Plant Control Systems Flashcards
How do plants respond to stimuli?
Unlike animals which respond by movement, plants respond to stimuli by altering growth and development
Response in plants is carried out via signal transduction
signal transduction
Reception, transduction, response
how plants respond to stimuli
Etiolation
is the morphological adaptation required for growing in the dark
Give an example of etiolation and explain
Like a potato that sprouts in a dark cellular
Little light and no evaporative pressure in the leaves means the plant doesn’t need extensive roots right away
Plant focuses its energy on growing the stems, thus reducing the energy it takes for shoots to break ground
de-etiolation
When a plant shoot reaches light (“greening”) commences
What happens when de-etolation starts?
Stem elongation slows
Shoot beings to produce chlorophyll to initiate photosynthesis
What happens during reception?
- Light signal is detected by a phytochrome receptor located in the cytoplasm
- This activates at least 2 signal transduction pathways
What is the first pathway of transduction?
cGMP
What is the second pathway of transduction?
Ca2+ ions
What happens during the first pathway of transduction?
Weak levels of light trigger the phytochrome and initiates the secondary messenger cGMP through the activation of the enzyme guanylyl cyclase
• cGMP then activates a protein kinases, which carries the signal into a response
secondary messagers
A small, nonprotein, water-soluble molecule or ion, such as a calcium ion (Ca2+) or cyclic AMP, that relays a signal to a cell’s interior in response to a signalling molecule bound by a signal receptor protein.
What happens during the second pathway of transduction?
Second pathway Ca2+ ions Phytochrome activation opens up Ca2+ channels, flooding the cytosol with increase Ca2+ This activates a different protein kinase to initiate a response
What must happen for full de- etiolation to occur?
Both pathways must be induced for full de- etiolation to occur
Response
Both pathways lead to the expression of genes for proteins that function in the de- etiolation process
Products of response are enzymes for photosynthesis, chlorophyll production, plant hormones levels
What are the two main mechanisms by which a signalling pathway can enhance an enzymatic step in a biochemical pathway?
Post-transcriptional modification
Transcriptional Regulation
Post-transcriptional modification
modification of proteins
Pre-existing proteins are phosphorylated (phosphate added), altering protein shape and function (review Fig. 11.10)
Protein phosphatases dephosphorylate enzymes, turning off the signals
Transcriptional Regulation
Transcription factors bind to specific regions of DNA (see concept 18.2) to control transcription of genes on DNA
Activators=increase transcription
Repressors=decrease transcription
What are plant hormones?
(plant growth regulators)
• Signalling molecule produced in minute amounts in one part of the plant and transported to another part to initiate responses in cells and tissues
Each hormone can have a multitude of effects depending which tissue it is acting in, its concentration, and the developmental stage of the plant
Transported in the phloem sap
Where is Auxin (IAA) Produced
or Found in Plant?
shoot apical meristems and young leaves are the primary sites of auxin synthesis
Root apical meristems also produce auxin, although the root depends on the shoot for much of its auxin
Developing seeds and fruits contain high levels of auxin, but it is unclear whether it is newly synthesized or transported from maternal tissues.
Where is Cytokinins Produced or Found in Plant?
synthesized primarily in roots and transported to other organs
there are many minor sites of production as well
Where is Gibberellins (GA) Produced or Found in Plant?
Meristems of apical buds and roots, young leaves, and developing seeds
Where is Abscisic acid (ABA) Produced or Found in Plant?
all plant cells have the ability to synthesize abscisic acid,
found in every major organ and living tissue
may be transported in the phloem or xylem
Where is Ethylene Produced or Found in Plant?
a gaseous hormone produced by most parts of the plant
produced in high concentrations during senescence, leaf abscission, and the ripening of some types of fruits
Synthesis is also stimulated by wounding and stress.
Where are Brassinosteroids Produced or Found in Plant?
present in all plant tissues, although different intermediates predominate in different organs
Internally produced brassinosteroids act near the site of synthesis.
Where are Jasmonates Produced or Found in Plant?
a small group of related molecules derived from the fatty acid linolenic acid
are produced in several parts of the plant and travel in the phloem to other parts of the plant
Where are Strigolactones Produced or Found in Plant
carotenoid-derived hormones and extracellular signals are produced in roots in response to low phosphate conditions or high auxin flow from the shoot
What are the major functions of Auxin?
Stimulates stem elongation (low concentration only);
promotes the formation of lateral and adventitious roots; regulates development of fruit;
enhances apical dominance; functions in phototropism and gravitropism; promotes
vascular differentiation; retards leaf abscission
used in horticulture:
What are the major functions of Cytokines?
Regulate cell division in shoots and roots
modify apical dominance and promote lateral bud growth
promote movement of nutrients into sink tissues
stimulate seed germination; delay leaf senescence
Anti-aging
What are the major functions of GA?
Stimulate stem elongation, pollen development, pollen tube growth, fruit growth, and seed development and germination
regulate sex determination and the transition from juvenile to adult phases
What are the major functions of ABA?
Inhibits growth
promotes stomatal closure during drought stress;
promotes seed dormancy and inhibits early germination;
promotes leaf senescence;
promotes desiccation tolerance
What are the major functions of Ethylene?
Promotes ripening of many types of fruit, leaf abscission, and the triple response in seedlings (inhibition of stem elongation, promotion of lateral expansion, and horizontal growth);
enhances the rate of senescence
promotes root and root hair formation
promotes flowering in the pineapple family
What are the major functions of Brassinosteroids?
Promote cell expansion and cell division in shoots; promote root growth at low concentrations; inhibit root growth at high concentrations; promote xylem differentiation and inhibit phloem differentiation; promote seed germination and pollen tube elongation
What are the major functions of Jasmonates?
Regulate a wide variety of functions, including fruit ripening, floral development, pollen production, tendril coiling, root growth, seed germination, and nectar secretion; also produced in response to herbivory and pathogen invasion
What are the major functions of Strigolactones?
Promote seed germination, control of apical dominance, and the attraction of mycorrhizal fungi to the root
phototropism
Plants generally grow towards the light
Tropism
plant organs curving toward or away from a stimulus
Usually as a result of stem elongation
Auxins (indoleacetic acid, IAA)
Promote growth/elongation of the coleoptiles (stems above the cotyledons)
• Produced predominantly in the shoot tips (SAM)
• Moves unidirectional shoot tip to shoot base -> polar transport (unrelated to gravity)
coleoptiles
stems above the cotyledons)
Brassinosteroids
Steroids similar to cholesterol
Induce cell elongation and division in stems
Slow down leaf abscission
Promote xylem differentiation
Strigolactones
Xylem-mobile chemicals
Stimulate seed germination, suppress adventious root development, helps with mycorrhizae, controls apical dominance
How were the Strigolactones ID?
First ID’d from Striga (witchweed), a rootless parasitic plant that penetrate the roots of host plant
Jasmonates
Plant defense and plant development
First ID’d in jasmine plants
• Also works with phytochromes,GA,IAA,ethylene
Describe the role of Auxin in plant development?
Auxin produced in the shoot tip controls spatial organization of the plants –>Affects size, shape, environment of branches and stems
When auxin production decreases, lateral branches allowed to develop
Involved in phyllotaxy -> local peaks in auxin determine the site of the leaf primordia
Polar transport in leaf margins affects the formation of leaf veins
• Less auxin, more secondary leaf veins and loosely organized main veins
Reduction in auxin at the end of the growing season stimulates the reduction in the vascular cambium activity
Describe the role of Auxin in Stem elongation?
Binds to receptors in the plasma membrane to initiate cell expansion
• Stimulates growth when concentration is between 10-8 to 10-4 M
Also stimulates gene expression to, produce proteins, increase cytoplasmic fluids, and cell wall material
What is the acid growth hypothesis?
Auxin stimulates proton (H+) pumps along plasma membrane, increasing the membrane voltage and lowering pH inside of the cell
Acidification of the cell wall activates expansins, proteins that break the linkages (hydrogen bonds) in the cell walls
Increase in water potential due to increased ion uptake due to increasing membrane potential -> higher turgor pressure the cell is free to expand and contribute to stem elongation
How is auxin used in horticulture?
are used in horticulture:
• Rooting hormone for vegetative propagation
• Synthetic auxins are used as herbicides (die
from hormonal overdose)
• Synthetic auxins increase fruit production
expansin
Plant enzyme that breaks the cross-links (hydrogen bonds) between cellulose microfibrils and other cell wall constituents, loosening the wall’s fabric
Cytokinins
Discovered in the early 1940s
Stimulate cytokinesis (cell division)
Produced in actively growing tissues, particularly in the roots
Works with auxins to promote cell division and differentiation
Describe the role of cytokinins in apical domianance?
Apical bud suppresses growth of axillary buds
The apical bud is a sugar sink and produces auxins
2) Auxin moves downward, producing strigolactones that repress lateral bud growth
3) Cytokinin’s from the roots antagonizes effect of auxins and strigolactone, allowing limited lateral bud growth
4) Removal of apical bud allows remaining buds to receive more sugar and allow topmost lateral buds to assume apical dominance
How does cytokinins work in anti aging?
Slow apoptosis in cells
Inhibits protein breakdown, stimulates RNA and protein synthesis
Mobilizes nutrients from surrounding tissues
How do cytokinin and auxin work together?
Works with auxins to promote cell division and differentiation.
If just auxin present, cells will grow large but not divide
If just cytokinins,there is no effect
ratio of cytokinins to auxin controls cell differentiation.
When the concentrations of these two hormones are at certain levels, the mass of cells continues to grow, but it remains a cluster of undifferentiated cells called a callus
If cytokinin levels increase, shoot buds develop from the callus. If auxin levels increase, roots form
What is apical dominance?
the ability of the apical bud to suppress the development of axillary buds, is under the control of sugar and various plant hormones, including auxin, cytokinins, and strigolactones.
What does cutting off the apical bud do?
removes apical sugar demand and rapidly increases sugar (sucrose) availability to axillary buds. This increase of sugar is sufficient to initiate bud release
Gibberellins (GAs)
• Discovered in the early 1900s
Plants grew spindly and toppled over(“foolish seedling disease”)
Caused by a fungus Gibberella,resulting in hyperelongation from a secreted chemical (gibberellin)
Plants also produce gibberellin
• Produced in young roots and leaves
Describe the role of GA in stem elongation
Stimulates both cell division and elongation (act in concert with auxins re: expansins)
Dwarf plants grow tall in the presence of gibberellins
Describe the role of GA in Fruit growth:
Both auxins and gibberellins must be present for fruit to develop
In grapes, commercially applied gibberellins makes the grapes grow
larger (yum!), and elongate internodes, allowing for more space between grapes (therefore more air flow!)
Describe the role of GA in germination:
• Signals seed to break dormancy, stimulates digestive enzymes for endosperm breakdown
Abscisic Acid (ABA)
Discovered in the early 1960s
• Chemical changes occurred during abscission(dropping off)of leaves and fruits
• Unlike other hormones, ABA slows growth
Describe the role of ABA in seed dormancy:
Increases likelihood that seeds will germinate only when the environment is suitable (ie enough light, water, nutrients)
ABA prevents seeds from germinating in the dark, moist interior of the fruit (100-fold concentration during seed maturation)
When ABA concentration decreases, seed germination occurs
• Decrease cause by water washing away ABA,light in activating ABA
Ratio of ABA:GA determines whether the seed is dormant of will germinate
Describe the role of ABA in drought tolerance:
ABA closes the stomata to prevent water loss by affecting Ca2+
secondary messengers, resulting in K+ channels to open in the guard cells
Ethylene
Plants produce ethylene gas in response to stresses such as drought, flooding, mechanical pressure, injury, and infection
Also produced during fruit ripening and programmed cell death
Auxin also induces the production of ethylene in plants
What are the 4 main effects of ethylene?
mechanical stress, senescence, leaf abscission, fruit ripening
Triple Response
shoots avoid obstacles via horizontal growth
What are the 3 parts of triple response?
Stem elongation
Thickening of the stem
Curvature of the stem
The response is greater with increased ethylene concentration
Senescence (ethylene)
(leaf/flower shedding)
Programmed death of cells and organs or the entire plant
A burst of ethylene initiates the cascade of apoptosis
Enzymes break down chemical components,cell organelles, DNA, RNA, chlorophyll, etc and recycles it back to the plant
Describe leaf abscission (ethylene)
Common in deciduous trees and plants
Helps manage climatic stress during seasonal changes
Essential nutrients are salvaged in the plant and stored in stem parenchyma cells
• Recycled back to developing leaves in the spring
The breaking point is called the abscission layer
Enzymes breakdown the cell walls of the cells on this layer
The weight of the leaf eventually causes the weak walls to break, and the leaf falls
Cork will form a protective scar to heal the wound (ie leaf scar on stems)
Aging leaves have less auxin, more ethylene
Fruit ripening (ethylene)
Fruit starts off tart and unappealing to herbivores
Protects the seeds until they are mature
When ready, a burst of ethylene triggers enzymatic breakdown of cell walls allowing the fruit to soften, convert starch to sugars to make it sweeter
Herbivores are now attracted to the fruit and will help disperse seeds
What happens during ripening?
A chain reaction though
Ethylene promotes ripening and ripening produces more ethylene
Can speed up fruit ripening by leaving them in a paper bag
or putting them in the fridge beside the apples
Commercial producers will store fruit in CO2 to slow the
production of ethylene or spray with ethylene to promote ripening
What happens during ripening?
A chain reaction through
Ethylene promotes ripening and ripening produces more ethylene Because ethylene is a gas, the signal to ripen spreads from fruit to fruit.
How can you speed up or slow ripening?
Can speed up fruit ripening by leaving them in a paper bag or putting them in the fridge beside the apples
Commercial producers will store fruit in CO2 to slow the
production of ethylene or spray with ethylene to promote ripening
Photomorphogenesis
is the effect of light on plant growth and development • Allows plants to measure day length, time of year, seasons
What can plants detect?
not only the presence of light but also its direction, intensity, and wavelength (colour)
Action spectra
depicts the relative effectiveness of different wavelengths of light on processes
• Can help determine what photo receptors are active in a response
What are the 2 main photoreceptors?
Blue-light photo receptors(450-500nm)
Phytochromes(red(660nm)and far-red(730nm)
Blue-light photo receptors
Phototropism, light-induced opening of stomata, light- induced hypocotyl growth reduction after breaking ground
Phytochromes
are essential for seed germination
Red and far-red have reversible, opposite effects
There are several kinds of phytochromes,even within the same plant
helps a plant keep track of the passage of days and season
Red-light=
germination
far-red=
inhibits germination
i
The light absorbing part is photoreversible…..changes in shape due to light exposure is reversible with exposure to other forms of light (ie red vs far-red)
What does Pr do?
absorbs red light and is converted to Pfr
• Red light promotes seed germination
What does Pfr do?
absorbs far-red and is converted back to Pr
• Far-red light inhibits germination
is the form of phytochrome that triggers many of a plant’s developmental responses to light
What initiates germination?
Higher ratio of Pfr:Pr forms initiates germination
How does phytochrome switching explain light-induced germination in nature?
Plants synthesize phytochrome as Pr, and if seeds are kept in the dark, the pigment remains almost entirely in the Pr form, inhibiting germination (see Figure 39.17). Sunlight contains both red light and far-red light, but the conversion to Pfr is faster than the conversion to Pr. Therefore, the ratio of Pfr to Pr increases in the sunlight. When seeds are exposed to adequate sunlight, the production and accumulation of Pfr will trigger their germination.
During the day…
the conversion between phytochrome states(Pr and Pfr) reach equilibrium
Shade avoidance
if plant is shaded, phytochrome ratio of Pr is higher
Leaves in canopy absorb red light in the chlorophyll, leaving behind far-red
Shift allows allocation of more resources for growing taller
What happens when ratio of Pfr is higher?
If ratio of Pfr is higher,lateral branches develop rather than height
circadian rhythms
A physiological cycle of about 24 hours that persists even in the absence of external cues.
Plants respond the daily changes in light, temperatures and relative humidity
• Some responses occur on a 24hr cycle, without a known underlying cause
What are circadian rhythms controlled by?
Controlled by gene transcription
Photoreversible
The light absorbing part
- changes in shape due to
light exposure is reversible with exposure to other forms of light (ie red vs far-red)
Photoperiodism
A physiological response to photoperiod, the relative lengths of night and day
example- is flowering
What is the flowering response?
Short-day plants->shorter photo period induces flowering
Long-day plants->longer photo periods induce flowering
Day-neutral plants -> flower when a certain stage of maturity is reached regardless of day length(are unaffected by photoperiod) - as tomatoes, rice, and dandelions,
Interestingly, controlled by night length, not day length
What happens when there is light in night?
If the nighttime is interrupted by light,flowers won’t develop since they don’t get the required amount of continuous dark to stimulate flowering
ii
Some plants need additional stimuli to promote flowering
• Vernalisation pre-treats the plant with a period of cold (<10C)
• Small amount of light on a leaf can trigger florigen (signalling molecule that promotes flowering)
short day plant
flowers when night exceeds a critical dark period a flash of light interrupting the dark period prevents flowering
long day plant
Flowers only if the night is shorter than the critical dark period a flash of light interrupts the long dark period and inducing flowering
Which light is most effective in interrupting night time?
Red light is the most effective colour in interrupting the nighttime portion of the photoperiod
if a flash of red (r) light during the dark period is followed by a flash of far-red (fr) light?
then the plant detects no interruption of night length
What distinguishes short and long day plants?
are not distinguished from short-day plants by an absolute night length but by whether the critical night length sets a maximum (long-day plants) or minimum (short-day plants) number of hours of darkness required for flowering
florigen
signalling molecule that promotes flowering
What other stimuli than light do plants respond to?
gravity
enviromental stress mechanical stimuli
gravitropism
allows the plant to grow towards the light, regardless of position
What type of gravitropism do roots and shoots display?
Roots display positive gravitropism (grow down)
Shoots display negative gravitropism (grow against)
Statoliths
are starch containing plastids in plant tissues that settle due to gravity
Roots contain these near the root cap
Settle near basal ends of the cells, triggering redistribution of calcium and lateral transport of auxin within the root
Auxin accumulates on the lower side of the zone of elongation
Higher concentrations inhibit elongation, allowing the top of the root to elongate and bend and reorient the root to growing down
Thigmomorphogenesis
refers to changes in morphology due to physical/mechanical perturbations
• Short, stocky trees in super windy areas
• Plants are super sensitive to touch
Thigmotropism
directional growth due to touch
Tendrils coil around supports to support the growing stem
Mimosa pudica (sensitive plant) results from a loss of turgor due to touch and action potentials in the leaf cells
What are some environmental stresses?
• Flooding
• Drought
• Salt Stress
- heat and cold
Flooding
Too much water suffocates plant roots
Mangroves have pneumatophores to help get air into the soil
Oxygen deprivation stimulates ethylene, which initiates apoptosis to kill off cells in the roots to make their own air spaces
Drought
Closing of stomata during the day/response to water deficit due to production of ABA
Rolling up of grass leaves reduces transpiration
Some species shed their leaves
Salt Stress
Excess salt lowers the water potential in the soil, resulting in less water uptake by the plant
Excess sodium and other ions can be toxic to plants at high concentrations (affects the selective permeability of root cells)
Can overcome this by producing their own solutes so they don’t acquire the toxic ones (but only short term)
Halophytes can pump out excess salt from the leaf epidermis
What are the different defences against pathogens?
physical barrier
chemicals
PAMP-triggered immunity and effector-triggered immunity
What is the first line of defence?
the epidermis, covered in the waxy cuticle
Periderm also first line in wood plants lacking the epidermis
However, pathogens can still enter via natural pores (stomata, lenticels)
What is the second line of defence?
chemicals
Plants produce many chemicals that are toxic to invaders or inhibit their growth within the plant
Ex. Pacific Yew produces paclitaxel, which inhibits fungal growth at injury sites
Heat stress
Excessive heat can denature plant proteins, disrupting metabolism
Transpiration can cool leaves, until water loss becomes overwhelming
Most plants can produce heat-shock proteins at temps >40C which help prevent protein denaturation in the plant body
• These are chaperone proteins that help proteins fold properly in excess heat
Cold Stress
Cooler temperatures change the plasma membrane fluidity since lipids become locked into crystalline structures
Alters solute transport across membranes and protein function
Plants can alter lipid composition in their membranes -> increase unsaturated fatty acids
• But it can take days to adjust so really fast cold snaps are still a problem
Why is freezing a problem?
Ice forms in the cell walls and intracellular spaces
Cytosol has lots of solutes though, so it has a lower freezing point
But ice in the cell walls lowers the water potential, resulting in water loss from the cytoplasm
- The concentration of solutes in the cytoplasm + dehydration can kill the cell
Cold adapted plants have anti-freeze proteins, which prevent the crystallisation of ice in large amounts within the cells
PAMP-triggered immunity
A chemical attack on the pathogen that isolates and prevents its spread from the site of infection
• The plant recognizes pathogen-associated molecular patterns (PAMPS) (used to be called elicitors)
Pathogen-associated molecular patterns (PAMPS)
• Ex bacterial flagellin is a PAMP that the plant recognises
• These PAMPS are recognised by Toll-like receptors on the plant that initiate the innate immune system
- Dominant immune system in plants, fungi, insects, and primitive multi-cellular organisms
- Plants do not have an adaptive immune response
• Do not produce T-cells or antibodies
PAMP recognition triggers what?
triggers signal transduction pathways to produce a response
• Production of antimicrobial chemicals called phytoalexins
• Toughing of plant cell walls
Phytoalexins
Production of antimicrobial chemicals
How can PAMP triggered immunity be overcome?
can be overcome by the evolution of pathogens over time
These pathogens deliver effectors, which are pathogen-encoded proteins that cripple the host immune system, directly into the plant host cell
• Ex. Some bacteria deliver effectors that block the perception of flagellin, suppressing PAMP-triggered immunity
What is the plant immune system made up of?
The plant immune system is made up of hundreds of disease-resistance genes (R)
• Each R gene codes for an R protein that is activated in the presence of the effector
What responses get initiated?
- Signal transduction then initiates a slew of responses
- Hypersensitive response
- Systemic acquired resistance
Hypersensitive response
Local cell and tissue death that occurs near or at the infection site, results in lesions
Increases production of lignin and cell-wall cross-linkages
Restricts the spread of the pathogen
Production of enzymes and chemicals (jasmonates) that impair the pathogen’s cell wall integrity, metabolism, or reproduction
Systemic acquired resistance
Plant-wide expression of defense genes, non-specific against a diversity of pathogens
Methylsalicylic acid is produced at the infection site, carried by phloem, and converted to salicylic acid which promote signal transduction and the production of more defence further in the plant
- Pathogens infect leaf cells and secrete effectors, by-passing PAMP-triggered immunity
- Hypersensitive response occurs in cells near and on the infection site, creating a lesion
- Before infected cells die, they release methylsalicylic acid which is carried via phloem throughout the plant body
- Cells in other areas convert methylsalicylic acid to salicylic acid, initiating biochemical responses that protect the plant from pathogens for several days
What does herbivory cause?
causes mechanical stress on the plant
Reduces plant size
Reduces photosynthetic capacity
Restricts growth as plants divert energy and resources to anti herbivory defense mechanisms
Opens sites for infection by pathogens(virus, bacteria, fungi)
How Do plants adapt to herbivory?
Physical defenses: thorns, trichomes
Chemical defenses: tastes horrible, toxic effects, hallucinogenic effects
Combination of both: burning sap, irritants
What do many plants contain?
Many plant chemicals contain anti-cancer properties
Toxins that interfere with cell division may have therapeutic potential
Ex Taxol isolated from Pacific yew
Ex Brown-eyed Susan (Gaillardia aristate) and Buffalo
bean (Thermopsis rhombifolia) have been grazed heavily and have developed toxic compounds, with potential use as anti-cancer drugs
What do many plant chemicals have?
Many plant chemicals have other effects as well
• Some chemicals have hallucinogenic effects
Ex. Ibogaine from the iboga plant has psychedelic properties and can be used to treat additions from other compounds
