LEC 15 Plant Transport - symplastic transport = Molecules ...
LEC 15 Plant Transport - symplastic transport = Molecules move thru ‘living’ space (interior of cells bounded by plasma mem) o Diffusion = move across mem o Facilitated trans. = use transport protein o Active trans. = transport protein + ATP gradient o Endo/exo cytosis = enter/exit by making vesicle o Osmosis = conc. gradient - apoplastic Transport = molecules move thru space around protoplast (external to plasma mem) - need semi permeable barrier for movement across membranes Water Movement Principles - water = living cell solvent - movement by 3 processes o Bulk Flow Water molecules move in mass in response to potential energy differences o Diffusion Spontaneous movement of water down a conc. Gradient (higher to lower) o Osmosis Movement of water/solvent across selectively permeable membrane Lower solute conc to higher (higher water conc to lower) Turgor Pressure Contributes to Cell Stiffness - Cell was counters turgor pressure - For plant cells to grow, water must increase & cell walls must expand Why is Turgor Pressure Important to Maintain? - driving force for cell expansion - provides support for cells and tissues. - drives opening of stomata & phloem transport. - tells plant about soil water status Water Potential (Ψ) - Refers to measurement predicting which way water will flow b/w plant cell and surroundings or between different parts of a plant (ex roots & leaves) - Defined as combination of osmotic potential and the pressure potential (the effect of cell wall pressure) - Measured in units represented by Greek letter psi (Ψ)
Water Potential - Pressure potential always + (cell wall can’t exert negative pressure) - Osmotic potential always zero or - Most living plant cells have water potential of negative or zero - If water potential is negative, cell has more capacity to take up water Hypotonic Solutions - solution surrounding cells has a lower conc. of solutes - Water flows into cell but cell wall prevents rupture Isotonic Solutions - solution surrounding cells has same conc. of solutes - Water at equilibrium but cell not turgid Hypertonic Solutions - The solution surrounding cells has a higher conc. of solutes - Water flows out of cells and plasma mem shrinks away from wall (plasmolysis) - Symplastic connections can be compromised and cause cell death Transport in Plants Bidirectional process: o From roots to shoot & from leaves to the rest of the plant o Mediated by vascular system made up of two complex tissues, xylem and phloem Overview of Water & Solute Transport
Transpiration - Serves two important functions o Cools leaves heated by sunlight o Pulls water and water-soluble minerals up from the root - Water evaporated thru stomata and stems by transpiration - design of xylem tissues and polar water facilitate transport of water upward thru plant Water Characteristics - Adhesion: attraction between different kinds of molecules (cellulose adhere to water, binds) - Cohesion: the attraction between molecules of the same kind (water binds to others w/ h-bonds) - Tension: negative pressure on water or solutions (created by evap from stomata causing a pull on water column that’s transmitted down. Tree trunk shrinks in trans.) - Tension-Cohesion Theory is used to explain water transport in the xylem Cavitation - Greater tension increases risk of breakage of water column - Formation of air bubbles/ice crystals can break water column - Breakage occurs less in tracheids than vessel elements because of anatomical differences Relative Water Potential o Becomes more and more negative as water moves from soil to leaves Water uptake from the soil - Root hairs take up water and minerals from soil - Roots compete directly with soil particles for water Water movement through the root - apoplasm = water flowing between cells - symplasm = water flowing through the cytoplasm - apoplasm/symplasm until it reaches endodermis, where symplastic route is used Root Pressure - Negative water potential of root cells causes enough water uptake to generate root pressure - Removal of stem does not perturb flow of water from roots - Root pressure is not sufficient to move water up plant more than a few feet - Root pressure is lowest during the day when transpiration rates are highest
Stomata play important roles in water relations and gas exchange - Turgor must be maintained in plant cells to prevent wilting - Wilting occurs when plasma membrane isn’t pushing against cell wall - Loss of turgor interrupts cell-to- cell communication and disrupts nutrient and hormone supplies Stomata - Cuticle allows little water loss (~ 5%) - 90% of plant water loss occurs thru the stomata - Stomata occur at highest density on underside of leaves Transport of sugars and other organic molecules - In flowering plants, sugar & organic molecules are transported in the phloem - Transport takes place in sieve-tube members and companion cells Sugars move from source to sink - Sugar source is part of plant producing sugars (leaves, green stems) - Sugar sink is a part of plant that mainly consumes or stores sugar (roots, stems, and fruits) - Sugar transport is driven by water uptake by osmosis Leaf to Root Transport - movement of sugar and other organic molecules can be apoplastic or symplastic - Symplastic transport is common in plants that live in warm climates - Apoplastic transport is most common in plants that live in cold/temperate climates Apoplastic Transport - Energy required to move sucrose molecules from apoplasm to the symplasm - H+ gradient, generated by a proton pump, used to cotransport one sucrose molecule with every H+ ion Pressure-flow Hypothesis - Sugar enters sieve tube member & drives water uptake due to osmotic potential that it generates - generated turgor pressure moves water and sugar down the phloem until sugar is unloaded into sink tissues such as roots Using Aphids to Understand Phloem Transport - Aphids feed on phloem sap by inserting their feeding stylet into phloem cells - Phloem flow and content can bemeasured by scientists using aphid stylets that exude pure phloem sap
Soil, Minerals and Plant Nutrition - Plants obtain needed minerals from the soil - Minerals taken up with water and transported through xylem - Soil made up of ground-up particles of rocks - Soil particles negatively charged and bind water and minerals Plant Require 17 Essential Elements - Plant require 17 chemical elements (macronutrients or micronutrients) - Macronutrients used for growth & physiological processes - Micronutrients necessary cofactors for enzymes and recycled by plant - Deficiency symptoms manifested by plants lacking 1+ essential nutrients - Plants concentrate some nutrients so they‘re required by organisms that eat them Soil Binding Properties - Soil particles display -ve charges & water forms rings around each particle - Mineral ions dissolve in the water as cations (+ve) or anions (-ve) - Some cations + bind directly to the soil Binding Properties of Soil - Three basic rules of ions binding to soil particles: o Cations with +ve charge bind first o Smaller ions bind before larger ones o Ions at higher concentrations bind before ions at lower concentrations The Role of Cation Exchange - displacement of mineral cations by H+ plays role in normal uptake by roots and is driven by cation exchange - Acidic soils, which occur in regions of high rainfall, are often nutrient poor for the same reason Mutualistic Associations - can assist plants in getting nutrients from environment - nitrogen is abundant and essential, but difficult to obtain o Plants absorb nitrogen compounds in the soil [as nitrate (NO3-), and ammonium, (NH4+)] o Nitrogen-fixing bacteria can convert nitrogen gas into ammonia, NH -
Some nitrogen-fixing bacteria invade the plant root and stimulate the formation of special structures called nodules where the bacteria live Bacteria enters thru root hair and signaling events stimulate cortical and pericycle cells to divide, forming a nodule Bacteria assume altered form (bacteroids) that live in vesicles inside root
Formation of Root Nodules
LEC 16 Plant Hormones - direct growth and development - Greek = “to stir up/stimulate” - stimulate and inhibit responses - Effects depend on concentration, location and timing - Key part of plant’s communication system (cell-cell or long distance) Plant Hormones - The concept of “hormones” originated from work in mammalian systems - Shares 3 basic elements o Synthesis in one body part o Transport to another body part o Induction of a chemical response to control physiological event - some hormones act in same tissues they’re produced in - Act with other hormones synergistically or antagonistically Plant Hormones -> Signal-transduction pathways - can act by binding a protein, initiating a response cascade - response can be +ve (turn something on) or -ve (turn something off) - Developmental/growth responses integrate activity of signaling pathways
Plant Hormones - Auxins - Cytokinins - Gibberellins Auxin -
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Abscisic Acid Ethylene Brassinosteroids
first hormone discovered cell elongation & new tissue formation vascular tissue differentiation Structural formula = indoleacetic acid (IAA: Modified a.a from tryptophan) Synthetic auxins more stable than IAA Produced in shoot apical meristem; young leaves, fruits & seeds moves by diffusion and polar transport polar transport is basipetal (in the shoot), and acropetal (in the root)
Auxin Transport - main polar route in stems and leaves is parenchyma around vascular bundles - leaves can be transported nonpolarly in the phloem sieve tubes - IAA In roottip redirected to the root-shoot junction in the epidermal & cortical parenchyma cells - auxin In root,moves thru the sieve tubes to tip and redirected to the epidermis & cortex and transported basipetally (toward the root-shoot junction) Auxin and Cell Elongation • Activates H+ATPase in cell membrane to pump proton into cell wall • Decrease in wall pH activates enzyme “expansins” • Expansins cleaves bonds between cellulose and hemicellulose. weakens cell wall • process is called the “acid growth hypothesis” Auxin - promotes lateral root development o stimulates pericycle & vasc + cork cambium - involved in apical dominance(the suppression of axillary bud growth) - Removaling shoot apex ‘releases’ axillary buds from apical dominance Auxin and Fruit Development - promotes fruit development - Parthenogenic fruit formation (fruitformed without fertilization) can be stimulated in by the exogenous application of auxins - Developing seeds are a source of auxin o without it promotes maturation of the ovary wall (strawberries)
Auxin synthesis - In leaves, the location of cells synthesizing auxin changes as the leaves mature reflecting a shift in cell maturation Synthetic Auxins - first herbicides developed (1946) - used to induce the formation of adventitious roots in cuttings and reduce fruit drop - basis for Agent Orange which was used in Vietnam as a defoliant of broad leaf plants (dicots Cytokinins - Usually modified adenine - Discovered by Overbeek (1941) as a growth promoter in coconut milk - Lay groundwork for in vitro methods for plant propagation (tissue culture) - Miller, Skoog identified purine compound called kinetin, named the group of growth regulators cytokinins bc of their involvement with cytokinesis - Zeatin, isolated from maize kernels, is the most active of the naturally occurring cytokinins Cytokinins - Synthesized actively dividing tissues such as seeds, fruits, leaves and roots - Transported thru xylem to other plant organs - Delay leaf senescence and direct amino acids higher cytokinin concentrations - Promote cell division in shoot apical meristem - Ratio of auxins : cytokinins regulate production of roots/shoots in tissue Cytokinins Alter Apical Dominance - induces cell division and promotes branching Auxin : Cytokinin Ratios - Cytokinins alone have little effect - Low ratios give rise to roots in tissue culture - Equal ratios give rise to undifferentiated cells (callus) - High ratios cause cells to divide and differentiate into shoot buds - Effects depend on tissue type and plant species
LEC 17 Gibberellins - Discovered by Kurosawa (1926) when studying rice pathogen - highest concentrations in immature & germinating seeds - synthesized in apical meristems, young leaves & embryos - promote stem elongation - embryo growth & seed germination - plant lowering & fruit formation GA Promotes Stem Elongation - stimulates stem elongation in dwarf plants - stem elongation by XET to modify of wall extensibility - enhances expansin synthesis - promotes transverse arrangement of microtubules Gibberellins and Seed Dormancy - vernalization = cold period experienced before seeds germinate, in others light is required to break dormancy - can substitute for vernalization or light-induction of germination Gibberellins and Germination - mobilize food reserves through hydrolytic enzymes in barley seeds Gibberellins and Fruit Growth - promote elongation of stem internodes and increase grape size Abscisic Acid (ABA) - no direct role in abscission - stimulates ethylene & seed storage protein production - Synthesized in cells with plastids (chloroplasts/amyloplasts) - Transported by phloem and xylem - Helps seed development & in root-to- shoot signaling - promotes seed dormancy and inhibits seed germination - without ABA no dormancy, wilty phenotype and must be grown in high humidity - Under water stress, roots release ABA into xylem, it closes of leaf stomata GA and ABA - have antagonistic effects in several important processes. - GA promotes these processes while ABA inhibits them o Seed germination o Floral transition o Fruit development
Ethylene - Effects discovered before auxin - synthesized from methionine -> ACC -> converted to ethylene - Touch/physical stress induces ethylene - produced in response to stress: wounding, flooding, temperature - Can be transported in intercellular air spaces, and outside the plant Ethylene and Abscission - Ethylene promotes shedding (abscission) of leaves, flowers and fruits - abscission controlled by interaction of auxin & ethylene - Auxin decreases sensitivity of abscission cells to ethylene Ethylene - enables plants to adapt to underground obstacles by initiating triple response o Slowing stem/root elongation o Thickening of stem/root o Curving to grow horizontally Ethylene: Cell Expansion - induces lateral cell expansion by changing microtubule from a transverse to vertical orientation - Shift in MTs lead to change in cellulose microfibril deposition Ethylene and Fruit Ripening - climacteric fruits = have rapid increase in ethylene production that precedes a sharp increase in cellular respiration (apples, avocados, bananas) - nonclimacteric fruits (cherries, citrus, grapes, pineapple, and watermelon) - Fruit growers use ethylene to control fruit ripening Brassinosteriods - Newly discovered, act like auxin - bind to plasma membrane receptor proteins. do not enter the cell - stimulate cell division & stem elongation - cause xylem differentiation, - promote pollen tube & slow root growth, - enhance ethylene synthesis, delays senescence Plant Hormones o Auxin binds to cellular factor used in protein degradation o Ethylene, cytokinins and brassinosteroids bind to membrane receptors and mediate signaling thru phosphorylation o Brassinosteroids and GAs indirectly regulate protein stability
Plant Hormones: Other Compounds - Polyamines (from a.a) o promote cell division and DNA, RNA & proteins synth o more abundant in plants than other hormones (GA, cytokinins) - Jasmonic acid (from fatty acids) o inhibits growth of seeds o active defense against pathogens o stimulates formation of flowers, fruits, and seeds o promotes accumulation of proteins during seed development - Methyl-jasmonate o signal stimulating interplant communication o fragrant in the perfume industry LEC 18 Plant Tropisms - Tropism: a growth response of bending toward/away from external stimulus that determines movement direction - Tropic growth occurs in response to o Light = phototropism o gravity = geotropism o touch = thigmotropism Light Perception - phototropism = growth toward or away from light o Growth toward light = positive phototropism (ex: shoots) o Growth away from light = negative phototropism (ex: roots) - Etiolation allows plants to grow rapidly toward a light source - photomorphogenic responses = Light triggered growth and developmental responses Light regulates many aspects of plant growth and development 1. Polar cell growth 2. Chloroplast movements 3. Elongation growth of tissues 4. Polarity of growth (differential expansion) 5. Germination 6. Flowering time 7. Seasonal responses
Growth Response - Light changes auxin distrubution - shoots: growth in side with more auxin - roots: high auxin concentrations inhibit growth; dark side of the root grows less Blue Light Perception - Phototropins I and II (NPH mutants) o light receptors that absorb blue light and tart signal transduction cascade making auxin movement to stem’s dark side Photochemistry Light and Protein conformation - The spectral changes by light exposure of phototropin proteins are fully reversible in darkness - Protein conformation induced by light between the LOV and chromophore - thought to activate the C-terminal kinase - This autophosphorylation initiates the signal transduction cascade Light Responses - Phototropism - Chloroplast Migration - Stomatal opening Chloroplast Migrations - Accumulation: low light response. chloroplasts spread evenly to max. light capture - Avoidance : high light response. chloroplasts redistribute to minimize photodamage Light Perception - Phytochromes: light receptors responding to red & infrared light - Phytochrome to Red light: converts to active form Pfr that absorbs infrared - Pfr to Infrared : converts back to inactive form, absorbing red light - Act as on/off switch Phytochromes - cause short-term and long- term effects modulated by signal transduction - regulated leaf greening genes and photosynthetic proteins - regulate seed germination and flowering time - function as dimers
Red Light Responses - Photodormancy overcome by exposure to red or far-red light, gibberellins, cold treatments or by exposing the embryo - Phytochromes alert sun-loving plants when they are shaded causing elongation growth Seasonal Responses - Plants can sense length of night and day (photoperiod) - photoperiods, can alter development & switch to flower production - Some flower when day length becomes short (short-day plants, SDP) - Some flower when day length become long (long-day plants, LDP) - Others are unaffected by day length (day-neutral) Seasonal Responses - determined that plants measure length of night - Some only need 1 inductive cycle - some need multiple inductive cycles (multiple nights of correct duration) - phytochromes and cryptochromes (blue light photoreceptors) play role in floral induction Flowering Time - Short-day plants flower when night exceeds critical length - Long-day plants flower when the night is shorter than critical length - Flash of red light will reverse the response - Phytochrome doesn’t measure night length (unknown) - End of night, Pfr -> pr The Flowering Signal - mature leaf is day-length sensing organ - inductive signal, florigen, transmitted from the leaf to the shoot apical meristem - floral development is initiated - signal is transmissible and is thought to move through the phloem What is florigen? – product of Flowering Time locus. Phloem mobile signal Transmitting the Message - FT mRNA synthesized in leaves and FT protein transmitted to the SAM - FT protein acts w/ transcription factor, FD, to activate genes in floral development
Circadian Rhythms - mediated by bio clocks - Plant biological cycles, ~24 hrs. o Leaf and flower opening and closing o Volatile emissions (floral scents) o Photosynthesis (for example, the CAB protein) o Auxin synthesis o Gene expression Biological Clocks - reset by the natural cycle of day/night - run under const. env. conditions (continuous light) - rhythm is free-running and behaves like oscillator - has temp compensating features; changes in temperature have little effect Lec 19 Gravitropism - Growth toward (+) or away from (-) gravity - Shoots grow away from gravity and show negative gravitropism - Roots grow toward gravity and therefore show positive gravitropism How do plants sense gravity? - Starch-statolithhypothesis o sedimentable amyloplasts play the role of statoliths o in shoot, starch cells surround vasc tissue o in root, cells in cap - Protoplast pressure hypothesis (gravitational pressure hypothesis) o Weight of protoplast involved with gravity perception - Tensegrity model (tensional integrity) o myloplast sedimentation disrupts the actin filaments causing influx of calcium o Gravitropic Response - after gravity perception, auxin is redistributed so concs are higher on the lower side of the root o results in differential cell elongation - Root cells sensitive to higher auxin concentrations, slowing growth Thigmotropism - Growth in response to touch - Plants produce ethylene in response to touch - Ethylene inhibits elongation growth and stimulates growth in width - Tendrils produce ethylene on side that touches object - untouched side is unaffected and continues to grow
Turgor Movement - pulvini special motor cells/swellings at the base of leaf - Leaf movement from changes in water pressures of pulvini initiated by contact with objects - electrical impulse sends the signal along the length of leaf causing the remaining leaflets to fold Defense Responses - Locally, molecules (elicitors) from pathogen bind to receptors in the plasma membrane leading to a hypersensitive response (HR) - HR limits the spread of infection by localized cell death and production of antimicrobial compounds - localized cell death is programmed and results in noticeable lesion on the leaf at the site of infection Defense Responses - Dying cells release salicylic acid and nitric oxide. it initiates a systemic response: - systemic acquired resistance (SAR) produces compounds that elevate levels of resistance at organism level Defense Responses - Herbivore and ozone damage results in production of ethylene - Ethylene reduces production of defense compounds and stimulates production of jasmonic acid - Damaged plant tissue releases products from breakdown of fatty acids that act as signals that can be perceived by other plants Community Interactions - volatiles are released into the atmosphere from leaves, flowers and fruits and into the soil from roots Plant Volatiles - constitute ~ 1%of plant secondary metabolites - fatty acid derivatives and amino acid derivatives - typically lipophilic (fat loving) liquids with high vapor pressures - can cross membranes freely - synthesized in epidermis
Plant Chat Room - Plants can share info about biological status with neighboring plants through volatiles - Plants can attract predators of attacking herbivores using volatiles - directly affects herbivore physiology and behavior - attracts predators of the herbivores or additional unwanted plant predators - Herbivore-infested plants can signal intact leaves on same plant or neighboring plants to launch defense responses - Defense = expression of defense genes & emission of volatiles - Ethylene and jasmonic acid signaling pathways mediate responses - strong Ca2+ dependent membrane depolarization occurs @ site of herbivore damage - Mechanical damage induces depolarization and kinase activation (without Ca2+) - Ethylene biosynthesisis induced in herbivore/mechanical damage and herbivore induced volatiles Language of Love - Floral scents o may be composed of up to 100 different volatiles o allow pollinators to discriminate between flowers among/ within species (reward potential) o facilitate pollinator discrimination - Within species, rate of scent can vary bc of time of day, flower age, temp, moisture and pollination status - Some bees sensitive to odor - certain moth species use odor quantity as cues Volatiles as Solutions? - might be possible to engineer plants that… - emit volatiles detering agricultural pests - are more effective at attracting pollinators - might engineer plants that emit volatiles impacting climate change Environmental Impacts? - Plants can modify troposphere - troposphere = layer between the earth’s surface and stratosphere where most of weather changes occur - some evidence that plants promote aerosol formation to reduce harmful UV irradiation - primitive land plants (mosses + ferns) volatiles made constitutively (volatiles usually made after induction in new arrivals like angiosperms)
Water Potential - Pressure potential always + (cell wall can’t exert negative pressure) - Osmotic potential always zero or - Most living plant cells have water potential of negative or zero - If water potential is negative, cell has more capacity to take up water Hypotonic Solutions - solution surrounding cells has a lower conc. of solutes - Water flows into cell but cell wall prevents rupture Isotonic Solutions - solution surrounding cells has same conc. of solutes - Water at equilibrium but cell not turgid Hypertonic Solutions - The solution surrounding cells has a higher conc. of solutes - Water flows out of cells and plasma mem shrinks away from wall (plasmolysis) - Symplastic connections can be compromised and cause cell death Transport in Plants Bidirectional process: o From roots to shoot & from leaves to the rest of the plant o Mediated by vascular system made up of two complex tissues, xylem and phloem Overview of Water & Solute Transport
Transpiration - Serves two important functions o Cools leaves heated by sunlight o Pulls water and water-soluble minerals up from the root - Water evaporated thru stomata and stems by transpiration - design of xylem tissues and polar water facilitate transport of water upward thru plant Water Characteristics - Adhesion: attraction between different kinds of molecules (cellulose adhere to water, binds) - Cohesion: the attraction between molecules of the same kind (water binds to others w/ h-bonds) - Tension: negative pressure on water or solutions (created by evap from stomata causing a pull on water column that’s transmitted down. Tree trunk shrinks in trans.) - Tension-Cohesion Theory is used to explain water transport in the xylem Cavitation - Greater tension increases risk of breakage of water column - Formation of air bubbles/ice crystals can break water column - Breakage occurs less in tracheids than vessel elements because of anatomical differences Relative Water Potential o Becomes more and more negative as water moves from soil to leaves Water uptake from the soil - Root hairs take up water and minerals from soil - Roots compete directly with soil particles for water Water movement through the root - apoplasm = water flowing between cells - symplasm = water flowing through the cytoplasm - apoplasm/symplasm until it reaches endodermis, where symplastic route is used Root Pressure - Negative water potential of root cells causes enough water uptake to generate root pressure - Removal of stem does not perturb flow of water from roots - Root pressure is not sufficient to move water up plant more than a few feet - Root pressure is lowest during the day when transpiration rates are highest
Stomata play important roles in water relations and gas exchange - Turgor must be maintained in plant cells to prevent wilting - Wilting occurs when plasma membrane isn’t pushing against cell wall - Loss of turgor interrupts cell-to- cell communication and disrupts nutrient and hormone supplies Stomata - Cuticle allows little water loss (~ 5%) - 90% of plant water loss occurs thru the stomata - Stomata occur at highest density on underside of leaves Transport of sugars and other organic molecules - In flowering plants, sugar & organic molecules are transported in the phloem - Transport takes place in sieve-tube members and companion cells Sugars move from source to sink - Sugar source is part of plant producing sugars (leaves, green stems) - Sugar sink is a part of plant that mainly consumes or stores sugar (roots, stems, and fruits) - Sugar transport is driven by water uptake by osmosis Leaf to Root Transport - movement of sugar and other organic molecules can be apoplastic or symplastic - Symplastic transport is common in plants that live in warm climates - Apoplastic transport is most common in plants that live in cold/temperate climates Apoplastic Transport - Energy required to move sucrose molecules from apoplasm to the symplasm - H+ gradient, generated by a proton pump, used to cotransport one sucrose molecule with every H+ ion Pressure-flow Hypothesis - Sugar enters sieve tube member & drives water uptake due to osmotic potential that it generates - generated turgor pressure moves water and sugar down the phloem until sugar is unloaded into sink tissues such as roots Using Aphids to Understand Phloem Transport - Aphids feed on phloem sap by inserting their feeding stylet into phloem cells - Phloem flow and content can bemeasured by scientists using aphid stylets that exude pure phloem sap
Soil, Minerals and Plant Nutrition - Plants obtain needed minerals from the soil - Minerals taken up with water and transported through xylem - Soil made up of ground-up particles of rocks - Soil particles negatively charged and bind water and minerals Plant Require 17 Essential Elements - Plant require 17 chemical elements (macronutrients or micronutrients) - Macronutrients used for growth & physiological processes - Micronutrients necessary cofactors for enzymes and recycled by plant - Deficiency symptoms manifested by plants lacking 1+ essential nutrients - Plants concentrate some nutrients so they‘re required by organisms that eat them Soil Binding Properties - Soil particles display -ve charges & water forms rings around each particle - Mineral ions dissolve in the water as cations (+ve) or anions (-ve) - Some cations + bind directly to the soil Binding Properties of Soil - Three basic rules of ions binding to soil particles: o Cations with +ve charge bind first o Smaller ions bind before larger ones o Ions at higher concentrations bind before ions at lower concentrations The Role of Cation Exchange - displacement of mineral cations by H+ plays role in normal uptake by roots and is driven by cation exchange - Acidic soils, which occur in regions of high rainfall, are often nutrient poor for the same reason Mutualistic Associations - can assist plants in getting nutrients from environment - nitrogen is abundant and essential, but difficult to obtain o Plants absorb nitrogen compounds in the soil [as nitrate (NO3-), and ammonium, (NH4+)] o Nitrogen-fixing bacteria can convert nitrogen gas into ammonia, NH -
Some nitrogen-fixing bacteria invade the plant root and stimulate the formation of special structures called nodules where the bacteria live Bacteria enters thru root hair and signaling events stimulate cortical and pericycle cells to divide, forming a nodule Bacteria assume altered form (bacteroids) that live in vesicles inside root
Formation of Root Nodules
LEC 16 Plant Hormones - direct growth and development - Greek = “to stir up/stimulate” - stimulate and inhibit responses - Effects depend on concentration, location and timing - Key part of plant’s communication system (cell-cell or long distance) Plant Hormones - The concept of “hormones” originated from work in mammalian systems - Shares 3 basic elements o Synthesis in one body part o Transport to another body part o Induction of a chemical response to control physiological event - some hormones act in same tissues they’re produced in - Act with other hormones synergistically or antagonistically Plant Hormones -> Signal-transduction pathways - can act by binding a protein, initiating a response cascade - response can be +ve (turn something on) or -ve (turn something off) - Developmental/growth responses integrate activity of signaling pathways
Plant Hormones - Auxins - Cytokinins - Gibberellins Auxin -
-
Abscisic Acid Ethylene Brassinosteroids
first hormone discovered cell elongation & new tissue formation vascular tissue differentiation Structural formula = indoleacetic acid (IAA: Modified a.a from tryptophan) Synthetic auxins more stable than IAA Produced in shoot apical meristem; young leaves, fruits & seeds moves by diffusion and polar transport polar transport is basipetal (in the shoot), and acropetal (in the root)
Auxin Transport - main polar route in stems and leaves is parenchyma around vascular bundles - leaves can be transported nonpolarly in the phloem sieve tubes - IAA In roottip redirected to the root-shoot junction in the epidermal & cortical parenchyma cells - auxin In root,moves thru the sieve tubes to tip and redirected to the epidermis & cortex and transported basipetally (toward the root-shoot junction) Auxin and Cell Elongation • Activates H+ATPase in cell membrane to pump proton into cell wall • Decrease in wall pH activates enzyme “expansins” • Expansins cleaves bonds between cellulose and hemicellulose. weakens cell wall • process is called the “acid growth hypothesis” Auxin - promotes lateral root development o stimulates pericycle & vasc + cork cambium - involved in apical dominance(the suppression of axillary bud growth) - Removaling shoot apex ‘releases’ axillary buds from apical dominance Auxin and Fruit Development - promotes fruit development - Parthenogenic fruit formation (fruitformed without fertilization) can be stimulated in by the exogenous application of auxins - Developing seeds are a source of auxin o without it promotes maturation of the ovary wall (strawberries)
Auxin synthesis - In leaves, the location of cells synthesizing auxin changes as the leaves mature reflecting a shift in cell maturation Synthetic Auxins - first herbicides developed (1946) - used to induce the formation of adventitious roots in cuttings and reduce fruit drop - basis for Agent Orange which was used in Vietnam as a defoliant of broad leaf plants (dicots Cytokinins - Usually modified adenine - Discovered by Overbeek (1941) as a growth promoter in coconut milk - Lay groundwork for in vitro methods for plant propagation (tissue culture) - Miller, Skoog identified purine compound called kinetin, named the group of growth regulators cytokinins bc of their involvement with cytokinesis - Zeatin, isolated from maize kernels, is the most active of the naturally occurring cytokinins Cytokinins - Synthesized actively dividing tissues such as seeds, fruits, leaves and roots - Transported thru xylem to other plant organs - Delay leaf senescence and direct amino acids higher cytokinin concentrations - Promote cell division in shoot apical meristem - Ratio of auxins : cytokinins regulate production of roots/shoots in tissue Cytokinins Alter Apical Dominance - induces cell division and promotes branching Auxin : Cytokinin Ratios - Cytokinins alone have little effect - Low ratios give rise to roots in tissue culture - Equal ratios give rise to undifferentiated cells (callus) - High ratios cause cells to divide and differentiate into shoot buds - Effects depend on tissue type and plant species
LEC 17 Gibberellins - Discovered by Kurosawa (1926) when studying rice pathogen - highest concentrations in immature & germinating seeds - synthesized in apical meristems, young leaves & embryos - promote stem elongation - embryo growth & seed germination - plant lowering & fruit formation GA Promotes Stem Elongation - stimulates stem elongation in dwarf plants - stem elongation by XET to modify of wall extensibility - enhances expansin synthesis - promotes transverse arrangement of microtubules Gibberellins and Seed Dormancy - vernalization = cold period experienced before seeds germinate, in others light is required to break dormancy - can substitute for vernalization or light-induction of germination Gibberellins and Germination - mobilize food reserves through hydrolytic enzymes in barley seeds Gibberellins and Fruit Growth - promote elongation of stem internodes and increase grape size Abscisic Acid (ABA) - no direct role in abscission - stimulates ethylene & seed storage protein production - Synthesized in cells with plastids (chloroplasts/amyloplasts) - Transported by phloem and xylem - Helps seed development & in root-to- shoot signaling - promotes seed dormancy and inhibits seed germination - without ABA no dormancy, wilty phenotype and must be grown in high humidity - Under water stress, roots release ABA into xylem, it closes of leaf stomata GA and ABA - have antagonistic effects in several important processes. - GA promotes these processes while ABA inhibits them o Seed germination o Floral transition o Fruit development
Ethylene - Effects discovered before auxin - synthesized from methionine -> ACC -> converted to ethylene - Touch/physical stress induces ethylene - produced in response to stress: wounding, flooding, temperature - Can be transported in intercellular air spaces, and outside the plant Ethylene and Abscission - Ethylene promotes shedding (abscission) of leaves, flowers and fruits - abscission controlled by interaction of auxin & ethylene - Auxin decreases sensitivity of abscission cells to ethylene Ethylene - enables plants to adapt to underground obstacles by initiating triple response o Slowing stem/root elongation o Thickening of stem/root o Curving to grow horizontally Ethylene: Cell Expansion - induces lateral cell expansion by changing microtubule from a transverse to vertical orientation - Shift in MTs lead to change in cellulose microfibril deposition Ethylene and Fruit Ripening - climacteric fruits = have rapid increase in ethylene production that precedes a sharp increase in cellular respiration (apples, avocados, bananas) - nonclimacteric fruits (cherries, citrus, grapes, pineapple, and watermelon) - Fruit growers use ethylene to control fruit ripening Brassinosteriods - Newly discovered, act like auxin - bind to plasma membrane receptor proteins. do not enter the cell - stimulate cell division & stem elongation - cause xylem differentiation, - promote pollen tube & slow root growth, - enhance ethylene synthesis, delays senescence Plant Hormones o Auxin binds to cellular factor used in protein degradation o Ethylene, cytokinins and brassinosteroids bind to membrane receptors and mediate signaling thru phosphorylation o Brassinosteroids and GAs indirectly regulate protein stability
Plant Hormones: Other Compounds - Polyamines (from a.a) o promote cell division and DNA, RNA & proteins synth o more abundant in plants than other hormones (GA, cytokinins) - Jasmonic acid (from fatty acids) o inhibits growth of seeds o active defense against pathogens o stimulates formation of flowers, fruits, and seeds o promotes accumulation of proteins during seed development - Methyl-jasmonate o signal stimulating interplant communication o fragrant in the perfume industry LEC 18 Plant Tropisms - Tropism: a growth response of bending toward/away from external stimulus that determines movement direction - Tropic growth occurs in response to o Light = phototropism o gravity = geotropism o touch = thigmotropism Light Perception - phototropism = growth toward or away from light o Growth toward light = positive phototropism (ex: shoots) o Growth away from light = negative phototropism (ex: roots) - Etiolation allows plants to grow rapidly toward a light source - photomorphogenic responses = Light triggered growth and developmental responses Light regulates many aspects of plant growth and development 1. Polar cell growth 2. Chloroplast movements 3. Elongation growth of tissues 4. Polarity of growth (differential expansion) 5. Germination 6. Flowering time 7. Seasonal responses
Growth Response - Light changes auxin distrubution - shoots: growth in side with more auxin - roots: high auxin concentrations inhibit growth; dark side of the root grows less Blue Light Perception - Phototropins I and II (NPH mutants) o light receptors that absorb blue light and tart signal transduction cascade making auxin movement to stem’s dark side Photochemistry Light and Protein conformation - The spectral changes by light exposure of phototropin proteins are fully reversible in darkness - Protein conformation induced by light between the LOV and chromophore - thought to activate the C-terminal kinase - This autophosphorylation initiates the signal transduction cascade Light Responses - Phototropism - Chloroplast Migration - Stomatal opening Chloroplast Migrations - Accumulation: low light response. chloroplasts spread evenly to max. light capture - Avoidance : high light response. chloroplasts redistribute to minimize photodamage Light Perception - Phytochromes: light receptors responding to red & infrared light - Phytochrome to Red light: converts to active form Pfr that absorbs infrared - Pfr to Infrared : converts back to inactive form, absorbing red light - Act as on/off switch Phytochromes - cause short-term and long- term effects modulated by signal transduction - regulated leaf greening genes and photosynthetic proteins - regulate seed germination and flowering time - function as dimers
Red Light Responses - Photodormancy overcome by exposure to red or far-red light, gibberellins, cold treatments or by exposing the embryo - Phytochromes alert sun-loving plants when they are shaded causing elongation growth Seasonal Responses - Plants can sense length of night and day (photoperiod) - photoperiods, can alter development & switch to flower production - Some flower when day length becomes short (short-day plants, SDP) - Some flower when day length become long (long-day plants, LDP) - Others are unaffected by day length (day-neutral) Seasonal Responses - determined that plants measure length of night - Some only need 1 inductive cycle - some need multiple inductive cycles (multiple nights of correct duration) - phytochromes and cryptochromes (blue light photoreceptors) play role in floral induction Flowering Time - Short-day plants flower when night exceeds critical length - Long-day plants flower when the night is shorter than critical length - Flash of red light will reverse the response - Phytochrome doesn’t measure night length (unknown) - End of night, Pfr -> pr The Flowering Signal - mature leaf is day-length sensing organ - inductive signal, florigen, transmitted from the leaf to the shoot apical meristem - floral development is initiated - signal is transmissible and is thought to move through the phloem What is florigen? – product of Flowering Time locus. Phloem mobile signal Transmitting the Message - FT mRNA synthesized in leaves and FT protein transmitted to the SAM - FT protein acts w/ transcription factor, FD, to activate genes in floral development
Circadian Rhythms - mediated by bio clocks - Plant biological cycles, ~24 hrs. o Leaf and flower opening and closing o Volatile emissions (floral scents) o Photosynthesis (for example, the CAB protein) o Auxin synthesis o Gene expression Biological Clocks - reset by the natural cycle of day/night - run under const. env. conditions (continuous light) - rhythm is free-running and behaves like oscillator - has temp compensating features; changes in temperature have little effect Lec 19 Gravitropism - Growth toward (+) or away from (-) gravity - Shoots grow away from gravity and show negative gravitropism - Roots grow toward gravity and therefore show positive gravitropism How do plants sense gravity? - Starch-statolithhypothesis o sedimentable amyloplasts play the role of statoliths o in shoot, starch cells surround vasc tissue o in root, cells in cap - Protoplast pressure hypothesis (gravitational pressure hypothesis) o Weight of protoplast involved with gravity perception - Tensegrity model (tensional integrity) o myloplast sedimentation disrupts the actin filaments causing influx of calcium o Gravitropic Response - after gravity perception, auxin is redistributed so concs are higher on the lower side of the root o results in differential cell elongation - Root cells sensitive to higher auxin concentrations, slowing growth Thigmotropism - Growth in response to touch - Plants produce ethylene in response to touch - Ethylene inhibits elongation growth and stimulates growth in width - Tendrils produce ethylene on side that touches object - untouched side is unaffected and continues to grow
Turgor Movement - pulvini special motor cells/swellings at the base of leaf - Leaf movement from changes in water pressures of pulvini initiated by contact with objects - electrical impulse sends the signal along the length of leaf causing the remaining leaflets to fold Defense Responses - Locally, molecules (elicitors) from pathogen bind to receptors in the plasma membrane leading to a hypersensitive response (HR) - HR limits the spread of infection by localized cell death and production of antimicrobial compounds - localized cell death is programmed and results in noticeable lesion on the leaf at the site of infection Defense Responses - Dying cells release salicylic acid and nitric oxide. it initiates a systemic response: - systemic acquired resistance (SAR) produces compounds that elevate levels of resistance at organism level Defense Responses - Herbivore and ozone damage results in production of ethylene - Ethylene reduces production of defense compounds and stimulates production of jasmonic acid - Damaged plant tissue releases products from breakdown of fatty acids that act as signals that can be perceived by other plants Community Interactions - volatiles are released into the atmosphere from leaves, flowers and fruits and into the soil from roots Plant Volatiles - constitute ~ 1%of plant secondary metabolites - fatty acid derivatives and amino acid derivatives - typically lipophilic (fat loving) liquids with high vapor pressures - can cross membranes freely - synthesized in epidermis
Plant Chat Room - Plants can share info about biological status with neighboring plants through volatiles - Plants can attract predators of attacking herbivores using volatiles - directly affects herbivore physiology and behavior - attracts predators of the herbivores or additional unwanted plant predators - Herbivore-infested plants can signal intact leaves on same plant or neighboring plants to launch defense responses - Defense = expression of defense genes & emission of volatiles - Ethylene and jasmonic acid signaling pathways mediate responses - strong Ca2+ dependent membrane depolarization occurs @ site of herbivore damage - Mechanical damage induces depolarization and kinase activation (without Ca2+) - Ethylene biosynthesisis induced in herbivore/mechanical damage and herbivore induced volatiles Language of Love - Floral scents o may be composed of up to 100 different volatiles o allow pollinators to discriminate between flowers among/ within species (reward potential) o facilitate pollinator discrimination - Within species, rate of scent can vary bc of time of day, flower age, temp, moisture and pollination status - Some bees sensitive to odor - certain moth species use odor quantity as cues Volatiles as Solutions? - might be possible to engineer plants that… - emit volatiles detering agricultural pests - are more effective at attracting pollinators - might engineer plants that emit volatiles impacting climate change Environmental Impacts? - Plants can modify troposphere - troposphere = layer between the earth’s surface and stratosphere where most of weather changes occur - some evidence that plants promote aerosol formation to reduce harmful UV irradiation - primitive land plants (mosses + ferns) volatiles made constitutively (volatiles usually made after induction in new arrivals like angiosperms)
