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

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 :

Acclimation of Several Cereals to cold

Summer oat

Winter oatWinter rye

No acclimation

With acclimation

Elec

trol

yt le

akag

e

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

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

(ROS)

Oxidative stress results from conditions that promote formation of ROS, which damage or kill cells

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 H2O2, ROS have a very short half life

v

Formation of ROS by Electron Transfer

Singlet-Oxygen

Molecular Oxygen

Superoxide

Hydrogen peroxide

Hydroxyl radical

Water

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

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 plantagineumTop: 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.

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.

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 kinasesABA enables PYR/RCAR proteins to bind and sequester PP2CThis 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

H2O

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

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 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).

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 -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 -8-12°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 :

Thickened root hypodermis to reduce O2 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)

root

le

ngth

Phenotypic plasticity also helps some plants to adapt to flooding

Agropyron smithii at a dry site

The (genetically) same plant at a wet site

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

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

Tropospheric Ozone is linked to Oxidative Stress in Plants Stratospheric ozone (O3) 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)

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

Ozone-damaged oat leaves

accl

imati

on

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

•  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 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

•  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

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