Plants and Their Environment II Flashcards
What are some constitutive abilities and adaptive abilities of plants to withstand abiotic environments?
Constitutive abilities:
-Hairy leaves (protect against high radiation and water loss).
-C4 or CAM
photosynthesis
-Waxy cuticle
-Folded leaves
-Photosynthetic stems with loss of leaves (especially used by cactus family).
-Swollen stems or leaves
-Thermonasty
Mostly developmental.
Hard wired in from the start- environmental factors mean that plants need these all the time.
Adaptive abilities:
-Stomatal closure
-Compatible solute production (counteract water potential in drought to keep water in).
-Heat shock protein production
-LEA (late embryo abundance) protein production
Detoxification/Sequestration
-Pigment production in response to high light levels.
Mostly by gene induction.
Turned on and off in response to stress.
How does high temperature affect plants?
Often coupled with low water stress.
The ability of RuBisCo to differentiate CO2 from O2 is reduced - losses from photorespiration increase faster than gains from photosynthesis. (CO2 combined with water to make sugars in photosynthesis. RuBisCo combines water with O2, then photorespiration occurs. Plant gets less productive).
Breakdown of thylakoid and granal stack structure.
Breakdown of chloroplast membrane organisation – photosynthesis stops
Induction of heat shock proteins is the main response. These stabilise granal stacks and RuBisCo.
How does water stress affect plants?
Loss of turgor.
Membrane damage.
Protein denaturation.
Increase in ABA synthesis but also ABA independent pathways.
Common effect of salt, drought and freezing stresses :
Drought – reduced water potential outside the cell due to lack of water in the environment.
Salt – reduced water potential outside the cell due to increased solute concentration in the apoplastic environment. Tends to cause protein denaturation.
Freezing – reduced water potential outside the cell due to ice crystal formation in the apoplast- disrupts physical cell structure.
Given sufficient warning plants can at least partially adapt to these conditions.
What is cold acclimation?
Process that most temperate perennials and overwintering annuals perform.
Plants can detect lowering of temperature over scales of hours to months.
Circadian clock allows plants to predict lowering of temperatures in the evening and adapt in anticipation. lant can express genes in advance if it can sense what time it is.
Uncertain how plants detect cold but likely to involve changes in membrane fluidity in the plasma membrane and chloroplast.
May use the cytoskeleton as a tension sensor.
Changes in fluidity can affect protein conformation.
Involves plasma membrane Ca2+ signalling pulses.
Detection of lowering temperatures over time indicates onset of winter.
Promotes process known as cold acclimation to allow survival of subsequent freezing conditions.
(Lipids- go lumpy when it gets to cold, melt when it gets too hot).
What damage can be caused by freezing?
Cellular membranes ruptured by ice crystals, ice formation outside of cells causing water to be drawn out of cells, protein and membrane denaturation.
Plants that have been acclimatised survive much better in freezing conditions that those that haven’t.
What is the signalling pathway involved in cold acclimation?
Plants detect rate and size of temperature change. Movement of proteins- if they move slower, it is a sign that membrane is more viscous- it is colder. Conformational changes in proteins. Ca2+ signal- binds to nucleus. ICE induction (bHLH)- transcription factor, binds to promoter of CBF. CBF 1/2/3 induction (AP2)- important transcription factors for stress. Chromatin remodelling and gene induction. Acclimate due to expression of COR genes.
What are the outputs of cold acclimation?
Compatible solutes-osmolytes and antifreeze:
- Proline
- Sugars
- Quaternary ammonium compounds
(COR genes have a wide variety of functions. Osmolytes keep water in the cell. Antifreeze prevents ice crystal formation).
Protein folding and stabilizing – prevent denaturation and aggregation:
- Induction of heat shock proteins
- Induction of LEA proteins
- Polyols
- Proline
Membrane stabilizing – prevent phase transitions and lysis:
- Increase phospholipids and reduce ceramides
- Induction of LEA proteins
How does drought stress affect plants? What are the similarities and differences to cold stress?
Similarities to cold stress:
- Production of compatible solutes, membrane and protein stabilizers, etc.
- Occurs through CRT motif genes but in this case is activated by DREB2A and B (closely related to CBFs).
Differences to cold stress:
- Large ABA signaling component.
- Promotes gene induction through ABRE promoter element containing genes.
- Promotes stomatal closure.
Activate a different set of COR genes, but use the same motif. Second set of transcription factors due to ABA signalling- ABF.
What are stomata?
Stomata are pores on the leaf surface that allow transpiration (water loss) and gas exchange (CO2 in).
How does stomatal closure work?
High water levels- stomata open because there is plenty water.
Low water levels- stomata close to prevent water loss. CO2 cannot get in.
What is the pathway that regulates stomatal closure?
Pumps regulate opening and closing of stomata.
Sense ABA being produced cause of drought stress.
Vacuole keeps Ca and K out of the way, but there if needed.
Outward (mem)Inward rectifying Ca channels (vacuole)- increased Ca in cytosol- inhibits outward rec. proton channels.
Accumulation of protons- increase acidity- hyperpolarisation of Plasma membrane. Inhibits In rec K channels- can’t get into guard cell.
Ca activates out rec K channel.
K leaves cell- draws water out of guard cell- changes turgor pressure- stomata snaps shut.
What is vernalisation?
How to flower at the right time of year.
Balancing act – need to have enough vegetative mass for photosynthesis but environmental conditions must also be right for seed set. Juvenile phase in perenial plants ensures sufficient size is reached to enable good seed set.
If cross pollination is required flowering synchrony is necessary
Transition controlled by environmental cues – day length, temperature (also drought, etc. bring on flowering).
Plants need to flower to set seed.
Don’t want to seed if there is not enough mass to photosythesise to support plant, or if conditions are not good for seed (esp. if seed doesn’t have dormancy).
Synchrony- all flower at same time.
What does vernalisation do?
Ensures that plants experience winter before flowering.
Prevents flowering that would cause late seed set, incomplete seed maturation and possible seedling lethality.
Overrides day length considerations eg. temperature, phytochromes.
Typically a long exposure needed (weeks-months) compared to day length input required for Phytochrome mediated transition to flowering.
A quantitative process – longer vernalization leads to faster flowering after returning to “normal” growth conditions.
Detected in shoot apex.
Winter wheat vs spring wheat
Spring wheat has had the vernalization requirement bred out.
Vernalisation has cellular “memory” – stable chromatin remodelling.
What genes are necessary for vernalisation?
Requires active copies of Flowering locus C (FLC) and Frigida (FRI) genes:
- Found in overwintering crops and many temperate annual and perennial species.
- Many early flowering varieties have mutations in FLC or FRI.
Long term process of cold exposure (compare to cold acclimation):
-1-3 months of cold, non-freezing temperatures.
Homologs of these genes are found in winter crops and temperate/perennial plants.
What happens when a plant does not need vernalisation?
Eg. Lab arabidopsis- usually come from warmer places- can flower and set seed whenever as they are not used to cold. No vernalisation.
Photoperiodic measurement-> circadian clock-> CO- activates FT, which activates AP1,LFY- flowering happens.
