Abiotic Stress in Plants PDF

ABIOTIC STRESS Chapter 24 Plants are sessile and must deal with stresses in place • Plants cannot avoid stress after germination • How plants deal with stress has implications in – Ecology: Stress responses help explain geographic distribution of species – Crop science: Stress affects productivit...

ABIOTIC STRESS Chapter 24 Plants are sessile and must deal with stresses in place • Plants cannot avoid stress after germination • How plants deal with stress has implications in – Ecology: Stress responses help explain geographic distribution of species – Crop science: Stress affects productivity – Physiology and biochemistry: Stress affects the metabolism of plants and results in changes in gene expression ABIOTIC STRESSES Environmental, nonbiological • Temperature (high / low) • Water (high / low) • Toxins (salt, heavy metals) • Radiation (light) • Nutrients • Wind • O2 BIOTIC STRESSES Caused by living organisms • Fungi • Bacteria • Insects • Herbivores • Other plants/competition Preferable! Heat-stressed wheat Stresses cause responses in metabolism and development Injuries occur in susceptible plants, can lead to impeding flowering, death Plants adjust homeostatically to minimize the negative impacts of stress and maintain metabolic equilibrium TRADE-OFFS! Environmental stimuli that affect plant growth Control systems in plants involve adaptations, adaptations, adaptations Plants need to monitor everything in order to optimize growth (i.e., to adapt) to environmental conditions, endogenous present & future In biology, stress is the driving force behind the process of adaptation and evolution Adaptation versus Acclimation • Adaptation – evolutionary changes that enable an organism to exploit a certain niche. These include inherited modification of existing genes, as well as gain/loss of genes => many generations – e.g., thermo-stable enzymes in organisms that tolerate high temperature • Acclimation – inducible responses that enable an organism to tolerate an unfavorable or lethal change in their environment. No new genetic modification. Nonpermanent change – e.g., heat shock response BOTH contribute to the plants’ overall tolerance of extremes in their abiotic environment Factors that determine plant stress responses • Most abiotic stresses result in ROS production • ROS signal metabolic processes to generate a response Plants respond to stresses as individual cells and as whole organisms – stress induced signals can be transmitted throughout the plant, making other parts more ready to withstand stress.. LIGHT STRESS • Too much for shade-adapted or shade-acclimated  overwhelms the photosynthetic machinery – Most shade-adapted/acclimated: more PSII units – If full sunlight: too much sunlight is converted to energy but there´s not enough machinery to process it • Generates ROS => cellular damage • Photoinhibition UV & OZONE STRESS • Both: growth suppression and agr. yields • O3 enters the plant through open stomata and is converted into ROS • Toxic effects of ROS causes injuries in leaves (PCD) • Thinning of ozone layer => increases in UV radiation • Affects photosynthesis and induces formation of ROS  PCD WATER STRESS • Too little water in soil: drought  water deficit • Too much water in soil: flooding 1. Drought • Plants reduce transpiration (closing stomata), slowing leaf growth, and reducing exposed surface area • Growth of shallow roots is inhibited, while deeper roots continue to grow Drought Osmotic adjustment: plant cells accumulate solutes. Mostly in vacuoles Plants have to be careful when accumulating solutes. Some can be toxic! During water stress, ABA increases in leaves which leads to stomatal closure. ABA depolarizes the membrane and causes a Ca2+ influx  outward flow of Cl- and malate Rose of Jericho Brassicaceae - Anastatica hierochuntica 2. Flooding • O2 levels decrease dramatically: all air is displaced by water  hypoxia • Respiration is suppressed and fermentation enhanced  energy depletion, acidification and toxicity (ethanol) – This can cause cell death within hours or days! • Recovery is also difficult and hazardous – Absence of O2 prevents ROS – If O2 increases rapidly: lots of ROS  oxidative damage to roots Shift carbohydrate metabolism from respiration to anaerobic glycolysis Plants vary in ability to tolerate flooding Plants can be classified as: • Wetland plants (e.g., rice, mangroves) • Flood-tolerant (e.g., Arabidopsis, maize) • Flood-sensitive (e.g., soybeans, tomato) • Enzymatic destruction of root cortex cells creates air tubes that help plants survive oxygen deprivation during flooding Involves developmental/structural, cellular and molecular adaptations. Pneumatophores in mangrove Ethylene triggers cell death and disintegration of cells in the root cortex Some tissues can tolerate weeks/months of flooding before developing aerenchyma (e.g., rice). They even expand leaves on anaerobic conditions Many wetland species suberize/lignify roots to prevent the escape of O2 TEMPERATURE STRESS • Too hot: heat shock • Too cold: freezing stress (chilling or freezing) Temperature disrupts the metabolism • Effects on protein stability and enzymatic reactions  accumulation of ROS – Destabilize and melt or overstabilize and harden RNA and DNA, transcription, translation – Block protein degradation: clumps that disrupt function of organelles and cytosol Heat Stress (or Heat Shock) Response • • • • Induced by T ~10-15 oC above normal Ubiquitous (conserved), rapid & transient Heat-shock proteins help protect others from heat stress Dramatic change in pattern of protein synthesis –most HSPs are chaperones: stabilize, reduce misfolding, prevent disaggregation/aggregation Cold Acclimation involves • Decrease in membrane fluidity • Altered lipid composition in membranes • Freezing  ice formation in plant’s cell walls and intercellular spaces  lethal. At -10 ºC symplast loses about 90% of its active water to the apoplast • Many plants have antifreeze proteins that prevent ice crystals from growing and damaging cells • Freezing stress has a lot in common with drought stress! • Increased accumulation of small solutes – retain water & stabilize proteins – e.g., proline, glycine betaine, trehalose Changes in gene expression [e.g., antifreeze proteins, proteases, RNA-binding proteins (?)] Chilling resistant plants: more unsaturated fatty acids that increase membrane fluidity Chilling sensitive: more saturated fats that tend to solidify at lower temps (like butter!) More bonds to fatty acids: acclimation to low temperatures Supercooling • Adaptation to super cold: cellular water won´t freeze even if at very low temperatures – Cells can supercool “only” at about -40ºC – Limits where plants (alpine, arctic) can survive • Special proteins: antifreeze. They lower freezing point – Synthesis is induced at cold temperatures – Bind ice crystals to prevent their growth • Sugars, polysaccharids, osmoprotectant solutes, dehydrins also have cryoprotective effects TOXIC IONS • Cadmium (Cd), arsenic (As), aluminum (Al), copper (Cu), nickel (Ni), zinc (Zn), sodium (Na), Selenium (Se): can lead to ROS accumulation, inhibition of photosynthesis, disruption of membrane structure and ion homeostasis, inhibition of enzymatic reactions, activation of PCD • They’re toxic because they mimic essential minerals and take their place, disrupting reactions • React directly with O2 to form ROS • Plants can exclude or tolerate them internally – Exclusion: block their uptake – Internal tolerance: biochemical adaptations to deal with high concentrations -> compartmentalize, chelate • Some plants can hyperaccumulate certain trace elements: limited number of spp. Very rare Chelation: binding of an ion with at least two ligating atoms in a chelating molecule. This makes the ion less chemically active: reduces toxicity Long-distance transport of chelated ions is important in hyperaccumulation Many aminoacids! SALINITY STRESS • Excess soil salinity: over-irrigation and poor soil drainage – 20% of irrigated land: affected by salinity stress • 2 main components: – Nonspecific osmotic stress: Causes water deficits – Specific ion effects: accumulation of toxic ions (interfere with nutrient uptake and cytotoxicity) • Salt can lower soil water potential and reduce water uptake – Plants respond to salt stress by producing solutes tolerated at high concentrations – This process keeps the water potential of cells more negative than that of the soil solution GLYCOPHYTE: less salt-tolerant plants, not genetically adapted to salinity HALOPHYTE: plants genetically adapted to salinity Reduce uptake or actively pump ions back into the soil Sequesters ions in vacuole and compartmentalize MINERAL DEFICIENCIES • Deficiencies occur when there are no nutrients • But even if there are nutrients. they may not be available: pH can change their solubility  unavailable • Nutrient and pH stress: Always results in suppression of plant growth and reproduction Combination of abiotic stresses • Plants are often subjected to several stressors simultaneously – Drought and heat tend to occur together Crop losses US, 1980-2004 Conflicting physiological response? If too hot: plants transpire more to cool down. With drought plants close stomata and deal with high leaf temperatures! Combination of heat and drought induces different patterns of gene expression and metabolite biosynthesis than either stress alone Heat+drought: 772 unique transcripts  acclimation is different compared to just one stress average losses A comparison of the record yields and the average yields: Most crops are only reaching 20% of their genetic potential due to biotic categories: disease, insect and weeds. The major reduction in yield (~ 70%) is due to abiotic stress. The most significant abiotic stress is water stress, both deficit stress (drought) and excess stress (flooding, anoxia). Crop record yield* average yield* disease insect weed other (abiotic) corn 19,300 4,600 750 691 511 12,700 wheat 14,500 1,880 336 134 256 11,900 soybean 7,390 1,610 269 67 330 5,120 sorghum 20,000 2,830 314 314 423 16,200 oats 10,600 1,720 465 107 352 7,960 barley 11,400 2,050 377 108 280 8,590 potatoes 94,100 28,300 8,000 5,900 875 50,900 sugar beets 121,000 42,600 6,700 6,700 3,700 61,300 21.6% 4.1% 2.6% 2.6% 69.1% % of record yield Combination of abiotic stresses • Exposure to abiotic stress can enhance the tolerance of plants to subsequent exposure to different abiotic stress => cross protection • Why? – Stress-response proteins accumulate: ROS-scavenging enzymes, molecular chaperones, osmoprotectants – They persist even after the plant is no longer under stress – “Memory”: plants seem to “remember” stress long after it is over and will respond much faster to its reoccurence Heat stress + salinity/heavy metal = heat stress + drought Nutrient stress + drought/salinity? Stress sensing mechanisms Physical, biophysical, metabolic, biochemical, epigenetic sensing Morphological responses Phenotypic plasticity: enable plants to avoid abiotic stress • Leaf shape modifications – – – – Leaf area: reducing leaf area  less water evaporates, less solar absorption Leaf orientation: protection against overheating Wilting, rolling Trichomes: reflect radiation  leaves cooler Cuticle: Reflects light. Restricts diffusion of water and gases, and pathogens • Root:shoot ratio: balance between water uptake by roots and photosynthesis by shoots • Recovery is dangerous to the plant because of ROS. Recovery is a synchronized process • All hormones are in general important when dealing with stress. ABA: One of the most rapid responses to abiotic stress Crops?? Stress responses are very important in the context of crops: Enhance tolerance to abiotic stress