Download Chapter 39 – Plant Responses to Stimuli Signal Transduction

Chapter 39 – Plant Responses to Stimuli
Signal Transduction Pathway (STP)
 Path from stimulus to response (stages = Reception, Transduction, & Response)
 Organisms have cell receptors that detect changes in environment
o For stimulus to elicit response, cell must have right receptor
 Stimulation of receptor initiates a specific STP
Potatoes
 Grown in darkness – make pale, leafless shoots, no elongated roots
o Etiolation – adaptations for growing in darkness
 Exposure to light – shoots & roots grow normally (de-etiolation)
Hormone
 Chemical signal that coordinates different parts of an organism
o Growth, development, & responses to stimuli
Reception
 Internal & external signals are detected by receptors
 Proteins that change in response to specific stimuli
o Can be in membrane or cytoplasm
Transduction
 Second messengers
o Transfer & amplify signals from receptors to proteins
 e.g. Ca2+, cyclic GMP (cGMP), cAMP
o Turns on/off proteins (leads to the response)
Response
 STP leads to regulation of one or more cellular activities
 Usually involves increased enzyme activity
Protein Review
 DNA → RNA → Protein; Transcription (DNA to mRNA); Translation (mRNA to protein)
Phosphorylation
 Addition of phosphate group to certain amino acids (in proteins); Done by Protein Kinase
 Removal of phosphate (de-phosphorylation) turns ‘off’; Done by Protein Phosphatase
Transcriptional Regulation
 Specific transcription factors (TFs) bind to specific parts of DNA to control gene transcription
o Positive TF – protein increases transcription of specific gene
o Negative TF – protein decreases transcription of specific gene
Post-Translational Modification of Proteins
 Modify existing proteins that are inactive
 Alters their properties or shape thereby activating them
Phytochrome
 Absorbs red & far-red light
 2 Geometric forms
o Pr – absorbs red light (650-670nm) to make Pfr (fast)
o Pfr – absorbs far-red light (705-740nm) to make Pr (slow)
 Pfr acts as a kinase
Plant Defense
 Caterpillar herbivory
o Caterpillar eats leaf
o Wounding & saliva initiate STP
 Plant creates & releases volatile chemicals
 Signals parasitoid wasp; lays eggs on caterpillar, who hatch & eat it
 Hypersensitive Response
o Pathogen enters plant
o Plant recognizes pathogen or its byproducts
o STP sends message to rest of plant (salicylic acid)
 Systemic Acquired Resistance – Plant makes defense proteins
o Region of infection dies
Tropism
 Any response resulting in curvature of organs toward or away from a stimulus
 Often caused by hormones
o Positive – towards stimuli
o Negative – away from stimuli
 Phototropism – response to light
 Thigmotropism – response to touch
 Gravitropism – response to gravity
o Root = positive gravitropism & Shoot = negative gravitropism
o Statoliths (plastid filled w/ starch, more dense than cytoplasm) sink to bottom of cell; detect gravity
Survey of Plant Hormones
 Control plant growth & development by affecting division, elongation, & differentiation of cells
 Produced in very low concentration, but a small amount can cause big effects
 Auxin
o Shoot apical meristems make it
o Enhances apical dominance
o Stem elongation
o Synthetic auxin – weed killer & fruit production (seedless tomato)
o Indoleacetic acid (IAA) – most common auxin in plants
o Responsible for photo- & gravitropism
o Acid Growth Hypothesis
 Auxin stimulates proton pumps in plasma membrane
 pH in cell wall lowered, activates expansin
 Expansin loosens cell wall fabric
 Voltage gradient (from pump) causes cations to enter cell
 Water follows, cell elongates
 Cytokinins
o Made in roots & travels up
o Stimulates cell division & cytokinesis (splitting of cytoplasm)
o Anti-aging effect – stimulate RNA & protein synthesis, inhibit protein breakdown
 Used on flowers
 Gibberellins (GAs)
o Made in roots, young leaves, developing seeds
o Stem & leaf elongation
 Bolting – fast growth floral stalk
o Fruit growth
o Breaks seed dormancy; starts germination
o Germination – after water absorbed, GAs released from embryo; signals germination
 Abscisic Acid (ABA)
o Produced by almost all cells
o Inhibits growth
o Seed dormancy ends when ABA removed or inactivated (Heavy rain, light, or prolonged cold)
o Closes stomata during times of water stress
 Ethylene
o Gaseous hormone made all over
o Promotes fruit ripening
o Produced in response to stress
 Facilitates apoptosis (Programmed cell death)
 Promotes leaf abscission (Leaf dies & falls off)
 Triple Response
o Triggered by ethylene burst
o Allows growing shoot to avoid obstacles (e.g. rocks)
 Slowing of stem elongation
 Thickening of stem
 Horizontal growth