whole plant
4. Stages resembled embryonic development of normal zygotes = "embryoids"

4. Regeneration in Nature
1. Common practice uses cuttings of plants to produce whole new plants
2. Formation of adventitious roots from mature root primordia
1. Produced at base of cut stem or from phloem tissue
2. Primordia differentiate into root meristems

3. Adventitious shoots do not readily form
1. Some plant cuttings root if simply placed in water or wet sand fig 39.4
2. Other plants do not readily produce roots
3. Stems with leaves form roots more readily than those without
4. Leaves and buds produce auxins that stimulate root growth

4. Regeneration from other tissues
1. Bits of succulent leaf tissue may produce entire plants
2. Tiny plantlets differentiate along leaf edges of some plants
3. Propagation from rhizomes, stolons or other modified stems or roots
4. Century plants may form plantlets among flowers
5. Can produce seeds with embryos from unfertilized eggs

5. Stems readily produce adventitious roots
6. Roots produce adventitious stems less frequently

1. Plant Hormones
1. Expression of Plant Genes Controlled by Plant Hormones
1. Differentiated tissue capable of expressing hidden genetic complement
2. Must provide suitable environmental signal

2. Chemical Nature of Hormones
1. Chemical substances produced in small quantities in one location
2. Transported to another location to effect physiological response
3. Response can be stimulatory or inhibitory
4. Animal hormones produced at definite sites, organs of hormone production
5. Plant hormones not produced in such specialized tissues
6. Five major kinds of plant hormones tbl 39.1
1. Auxin
2. Cytokinins
3. Gibberellins
4. Ethylene
5. Abscisic acid

7. Study of hormones an active field of research fig 39.5

2. Auxins
1. Basic Effects of Auxins
1. Increases plasticity of plant cell walls
2. Regulates stem elongation along with gibberellins

2. Discovery of Auxin
1. Experiments by Charles and Francis Darwin
1. Observed phototropism: Bending of seedlings toward light
2. Response occurred if tip of seedling covered with glass rod
3. Response prevented in seedling tips covered with foil fig 39.6
4. Response occurred if stem below tip covered with opaque collar
5. Conclusion: Substance produced in response to light was transmitted downward causing shoot to bend toward the light

2. Experiments by Boysen-Jensen and Paal
1. Identified substance as a chemical
2. Normal response if tip separated from shoot by agar block
3. In darkness or normal illumination chemical passed down shoot evenly on all sides, thus no bending occurred

3. Experiments by Went fig 39.7
1. Cut tips from illuminated seedlings, placed them on agar
2. Cut tips from seedlings grown in the dark
3. Placed tiny agar blocks off-center on tipless seedlings grown in dark
4. Seedlings bent away from side on which block was placed
5. Conclusion: Substance diffused from agar, enhanced cell elongation fig 39.8
6. Named substance auxin from Greek "to increase"

4. Dark side of seedling has more auxin, its cells elongate more, which bends the seedling
5. Auxin acts to adapt plant to its environment in advantageous way
1. Environmental signals directly influence distribution of auxin in plant
2. May exert this influence in several ways
1. Light might destroy auxin
2. Light may decrease cells' sensitivity to auxin
3. Light might cause auxin to migrate away from light into shaded part of shoot

3. Last possibility is the actual mode of influence

3. How Auxins Work
1. Experiments by Briggs fig 39.9
1. Vertical mica sheet separated light and dark sides of the tip
2. No bending, same amount of auxin on both sides of barrier
3. Conclusion: Auxin migrates laterally from light side to dark side

2. Chemical nature of auxin
1. Only naturally occurring compound is indoleacetic acid (IAA) fig 39.10
2. Resembles and is probably synthesized from tryptophan

3. Three other auxinlike substances occur, not converted to IAA
1. Phenylacetic acid (PAA) is abundant but less active than IAA
2. Indolebutyric acid (IBA) is present in leaves of corn and various dicots
3. 4-chloroindoleacetic acid (4-chloroIAA) found in legume germinating seeds

4. Auxin used to describe natural and synthetic products that produce similar effects

4. Auxins and Plant Growth
1. Mechanism of action: Increases plasticity of cell wall
1. Plastic wall stretches with protoplast swelling during active cell growth
2. Promoted by very low auxin concentrations
3. Must rapidly degrade hormone to prevent its accumulation

2. Monoclonal antibodies used to determine auxin transport sites
1. Auxin moves downward from shoot apex through plant
2. Sites are in plasma membranes of cells transporting auxins

3. Speed of reaction makes determination of chemical basis difficult
1. Grass seedlings may begin to bend within ten minutes
2. Unlikely that reaction results from transcription/translation of genes
3. Must effect already existing system
1. Changes in polysaccharides of plant cell walls
2. Increase in concentration of H⁺ ions
3. Mediates stimulation of mRNA transcription for long-term growth changes

4. Additional effects
1. Promotes growth of vascular tissue and vascular cambium
2. Present in large quantities in pollen, key role in fruit development
1. Fruit fails to develop without fertilization and in absence of seeds
2. Fruit will grow when auxins applied

5. Synthetic auxins
1. Examples: Naphthalene acetic acid (NAA) and IBA
2. Primarily used to prevent abscission, separation of organ from plant
3. Commercial applications
1. Prevent fruit drop
2. Promote flowering and fruiting in pineapples fig 39.11
3. Induce formation of roots on cuttings
4. Herbicides to control weeds: 2,4-D fig 39.10c
1. Selectively eliminates broad-leaved dicots
2. Stems of dicot weeds halt axial growth

5. Herbicide 2,4,5-T used as broad spectrum herbicide
1. Defoliant Agent Orange during Vietnam War
2. Contaminated with dioxin a toxic by-product
3. Dioxin produces serious diseases, birth defects, death

3. Cytokinins
1. Actions of Cytokinins
1. Promote differentiation of organs in masses of cultured plant tissue
1. Induces parenchyma cells to become meristematic
2. Induce differentiation of cork cambium
3. Causes differentiation of callus tissue

2. Mechanism of action
1. When combined with auxin, cell division stimulated and differentiation induced
2. Mostly produced in roots and transported throughout plant, also by fruit
1. Cause formation of vegetative buds on moss protonemata
2. Regulate growth patterns in concert with other hormones

2. Formation of Cytokinins
1. Chemically derived from or related to adenine fig 39.12
2. Other similarly acting chemicals exist, but are not produced naturally
3. Act opposite of auxin
1. Promote growth of lateral buds into branches fig 39.13
2. Play role in apical dominance, suppress lateral bud growth
3. Suppress formation of lateral roots
4. In balance with auxin determines appearance of mature plant
5. Prevents yellowing of leaves detached from plant

4. Action studied in terms of effects on masses of tissues grown in culture
1. Coconut needed for early experiments contains cytokinins
2. Appear to be necessary for mitosis and cell division
3. Promote synthesis or activation of proteins needed for mitosis

4. Gibberellins
1. Discovery of Gibberellin
1. Named for fungus that causes "foolish seedling" disease in rice
1. Isolated from fungal filtrate
2. Causes infected plant to grow abnormally tall

2. Large class of chemicals additionally found in normal plants
1. Acidic compounds abbreviated GA, for gibberellic acid
2. Subscripts added to distinguish individual compounds

2. Mode of Action
1. Synthesized in apical portions of stems and roots
2. Promotes internodal elongation
1. Enhanced by auxin
2. Restored normal growth to dwarf plant mutants fig 39.14

3. Stimulate hydrolytic enzyme production in germinating grain seed
1. Initiates burst of mRNA and protein synthesis
2. Enhances DNA binding proteins, allow DNA transcription of a gene
3. Occurs when radicle has grown through seed coats

4. Induce biennial plants to flower fig 39.15
5. Speeds seed germination
6. Only gibberellin GA1 is active in shoot elongation

5. Ethylene
1. Action of Ethylene
1. Initial observation of ethylene gas inducing defoliation
2. Acts alone and interacts with other plant hormones
1. Suppresses lateral bud formation when combined with auxin
2. Suppresses stem and root elongation

3. Plays major role in ripening of fruit
1. Auxins from pollen stimulate ethylene production
2. Hastens fruit ripening

4. Higher concentrations of ethylene eventually counter effects of auxin
1. Fruit forms separation layer at base of leaf petioles
2. Abscission does not occur if present or applied to flowers or fruit

5. Produced in large quantities during climacteric of fruit ripening
1. Hastens ripening, respiration proceeds at much more rapid rate
2. Complex carbohydrates broken down into simple sugars
3. Chlorophylls broken down
4. Cell walls become soft
5. Volatile chemicals produced, associated with flavor and scent of ripe fruit

6. Recognition of ethylene as plant hormone associated with premature ripening
1. Bananas ripened early in presence of oranges
2. Lead to commercial uses of ethylene
1. Tomatoes picked green, artificially ripened with ethylene
2. Speed ripening or oranges, limes
3. Carbon dioxide has opposite effect, fruit shipped in CO₂ atmosphere

2. Ecological Role
1. Ethylene production increased after exposure to adverse conditions
1. Includes exposure to ozone and toxic chemicals
2. Includes temperature extremes, drought, attack by pathogens and herbivores

2. Can accelerate abscission of leaves or fruit damaged by stresses fig 39.16
3. Damage from exposure to ozone due to ethylene production

6. Abscisic Acid
1. Discovery of Abscisic Acid
1. Synthesized primarily in mature green leaves, fruit and root caps
1. Application seemed to stimulate leaf senescence and abscission
2. Little evidence it plays an important role in the processes

2. May cause ethylene synthesis, aging effects due to ethylene

2. Actions of Abscisic Acid
1. Application on leaves causes yellow spots (opposite effect as cytokinins)
2. May induce formation of winter buds
1. Suppresses growth
2. Followed by conversion of leaf primordia into bud scales fig 39.17a

3. Suppresses growth of dormant lateral buds, along with ethylene
1. Counters effects of gibberellins
2. Promotes senescence by countering auxin

4. Causes dormancy of seeds
5. Controls opening and closing of stomata fig 39.17b
6. Physiological effects are extremely rapid
1. Partly independent of gene expression
2. Long term effects involve regulation of gene expression

1. Tropisms
1. Orientation in Response to External Stimuli
1. Influence growth patterns of plants, influence appearance
2. Produce adjustments to conditions of environment
3. Three major classes
1. Phototropism
2. Gravitropism
3. Thigmotropism

2. Phototropism
1. Bending of plants toward unidirectional sources of light fig 39.18
1. Stems grow toward light and are positively phototropic
2. Roots are slightly negatively phototrophic or neutral

2. Response is adaptive for leaves to capture greater amounts of light
3. Important in determining development of plant organs, appearance of plant
4. Most phototrophic responses mediated by auxins

3. Gravitropism
1. Response is to gravity, formerly known as geotropism
2. Causes stems to grow upward and roots downward
3. Obviously adaptive to both roots and stems
4. Differential in auxin concentration develops in horizontal stems
1. More auxin on lower side causes these cells to elongate, stem rises
2. A negative gravitropic response fig 39.19
3. Concentration gradient not well documented in roots

5. Upper sides of horizontal roots grow more rapidly than lower sides
1. Root ultimately grows downward
2. A positive gravitropism
3. May be related to stimulation of root cap cells by amyloplasts
4. Amyloplasts drift in direction of gravity, new growth is downward
5. Roots lacking amyloplasts grow randomly

6. Roots in tropical rainforests often grow upward
1. Soil is very nutrient poor
2. Precipitation is more reliable source of nutrients

4. Thigmotropism
1. Response of plants to touch fig 39.20a
1. Specialized cells of tendril perceive contact
2. Promote uneven growth

2. Causes curling of tendrils, twining of vines
1. Initial coiling may be in one direction ,then opposite fig 39.20b
2. Other coiling occurs in only one direction

3. Coiling of tendrils is associated with auxin and ethylene

5. Other Tropisms
1. Electrotropism is a response to electricity
2. Chemotropism is a response to chemicals
3. Traumatropism a response to wounding
4. Thermotropism is a response to temperature
5. Aerotropism is a response to oxygen
6. Skototropism is a response to dark
7. Geomagnetotropism is a response to magnetic fields
8. Hydrotropism is not a true response, although roots do grow toward water

2. Turgor Movements
1. Movement Via Reversible Turgor Pressure Changes in Specific Cells
1. Water leaving cells may cause them to collapse
2. Water entering limp cells cause them to become turgid

2. Types of Movements
1. Changes in position of flowers and leaves
1. Seen in flowers that open during the day and close at night
2. Prayer plants leaves are horizontal in day, vertical at night fig 39.21

2. Movement associated with pulvinus, turgor of motor cells
1. Example: Touch sensitive plants like Mimosa fig 39.22
2. Movements are extremely rapid
3. Touch generates electrical signal
4. Electric signal changed to chemical signal
5. Potassium ions and water migrate from one half of pulvinus to other
6. Loss of turgor causes leaf to fold
7. In time water flows back and leaf returns to original position

3. Leaves of some plants track the sun
1. Blades held at right angles to sun
2. Cause for orientation largely unknown
3. Sunflowers do not follow sun, face east

4. Movements in carnivorous plants like Venus flytrap
1. Not caused by changes in turgor pressure as leaves do not have pulvini
2. With stimulation of two of six trigger hairs, certain cells irreversibly enlarge
1. Initiated by drop in pH in cell walls
2. Walls most flexible at ph 3 to 4
3. Expends ATP

3. Cells on opposite side expand slowly to open leaf

1. Photoperiodism
1. Mechanism to Measure Seasonal Changes in Day and Night Length

2. Flowering Responses
1. Pronounced Changes in Day Length Associated with Seasons
1. Greater variation in change in day length when farther from equator
2. Short-day plants flower when days get shorter than critical length fig 39.23
3. Long-day plants flower when days get longer than critical length
4. Day neutral plants
1. Produce flowers whenever environmental conditions are suitable
2. No reference to day length

5. Intermediate-day plants
1. Some grasses have two critical photoperiods
2. Respond to various segments of day length

6. Significant stimulus is length of darkness not length of day
1. Long-day plants flower if day length is between 12 and 16 hours
1. Bloom in spring and early summer
2. Include clover, iris, lettuce, spinach, hollyhocks

2. Short day plants flower when day length is less than 14 hours
1. Bloom in late summer and autumn
2. Include chrysanthemums, goldenrods, poinsettias, soybeans, many weeds

7. Light artificially controlled to force plants to flower out of season
8. Helps control distribution of plants

3. The Chemical Basis of the Photoperiodic Response
1. Interruption of Normal Responses
1. Brief period of light within dark period cancels flowering response
2. Effective wavelength is at 660 nanometers, red light
3. Effect canceled if followed by far-red light at 730 nanometers

2. Chemical Basis of Effect
1. Presence of two forms of phytochrome: P_r and P_fr
1. Pr absorbs red light and is converted to P_fr
2. P_fr absorbs far-red light and is converted to P_r
3. P_fr is biologically active, Pr is biologically inactive

2. In short-day plants P_fr leads to suppression of flowering fig 39.24
1. In darkness P_fr is converted to P_r
2. When darkness is long enough, suppression is removed, plants flower
3. Single flash of red light converts Pr to P_fr, flowering blocked
4. Conversion of P_r and P_fr not sole factor controlling flowering

3. Chemical Nature of Phytochrome
1. Composed of small, light sensitive part and large protein part
2. Pigment is blue, similar to phycobilins in algae and cyanobacteria
3. Phytochrome involved in other growth responses
1. Seed germination inhibited by far-red light, stimulated by red light
2. Slender, colorless seedlings exposed to red light regain shoot length fig 39.25
3. Effects canceled by far-red light

4. Etiolated seedlings may act light a bundle of fiber optic strands
1. Guide light over distance of several centimeters
2. Light carried through cells and junctions between them
3. Light may go to and affect plant portions below ground

4. The Flowering Hormone: Does It Exist?
1. Removal of leaves affects response to day length and inhibits flowering
1. Presence of single leaf or exposure of that leaf initiates flowering
2. Leaf removed immediately after exposure results in no flowering
3. Leaf left on for a few hours then removed, results in flowering

2. Substance produced in leaves passes to apices to promote flowering
3. Substance does not pass through agar block, requires presence of plant parts
4. Substance not identified after 50 years of searching

4. Dormancy
1. In Temperate Climates
1. Associate dormancy with winter
2. Low temperatures and unavailability of water prevent growth
3. Tree buds are dormant, perennials reduced to underground parts, other plants exist as only seeds

2. In Seasonally Dry Climates
1. Dormancy occurs during dry season
2. Strategies similar to temperate plants

3. In Areas with Seasonal Drought
1. Predominance of annual plants
2. Seeds are capable of surviving indeterminate dry seasons
3. Seeds may contain chemicals that must leach out with sufficient water
4. Rapidly germinate, grow and flower when water becomes available fig 39.26

4. Seed Dormancy
1. Remain viable for long periods of time, especially legumes
2. Period of cold may be required to initiate germination of some seeds
3. Other seeds require only adequate water and high temperatures
4. Cold may be needed for some buds to break dormancy