Download Ch. 39

AP Biology
Chapter 39 –
Plant Responses to
Internal and
External Signaling
Signal Transduction
• Etiolation
• Plant Morphological Adaptations for Growing in Darkness
• De-etiolation (Greening)
• Morphological Change to “Normal” Plant
*
Signal Transduction
• Transcriptional Regulation
• Up-Regulation
• ↑ Light, Ca2+, cGMP, TF-1, TF-2
• Down-Regulation
• ↑ Greening Response Proteins* (Feedback) or TF
Inhibitory Proteins
• *Auxin (Indoleacetic Acid) - Growth Hormone
• *Brassinosteroids – Growth hormone
• Post Translational Modifications
• Phosphorylation (Kinases)
• De-phosphorylation (Phosphatases)
Plant Response to Hormones
• Hormones
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Chemical Signals that Bind to Specific Receptors
Trigger Responses in Targets Cells and Tissues
Produced at Very Low Concentrations
Signal Transduction Pathways Amplify the Signal
Alter Expression of Genes by Affecting the Activity of
Enzymes
• Response Depends On Relative Concentration
Compared to Concentration of Other Hormones
• Hormonal Balance
Plant Response to Hormones
• Tropism
• Growth Response Results in Curvatures of Plant
Toward or Away from Stimuli
• Phototropism
• Tropism Caused by Light Stimulus
• Positive or Negative
• Controlled by an Asymmetrical Distribution
of Growth Hormones
• Auxin
• Cytokinins
Plant Response to Hormones
Plant Response to Hormones
• Auxin
• Indoleacetic Acid (IAA)
• Produced from Tryptophan at Apical Meristem of
Shoot
• Stimulates Elongation of Cells at Concentrations from
10-8 to 10-4 M
• Inhibit Cell
Elongation
at Higher
Concentrations
Plant Response to Hormones
• Auxin
• Stimulates Sustained Growth of More Cytoplasm and
Cell Wall Material Required by Elongation
• Alters Gene Expression
• Activates TFs that Repress or Activate Expression of
other Genes
• Commercial Use
• Rooting Powder Containing Auxin Causes Roots to
Form near Cutting Surface
• Selective Herbicide
• Monocots Rapidly Inactivate Synthetic Auxins
• Dicots Die from Hormonal Overdose
Plant Response to Hormones
• Cytokinins
• Stimulate Cytokinesis & Root Growth
• Zeatin
• Produced in Roots, Embryos, & Fruits
• Travels Upward in Xylem Sap
• Causes Cell Division & Differentiation (in vitro)
• Balanced Conc. of Cytokinins & Auxin
• Callus Formation
• Higher Conc. of Cytokinins = Shoot Formation
• Higher Conc. of Auxin = Root Formation
Plant Response to Hormones
• Cytokinins
• Control Apical Dominance
• Ability of Terminal Bud to Suppress Development of
Axillary Buds
• Zeatin (Aux) / IAA (Shoot) Balance
• Exact Process Still Unknown
• Anti-Aging Effects
• Inhibits Protein Degradation by Stimulating RNA and
Protein Synthesis
• Florists use cytokinin sprays to keep cut flowers fresh
Plant Response to Hormones
• Gibberellins
• Growth Hormone
• >100 Different Types Occur in Plants
• Stimulate Stem Elongation, Leaf Growth,
Fruit Growth, and Seed Germination
• Produced in Roots & Leaves
• Stimulate Cell Wall Loosening
Enzymes that Facilitate Penetration
of Expansin Proteins into the Cell Wall
• So, Auxin, by Acidifying the Cell Wall and Activating
Expansins, and Gibberellins, by Facilitating Penetration
of Expansins, Act Together to Promote Cell Elongation
Plant Response to Hormones
• Brassinosteroids
• Chemically Similar to Cholesterol
• Induce Cell Elongation and Division in Stems &
Seedlings
• Inhibit Leaf Loss
• Promote Xylem Differentiation
Plant Response to Hormones
• Abscisic Acid (ABA)
• Slows Plant Growth and Inhibits Seed Germination
(Leading to Dormancy)
• Ratio of ABA to Growth Hormones (Auxins, Cytokinins,
and Gibberellins) Determines Growth Rate
• Removal of ABA Leads to Seed Germination
• Drought Resistance
• Increased Concentration Causes Increase in Opening
of Outwardly Directed Potassium Channels in the
Plasma Membrane of Guard Cells which Results in
Reduction of Guard Cell Turgor and Stomata Closure
Plant Response to Hormones
• Ethylene
• Produced in Response to Stresses
• Drought, Flood, Injury, Infection
• Also During Fruit Ripening and Apoptosis
• Triple Response
• Enables a Seedling to Circumvent an Obstacle
1. Shoot Elongation Slows
2. Shoot Thickens
3. Curvature Causes
Horizontal Shoot Growth
Plant Response to Hormones
• Ethylene
• Triple Response
Experimental Results
Ethylene-Insensitive
Mutant
Constitutive
Triple Response
Mutant
Plant Response to Light
• Detect Direction, Intensity, & Wavelengths
• Light Receptors
• Blue-Light Photoreceptors
• Phototropin
• Stimulates Phototropism
• Zeaxanthin
• Stomata Opening
• Cryptochromes
• Slowing of Hypocotyl Elongation
• Phytochromes
• Absorb Red Light
• 660nm Induce Seed Germination
• 730nm Inhibit Seed Germination
Plant Response to Light
• Phytochromes
• Pr Conformation - 660nm Pfr Conformation - 730nm
• Direct Sunlight Alters Balance Toward Pfr
• Sun > Shade
• Stimulate Growth in Girth > Height
• Day vs. Night (Night Length Critical)
• Promotes Circadian Rhythm Behaviors
• Summer vs. Winter
• Photoperiodism
» Physiological Response to Photoperiod
• Recognition of Length of Day Allows Plants to
Flower When Appropriate
» Florigen – Flowering Hormone Transduced from Leaves to Buds
Plant Response to Other Stimuli
• Gravitropism
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Roots – Positive Gravitropism
Shoots – Negative Gravitropism
Stimulated by Auxin
Statoliths
• Plastids with Dense Starch Grains
• Thigmomorphogenesis
• Change of Form Due to Mechanical Pressures
• Stimulation Activates Signal-transduction
Pathway that Affect Cell Wall Properties
• Vines
• Leaf Action Potentials
• Signals Transmitted from Leaf to Leaf
Plant Response to Other Stimuli
• Drought
• Reduce Transpiration
• Soil Dries from Top Down So Deeper Roots In Soil that is
Still Moist Continue to Grow
• Flood
• Flooded Soil Lacks Air Spaces that Provide Oxygen for
Cellular Respiration
• Mangroves – Aerial Roots
• Other Plants
• Ethylene Produced in Roots Cause Some Cells to
Undergo Apoptosis which Creates Air Tubes that
Function as Snorkels
Plant Response to Other Stimuli
• Salt
• Some Plants Produce Organic Compounds that Keep
Water Potential of the Cell More Negative Than the Soil
Without Admitting Toxic Quantities of Salt
• Heat
• Heat-Shock Proteins (HSP)
• Scaffolds Help Proteins Properly Fold
• Prevent Denaturation of Proteins / Enzymes
• Cold
• Increase Proportion of Unsaturated FAs Which help
Keep Cell Membranes Fluid at Lower Temperatures
• Increase Cytoplasmic Levels of Sugars Which Reduce
Water Loss During Extracellular Freezing
Plant Response to Herbivores
• Thorns Protect from Herbivorous Animals
• Canavanine
• Resembles Arginine
• Insects Eat Plants Containing Canavanine Which is
Incorporated Into the Insect’s Proteins in Place of
Arginine
• Insect Dies N
• Recruitment of Predatory Animals
• Early Warning Alarm
• Signal Hormones Released
• Nearby Plants Recruit
Predatory Animals
Plant Response to Pathogens
• 1st Line of Defense – Epidermis
• 2nd Line of Defense – Chemicals
• Virulent Pathogen – Plant has
Little Specific Defense
• Avirulent Pathogen – Plant Has
Specific Defense
• Gene-for-Gene Recognition
• Plant with Resistance Alleles
(R) Recognizes Pathogens with
Complementary Avirulence
(Avr) Alleles
• Recognition Induces Expression
of Gene Products which Defend
Against the Pathogen
Plant Response to Pathogens
• Chemical Attack
• Elicitors – Oligosaccharins Induce the Production of
Antimicrobial Phytoalexins
• PR (Pathogenesis-Related) Proteins
• Enzymes Attack Bacterial Cell Walls and Signal Other
Parts of the Plant
• Hypersensitive Response (HR)
• Increased Response of Phytoalexins and PR Proteins
to Avirulent Infection Recognized by Gene-for-Gene
• Plant Cell Death Near Infected Site Prevents Spread of
Infection
Plant Response to Pathogens
System Acquired Resistance
Activated by Salicylic Acid
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G-to-G Recognition
Signal Transduction
Pathway Initiates
Hypersensitive Response
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PR Proteins & Phytoalexins
STP Also Initiates
Production of Salicylic
Acid Released to Plant
Other Cells Initiate Their
Own STP (2o Response)