Lecture 28 - Plant Control Systems (part 1) Flashcards
Unlike animals which respond by movement, plants respond to stimuli by what?
altering growth & development
What is response in plants carried out via?
signal transduction
- reception, transduction, response
What is Etiolation?
is the morphological adaptation required for growing in the dark
• 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
When a plant shoot reaches light, what commences?
de-etiolation (“greening”)
What happens in de-etiolation (“greening”)?
- STEM elongation SLOWS
* SHOOT beings to PRODUCE CHLOROPHYLL to initiate photosynthesis
Reception
Light signal is DETECTED by a phytochrome receptor located in the cytoplasm
What does Reception activate?
This activates at least 2 signal transduction pathways
Transduction
- First pathway: cGMP
- Second pathway Ca2+ ions
- Both pathways must be induced for full deetiolation to occur
First pathway: cGMP (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
Second pathway Ca2+ ions (of transduction)
- Phytochrome activation OPENS up Ca2+ channels, flooding the cytosol with increase Ca2+
- This ACTIVATES a different protein KINASE to initiate a response
Response
• Both pathways lead to the expression of genes for proteins that function in the deetiolation process
- Post-transcriptional modification of proteins
- Transcriptional Regulation
• Products of response are enzymes for photosynthesis, chlorophyll production, plant hormone levels
Post-transcriptional modification of proteins
- PRE-existing proteins are PHOSPHORYLATED (phosphate added), altering protein shape and function
- Protein phosphatases DEphosphorylate ENZYMES, turning off the signals
Transcriptional Regulation
- Transcription factors BIND to specific regions of DNA TO CONTROL transcription of genes on DNA
- Activators = increase transcription
- Repressors = decrease transcription
What do Activators do?
increase transcription
What do Repressors do?
decrease transcription
What are products of response?
are enzymes for photosynthesis, chlorophyll production, plant hormones levels
What do plants produce?
hormones (plant growth regulators)
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
What can each hormone have?
Each hormone can have a multitude of effects depending which tissue it is acting in, its concentration, and the developmental stage of the plant
Where are plant hormones transported?
in the phloem sap
Phototropism
Plants generally grow towards the light
Tropism
plant organs curving toward or away from a stimulus
What is Phototropism usually a result of?
stem elongation
What does Auxins (indoleacetic acid, IAA) promote?
growth/elongation of the coleoptiles (stems above the cotyledons)
Where are Auxins (indoleacetic acid, IAA) produced?
predominantly in the shoot tips (SAM)
How does Auxins (indoleacetic acid, IAA) move?
Moves unidirectional shoot tip to shoot base -> polar transport (unrelated to gravity)
Polar transport
Moves unidirectional shoot tip to shoot base (unrelated to gravity)
Describe the Plant Development function of Auxins (indoleacetic acid, IAA)?
• Auxin produced in the shoot tip controls SPATIAL ORIENTATION of the plants
- Affects size, shape, environment of branches and stems
• When auxin production DECREASES, lateral branches allowed to develop
• Involved in phyllotaxy
• Polar transport in leaf margins affects the formation of leaf veins
- Less auxin, more secondary leaf veins and loosely organised main veins
• Reduction in auxin at the end of the growing season stimulates the reduction in the vascular cambium activity
Phyllotaxy
local peaks in auxin determine the site of the leaf primordia
Describe the Stem Elongation of Auxins (indoleacetic acid, IAA)
- Binds to RECEPTORS in the plasma membrane to initiate cell expansion
- Stimulates growth when concentration is between 10-8 to 10-4 M
- Acid-growth hypothesis
- 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
- 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
Expansins
proteins that BREAK the linkages (hydrogen bonds) in the cell walls
What are Auxins used in? DESCRIBE
horticulture
• Rooting hormone for vegetative propagation
• Synthetic auxins are used as herbicides (die from hormonal overdose)
• Synthetic auxins increase fruit production
When were Cytokinins discovered & what do they stimulate?
- Discovered in the early 1940s
* Stimulate cytokinesis (cell division)
Where are Cytokinins produced?
in actively growing tissues, particularly in the roots
Cytokinins works with what? Describe further dets
• 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
Apical Dominance (in Cytokinins)
Apical bud suppresses growth of axillary buds
Anti-aging (in Cytokinins)
- Slow apoptosis in cells
- Inhibits protein breakdown, stimulates RNA production and protein synthesis
- Mobilises nutrients from surrounding tissues
Describe the 4 steps of Cytokinins
1) The apical bud is a sugar sink and produces auxins
2) Auxin moves downward, producing strigolactones that repress lateral bud growth
3) Cytokinins from the roots antagonises 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
When were Gibberellins (GAs) discovered & describe background
• 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
Where are Gibberellins (GAs) produced?
in young roots and leaves
What do Gibberellins (GAs) function in?
- Stem elongation
- Fruit growth
- Germination
Stem elongation in Gibberellins (GAs)
- Stimulates BOTH cell division and elongation (act in concert with auxins re: expansins)
- Dwarf plants grow tall in the presence of gibberellins
Fruit growth in Gibberellins (GAs)
- 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!)
Germination in Gibberellins (GAs)
Signals seed to break dormancy, stimulates digestive enzymes for endosperm breakdown
When were Abscisic Acid (ABA) discovered & describe background
• Discovered in the early 1960s
• Chemical changes occurred during abscission (dropping off) of leaves
and fruits
Unlike other hormones, ABA ______ growth
slows
Seed dormancy in Abscisic Acid (ABA)
- 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 inactivating ABA
• Ratio of ABA:GA determines whether the seed is dormant of will germinate
Drought tolerance in Abscisic Acid (ABA)
ABA CLOSES the stomata to PREVENT water loss by affecting Ca2+ secondary messengers, resulting in K+ channels to OPEN in the guard cells
Plants produce ETHYLENE gas in response to what?
in response to stresses such as drought, flooding, mechanical pressure, injury, and infection
Ethylene is also produced during what?
fruit ripening and programmed cell death
What also induces the production of ethylene in plants?
auxin
What are 4 main effects of Ethylene?
- mechanical stress
- senescence
- leaf abscission
- fruit ripening
Triple Response in Ethylene
Triple Response
• Stem elongation
• Thickening of the stem
• Curvature of the stem
Triple Response
shoots avoid obstacles via horizontal growth
Senescene (leaf/flower shedding) within Ethylene
- 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
What is leaf abscission?
the loss of leaves
Leaf abscission details
is the loss of leaves
• 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 break down 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 (within 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
How is fruit ripening a chain reaction?
- 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
Brassinosteroids
- Steroids similar to cholesterol
- Induce cell elongation and division in stems
- Slow down leaf abscission
- Promote xylem differentiation
Jasmonates
- Fatty-acid derived molecules
- Plant defence and plant development
- First ID’d in Jasmine plants
- Produced in wounded plants to serve as defence for pathogens and herbivores
• Also key roles in nectar secretion
- fruit ripening
- pollen production
- flowering time
- root growth
- seed germination
- tuber formation
- promoting mycorrhizal associations
- tendril coiling
• Also works with phytochromes, GA, IAA, ethylene
Strigolactones
• Xylem-mobile chemicals
- Stimulate seed germination
- suppress adventious root development
- helps with mycorrhizae
- controls apical dominance
• First ID’d from Striga (witchweed), a rootless parasitic plant that penetrate the roots of host plants
