CHAPTER 39. PLANT RESPONSES TO INTERNAL AND EXTERNAL SIGNALS. Plants respond to a wide array of stimuli throughout its lifecycle Hormonal signals Gravity Direction of light Plant interactions between environmental stimuli and internal signals. Responses to Stimuli.
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PLANT RESPONSES TO INTERNAL AND EXTERNAL SIGNALS
Direction of light
Plant interactions between environmental stimuli and internal signals.Responses to Stimuli
Patterns of growth & development
Responses to Stimuli
Cellular receptors detect environmental changes
Responses to Stimuli
Potato grown in dark Potato grown in lightSignal-transduction pathways link internal and environmental signals to cellular responses.
The shoot does not need a thick stem.
Leaves would be damaged as the shoot pushes upward.
Don’t need an extensive root system
No chlorophyll produced
Energy allocated to stem growth
The elongation rate of the stems slow.
The leaves expand and the roots start to elongate.
The entire shoot begins to produce chlorophyll.
The receptor is called a phytochrome: a light-absorbing pigment attached to a specific protein.
Located in the cytoplasm.
Sensitive to very weak environmental and chemical signals
Signal is then amplified by a second messenger
Second messenger produced by the interaction between phytochrome and G-protein
G-protein activates enzyme with produces Cyclic GMP (2nd messenger)
Ca2+-calmodulin is also a 2nd messenger
Cyclic GMP and Ca2+-calmodulin pathways lead to gene expression for protein that activates greening response
Response ends when “switch-off” is activated (protein phosphatases)
Only small amts are needed
Often the response of a plant is governed by the interaction of two or more hormones.
Phototropism and Negative phototropismHormone
Cytokinins- root growth
Abscisic acid- inhibits growth
Ethylene- promote fruit ripening
Brassinosteroids- inhibits root growth
Many function in plant defense against pathogens
Auxins are used commercially in the vegetative propagation of plants by cuttings.
Synthetic auxins are used as herbicidesAuxin
The active ingredient is a modified form of adenine
They are produced in actively growing tissues, particularly in roots, embryos, and fruits.
Cytokinins interact with auxins to stimulate cell division and differentiation.
A balanced level of cytokinins and auxins results in the mass of growing cells, called a callus, that remains undifferentiated.
High cytokinin levels shoot buds form from the callus.
High auxin levels roots form.Cytokines
The direct inhibition hypothesis - proposed that auxin and cytokinin act antagonistically in regulating axillary bud growth.
Auxin levels would inhibit axillary bud growth, while cytokinins would stimulate growth.
If the terminal bud, the primary source of auxin, is removed, the inhibition of axillary buds is removed and the plant becomes bushier.
This can be inhibited by adding auxins to the cut surface.
However, experimental removal of the terminal shoot (decapitation) has not demonstrated this.
In fact, auxin levels actually increase in the axillary buds of decapitated plants.
They inhibit protein breakdown by stimulating RNA and protein synthesis, and by mobilizing nutrients from surrounding tissues.
Leaves removed from a plant and dipped in a cytokinin solution stay green much longer than otherwise.
Cytokinins also slow deterioration of leaves on intact plants.
Florists use cytokinin sprays to keep cut flowers fresh.
Dwarf pea plants treated with gibberellins.
After treatment dwarf pea plant grew to normal height.
In many plants, both auxin and gibberellins must be present for fruit to set.
Individual grapes grow larger the internodes of the grape bunch elongate.
Seeds treated with gibberellins will break dormancy.
It’s produced in response to stresses such as drought, flooding, mechanical pressure, injury, and infection.
Ethylene production also occurs during fruit ripening and during programmed cell death.
Ethylene is also produced in response to high concentrations of externally applied auxins.
Ethylene produced during apoptosis (programmed cell death)Ethylene
Ethylene production is induced by mechanical stress on the stem tip.
In the triple response, stem elongation slows, the stem thickens, and curvature causes the stem to start growing horizontally.
Some lack a functional ethylene receptor.
Essential elements are salvaged prior to leaf abscission and stored in stem parenchyma cells.
These nutrients are recycled back to developing leaves the following spring.
Aged leaves produce less auxin
Cells become more sensitive to ethylene
The cells in the abscission layer produce enzymes that digest the cellulose and other components of cell walls.
Hormones responsible for leaf abscission
Ethylene production helps ripen fruit
The production of new scents and colors helps advertise fruits’ ripeness to animals, who eat the fruits and disperse the seeds.
Fruit ripens quickly in closed paper bag
Prevent ripening in produce by spraying CO2
Brassinosteroids induce cell elongation and division in stem segments and seedlings.
They also retard leaf abscission and promote xylem differentiation.
Brassinosteroids are nonauxin hormones.Brassinosteroids
Cue many key events in plant growth and development.
Photomorphogenesis- the effects of light on plant morphology.
Light reception – circadian rhythms.The effect of light on plants
For example, the measure of physiological response to light wavelength, the action spectrum, of photosynthesis has two peaks, one in the red and one in the blue.
These match the absorption peaks of chlorophyll.Action Spectrum
Analysis Arabidopsis mutants found three completely different types of pigments that detect blue light.
cryptochromes (for the inhibition of hypocotyl elongation)
phototropin (for phototropism)
zeaxanthin (for stomatal opening) a carotenoid-based photoreceptor called
Seed germination needs optimal environmental conditions, especially good light.
Such seeds often remain dormant for many years until a change in light conditions.
For example, the death of a shading tree or the plowing of a field may create a favorable light environment.Phytochromes function as photoreceptors in many plant responses to light
Seeds were exposed to a few minutes of monochromatic light of various wavelengths and stored them in the dark for two days and recorded the number of seeds that had germinated under each light regimen.
While red light increased germination, far red light inhibited it and the response depended on the last flash.
The Pfr form triggers many of the plant’s developmental responses to light.
Exposure to far-red light inhibits the germination response.
If the seeds are illuminated with sunlight, the phytochrome is exposed to red light (along with other wavelengths) and much of the Pr is converted to (Pfr), triggering germination.
During the day, with the mix of both red and far-red radiation, the Pr <=>Pfr photoreversion reaches a dynamic equilibrium.
Plants can use the ratio of these two forms to monitor and adapt to changes in light conditions.
synthesis of certain enzymes
opening and closing stomata
Response to changes in environmental conditions
Relative humidityBiological clocks control circadian rhythms in plants and other eukaryotes
24 hr day/night cycle
These movements will be continued even if plants are kept in constant light or constant darkness.
circadian rhythms- internal clock; no environmental cues
Desynchronization can occur when denied environmental cues.
Humans experience jetlag.
Eventually, our circadian rhythms become resynchronized with the external environment.
Plants are also capable of re-establishing (entraining) their circadian synchronization.Light entrains the biological clock
The phytochrome system involves turning cellular responses off and on by means of the Pr <=> Pfr switch.
In darkness, the phytochrome ratio shifts gradually in favor of the Pr form, in part from synthesis of new Pr molecules and, in some species, by slow biochemical conversion of Pfr to Pr.
When the sun rises, the Pfr level suddenly increases by rapid photoconversion of Pr.
This sudden increase in Pfr each day at dawn resets the biological clock.
Seed germination, flowering, and the onset and breaking of bud dormancy.
The environmental stimulus that plants use most often to detect the time of year is the photoperiod, the relative lengths of night and day.
A physiological response to photoperiod, such as flowering, is called photoperiodism.Photoperiodism synchronizes many plant responses to changes of season
Long-day plants will only flower when the light period is longer than a critical number of hours.
Examples include spinach, iris, and many cereals.
Day-neutral plants will flower when they reach a certain stage of maturity, regardless of day length.
Examples include tomatoes, rice, and dandelions.
Night length, not day length, controls flowering and other responses to photoperiod
Cocklebur is actually unresponsive to day length, but it requires at least 8 hours of continuous darkness to flower.
A long-day plant grown on photoperiods of long nights that would not normally induce flowering will flower if the period of continuous darkness are interrupted by a few minutes of light.
Action spectra and photoreversibility experiments show that phytochrome is the active pigment.
If a flash of red light during the dark period is followed immediately by a flash of far-red light, then the plant detects no interruption of night length, demonstrating red/far-red photoreversibility.
The flowering signal may be hormonal
While light is one important environmental cue, other environmental stimuli also influence plant development and physiology.Introduction
Roots demonstrate positive gravitropism
Shoots exhibit negative gravitropism
Auxin plays a major role in gravitropic responses
Statoliths- specialized plastids containing dense starch grains, play a role in gravitropismPlants respond to environmental stimuli through a combination of developmental and physiological mechanisms
Differences seen in members of the same species grown in different environments
Windy mtn ridge
taller, slenderer tree
For example, most vines and other climbing plants have tendrils that grow straight until they touch something.
Contact stimulates a coiling response, thigmotropism, caused by differential growth of cells on opposite sides of the tendril.
This allows a vine to take advantage of whatever mechanical support it comes across as it climbs upward toward a forest canopy.
Mimosa’s leaflets fold together when touched.
This occurs when pulvini, motor organs at the joints of leaves, become flaccid from a loss of potassium and subsequent loss of water by osmosis.
It takes about ten minutes for the cells to regain their turgor and restore the “unstimulated” form of the leaf.
Response to Stress
fungi in mycorrhizae
Attack by herbivores
Attacks by pathogenic viruses, bacteria, and fungi.Plant Interactions
A corn leaf recruits a parasitoid wasp as a defensive response to a herbivore
viruses, bacteria, and the spores and hyphae of fungi can through injuries or through natural openings in the epidermis, such as stomata.
Once a pathogen invades, the plant mounts a chemical attack as a second line of defense that kills the pathogens and prevents their spread from the site of infection.Plants defense against pathogens