ACCLIMATIONS TO ABIOTIC STRESS (2) Flashcards
Explain the processes used for reactive oxygen species scavenging. Describing specific acclimations to challenging environmental conditions.
STRESS-INDUCED PRODUCTION OF ROS: REMINDER
Reactive oxygen species are produced in high quantities:
. in chloroplasts under high light, especially if combined with water stress and/or cold
. in peroxisomes under drought and heat stress
. in mitochondria under heat and cold stress
ROS scavenging
Superoxide dismutase:
. catalyses the dismutation of superoxide into hydrogen peroxide…
…to limit the formation of hydroxyl radicals through the Haber-Weiss reaction
. present in most cellular compartments
. different forms in different compartments, replying on a variety of metals (Cu/Zn, Mn, Fe)
Catalase:
. converts hydrogen peroxide into water
. present mostly in peroxisomes and other small organelles
. low affinity for hydrogen peroxide: efficient under high (H2O2)
. no need for reducing power
Ascorbate glutathione cycle:
. converts hydrogen peroxide into water
APX = ascorbate peroxidase
MDAR = monodehydroascorbate (MDA) reductase
DHAR = dehydroascorbate (DHA) reductase
GR= glutathione reductase
AsA = ascorbic acid
GSH = glutathione
GSSG= oxidised glutathione
. requires reducing power provided by NAD(P)H
. high affinity for hydrogen peroxide: efficient under low (H2O2)
The water-water cycle:
. specific to chloroplasts (stroma)
. integrated into photosynthesis
. allows electron flow without accumulation of ROS even when NADP+ limiting
SOD = Cu/Zn superoxide dismutase
tAPX = thylakoid ascorbate peroxidase
MDA = monodehydroascorbate
AsA = ascorbic acid
Fd = ferredoxin
ROS DETOXIFICATION
Peroxiredoxins reduce peroxides (e.g. in lipids, proteins and DNA) into alcohols and H2O2 into water; they are found in most cellular compartments.
Reductants (electron donors) regenerating the active peroxiredoxin include NAS(P)H, thioredoxins and glutaredoxins.
ACCLIMATION TO LIGHT: LEAF MOVEMENT
Plants displaying paraheliotropism can make their leaves more vertical so that they receive less light and avoid photoinhibition. This behaviour is mostly found in legumes.
ACCLIMATION TO LIGHT: CHLOROPLAST MOVEMENT
Chloroplasts can move to areas of the cell with limited irradiation under high light conditions.
Chloroplasts movement under varying light intensity could depend on the relocation of actin filaments around the chloroplast.
ACCLIMATION TO LIGHT: NON-PHOTOCHEMICAL QUENCHING
Non-photochemical quenching (NPQ) can be induced in a matter of seconds or minutes by the acidification of the thylakoid lumen.
Protons accumulate in the lumen under excess light due to proton pumping and reduced use of proton by the ATPase.
Under excess light, violaxanthin epoxidase is activated by low lumen pH and converts violaxanthin to antheraxanthin and zeaxanthin, both of which can dissipate chlorophyll excitation heat.
Low lumen pH also triggers changes in the conformation of light harvesting complex (LHC) proteins that promote de-excitation (quenching) of chlorophylls through the xanthophylls.
The PsbS proteins act like a pH sensor that initiates the changes in antenna organisation.
ACCLIMATION TO LIGHT: PHOTOSYSTEM II REPAIR
D1 repairs cycle occurs at all light levels, but under excess light, it speeds up and relies on a pool of replacement D1. The repair machinery (including FtsH) can be damaged by ROS. ROS scavenging is therefore essential to PSII repair.
ACCLIMATION TO COLD
Cold acclimation (= hardening) occurs when plants experience chilling temperatures. It involves cellular changes that will limit frost induced damages.
Cold acclimation includes general water stress and oxidative acclimations, as well as more specific changes.
ACCLIMATION TO COLD: CHANGE IN MEMBRANE COMPOSITION
During cold acclimation, lipids prone to forming hexagonal II phases decrease, while lipids stabilising bilayer configuration increase in chloroplast and plasma membranes.
Changes in membrane lipid composition depend on species.
ACCLIMATION TO COLD: INCREASE IN MEMBRANE LIPID UNSATURATION
An increase in lipid unsaturation counterbalances the increase in membrane rigidity due to low temperatures. More unsaturated phosphatidylcholine reduces the risk of expansion-induced lysis.
ACCLIMATION TO WIND
Strategies:
. reduce wind strain (drag) by decreasing the surface offered to the wind (avoidance)
. increase resistance to breakage (tolerance)
. increase anchorage
The growth response of plants exposed to wind is called thigmomorphogenesis.
Reduce wind strain:
. decrease stem/trunk growth rate
. leaf/branch shedding
. reconfiguration: temporary or permanent
. leaf rolling/re-orientation (adaption?)
Increase resistance to breakage:
. thicker/reinforced stem/trunk (wood ≠ non woody plants)
. flexible petioles and branches
. flexure wood
Increase anchorage
. higher root to shoot ratio
