ABIOTIC STRESS

Question 1

MECHANICAL DAMAGES CAUSED BY WIND

Answer 1

> stem lodging in barley
root lodging in wheat
stem breakage in Norway spruce
uprooting in Sitka spruce

Question 2

TEMPERATURE AFFECTS MEMBRANE FLUIDITY

Answer 2

phospholipids move around the membrane; this movement increases with temperature

Question 3

HEAT INCREASES MEMBRANE FLUIDITY

Answer 3

increase in membrane fluidity affects the activity of transmembrane proteins.
> high temperature disrupt membrane structure
> the LIGHT reactions take place in the lumen of the thylakoids
—»> THYLAKOIDS = sheets of inner membrane (when in stacks of disks, called grana)

> high temperatures disrupt chloroplast internal organisation
electron micrographs of broad bean chloroplasts incubated at 35°C and 45°C for 5 min prior to fixation at 20°C .

Question 4

COLD-INDUCED MEMBRANE DAMAGE

Answer 4

—»> FREEZE-INDUCED OSMOTIC CONTRACTION
Close proximity between plasma and chloroplast membranes in cold conditions can lead to fracture-jump lesions and hexagonal II phases.
Some consequences of cold stress occur upon thawing when membrane damage prevents the expansion of the cell.
—»> exocytic extrusions and membrane apposition: fracture jump lesion

Question 5

FREEZING-INDUCED MEMBRANE AND CELL WALL DAMAGE

Answer 5

freezing —»> ice nucleation and spreading in the apoplast —»> which lead to either water efflux to apoplast or ice spreading into cell

ice spreading into cell leads to cell death

water efflux to apoplast leads to cell dehydration or osmotic contraction which causes membrane disruption and eventually cell death

LOSS OF CELL TURGOR = COLLAPSE OF THE CELL WALL

Question 6

TEMPERATURE AFFECTS PROTEIN FOLDING

Answer 6

the thermal unfolding of apomyoglobin with and without glycerol. Below 0°C the protein unfolds, as well as above 60°C.

high temperatures lead to protein aggregation

Question 7

PRINCIPLES OF WATER MOVEMENT: water potential

Answer 7

ψ = ψπ + ψp + ψm + ψs
ψ: water potential
ψπ: osmotic potential
ψp: pressure potential
ψm: matric potential
ψs: gravimetric potential

water moves from areas of high potential to areas of low potential

Question 8

DESICCATION AFFECTS PROTEIN STABILITY

Answer 8

water stresses (drought, salt stress, and cold) can lead to low water potential and cellular desiccation

during cellular desiccation, the binding between proteins and other molecules increases, favouring destabilisation of proteins

Question 9

REACTIVE OXYGEN SPECIES: a major source of stress damage

Answer 9

Abiotic stresses such as salt, UV, drought, heavy metals, air pollutants, etc AND chloroplasts, mitochondria, peroxisomes and other sources in plant cell

ROS
=
Oxidative damage

UNDERSTAND generation of ROS by energy transfer

Question 10

SITES OF PRODUCTION OF ROS

Answer 10

MITOCHONDRIA:
complex I: NADH dehydrogenase segment
complex II: reverse electron flow to complex I
complex III: ubiquinone-cytochrome region
enzymes: aconitase, 1-galactono-y lactone, dehydrogenase (GAL)

CHLOROPLAST:
PSI: electron transport chain Fd, 2Fe-2S, and 4Fe-4S clusters
PSII: electron transport chain QA and QB
chlorophyll pigments

PLASMA MEMBRANE:
electron transporting oxidoreductases
NADPH oxidase, quinone oxidase

ENDOPLASMIC RETICULUM:
NAD(P)H-DEPENDENT ELECTRON TRANSPORT SYSTEM
> flavoproteins
> cyt b5
> cyt P450

APOPLAST:
cell-wall-associated oxalate oxidase
amine oxidases

CELL WALL:
cell-wall-associated peroxidase
diamine oxidases

PEROXISOME:
matrix: xanthine oxidase (XOD)
metabolic processes: glycolate oxidase, fatty acid oxidation, flavin oxidases, disproportionation of O2- radicals

Question 11

STRESS-INDUCED PRODUCTION OF ROS IN CHLOROPLASTS

Answer 11

light response curve for photosynthesis compared with the rate of light absorption.
ROS are generated when the light reactions are running at a higher rate than the carbon fixing reactions: there isn’t enough NADP+ for electrons to reduce, and the electron transport chain overflows.

DROUGHT > STOMATAL CLOSURE > CO2 LIMITATION

Question 12

STRESS-INDUCED PRODUCTION OF ROS IN PEROXISOMES

Answer 12

Peroxisomes appear to have a ROS-mediated role in abiotic stress situations induced by the heavy metal cadmium (Cd) and the xenobiotic 2,4-D, and also in the oxidative reactions of leaf senescence. The toxicity of Cd and 2,4-D has an effect on the ROS metabolism and speed of movement (dynamics) of peroxisomes.

Question 13

STRESS-INDUCED PRODUCTION OF ROS IN MITOCHONDRIA

Answer 13

both low and high temperature decrease the efficiency of ROS scavenging enzymes in mitochondria, resulting in increased ROS concentrations

Question 14

ROS DAMAGE: LIPID PEROXIDATION

Answer 14

Poly-unsaturated fatty acids (PUFAs) get oxidised by ROS.
Lipid radicals can react with and damage other cellular components.
Phospholipid peroxidation decreases membrane fluidity, increases membrane permeability and impairs the activity of transmembrane proteins.

Lipid peroxide damages the cell membrane, resulting in ion leakage.
Electrolyte leakage is measured by placing leaf disks in a water bath and measuring the conductance of the solution.

Question 15

ROS DAMAGE: PROTEIN OXIDATION

Answer 15

oxidation affects protein biological activity and increases chances of degradation

Question 16

SPECIFIC DAMAGES OF ROS ON PHOTOSYNTHESIS: PHOTOINHIBITION

Answer 16

Among the first victims of ROS production in chloroplasts is the photosynthetic apparatus.
The D1 protein in the core of PSII is a major target of photodamage.
The decrease in photosynthetic efficiency due to PSII damage is called photoinhibition

Question 17

SPECIFIC DAMAGES OF ROS ON PHOTOSYNTHESIS

Answer 17

Chlorophyll and carotene concentration in leaf disks of cucumber leaves after varying times at 1°C in dark or high light conditions.
Photosynthetic pigments are damage by oxidative stress and degraded.

Question 18

ROS DAMAGE: DNA DAMAGE

Answer 18

oxidation damage to DNA leads to mutations

Question 19

SODIUM-SPECIFIC INDIRECT DAMAGE

Answer 19

The reasons for the specific toxicity of Na+ are currently being debated.
. it is agreed that Na+ disrupts K+ homeostasis, indirectly leading to K+ deficiency
. it has been claimed that low K+/Na+ ration impairs the functionality of enzymes depending on K+ but there is evidence against this.

Question 20

CELLULAR DAMAGE OF ABIOTIC STRESS

Answer 20

At the cellular level, extreme tissue temperature, cellular desiccation, and photosynthetic imbalance will affect:
> membrane structure and integrity
> protein folding
> DNA
> cell walls

In the acclimation lectures, we will see:
>how the strains that cause these damages can be avoided
> how cellular components can be protected from damages and repaired (tolerance)