LECTURE NOTES CHAPTER 35

 

Control systems in plants are adaptations that evolved over time in response to interactions with their environment. Plants respond to environmental stimuli by:

·    Sending signals between different parts of the plant.

·    Tracking the time of day and the time of year.

·    Sensing and responding to gravity and direction of light, etc.

 

I. Research on how plants grow toward light led to the discovery of plant hormones: science as a process

 

Hormone = A compound produced by one part of an organism that is transported to other parts where it triggers a response in target cells and tissues.

 

Phototropism = Growth toward or away from tight.

•    Growth of a shoot toward light is positive phototropism; growth away from the light is negative phototropism.

•    Results form differential growth of cells on opposite sides of a shoot or, in the case of a grass seedling, coleoptile.

•    Cells on the darker side elongate faster than those on the light side.

 

Experiments on phototropism led to the discovery of a plant hormone.

 

1.   Charles and Francis Darwin removed the tip of the coleoptile from a grass seedling (or covered it with an opaque cap) and it failed to grow toward light. They concluded that:

 

·         The coleoptile tip was responsible for sensing light.

·         Since the curvature occurs some distance below the tip, the tip sends a signal to the elongating region.

 

2.   Peter Boysen-Jensen separated the tip from the remainder of the coleoptile by a block of gelatin, preventing cellular contact but allowing chemical diffusion.

 

·         Seedlings behaved normally.

 

·         If an impenetrable barrier was substituted, no phototropic response occurred.

 

·         These experiments demonstrated that the signal was a mobile substance.

 

3.      F.W. Went removed the coleoptile tip, placed it on an agar block, and then put the agar (without the tip) on decapitated coleoptile kept in the dark.

·        A block centered on the coleoptile caused the stem to grow straight up.

·        If the block was placed off center, the plant curved away from the side with the block.

·        Went concluded the agar block contained a chemical that diffused into it from the coleoptile tip, and that this chemical stimulated growth

·        Went called this chemical an auxin.
      

4.     Kenneth Thimann later purified and characterized auxin.

 

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

 

Plant hormones may function in one of many ways:

 

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

~   Effects depend on site of action, stage of plant growth and hormone concentration.

·         The hormonal signal is amplified, perhaps by affecting gene expression, enzyme activity, or membrane properties.

·         Reaction to hormones depends on hormonal balance (relative concentration of one hormone compared with others).

 

Five classes of plant hormones have been identified. (See Campbell, Table 35.1)

I.          Auxin (such as IAA)

2.         Cytokinins (such as zeatin)

3.         Gibberellins (such as GA3)

4.         Abscisic acid

5.         Ethylene

 

A. Auxin

 

Auxin = A hormone that promotes elongation of young developing shoots or coleoptiles.

·         The natural auxin found in plants is a compound named indoleacetic acid (IAA).

 

The apical meristem is a major site of auxin production.

·         Stimulates cell growth only at concentrations between 10-8 to l& M.

·         Moves from the apex down to the zone of cell elongation at a rate of about 10 mm per hour.

~ This is faster than would be found in diffusion but much slower than in phloem translocation.

 

Polar transport of auxin is unidirectional and requires metabolic energy. (See Campbell, Figure 35.4)

·         IAA is actively transported down a stem by auxin carriers located on the basal ends of cells (carriers are absent on the apical ends).

·         Energy for active transport is provided by chemiosmosis.

·         Movement of auxin is aided by the differences in pH between the acidic cell wall

        and the neutral cytoplasm.

~    ATP-driven pumps maintain a proton gradient across the plasma membrane.

~    As auxin passes through the acidic cell wall, it picks up a proton to become electrically neutral, which allows it to pass through the plasma membrane.

~    Auxin is ionized in the neutral intracellular environment which temporarily traps it within the cell since the plasma membrane is less permeable to ions.

                                 ~    Auxin can only exit the cell by the basal end, where specific carrier

proteins are built into the membrane. The proton gradient contributes to auxin efflux by favoring the transport of anions out of the cell.

 

The acid-growth hypothesis states that cell elongation is due to stimulation of a proton pump which acidifies the cell wall.

·         Acidification causes the crosslinks between the cellulose myofibrils of the cell walls to break.

·         This loosens the wall, allowing water uptake which results in elongation of the cell.

 

In addition to stimulating cell elongation, auxin also:

·         Affects secondary growth by inducing vascular cambium cell division and differentiation of secondary xylem.

·         Promotes formation of adventitious roots.

·         Promotes fruit growth in many plants.

 

Auxins are used as herbicides. 2,4-fl is a synthetic auxin which affects dicots selectively, allowing removal of broadleaf weeds from a lawn or grain field.

 

B. Cytokinins

 

Cytokinins = Modified forms of adenine that stimulate cytokinesis.

·         Move from the roots to target tissues by moving up in the xylem sap.

·         Stimulate RNA and protein synthesis. The new proteins produced by stimulation of RNA appear to be involved in cell division.

 

Cytokinins function in several areas of plant growth:

·         Cell division and differentiation.

·         Apical dominance.

·         Anti-aging hormones.

 

Cytokinins, in conjunction with auxin, control cell division and differentiation.

·         Stem parenchyma cells cultured without cytokinins grow very large and do not divide.

·         Cytokinins added alone have no effect on cells grown in tissue culture.

             ~     Equal concentrations of cytokinins and auxins stimulate cells to grow and divide, but they remain an undifferentiated callus.

~    More cytokinin than auxin causes shoot buds to develop from the callus.

~   More auxin than cytokinin causes roots to form.

 

Cytokinins and auxin contribute to apical dominance through an antagonistic mechanism.

·         Auxin from the terminal bud restrains axillary bud growth causing the shoot to lengthen.

·         Cytokinins (from the roots) stimulate axillary bud growth.

·         Auxin cannot suppress axillary bud growth once it has begun.

·         Lower buds thus grow before higher ones since they are closer to the cytokinin source than the auxin source.

·         Auxin stimulates lateral root formation while cytokinins restrain it.

·        This stimulation-inhibition action may help balance plant growth since an increase in the root system would signal the plant to produce more shoots.

 

             Cytokinins can retard aging of some plant organs, perhaps by inhibiting protein breakdown, stimulating RNA and protein synthesis, and mobilizing nutrients.

·         May slow leaf deterioration on plants since detached leaves dipped in a cytokinin solution stay green longer.

 

C. Gibberellins

 

         More than 70 different gibberellins, many naturally occurring, have been identified.

 

         Gibberellins are produced primarily in roots and young leaves. They:

·         Stimulate growth in leaves and stems but show little effect on roots.

·         Stimulate cell division and elongation in stems, possibly in conjunction with auxin.

·         Gibberellins cause bolting (rapid growth of floral stems, which elevates flowers).

 

        Fruit development is controlled by both gibberellins and auxin.

·         In some plants, both must be present for fruit set.

·         Commercial application of gibberellins is the spraying of Thompson seedless grapes. The hormones cause the grapes to grow larger and farther apart after treatment.

        The release of gibberellins signals seeds to break dormancy and germinate.

·         A high concentration of gibberellins is found in many seeds, especially in the embryo.

·         Imbibed water appears to stimulate gibberellin release.

·         Environmental cues may also cause gibberellin release in seeds which require special conditions to germinate.

·         In cereal grains, gibberellins stimulate germination and support growth by stimulating synthesis of mRNA which codes for a-amylase. The a-amylase then digests the stored nutrients, making them available to the embryo and seedling.

 

         In breaking both seed dormancy and apical bud dormancy, gibberellins act antagonistically with abscisic acid, which inhibits plant growth.

 

D. Abscisic Acid (ABA)

 

             Abscisic acid is produced in the terminal bud and helps prepare plants for winter by suspending both primary and secondary growth.

·         Directs leaf primordia to develop scales that protect dormant buds.

·         Inhibits cell division in vascular cambium.

 

            The onset of seed dormancy is another time it is advantageous to suspend growth.

·         In most cases, the ratio of ABA: gibberellin determines whether seeds remain dormant or germinate.

·         In other plants, seeds germinate when ABA is washed out of the seeds (desert plants) or degraded by some other stimulus such as sunlight.

 

          ABA also acts as a stress hormone, closing stomata in times of water-stress thus reducing  

          transpirational water loss.

 

E. Ethylene

           

            Ethylene = A gaseous hormone that diffuses through air spaces between plant cells.

·         Ethylene can also move in the cytosol, traveling from cell to cell in the phloem or symplast.

·         High auxin concentrations induce release of ethylene, which acts as a growth inhibitor.

 

Senescence (aging) is a natural process in plants that may occur at the cellular, organ, or whole plant level. Ethylene probably plays an important role at each level.

·         Examples include: xylem vessel elements and cork cells which die before becoming fully functional; leaf fall in the autumn; withering of flowers; death of annuals after flowering.

·         The best studied forms of senescence are fruit ripening and leaf abscission.

 

During fruit ripening, ethylene triggers senescence, and then the aging cells release more ethylene.

·         The breakdown of cell walls and loss of chlorophyll are considered aging processes.

·         The signal to ripen spreads from fruit to fruit since ethylene is a gas.

 

Leaf abscission is an adaptation that prevents deciduous trees from desiccating during winter when roots cannot absorb water from the frozen ground.

·         Before abscission, the leafs essential elements are shunted to storage tissues in the stem from which they are recycled to new leaves in the spring.

·         Environmental stimuli are shortening days and cooler temperatures.

 

When a leaf falls, the breakpoint is an abscission layer near the petiole base.

·         Weak area since the small parenchyma cells have very thin walls and there are no fiber cells around the vascular tissue.

·         Mechanics of abscission are controlled by a change in the balance of ethylene and auxin.

             ~ Auxin decrease makes cells in the abscission layer more sensitive to ethylene.   

                Cells then produce more ethylene which inhibits auxin production.

             ~ Ethylene induces synthesis of enzymes that digest the polysaccharides in the

                 cell walls, further weakening the abscission layer.

·         Wind and weight cause the leaf to fall by causing a separation in the abscission layer.

·         Even before the leaf falls, a layer of cork forms a protective scar on the twigs side of the abscission layer. The cork prevents pathogens from entering the plant.

 

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

 

       Tropisms = Growth responses that result in curvatures of whole plant organs toward or away

            from stimuli.

 

•    Mechanism is a differential rate of cell elongation on opposite sides of the organ.

 

Three primary stimuli which result in tropisms are: light (photo), gravity (gravit), and touch (thigmo).

1.  Photo tropism is a response to light.

2.  Gravitropism is a response to gravity.

3.  Thigmotropism is a response to touch.

 

A.       Phototropism

 

Phototropism = Growth either toward or away from light.

•     It is generally accepted that cells on the darker side of a grass coleoptile elongate faster than cells on the bright side due to asymmetric distribution of auxins moving down from the shoot tip.

•     For organs, other than grass coleoptiles, the mechanism may be different.

                                   ~ No evidence exists that unilateral light causes an asymmetric   

                                      distribution of auxins in the stems of many dicots.

                                    ~ There is evidence that other substances which act as growth inhibitors

                                      do have an asymmetric distribution toward the lighted side of the      

                                   stem.

•     Regardless of the mechanism, the shoot tip is the site of the photoreception that triggers the growth response.

                                  ~ A photoreceptor sensitive to blue light is present in the shoot tip; this

                                     receptor is believed to be a yellow pigment related to riboflavin.

                                  ~ The same receptor may be involved in other plant responses to light.

 

B.       Gravitropism

 

Gravitropism = Orientation of a plant in response to gravity.

•     Roots display positive gravitropism (curve downward).

•     Shoots display negative gravitropism (bend upward).

 

The possible mechanism of gravitropism in roots:

•     Specialized plastids containing dense starch grains (statoliths) aggregate in the low points of plant cells.

•     In roots, statoliths occur in certain root cap cells.

                                 ~ Aggregating statoliths trigger calcium redistribution which results in

                                    lateral transport of auxin in the root.

                                 ~ Calcium and auxin accumulate on the lower side of the elongation zone.

                                      ~ Roots curve down because, at high concentrations, auxin inhibits root

                                          cell elongation, so cells on the upper side elongate faster than those on    

                                          the lower side.

 

Researchers are challenging the falling statolith hypothesis for positive gravitropism in root growth.

·         Insufficient energy is released by starch grains settling to the bottom of cells to

account for gravitational detection.

·         Many plants lacking starch grains distinguish up from down.

·        Studies on Chara, a green alga closely related to plants, indicate the settling of the entire protoplasm provides a cell with its up-down orientation.

~ The protoplast is attached to the inside of the cell wall by proteins.

                        ~ When the protoplast settles, the protein tethers at the top of the cell are

                 stretched and those at the bottom are compressed.

             ~ The sense of up and down is related to this stretching and compressing of the

                 proteins.

             ~ Experiments where Chara was placed in a solution more dense than the

                protoplast resulted in the protoplast floating upward and an upside down

                growth pattern.

             ~ Whether this mechanism is at work in true plants is currently under

                investigation.

 

C. Thigmotropism

 

Thigmotropism = Directional growth in response to touch.

·         Contact of tendrils stimulates a coiling response caused by differential growth of cells on opposite sides of the tendril.

 

Thigmomorphogenesis = Developmental response to mechanical perturbation.

·         Usually results from increased ethylene production in response to chronic mechanical stimulation.

·         Stem lengthening is decreased while stem thickening increases.

 

IV. Turgor movements are relatively rapid, reversible plant responses

 

       Turgor movements = Reversible movements caused by changes in turgor pressure of

               specialized cells in response to stimuli.

 

A. Rapid Leaf Movements

 

Rapid leaf movements occur in plants such as Mimosa.

·         When the compound leaf is touched, it collapses and folds together.

·         Results from rapid a loss of turgor within pulvini (special motor organs located in leaf joints).

·         Motor cells lose potassium, which causes water loss by osmosis.

·         Turgor pressure is regained and natural leaf form restored in about 10 minutes.

Rapid leaf movements travel from the leaf that was stimulated to adjacent leaves along the stem.

·         May be a response to reduce water loss or protect against herbivores.

·         The stimulus and response travel wavelike through the plant at 1 cm/sec.

·         This transmission is correlated with action potentials (electrical impulses) resembling those in animals but thousands of times slower.

·         Action potentials may be widely used as a form of internal communication since they have been found in many algae and plants.

 

B. Sleep Movements

 

Sleep movements = Lowering of leaves to a vertical position in evening and raising of

leaves to a horizontal position in morning.

·         Occurs in many legumes.

·         Due to daily changes in turgor pressure of motor cells of pulvini.

·         Cells on one side of the pulvinus are turgid while those on the other side are flaccid.

·         Migration of potassium ions from one side of the pulvinus to the other is the osmotic agent leading to reversible uptake and loss of water by motor cells.

 

V. Biological clocks control circadian rhythms in plants and other eukaryotes

     

      Biological clocks (internal oscillators that keep accurate time) are common in all eukaryotes

      and control many rhythmic phenomena.

 

·         Many human features (blood pressure, temperature, metabolic rate, etc.) fluctuate with the time of the day.

·         Certain fungi produce spores for only certain hours during the day.

·         Plants display sleep movements and a rhythmic pattern of opening and closing stomata.

 

Circadian rhythm = A physiological cycle with a frequency of about 24 hours.

·         Persists even when an organism is sheltered from environmental cues.

·         The oscillator is probably endogenous and is set to a 24-hour period by daily signals from the environment.

·         When the organism is sheltered from environmental cues, rhythm may deviate from 24 hours (called free-running periods) and can vary from 21 to 27 hours.

 

      Deviation of a free-running period from 24 hours does not indicate erratic drift of a biological clock, just absence of a synchronizing cue.

·         Most biological clocks are cued to the light-dark cycle resulting from the Earth’s rotation.

~ The clock may take days to reset once the cues change.

             ~ Jet lag is a human condition resulting from a lack of synchronization of the internal

                clock to the time zone.

 

 

 

The nature of the internal oscillator is still unknown.

 

·Most researchers believe biological clocks are present at the cellular level; either in membranes or in the machinery of protein synthesis.

                                 ~ Researchers found that the sleep movements of legumes are correlated with the

                           rhythmic opening and closing of potassium channels in motor cell membranes.

 

 

VI. Photoperiodism synchronizes many plant responses to changes of season

 

Photoperiodism = A physiological response to day length.

·Seasonal events (seed germination, flowering) are important in plant life cycles.

·Plants detect the time of year by the photoperiod (relative lengths of night and day).

      

        A. Photoperiodic Control of Flowering

          W.W. Garner and H.A. Allard (1920) postulated that the amount of day length controls flowering. Based on their studies, they classified plants into three categories:

 

·         Short-day plants require a light period shorter than a critical length and generally flower in late summer, fall and winter.

·         Long-day plants flower only when the light period is longer than a certain number of hours, generally in late spring and summer.

·         Day-neutral plants are unaffected by photoperiod and flower when they reach a certain stage of maturity.

 

It was discovered in the 1940’s that a critical night length, not day length, actually controls flowering and other responses to photoperiod.

 

·         If the daytime period is broken by a brief exposure to darkness, there is no effect on flowering.

·         If the nighttime period is interrupted by short exposure to light, photoperiodic responses are disrupted and the plants do not flower.

·         Therefore, short-day plants flower if night is longer than a critical length and long-day plants need a night shorter than a critical length.

Some plants flower after a single exposure to the proper photoperiod.

 

·         Some require several successive days of the proper photoperiod to bloom.

·         Others respond to photoperiod only if they have been previously exposed to another stimulus. For example, vernalization is a requirement for pretreatment with cold before flowering.

There is evidence that a flowering hormone is present in plants since leaves detect the photoperiod while buds produce flowers.

 

·         Only requires one leaf for a plant to detect photoperiod and floral buds develop.

·         If all leaves are removed, no photoperiod detection occurs.

·         Most plant physiologists believe an as yet unidentified hormone is produced in the leaves and moves to the buds.

                 ~ The hormone (or mixture of hormones) appears to be the same in both long-day and                                        short-day plants.

 

VII.      Phytochrome functions as a photoreceptor in many plant responses to light and photoperiod

 

A pigment named phytochrome helps plants measure the length of darkness in a photoperiod.

 

Phytochrome = A protein containing a chromophore (light-absorbing component) responsible for a plant’s response to photoperiod.

 

·         It was discovered during studies on how different colors of light affect responses to photoperiod.

·         Red light (X of 660 nm) is most effective in interrupting night length.

~ Brief exposure of short-day plants to red light prevents flowering even if the plant is kept at critical night-length conditions.

~ A long-day plant is induced to flower by a brief exposure to red light even if kept at a night length exceeding the critical number of hours.

·         If a flash of red light (R flash) is followed by a flash of far-red (ER) light (A of 730 nm), the plant perceives no interruption of night length.

·         Only the wavelength of the last flash affects the plant’s measurement of night length, regardless of the number of alternating flashes.

 

Phytochrome alternates between two photoreversible forms: ~r (red absorbing) and Pfr (far-red absorbing). The P, Pfr interconversion is a switching mechanism controlling various plant events.

 

       A. Ecological Significance of Phytochrome as a Photoreceptor

 

Phytochrome functions as a photodetector that tells the plant if light is present.

·         Plants synthesize phytochrome as P, and, if kept in dark, it remains as ~r, but if the phytochrome is illuminated, some ~r is converted to Pfr.

·         Pfr triggers many plant responses to light (e.g. seed germination).

·         A shift in the ~r 4Pfr equilibrium indicates the relative amounts of red and far-red light present in the sunlight.

·         Shifts in the P, t~fr ratio may cause changes (e.g. increased growth) which would adjust a plant’s growth and development in response to some environmental changes.

 

Photoreception by phytochrome has a large effect on the whole plant even though very little of the pigment is present in plant cells.

·         This fact implies the photoconversion from P, to Pfr produces a signal that is amplified in some way.

·         The amplification may be by either an alteration of membrane permeability and/or by affecting gene expression.

~ Photoconversion of phytochrome triggers the potassium fluxes in cells of the pulvini that produces the sleep movements in legumes.

                                         ~ Light induces the synthesis of starch digesting a-amylase required

                                            for seed germination in some species.

 

 

Complementing phytochrome’s effect, other photoreceptors help coordinate a plant’s growth and development with its environment.

 

B. Interaction of Phytochrome and the Biological Clock in Photoperiodism

 

Pfr gradually reverts to P~r.

·         This occurs every day after sunset.

·         The pigment is synthesized as P~ and degradative enzymes destroy more P~, than Ps..

·       At sunrise, the Pfr level increases due to photoconversion of ~r•

 

Plants do not use the disappearance of P~ to measure night length since:

·         The conversion is complete within a few hours after sunset.

·         Temperature affects the conversion rate, thus, it would not be reliable.

 

Night length is measured by the biological clock, not by phytochrome.

·         Perhaps phytochrome synchronizes the clock to the environment.

·         The clock measures night length very accurately (some short-day plants will not flower if night is even 1 minute shorter than the critical length).

 

 

VIII. Control systems enable plants to cope with environmental stress

 

A plant must adjust to environmental fluctuations every day of its life. Severe fluctuations may put plants under stress.

 

     Stress = An environmental condition that can have an adverse effect on a plant’s growth, reproduction, and survival.

 

Some plants have evolutionary adaptations that enable them to live in environments that are stressful to other plants. For example,

·         Halophytes have special anatomical and physiological adaptations which permit them to grow best in salty soils.

       ~ Salt glands on the leaves eliminate excess salt from the plants — thus the saline environment is not an environmental stress.

 

A. Responses to Water Deficit

 

 

Plants have control systems in both the leaves and roots that help them cope with water deficits.

 

Most control systems in leaves help the plant conserve water by reducing transpirational water loss.

·      Guard cells lose turgor and the stomata close when a leaf faces a water deficit.

·      Mesophyll cells in the leaf are also stimulated to increase synthesis and release abscisic acid which acts on guard cell membranes to help keep the stomata closed.

·      Growth of young leaves is inhibited by a water deficit since cell expansion is a turgor-dependent process.

~ This reduces transpiration by slowing the increase in leaf surface area.

·         The leaves of many grasses and other plants wilt; they roll into a shape that reduces the surface area exposed to the sun, thus reducing transpiration.

 

Roots respond to water deficits by reducing growth.

·         Drying of the soil from the surface down inhibits the growth of shallow roots.

                              ~ The cells can not retain the turgor necessary for elongation.

·         Deeper roots surrounded by moist soil continue to grow.

                              ~ This maximizes root exposure to soil moisture.

B. Responses to Oxygen Deprivation

 

             Waterlogged soil lacks the air spaces that provide oxygen for cellular respiration in the roots.

·         Some plants form air tubes that extend from submerged roots to the surface, thus oxygen can reach the roots.

·         Mangroves are structurally adapted to their coastal marsh environments in that their submerged roots are continuous with aereal roots that provide access to oxygen.

 

C. Responses to Salt Stress

 

           Excess salts (sodium chloride or others) in the soil may:

·         Lower the water potential of the soil solution causing a water deficit even though sufficient water is present.

                 ~ A water potential in the soil that is more negative than that of the root tissue will       

                    cause roots to lose water instead of absorb it.

·         Have a toxic effect on the plant at relatively high concentrations.

                        ~ The uptake of most harmful ions is impeded by the selectively permeable

                     membranes of root cells.

                  ~ This causes a problem with acquiring water from solute rich soils.

·         Many plants produce compatible so/ides in response to moderately saline soils.

 

Compatible solute = An organic compound that keeps the water potential of cells

more negative than the soil solution without admitting toxic quantities of salt.

 

D. Responses to Heat Stress

 

             Transpiration is one mechanism that helps plants respond to excessive heat and prevent the denaturing of enzymes and damage to metabolism.

·         The evaporative cooling associated with transpiration keeps the temperature of the leaf 30 to 1 00C lower than ambient temperature.

·         Cooling via transpiration will continue while stomata remain open; however, if a water deficit occurs, the stomata close and the cooling function is lost in order to conserve water.

 

Most plants will begin producing heat-shock proteins when exposed to excessive

temperatures (40W or above for temperate zone plants).

·         This is a back-up system to transpiration.

·         Some heat-shock proteins are identical to chaperone proteins found in unstressed cells.

~ Chaperone proteins serve as temporary supports which help other proteins fold into their functional conformations.

~ Heat-shock proteins may help enzymes and other proteins maintain their conformation, thus preventing denaturation.

 

E. Responses to Cold Stress

 

Chilling of a plant (reduction of ambient temperature to a non-freezing level) causes

change in the fluidity of cell membranes.

 

Fluidity = The lateral drifting of proteins and lipids in the plane of the membrane; a result of the fluid mosaic structure of membranes.

 

·         At a critical point, lipids become locked into crystalline structures causing a loss of fluidity.

·         Solute transport and membrane protein function are adversely affected by the loss of fluidity.

·         Plants respond to the cold stress of chilling by altering the lipid composition of their membranes.

·         The proportion of saturated fatty acids in the membrane is increased.

~ The shape of the fatty acids reduces crystal formation, thus maintaining fluidity at lower temperatures.

·         This modification works best for gradual temperature changes as it takes several hours to days to occur.

 

Subfreezing temperatures are the most severe form of cold stress because ice crystals begin to form in the plant.

·         Less threat to plant survival occurs if the ice crystals form only in the cell walls and intracellular spaces.

·         When ice crystals form in the protoplasts, the cell usually dies.

~ The ice crystals perforate the membranes and organelles.

·         Wood plants which are native to regions where cold winters occur have adaptations to cope with the stress of freezing.

~ The solute composition of live cells is changed in a way that prevents ice crystal formation even when the cytosol is supercooled.

           ~ Effective even when ice crystals form in the cell walls.

 

F. Responses to Herbivores

 

Plants counter excessive grazing by herbivores with both physical and chemical defense measures.

·         Physical defenses include structures such as thorns and spines.

·         Chemical defenses take the form of distasteful or toxic compounds such as canavanhne.

~ Similar to arginine, canavanine is an unusual amino acid produced by some plants.

~ When ingested by insects, it is incorporated in place of arginine in the insect’s proteins.

             ~ Incorporation of canavanine disrupts protein conformation and the insect dies.

 

G. Defense Against Pathogens: Systemic Acquired Resistance

 

The epidermis of the primary plant body and the periderm of the secondary plant body serve as surface barriers against pathogenic microorganisms.

 

·         Pathogens can cross this barrier and invade the plant through injuries or natural epidermal openings such as stomata.

 

When a pathogen invades the plant, a variety of compounds known as phytoalexins are produced as a defense measure.

 

Phytoalexin = An antibiotic that destroys or inhibits the growth of microorganisms.

 

Plants also exhibit a systemic acquired resistance (SAR) response to help protect uninfected tissue from a pathogen that might spread from its point of invasion.

·         When a pathogen invades a plant, molecules from the pathogen and compounds from the injured plant tissue serve as “alarm substances”.

·         The alarm substances cause rapid responses at the infection which slows the spread of the pathogen to other parts of the plant.

~ One such response is the cross-linking of molecules in cell walls which form a local barrier.

·         The alarm substances also stimulate cells in infected areas to synthesize and release salicylic acid, a hormone that is transported throughout the plant. (A form of salicylic acid is the active ingredient in aspirin.) ~

·         The hormone stimulates phytoalexin production and other defense measures in cells throughout the plant, even those far from the infection.

 

IX. Signal-transduction pathways mediate the responses of plant cells to environmental and hormonal stimuli

 

     Plant cell responses to hormones and environmental stimuli are mediated by intracellular signals (signal-transduction pathways).

 

         Signal-transduction pathway = A mechanism linking a mechanical or chemical stimulus to a cellular response.

·         Three steps are involved in each pathway: reception, transduction and induction. (See Campbell, Figure 35.19)

 

Reception is the detection of a hormone or environmental stimulus by the cell.

·         May take various forms depending on the stimulus. For example,

         ~ Absorption of a particular wavelength of light by a pigment within a cell.

         ~ The binding of a hormone to a specific protein receptor in the cell or on its membrane.

·         Reception of a hormone only occurs in target cells for that hormone.

          ~ Target cells possess the specific protein receptor to which the hormone must bind; other cells do not possess the receptor.

 

Transduction in the pathway results in an amplification of the stimulus and its conversion into a chemical form which can activate the cell’s responses.

·         The hormone (first messenger) binds to a specific receptor and the hormone-receptor combination stimulates the second messenger (a substance that increases in concentration within a cell stimulated by the first messenger).

·         The receptor may be bound to the cell membrane and its activation results in a chemical change to the cell.

         ~ Calcium ions appear to be important second messengers in many plant responses. Calcium ion concentration increases in the cell and the ions bind to the protein calmodulin.

                                ~ The calmodulin-calcium complex then activates other target molecules

                 within the cell.

·         Amplification of the signal results from a single first messenger molecule binding to its receptor giving rise to many second messengers which activate an even larger number of proteins and other molecules.

·         A second part of transduction that is important is the specificity of the responses.

~ Two cell types may both have receptors for a hormone but respond differently because each contains different target proteins for the second messenger.

 

Induction is the pathway step in which the amplified signal induces the cell’s specific

response to the stimulus.

·         Some responses occur rapidly. For example, r~ ABA stimulation of stomatal closing.

~ Auxin-induced acidification of cell walls during cell elongation.

·         Other responses take longer, especially if they require changes in gene expression (thigmomorphogenesis).