1 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)
4 Why bother about plant stress? As human population increases, agriculture must feed more people while competing with urban development for premium arable landIf 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
5 Resistance and Acclimation Resistance is composed of :– Avoidance: mechanisms that prevent exposure to stress– Tolerance: mechanisms that permit the plant to withstand the stressResistance:Saguaro: succulent photo-synthetic stem (highy drought tolerant)Honey mesquite: deep roots (drought avoiding)Mohave desert star: wet-season life cycleAcclimation:Spinach: osmotic adjustmentBlack spruce: freezing tolerance
6 Acclimation of Several Cereals Adaptation andAcclimationAcclimation of Several Cerealsto coldSummer oatWinter oatWinter ryeNo acclimationWith acclimationElectrolyt leakageAdaptation (to stress): An inherited level of stress resistance acquired by a process of selection over many generations. Contrary with acclimationAcclimation (hardening): The increase in plant stress tolerance due to exposure to prior stress. May involve gene expression. Contrary with adaptation
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
8 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)
9 Reactive Oxygen Species (ROS) These reactive molecules are highly destructive to lipids, nucleic acids and proteinsExcept H2O2, ROS have a very short half lifePlants scavenge and dispose of these reactive molecules by use of antioxidant defense systems present in several subcellular compartmentsWhen these defenses fail to halt the self-propagating auto-oxydation reactions associated with ROS, cell death ultimately ensues
10 Formation of ROS by Electron Transfer vSinglet-OxygenHydrogen peroxideMolecular OxygenHydroxyl radicalSuperoxideWater
12 Disfunction of the Photosynthetic Transport Chain provides a major source of electrons for ROS formation
13 ROS Formation during Photosynthesis If light absorption and CO2-fixation are in balance: minimal ROS-formationPGA : phospho glycerate
14 Electron Transfer from Reduced PSI to O2 is an electron sink creating superoxide ions
15 Photoinhibition of PSII leads to singlet oxygen buil up
16 CO2-Limitation: Enhancement of Photorespiration and H2O2-Formation Glycolate cannot be used i n the Calvin Cycle and this reduces photosynthesis efficacy. Photo respiration rescues this.
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 exteriorw= water potential = s + ps = solute potential (number of particles dissolved in water)p = pressure potential (physical forces exerted on water by environment)
18 Resurrection plantsCraterostigma plantagineumTop: dried out plants are extremely fragile and crumble when touchedBottom: same plant, 24 hours after wateringPlants that “completely” dry out (no physiological activity detectable) and have the ability to green and resume physiological activity after watering are called resurrection plants.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.
19 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” proteinsAlterations in metabolism:accumulation of protective solutes such as sucrose, trehalose, and proline that stabilize proteins and cellular membranes,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.
20 Role of abscisic acid (ABA) in stomata closure and water retention H2O-ABA+ABAIn the absence of ABA, the phosphatase PP2C is free to inhibit autophosphorylation of SnRk kinasesABA receptor (PYR/RCAR) ony cloned in 2010!ABA enables PYR/RCAR proteins to bind and sequester PP2COther ABA receptor likely exist.This relieves inhibition of SnRk, which becomes autoactivated and phosphorylate ABF transcription factors
21 Freezing StressCold 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
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 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).
23 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 -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)
24 Flooding and Oxygen Deficit Plants are obligate aerobes: oxygen is the terminal electron acceptor in the mitochrondrial electron transfer chainDuring flooding, too much water blocks the entry of O2 into the soil so that roots and other organs cannot carry out respirationPlant species generally can be classified as wetland, flood-tolerant, or flood sensitive, according to their ability to withstand periods of oxygen deficitWetland plants have specific anatomical adaptations:- aerenchyma facilitate O2 transport- lenticels in periderm for gas exchange- pneumatophores: shallow roots with negative geotropy
25 Wetland plants are fully adapted : Thickened root hypodermis to reduce O2 lossPneumatophore: shallow roots that grow with negative geotrophy out of the aquatic environment prevent oxygen deficit stress for instance in mangrovesZea mays: root cortex under aerobic(A) and anaerobic (B) conditionsAerenchyma (continuous, columnar intracellular spaces) faciliate transport of O2 from aerial structures to submerged rootsAdventitious roots from the hypocotyl or stem, lenticells (openings in the periderm)
26 Phenotypic plasticity also helps some plants to adapt to flooding The (genetically) same plant at a wet siteAgropyron smithii at a dry siteroot length
27 Phenotypic plasticity also helps some plants to adapt to flooding Oryza sativa L. var. Indica: growth response of seedlings to floodingRice reduces root growth and increase coleoptile growth or internodal growth
28 Flood sensitive plants Flood Tolerant PlantsFlood 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 alteredFlood sensitive plantsThey show injury response to anoxia, causing death within 24 hrsGenerally sensing of oxygen deprivation is very poorly understood
29 Tropospheric Ozone is linked to Oxidative Stress in Plants Stratospheric ozone (O3) is beneficial because it shields the earth from UV irradiationBut 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 yieldBUT: ozone exposure induces pathogen resistance
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 mechanismsOzone-damaged oat leavesacclimation
32 Heat StressHeat 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 irradiationPlants 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
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) .28°C40°C, 2hrs -> 45°C45°C
34 Heat shock proteins• A and C: silver staining• B and D: fluorography of newly synthesised proteinsFluorography 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.
35 5 Classes of HSPs are Defined According to Size – 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 ratherthan prevent protein aggregation and misfolding during exposure to high temperatures• HSP90– are found in bacteria and in the cytosolic, nuclear, and endoplasmicreticulum compartments of eukaryotic cells, where they may function as molecular chaperones• HSP70– are essential for normal cell function. Some members are expressedconstitutively; others are induced by heat or cold. Localised to the nucleolus during heat stress, HSP70 is redistributed to the cytoplasm during stress recovery
36 5 Classes of HSPs are Defined According to Size – are thought to function as molecular chaperones. HSP60 proteinsare 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 thechloroplast, 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 proteinaggregation in response to high temperature in vitro
37 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
38 1) Monomeric form of heat shock factor (HSF) + HSP70 2) Trimeric form upon heat shock3) Binding of HSF to heat shock promoter4) HSP – gene transcription5) HSF - phosphorylation6) Interaction of phosphoryl. HSF with HSP707) Release of HSF from DNA8) Conversion to non DNA – binding monomer