Adaptive And Stress Physiology (Plant Section)

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Adaptation

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Shiftin genotype and phenotype over generations as a result of environmentalpressure

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Acclimation

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Process of an individualorg adjusting physiological processes to respond to stress (then correcting back tonormal under normal conditions)

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Definition of stress in plants

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Environmentalconditions that limit plants’ intrinsic genetic potential

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Osmotic stress

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Limited osmotic uptake of water

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Plant water status

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Degreeto which tissues are hydrated

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Water potential

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Measure of potential energy in water, or the difference in potential energybetween a given water sample and pure water (0). Influenced by solute concentration, pressure, gravity. Highfree energy = low solute concentration. Water always moves from high to low potential.

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Plasmolysis

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Condition in which plasmamembrane releases from cell wall and cell begins shrinking due to beingsurrounded by low-water potential fluid (high solute)

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Relative water content

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Amount of water in a leaf at the time of sampling relative to the maximal water a leaf can hold. A more practical value than water potential.

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Main strategies for long-term adaptation to water deficit

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1. Limit water loss (limit leaf surface area, increase cuticular wax, find ways of leaf cooling other than transpiration)

2. Ensure sufficient water uptake (increase root growth, direct root growth toward water, grow deeper roots, limit air bubbles in xylem)

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Main strategies for short-term adaptation to water deficit

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- Limit transpiration by closing stomata

- Induce CAM metabolism

- Produce osmolytes to reduce osmotic potential so water enters plant tissue

- Produce protective proteins like LEA proteins and osmotin

- Changing expression of ABA-dependent and -independent genes

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Root hydrotropism

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Active, directed root development toward water

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Two main mechanisms of stomata closing

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1. Hydro-passive (loss of turgor pressure in guard cells causes them to collapse, closing stomata)

2. Hydro-active closing (depends on changes in guard cell metabolism)

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Terpenes

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Hydrocarbon compounds responsible for the way plants smell. Important mediators of ecological interactions (e.g. discourage herbivory or attract pollinators)

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Synthesis of terpenes

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Produced from acetyl co-A in mevalonate pathway in cytoplasm and methyl-erythritolphosphate (MEP) pathway in plastids. From C5 isoprene units that have been linked end-to-end.

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Anion trap concept

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Mechanism explaining storage of ABA during no water stress and release during water stress. ABA is weak acid and can't cross plasma membrane in normally alkaline stroma (inner fluid) of chloroplast. When photosynthesis stops, stroma no longer alkaline, so ABA can cross membrane and be distributed where it's needed.

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Apoplast

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Space outside the plasma membrane within which material can diffuse freely. Comprises intercellular space, cell walls, and xylem.

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CAM metabolism

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Carbon fixation pathway that improves water-use efficiency by shifting all or part of CO2 uptake from the day to the night and store it as malic acid (take up C at night, but use during day).

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Calvin cycle

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Light-independent reactions in photosynthesis that take place in three key steps (fixation, reduction, regeneration). Uses products of light-dependent reactions (ATP and NADH).

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Two main mechanisms by which photosynthesis causes yield and water loss

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1. Plants must open stomata to take in CO2, losing water

2. Rubisco (involved in CO2 fixation) has high affinity for O2, and O2 competes w/CO2 for binding.

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How plants deal with problems associated with photosynthesis (e.g. water loss, O2 competing w/CO2 for binding to rubisco)

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1. Use C4 metabolism (physical separation of carbon fixation and Calvin cycle) because CO2 not fixated by rubisco but diff. enzyme. Raises photosynthetic rate.

2. Use CAM metabolism (temporal separation of carbon fixation and Calvin cycle).

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Osmotic adjustment

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Strategy for dealing with osmotic stress - plant actively produces solutes to decrease osmotic potential so water flows into cell.

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Osmolytes

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Solutes that plants produce to decrease osmotic potential and protect against harmful agents like ROS.

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Compatible solutes

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Types of osmolytes that are highly soluble and don't interfere w/cellular metabolism, even at high concentrations. Example: proline

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Properties of osmolytes

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- Organic compounds (not minerals)

- Neutral charge at physiological pH

- Non-ionic

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Osmoprotectants

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Group of compatible solutes that protect cells under extreme osmotic stress by protecting structures of cells and acting as antioxidants. Example: trehalose

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LEA proteins

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Protein synthesized by some plants to protect against protein aggregation due to desiccation or osmotic stresses associated with low temperature

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Osmotin

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Protein that functions as osmolyte and as an antifungal agent

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ABA response element

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Sequence found on promoter elements involved in regulating water stress genes (alternative: dehydration response element that is ABA-independent)

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Timing of physiological changes during plant dehydration

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Plants stop growing, then stomata close and photosynthesis drops, then ABA and osmolytes accumulate

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Salinity

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High total salt concentration (e.g. Ca2+, Mg2+)

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Sodicity

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High concentration of Nacl specifically

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Main effects of salt stress

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- Osmotic stress (lowers osmotic potential in soil, drawing out water from plant)

- Ion stress (toxic to plant, mainly accumulation of Na+ and Cl-)

- Secondary effects on membrane permeability, metabolism, ROS

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Main adaptations to salt stress

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1. Limiting osmotic stress (osmolyte production)

2. Limiting exposure to salt stress, esp. in meristems and leaves. Can involve compartmentalizing ions like Na+ so they do not reach vasculature.

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Antiporters

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Molecules driven by electrochemical potential generated by proton pumps, serve to transport Na+ and K+ across membranes.

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Casparian strips

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Specialized structures on endodermis, comprised mainly of lignan, which is used to block the passive flow of materials like water and ions

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Toxic spore elements

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Elements like cadmium, copper, arsenic, and zinc that are found in soil and are toxic to plants

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Main strategies to cope with toxic spore elements

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Exclusion (avoiding altogether), compartmentalizing them in vacuoles, or binding them to organic acids so they are not accessible

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Main effects of cold/freezing stress

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1. Ice formation dropping osmotic potential, so water is pulled out of cells - dehydration

2. Physical damage by ice crystals

3. Effect on enzyme activities

4. Effects on membrane function (become more rigid), which results in failure of electron transport chains and causes oxidative stress

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Main strategies to deal with cold/freezing stress

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1. Membrane stabilization by increasing desaturated fatty acids in membrane

2. Formation of extracellular ice to prevent ice crystal formation inside cell

3. Accumulation of compatible solutes/osmolytes (prevents osmotic stress and lowers freezing point)

4. Accumulate anti-freeze proteins in apoplast

5. Induce expression of cold-responsive genes, which often have ABA-independent response elements

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Main effects of heat stress

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Changes in membrane stability - membranes become less solid, more gel-like. Instability causes issues with electron transport chain and production of ROS (oxidative stress)

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Main strategies for dealing with heat stress

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1. Build resistance to drought stress (e.g. limit transpiration and ensure sufficient water uptake)

2. Decrease absorption of solar radiation by changing leaves

3. Altering metabolism because photosynthesis declines faster than respiration

4. Ensure membrane stability - add saturated fatty acids to make membranes less fluid.

5. Produce heat shock proteins (HSPs), which assist in protein folding and prevent aggregation. Plants have many small HSPs.

6. Produce hormones like ABA and ethylene or use calcium signaling to produce GABA, which controls pH and redox status

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Role of GABA in heat stress

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pH: During heat stress, Ca2+ levels in cytosol increase due to its release from vacuole and ER and flow in from apoplast. Ca2+ is sensed by calmodulin and Ca2+-loaded calmodulin binds to target enzyme GAD. GAD uses protons to produce GABA, which lowers proton levels in cytosol and restores normal pH.

Redox status (balance of oxidants and anti-oxidants): GABA shunt. Metabolization of GABAin mitochondria allows bypass of three NADH-producing steps in the Krebs cycle. NADH is reduced form of NAD+.

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Main effect of oxygen stress

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Lack of ability to produce ATP via respiration because O2 is needed as the final electron acceptor in the electron transport chain

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Main strategies for dealing with oxygen stress

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1. Changes in root growth and development - in roots, produce thicker epidermis to reduce O2 loss or make cavities to facilitate O2 transport from other parts of plant. In shoots, stem elongation mediated by gibberellins, whose expression stimulated by rising ethylene levels.

2. Switch to glycolysis and induce fermentation to recycle NAD+. Ethanol fermentation most efficient, prevents acidification.

3. Initiate changes in gene expression - selective transcription of genes and more efficient translation of mRNA that is transcribed. Genes that respond to low-energy stress have anaerobic response elements.

4. Rely on ethylene: antagonizes ABA to promote gibberellin stimulation and stem elongation. Also promotes formation of holes in stem by initiating enzymes that break down cell walls.

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Anaerobic response element

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Sequence found on promoters of genes that respond to low-energy stress caused by O2 depletion

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Ethylene

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Major plant hormone that is involved mainly in low-energy (oxygen) stress. Synthesis occurs via two main enzymes: ACC synthase and ACC oxidase.

Effects: blocking ABA to promote gibberellinstimulation and stem elongation. Initiating enzymes that breakdown cell walls to help form pores in roots to improve O2 uptake.

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Reactive oxygen species (ROS)

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Forms of oxygen that react aggressively with bio-organic molecules likeDNA, proteins, or membrane lipids and damage them. Produced by uptake of energyor e-. Something may go wrong during electron transport chain, causing ox totake up extra electron. Produced during both respiration and photosynthesis.

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Main strategies to protect against high levels of reactive oxygen species (oxidative stress)

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1. Use enzymes like superoxide dismutase and catalase to convert ROS to H2O2 and eventually O2 and H2O.

2. Use non-enzyme methods like vitamin C to neutralize ROS by putting the e- on NADPH.

3. Produce glutathione, a main scavenger of ROS.

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Ozone (O3)

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In atmosphere, main cause of oxidative stress in plants. Reduces photosynthetic activity, causes leaf damage, and reduces growth.

Coping strategies: close stomata, use antioxidants and repair mechanisms.

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Effect of O3 on hormone signaling in plants

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Produces H2O2, which triggers synthesis of salycylic acid. This then triggers defense response, such as producing lignans to fortify cell wall and anti-pathogenic proteins. O3 can also increase activity of enzymes that make ethylene (ACC synthase and ACC oxidase) to increase ethylene production and thus breakdown of cells.

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Snrk1

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Protein kinase complex that regulates energy homeostasis in plants. Activatedby energy depletion (lack of ATP), which is caused by almost all kinds of stress.

Effects: leads to expression of enzymes important forfermentation and starch mobilization to aid survival. Also induces expression of enzyme synthesizing asparagine, which is used in dark conditions to incorporate N into biomolecules.

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GS/GOGATsystem

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System that incorporates N into biomolecules like amino acids. Glutamate (an amino acid)gets an extra amino group from NH4+ to form glutamine. Glutamine converted back toglutamate and second glutamate is produced using an acceptor molecule producedin the Krebs cycle

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Asparagine

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Amino acid that is used in place of glutamine under low-carbon (i.e. dark) conditions to incorporate N into biomolecules.

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Nitrogenase enzyme complex

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Very large protein complex in bacteria that catalyzes the ATP-dependent reduction of atmospheric dinitrogen (N2) to ammonia (NH3)

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Flavonoids

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Specialized metabolites that produce color, are involved in UV filtration, N fixation. Mainly used to signal to attract N-fixing bacteria to establish symbiosis.

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Nodulins

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Series of proteins in plant genome expressed during root nodule development in response to bacterial differentiation. Expressed in infected and non-infected cells

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Main types of secondary metabolites (used for interaction w/other orgs)

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1. Terpenoid compounds (terpenes) e.g. ABA, gibberellins

2. Phenolic compounds (many types, made from phenylalanine) e.g. phenylpropanoids like caffeic acid and salicylic acid

3. N-containing compounds

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Allelopathy

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Leaves, roots and dying tissues release primary and secondary metabolitesin the soil, affecting plants and other orgs around them

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Lignin

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Polyphenol made from phenylpropanoids. Most abundant substance in plants after cellulose. Fortifies cell walls.

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Jasmonic acid

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Main plant hormone produced in response to insect herbivory. Stimulates production of defense proteins.It is detected by COI1 receptor and triggers degradation of repressorsrepressing transcription of target genes, like enzymes and proteins that interferew/animal metabolism.

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How jasmonic acid is produced in response to insect herbivory

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Hebivory injury activatesglutamate receptor-like ion channels and start wave of plasma membranedepolarizations – electrical signal that stimulates JA production