Biotic & abiotic stresses Flashcards
Plants lecture 4
What is stress?
Change in enviro conditions that might reduce a plant’s survival, growth, development or reproduction
What are the 3 types of biotic stress?
Pathogens
Herbivores
Competitors
What are the 4 types of abiotic stress?
Water
Temp
Light
Chemicals
What are the 2 forms of water stress?
Drought
Flooding –> anoxia
What are the 2 forms of light stress?
Photo-inhibition
UV damage
What are the 4 types of chemical stress?
Salinity
Heavy metals
Mineral deficiency
Acid soils
Describe the issues with plant-human interactions
Stress impacts agri –> impacts food
Humans impact enviro —> impacts stress on plants
What are the 4 ways in which humans are exacerbating stresses on plants?
> natural resource depletion
pollution - soil, water & atmosphere
irrigation - salinity
climate change - temp, rain, sea level & weather
What are the 3 types of plant responses to stress?
> injury/death
avoidance - spatially or temporally
tolerance - alter metabolism to tolerate impact of stress
Give an example of how plants avoid stress
Desert plants have v short life cycles & flower v rapidly only after rain
Give an example of how plants tolerate stress
Drought tolerance in resurrection plants (can survive desiccation to 7% water w/out injury
What are adaptations to stress?
Heritable modifications of structure/function that enable plants to survive & reproduce in otherwise adverse environments
Allows for tolerance or avoidance
What happens to plants during a water deficit?
Water loss via transpirations exceeds uptake from roots
–> leads to stress
What mechanisms can plants have to conserve water?
> close stomata via loss of turgor pressure in guard cells
ABA hormone acts on guard cell membrane
slow leaf growth
rolled grass leaves
shed leaves
What is the ABA hormone?
Abscisic acid
What is the issue w/ shedding leaves?
Reduces photosynthesis –> diminishes crop yield
How is Cassava (Manihot esculenta) tolerant to drought?
Sheds all leaves in drought
–> regrow rapidly after rain
How do roots respond to water deficit?
Shallow roots in dry, surface soil lose turgor = inhibited growth
Deep roots in moist soil = continue to grow
THUS root system proliferates to maximise exposure to water
Give an example of plants adapted to excess water
Mangroves have aerial roots exposed to air
–> can aerobically respire
How do plants respond to water logging?
O2 deprivation stimulates ethene production
–> causes some cells of root cortex to undergo apoptosis
–> creates air tubes (function as snorkels)
= provide O2 to submerged roots
What can excess salts in soil lead to?
Lower water potential of soil –> water deficiency of plant
Sodium & other ions = toxic when concs high enough to overwhelm selective permeability capabilities of root
How can plants respond to excess salts in soil?
Produce ‘compatible solutes’ = proline or glycine betaine
–> lower WP of cells w/out admitting toxic quantities of salts
What is the name of plants that can survive salt stress for long periods?
Halophytes
How does heat stress affect plants?
Kill them by denaturing proteins & disrupting metabolism
What do plants do above 40 degrees C?
Synthesise heat-shock proteins
(include chaperonin that prevent denaturation)
- protect other proteins from stress
How does cold stress affect plants?
Membranes lose fluidity
= crystalline
= alters solute transport
How do plants respond to cold stress?
Alter lipid composition
- increase proportion of unsaturated fatty acids
–> increases membrane fluidity
(takes hours/days)
How does freezing affect plants?
Ice forms in cell walls & intercellular spaces
–> H20 leaves cytoplasms due to reduced WP in cell wall
= increase in ion conc in cytoplasm
= death
Why doesn’t cytoplasm freeze?
High solute conc
How are cold-tolerant plants adapted?
Accumulate higher levels of solutes (e.g. sugars)
& unsaturation of fatty acids
What do antifreeze proteins do?
Prevent ice crystals growing so large
What defences have plants evolved against herbivores?
> Thorns
Trichomes = hairs that interfere w/ feeding systems of small herbivores
Chemicals - toxic
e.g. Canavanine
How does canavanine affect insects?
- insect eats plant
- canavanine incorporated into insect’s proteins instead of arginine
- alters structure & function of proteins
- insect dies
How do plants recruit predatory animals?
- Caterpillar eats plant
- Wounding & chemicals in saliva triggers signal induction pathway in plant
- synthesis & release of volatile attractants
- attracts parasitoid wasps that lay eggs in caterpillar
What do plants use volatile signals for?
Signalling between plants
Describe how plants use volatile signals to signal to one another
- Lima beans infested by spider mites release volatiles - including methyl-jasmonic acid
- Signals the attack to neighbouring plants
- Neighbouring plants prepare for attack
How can neighbouring plants prepare for an attack once they receive volatile signals?
> biochemical changes that make them less susceptible to attack
release other volatiles that attract predatory mites that feed on spider mites
What 3 things must occur for an interaction to turn into a disease?
- pathogen must be able to overcome plant’s defences
- host plant must be susceptible to pathogen
- environment must be in favour of pathogen
How have Pseudomonas syringae evolved to invade plants?
Evolved coronatine which causes stomata to open
What’s special about Puccinia monoica?
Fungal pathogen mimics flowers = pseudoflowers
What is a plant’s 1st line of defence?
Epidermis & periderm
- but pathogens can enter via wounds & natural openings
What is a plant’s 2nd line of defence?
What is this defence enhanced by?
Chemical attack
- destroy pathogen & prevents spread
Ability to recognise pathogen
Describe the zig-zag model of plant-pathogen interactions
- pathogen recognised = pattern triggered immunity
- pathogen effectors suppress defence response
= effector triggered susceptibility - effector recognised
= effector triggered immunity
What is a virulent pathogen?
One that plant’s have little specific defence against
What is an avirulent pathogen?
One that cannot infect & colonise the host plant due to effective plant immunity
What do avirulent pathogens produce?
What are these coded for by?
Effector molecules
- recognised by plants
Avirulence genes = Avr genes
Avirulent effector molecules are recognised by what?
What are these coded for by?
Plant resistance proteins
Resistance genes = R genes
What happens if plant resistance proteins detect effector molecules?
Activate plant defences by triggering signal transduction pathways
What are the 2 signal transduction pathways?
Hypersensitive response
Systemic acquired resistance
How do virulent proteins evade R protein recognition?
Produce molecules not recognised by R proteins
What type of evolutionary relationship occurs between plants & pathogens?
Arm’s race
Which gene combination results in host plant resistance?
Dominant R1 + Avr1
Describe R-Avr recognition
- Pathogen Avr effector molecules bind to specific plant R proteins
- Triggers signal transduction pathway
- Hypersensitive response
Describe the hypersensitive response
Plant cells produce anti-microbial molecules & PR molecules
- -> seal ff infected areas by modifying cell walls by synthesis of lignin & cross-linking molecules
- -> then destroy themselves
Describe systemic acquired resistance
Infected cells secrete signalling molecules methylsalycilic acid
- -> distributed to rest of plant via phloem
- -> converted to salicylic acid = initiates signal transduction pathway
- -> activate systemic acquired resistance
What are the antimicrobial compounds compounds in the hypersensitive response?
Phytoalexins
What are PR proteins?
involved in the hypersensitive response
Pathogenesis related proteins
= enzymes that attack pathogen cell walls
Briefly describe the process of defence responses against a virulent pathogen
- Avirulent protein infects plant
- Avr effector protein binds to R protein
- Signal transduction pathway
- Hypersensitive pathway
- Methylsalycilic acid secreted
- Distribution via phloem
- Signal transduction pathway
- Acquired resistance
