Abiotic Environmental Interactions
- Abiotic Stress Stress is usually defined as an external factor that
exerts a disadvantageous influence on the plant.
These can be environmental or abiotic factors that
produce stress in plants, although biotic factors such
as weeds, pathogens, and insect predation can also
produce stress. In most cases, stress is measured in
relation to plant survival, crop yield, growth
(biomass production), or the primary assimilation
processes (CO2 and mineral uptake), which are
related to overall growth.
Taiz, Zeiger (Plant Physiology)
Abiotic and Biotic environmental stress factors
Stress: live and let die?
Why bother about plant stress?
As human population increases, agriculture must feed more
people while competing with urban development for premium
arable land
If record yields can be assumed to represent plant growth under
ideal conditions, then the losses associated with biotic and
abiotic stresses can reduce the average productivity by 65-87%,
depending on the crop
Resistance and Acclimation
Resistance is composed of :
– Avoidance: mechanisms that prevent exposure to stress
– Tolerance: mechanisms that permit the plant to withstand the stress
Resistance:
Saguaro: succulent photo-synthetic
stem (highy drought tolerant)
Honey mesquite: deep roots (drought
avoiding)
Mohave desert star: wet-season
cycle
Acclimation:
Spinach: osmotic adjustment
Black spruce: freezing tolerance
life
Adaptation (to stress): An
inherited
level
of
stress
resistance acquired by a
process of selection over
many generations. Contrary
with acclimation
Acclimation (hardening): The
increase in plant stress
tolerance due to exposure to
prior stress. May involve
gene expression. Contrary
with adaptation
Acclimation of Several Cereals
to cold
Electrolyt leakage
Adaptation and
Acclimation
Summer oat
No acclimation
Winter oat
With acclimation
Winter rye
Gene Expression Patterns often
Change in Response to Abiotic Stress
Stress-induced changes in metabolism and development can often be attributed
to altered patterns of gene expression
Reactive oxygen species production as is a common, first response
to many stresses
Oxidative stress results from conditions that promote formation of ROS, which damage or
kill cells
(ROS)
Reactive Oxygen Species (ROS)
These reactive molecules
are highly destructive to
lipids, nucleic acids and
proteins
Except H2O2, ROS have a
very short half life
Plants
scavenge
and
dispose of these reactive
molecules by
use of
antioxidant
defense
systems present in several
subcellular compartments
When these defenses fail
to halt the self-propagating
auto-oxydation
reactions
associated with ROS, cell
death ultimately ensues
Formation of ROS by Electron Transfer
v
Singlet-Oxygen
Hydrogen peroxide
Molecular Oxygen
Hydroxyl radical
Superoxide
Water
Enzymatic Scavenger Reactions that Eliminate ROS
• OS: oxidative stress
• CS: cold stress
• HS: heat stress
• Salz: salt stress
• WS: water deficiency stress • HL: high light stress
Non Enzymatic Antioxydant Defense systems
• ascorbate (Vitamin C)
• reduced glutathione (GSH)
• polyamines (cabbage, broccoli, cauliflower)
• -tocopherol (vitamin E)
• flavonoids
Disfunction of the Photosynthetic Transport Chain provides a major
source of electrons for ROS formation
ROS Formation during Photosynthesis
If light absorption and CO2-fixation are in balance:
minimal ROS-formation
Electron Transfer from Reduced
PSI to O2 is an electron sink creating superoxide ions
Photoinhibition of PSII leads to singlet oxygen buil up
CO2-Limitation: Enhancement of
Photorespiration and H2O2-Formation
Water Deficit Stress
• Periods of little or no rainfall can lead to a meteorological condition called drought
• Transient or prolonged drought conditions reduce the amount of water available for plant growth
(saline habitats or low temperatures can have the same effect)
• Water deficit stress is measured by the water potential of the cell interior an cell exterior
w= water potential = s + p
s = solute potential (number of particles dissolved in water)
p = pressure potential (physical forces exerted on water by environment)
Resurrection plants
Plants that “completely” dry out (no physiological activity
detectable) and have the ability to green and resume
physiological activity after watering are called resurrection plants.
Craterostigma plantagineum
Top: dried out plants are extremely fragile and crumble when
touched
Bottom: same plant, 24 hours after watering
The Rose of Jericho (Asteriscus pygmaeus and Anastatica
hierochuntia) is not, though often claimed, a resurrection plant.
These plants die when they dry out; after watering, they simply
swell and release the seeds which germinate and form a new
plant.
Resurrection plants as a model for water-deficit stress adaptation
Dehydration –> Activation of “desiccation-related” genes
–> (1) Alterations in metabolism and
–> (2) Production of “protective” proteins
(1) Alterations in metabolism:
(a) accumulation of protective solutes such as sucrose, trehalose, and
proline that stabilize proteins and cellular membranes,
(b) production of antioxidant compounds (such as galloylquinic acids),
(c) biochemical alterations in membrane and cell wall composition.
(2) Production of “protective” proteins such as “dehydrins” and “expansins”
that help preserve the structural integrity of intracellular organelles and the
cell walls.
Role of abscisic acid (ABA) in stomata closure and water retention
-ABA
H2O
+ABA
In the absence of ABA, the phosphatase PP2C is
free to inhibit autophosphorylation of SnRk kinases
ABA receptor (PYR/RCAR) ony
cloned in 2010!
ABA enables PYR/RCAR proteins to bind and
sequester PP2C
Other ABA receptor likely exist.
This relieves inhibition of SnRk, which becomes
autoactivated and phosphorylate ABF transcription
factors
Freezing Stress
Cold temperatures can cause a type of water deficit stress:
as ice formation is initiated in the intercellular spaces, cellular water moves down the water
potential gradient, across the plasma membrane, and toward the extracellular ice. Therefore, a
water deficit develops within the cell in response to freezing.
Injury symptoms become apparent after transfer to normal temperatures:
- cellular dehydration
- alteration of membrane structure
- plasma membrane destabilization (lipid-lipid demixing)
- loss of interactions between cell wall and plasma membrane
- acidification of cytoplasm (vacuole rupture?)
Candidate sensing proteins may monitor changes in membrane fluidity
The Miracle of the Blue Orchid
Membrane lipids maintain fluid phase (liquid crystalline) at normal, warm temperatures, which
ensures maintenance of cellular function.
When temperature decreases, however, lipids with high melting temperatures begin to solidify
(gel phase) and become phase-separated within the membrane, resulting in the membranes
becoming leaky and/or dysfunctional, including the membrane of the vacuole (tonoplast)
Intracellular water and solutes are lost and membrane-associated reactions such as carriermediated transport, enzyme-mediated processes, and receptor function are inactivated.
In Prague, during the cold winter of 1875/76, Herrmann Müller
placed the flowers of the orchid Calanthe triplicata on the window
sill over night. On the next morning he took the frozen flowers back
into the room. After de-freezing, the flowers turned blue. What
happened?
A chromogenous secondary metabolite, the colorless indican, came
into contact with glycosidase after the disruption of the tonoplast
and was converted to indoxyl (yellow) which was then oxidized to
indigo (blue).
Cold acclimation (or freezing tolerance)
The ability to survive temperatures below freezing is genotype-specific.
Among the genotypes unable to withstand temperatures below freezing are
many important crop plants, including corn, tomato, and rice.
Other plants are able to survive temperatures below freezing; some can survive
temperatures lower than -40°C .
Freezing tolerance develops in a process known as cold acclimation, a response
to low, but non freezing temperatures that occur before freezing.
Arabidopsis, after exposure to temperatures in the range of
1-5°C for one to five days, can survive temperatures of -812°C.
- alteration of membrane composition (increase in phospholipids)
- accumulation of compatible solutes (membrane protection)
- improved water retention at the membrane surface
- change in plasma membrane protein composition (phospholipases,
synaptotagmin 1  membrane trafficking)
Flooding and Oxygen Deficit
Plants are obligate aerobes: oxygen is the terminal electron acceptor in the
mitochrondrial electron transfer chain
During flooding, too much water blocks the entry of O2 into the soil so that roots
and other organs cannot carry out respiration
Plant species generally can be classified as wetland, flood-tolerant, or flood
sensitive, according to their ability to withstand periods of oxygen deficit
Wetland plants have specific anatomical adaptations:
- aerenchyma facilitate O2 transport
- lenticels in periderm for gas exchange
- pneumatophores: shallow roots with negative geotropy
Wetland plants are fully adapted :
Pneumatophore: shallow roots that grow
with negative geotrophy out of the
aquatic environment prevent oxygen
deficit stress for instance in mangroves
Thickened root hypodermis to reduce
O2 loss
Zea mays: root cortex under aerobic
(A) and anaerobic (B) conditions
Aerenchyma (continuous, columnar intracellular spaces) faciliate
transport of O2 from aerial structures to submerged roots
Adventitious roots from
the hypocotyl or stem,
lenticells (openings in the
periderm)
Phenotypic plasticity also helps some plants to adapt to flooding
The (genetically)
same plant at a
wet site
root
length
Agropyron smithii
at a dry site
Phenotypic plasticity also helps some plants to adapt to flooding
Oryza sativa L. var. Indica: growth response of seedlings to flooding
Rice reduces root growth and increase coleoptile growth or internodal growth
Flood Tolerant Plants
Flood tolerant plants can endure anoxia temporarily but not for prolonged
periods.
Like wetland species, these plants generate ATP through anaerobic metabolism
during short term flooding.
In most cases, root elongation is inhibited, overall rates of protein synthesis
diminish, and patterns of gene expression are altered
Flood sensitive plants
They show injury response to anoxia, causing death within 24 hrs
Generally sensing of oxygen deprivation is very poorly understood
Tropospheric Ozone is linked to Oxidative Stress in Plants
Stratospheric ozone (O3) is beneficial
because it shields the earth from UV
irradiation
But tropospheric ozone is harmful to life
because it is a highly reactive oxidant…
…and is produced by human activity…
… which also destroys Stratospheric ozone
mostly by emission of CFCs (ozone hole)
The negative effects of ozone on plants
include:
-decreased photosynthesis rates
-leaf injury
-reduced growth of shoots and roots
-accelerated senescence
-reduced crop yield
BUT: ozone exposure induces pathogen
resistance
Ozone causes Oxidative Damages to Biomolecules
Plants vary Greatly in their Ability to Survive in High-Ozone
Environments
Their resistance to ozone utilizes either avoidance or tolerance mechanisms:
- Avoidance by physically excluding the pollutant by closing the stomata
- Tolerance via biochemical responses that induce or activate the antioxidant
defense system and DNA repair mechanisms
acclimation
Ozone-damaged oat leaves
Heat Stress
Heat stress may arise under numerous temporal and developmental
circumstances:
– in leaves when transpiration is insufficent (water limitation, high temperature)
or with closed stomata and high irradiance
– in germinating seedlings when the soil is warmed by the sun
– in organs with reduced capacity for transpiration (e.g. fruits)
– in general from high temperature or irradiation
Plants exposed to excess heat exhibit a characteristic set of cellular and
metabolic responses including:
– damage of cellular structures, including organelles and the cytoskeleton, and
impairment of membrane function
– decrease in the synthesis of “normal” proteins, denaturation of proteins
– transcription and translation of a new set of proteins known as heat shock
proteins (HSPs)
This response is observed when plants are exposed to temperatures at least 5°C
above their optimal growing conditions
Plants can Acclimate to Heat Stress
Plants can acquire thermotolerance if subjected to a nonlethal (permissive) high
temperature for a few hours before encountering heat shock conditions .
An acclimated plant can survive exposure to a temperature that would
otherwise be lethal (there is, of course, a limit to how much heat a plant can
withstand) .
28°C
40°C, 2hrs -> 45°C
45°C
Heat shock proteins
• A and C: silver staining
• B and D: fluorography of newly synthesised proteins
Fluorography is a method used to visualize substances present in gels, blots, or other biochemical separations. Radioactively
labeled substances emit radiation that excites a molecule known as a fluor or scintillator that is present in the gel. When the
excited molecule relaxes to its ground state, it emits a photon of visible or ultraviolet light that is detected by photographic
film. The developed film indicates which bands in the gel contain radioactively labeled material (Waterborg et al., 1994).
Actively synthesized proteins are labelled with S35 incorporated into methionin.
5 Classes of HSPs are Defined According to Size
• HSP100
– contain 2 conserved ATP domains. 3 subfamilies: ClpB is heat inducible;
members of this familiy are required for thermotolerance in plants and yeast
– The HSP100 familiy of chaperones may function in disaggregation rather
than prevent protein aggregation and misfolding during exposure to high
temperatures
• HSP90
– are found in bacteria and in the cytosolic, nuclear, and endoplasmic
reticulum compartments of eukaryotic cells, where they may function as
molecular chaperones
• HSP70
– are essential for normal cell function. Some members are expressed
constitutively; others are induced by heat or cold. Localised to the nucleolus
during heat stress, HSP70 is redistributed to the cytoplasm during stress
recovery
5 Classes of HSPs are Defined According to Size
• HSP60
– are thought to function as molecular chaperones. HSP60 proteins
are abundant even at normal temperatures. Their major role is thought to involve
protein assembly. In vitro, HSP60 proteins prevent other proteins from
aggregating at physiologically relevant temperatures and are important in protein
refolding as temperatures increase
• smHSPs
– Plants contain 5-6 classes of smHSPs (2 in the cytosol, 1 in the
chloroplast, 1 in the ER, 1 in the mitochrondrion, and possibly another in a
membrane compartment that has not been defined), whereas other eukaryotes
have only one single class of smHSP
– HSP18.1 from pea has been demonstrated to prevent protein
aggregation in response to high temperature in vitro
Heat Shock Transcription Factor (HSF)
• The heat shock transcription factor (HSF) is expressed constitutively but must
be activated via trimerization during heat stress to recognise its DNA target, the
heat shock element (HSE).
• The HSE is made up of 5-bp repeats in alternating orientations with the
consensus nGAAn. An HSF-regulated promotor may contain 5 to 7 of these
repeats close to the TATA box
1) Monomeric form of heat shock factor (HSF) + HSP70
5) HSF - phosphorylation
2) Trimeric form upon heat shock
6) Interaction of phosphoryl. HSF with HSP70
3) Binding of HSF to heat shock promoter
7) Release of HSF from DNA
4) HSP – gene transcription
8) Conversion to non DNA – binding monomer