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 (CO 2 and mineral uptake), which are related
to overall growth. Taiz, Zeiger (Plant Physiology) Slide 2 Abiotic
and Biotic environmental stress factors Slide 3 Stress: live and
let die? Slide 4 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 Slide 5
Resistance and Acclimation Acclimation: Spinach: osmotic adjustment
Black spruce: freezing tolerance Resistance: Saguaro: succulent
photo-synthetic stem (highy drought tolerant) Honey mesquite: deep
roots (drought avoiding) Mohave desert star: wet-season life cycle
Avoidance: mechanisms that prevent exposure to stress Tolerance:
mechanisms that permit the plant to withstand the stress Resistance
is composed of : Slide 6 Acclimation of Several Cereals to cold
Summer oat Winter oat Winter rye No acclimation With acclimation
Electrolyt leakage Adaptation and Acclimation Acclimation
(hardening): The increase in plant stress tolerance due to exposure
to prior stress. May involve gene expression. Contrary with
adaptation Adaptation (to stress): An inherited level of stress
resistance acquired by a process of selection over many
generations. Contrary with acclimation Slide 7 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 Slide 8 Reactive oxygen species
production as is a common, first response to many stresses (ROS)
Oxidative stress results from conditions that promote formation of
ROS, which damage or kill cells Slide 9 Reactive Oxygen Species
(ROS) These reactive molecules are highly destructive to lipids,
nucleic acids and proteins 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 Except H 2 O 2, ROS have a very short
half life Slide 10 v Formation of ROS by Electron Transfer
Singlet-Oxygen Molecular Oxygen Superoxide Hydrogen peroxide
Hydroxyl radical Water Slide 11 Enzymatic Scavenger Reactions that
Eliminate ROS OS: oxidative stress HS: heat stress WS: water
deficiency stress CS: cold stress Salz: salt stress HL: high light
stress Non Enzymatic Antioxydant Defense systems ascorbate (Vitamin
C) reduced glutathione (GSH) -tocopherol (vitamin E) polyamines
(cabbage, broccoli, cauliflower) flavonoids Slide 12 Disfunction of
the Photosynthetic Transport Chain provides a major source of
electrons for ROS formation Slide 13 ROS Formation during
Photosynthesis If light absorption and CO2-fixation are in balance:
minimal ROS-formation Slide 14 Electron Transfer from Reduced PSI
to O 2 is an electron sink creating superoxide ions Slide 15
Photoinhibition of PSII leads to singlet oxygen buil up Slide 16
CO2-Limitation: Enhancement of Photorespiration and H 2 O 2
-Formation Slide 17 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) Slide 18 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.
Slide 19 Dehydration > Activation of desiccation-related genes
> (1) Alterations in metabolism and > (2) Production of
protective proteins Resurrection plants as a model for
water-deficit stress adaptation (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. Slide 20 Role of
abscisic acid (ABA) in stomata closure and water retention In the
absence of ABA, the phosphatase PP2C is free to inhibit
autophosphorylation of SnRk kinases ABA enables PYR/RCAR proteins
to bind and sequester PP2C This relieves inhibition of SnRk, which
becomes autoactivated and phosphorylate ABF transcription factors
ABA receptor (PYR/RCAR) ony cloned in 2010! Other ABA receptor
likely exist. +ABA -ABA H2OH2O Slide 21 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. -
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?) Injury symptoms become apparent
after transfer to normal temperatures: Candidate sensing proteins
may monitor changes in membrane fluidity Slide 22 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 carrier- mediated transport, enzyme-mediated processes, and
receptor function are inactivated. In Prague, during the cold
winter of 1875/76, Herrmann Mller 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). Slide
23 The ability to survive temperatures below freezing is
genotype-specific. Cold acclimation (or freezing tolerance) 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 -40C. 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 1-5C for one to five
days, can survive temperatures of -8- 12C. - 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)
Slide 24 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 Slide 25 Wetland plants are
fully adapted : Thickened root hypodermis to reduce O 2 loss
Pneumatophore: shallow roots that grow with negative geotrophy out
of the aquatic environment prevent oxygen deficit stress for
instance in mangroves 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) Slide 26 root length
Phenotypic plasticity also helps some plants to adapt to flooding
Agropyron smithii at a dry site The (genetically) same plant at a
wet site Slide 27 Oryza sativa L. var. Indica: growth response of
seedlings to flooding Phenotypic plasticity also helps some plants
to adapt to flooding Rice reduces root growth and increase
coleoptile growth or internodal growth Slide 28 Flood tolerant
plants can endure anoxia temporarily but not for prolonged periods.
Flood Tolerant Plants 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 Slide 29 Tropospheric Ozone is linked to
Oxidative Stress in Plants Stratospheric ozone (O 3 ) is beneficial
because it shields the earth from UV irradiation 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 and is produced by human activity But tropospheric ozone
is harmful to life because it is a highly reactive oxidant which
also destroys Stratospheric ozone mostly by emission of CFCs (ozone
hole) Slide 30 Ozone causes Oxidative Damages to Biomolecules Slide
31 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 Ozone-damaged oat leaves acclimation
Slide 32 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 5C above their optimal growing conditions Slide 33 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). 28C40C, 2hrs -> 45C 45C Slide 34 A and C: silver
staining B and D: fluorography of newly synthesised proteins Heat
shock 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 S 35
incorporated into methionin. Slide 35 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 Slide 36 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 Slide 37 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). Heat Shock
Transcription Factor (HSF) 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
Slide 38 1) Monomeric form of heat shock factor (HSF) + HSP70 2)
Trimeric form upon heat shock 3) Binding of HSF to heat shock
promoter 4) HSP gene transcription 5) HSF - phosphorylation 6)
Interaction of phosphoryl. HSF with HSP70 7) Release of HSF from
DNA 8) Conversion to non DNA binding monomer
