Lecture 29 - Plant Control Systems (part 2) Flashcards
Photomorphogenesis
is the effect of light on plant growth and development
What does Photomorphogenesis allow?
Allows plants to measure day length, time of year, seasons
Action spectra
depicts the relative EFFECTIVENESS of different wavelengths of light on processes
What can Action Spectra help determine?
what photoreceptors are active in a response
What are the 2 main photoreceptors?
- Blue-light photoreceptors (450-500 nm)
* Phytochromes (red (660nm) and far-red (730nm)
Blue-light photoreceptors (450-500 nm)
Phototropism, light-induced opening of stomata, light-induced hypocotyl growth reduction after breaking ground
Phytochromes (red (660nm) and far-red (730nm)
- Red and far-red have reversible, opposite effects
* Red light = germination, far-red = inhibits germination
Red light =
germination
Far-red =
inhibits germination
What are Phytochromes essential for?
seed germination
Phytochromes
- a type of light receptor in plants that mostly absorbs red light & regulates many plant responses, such as seed germination & shade avoidance
- There are several kinds of phytochromes, even within the same plant
- The light absorbing part is photoreversible
- Higher ratio of Pfr:Pr forms initiates germination
Photoreversible
The light absorbing part (of phytochromes)
- changes in shape due to light exposure is reversible with exposure to other forms of light (ie red vs far-red)
Higher ratio of Pfr:Pr forms…
initiates germination
What do Phytochromes assess?
the quality of light
During the day, the conversion between phytochrome states (Pr and Pfr)…
reach equilibrium
What does the ratio of Pr & Pfr help?
helps the plant assess relative amounts of light wavelengths
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
If ratio of Pfr is higher…
lateral branches develop rather than height
Circadian Rhythms
• Plants respond the daily changes in light, temps and relative humidity
• Some responses occur on a 24hr cycle, without a known underlying cause -> circadian rhythms
- Controlled by gene transcription
What are Circadian Rhythms controlled by?
gene transcription
Photoperiodism and response to seasons
a physiological response to photoperiod, the relative lengths of night & day
- ex: flowering response
Flowering response
- Short-day plants
- Long-day plants
- Day-neutral plants
- Interestingly, CONTROLLED BY NIGHT LENGTH, not day length
Short-day plants
shorter photoperiod induces flowering
Long-day plants
longer photoperiods induce flowering
Day-neutral plants
flower when a certain stage of maturity is reached RATHER than day length
Gravitropism
allows the plant to grow towards the light, REGARDLESS OF POSITION
Roots display…
positive gravitropism (grow down)
Shoots display…
negative gravitropism (grow against)
Positive gravitropism
grow down
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
Thigmomorphogenesis
refers to changes in morphology due to physical/mechanical perturbations
• Short, stocky trees in super windy areas
- a mechanical stimuli
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)
Environmental stresses (abiotic)
- Flooding
- Drought
- Salt Stress
- Heat Stress
- Cold Stress
Flooding
- an environmental stress (abiotic)
• Too much water suffocates plant roots
• Mangroves have pneumatophores to help get air into the soil
• Oxygen deprivation stimulates ethylene
Drought
- an environmental stress (abiotic)
• 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
- an environmental stress (abiotic)
• 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
• Can overcome this by producing their own solutes
• Halophytes
Halophytes
can pump out excess salt from the leaf epidermis
Heat Stress
- an environmental stress (abiotic)
• 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
Heat-stock 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
- an environmental stress (abiotic)
• 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
• But it can take days to adjust so really fast cold snaps are still a problem
• Freezing also is a problem
Plants can alter lipid composition in their membranes –>
increase unsaturated fatty acids
Freezing also is a problem (in cold stress)
• 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
1st line of defences against pathogens
The first line of defence is 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)
2nd line of defences against pathogens
The second line of defence is 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
3rd line of defence against pathogens
Third line of defence is PAMP-triggered immunity and effector-triggered immunity
PAMP-triggered immunity
- A chemical attack on the pathogen that isolates and prevents its spread from the site of infection
- The plant recognises 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?
signal transduction pathways to produce a response
• Production of antimicrobial chemicals called phytoalexins
• Toughing of plant cell walls
Phytoalexins
Production of antimicrobial chemicals
Effector-triggered immunity
• PAMP-triggered immunity 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
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
• Signal transduction then initiates a slew of responses
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 DEFENCE 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
Effector-triggered immunity steps
- 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
Herbivory causes…
mechanical stress on the plant
Herbivore defence
- Reduces plant size
- Reduces photosynthetic capacity
- Restricts growth as plants divert energy and resources to antiherbivory defence mechanisms
- Opens sites for infection by pathogens (virus, bacteria, fungi)
Plants adapt via several mechanisms:
- PHYSICAL defences: thorns, trichomes
- CHEMICAL defences: tastes horrible, toxic effects, hallucinogenic effects
- COMBO of both: burning sap, irritants
Physical defences:
thorns, trichomes
Chemical defences:
tastes horrible, toxic effects, hallucinogenic effects
Combination of both (physical & chemical defences):
burning sap, irritants
Many plant chemicals contain…
anti-cancer properties
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
- Dr. Roy Golsteyn (U. of Lethbridge) and the Prairie to Pharmacy Project
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
- Dr. Jake Stout (U. of Manitoba, Dept. of Bio. Sci) is trying to determine the biosynthetic pathways and genes involved in ibogaine production
What happens if the night time is interrupted by light (during flowering response)?
flowers won’t develop since they don’t get the required amount of continuous dark to stimulate flowering
Pr absorbs red light and is converted to Pfr…
Red light promotes seed germination
Pfr absorbs far-red and is converted back to Pr…
Far-red light inhibits germination
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
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)
Florigen
signalling molecule that promotes flowering
Mimosa pudica (sensitive plant):
results from a loss of turgor due to touch and action potentials in the leaf cells
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
Excess sodium and other ions can be toxic to plants at…
high concentrations (affects the selective permeability of root cells)
Can overcome this (salt stress) by…
producing their OWN solutes so they don’t acquire the toxic ones (but only short term)
Cold adapted plants have anti-freeze proteins…
which prevent the crystallisation of ice in large amounts within the cells
