Chapter 39: Plant Responses to Internal and External Signals Jay Swan Cincinnati, Ohio.
Plant Responses to Internal & External Signals
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Transcript of Plant Responses to Internal & External Signals
CAMPBELL & REECE
CHAPTER 39
Plant Responses to Internal & External
Signals
Stimuli & a Stationary Life
observation about plants:1 part of plant sends signals to anothersense gravity & direction of lightresponds to environmental stimuli & internal
signals
Stimuli & the Stationary Life
animals have behavioral responses to stimuliplants generally respond to environmental
cues by adjusting its patterns of growth & development as result: plants of same species vary in body
form much more than animals
all organisms have ability to receive specific environmental & internal signals & respond to them in ways that enhance survival & reproductive success.
Plant cells have cellular receptors used to detect important changes in their environment Change in daylight hours Insect eating their leaves
Signal Transduction Pathways
@ cellular level plants & all other eukaryotes are surprisingly similar in signaling mechanisms
in order to respond to any stimuli cell must have receptor molecule that is sensitive to & affected by specific stimuli
Etiolation / De-etiolation
morphological adaptations for growing in the dark:
plant in dark allocates as much nrg as possible to elongation of stems break ground b/4 exhausts nutrients in tubers
After a week’s exposure tonatural daylight. The potatoplant begins to resemble a typical plant with broad greenleaves, short sturdy stems, andlong roots. This transformationbegins with the reception oflight by a specific pigment,phytochrome.
Signal Transduction
Step 1:Reception
proteins that change shape in response to specific stimuli
usually a weak signal binds to a receptor causing it to undergo a conformational change
Signal Transduction
Step 2:Transduction
amplification of message thru 2nd messengers
2nd messengers: small molecules or ions transfer signal from receptor to other
proteins that carry out the response
Signal Transduction
Step 3: Response usually involves increasing activity of an
enzyme by:1. post-translational modification of pre-
existing proteins2. transcriptional regulation
Signal Transduction in Plants
Post-Translational Modification of Pre-Existing Proteins
mostly involves phosphorylation of specific a.a. alters protein’s hydrophobicity & activity cAMP & Ca++ activate protein kinases
which then phosphorylates another protein phosphorylation of a transcription factor
Protein phosphatases dephosphorylate specific protein = off switch for activated proteins
Transcriptional Regulation
changing transcription factors turns genes on (or off) involves transcription factors or
repressors probably mechanism used for
developmental changes
De-Etiolation (“Greening”) Proteins
proteins involved in making chlorophyll precursors or certain plant hormones are either synthesized or activated
Plant Hormoneshormone (Gr): to excite
chemical messengers produced in 1 part of organism & transported to other parts
bind to specific receptors in target cells trigger responses
some scientists consider plant hormones as plant growth regulators to describe organic cpds (natural or synthetic) that modify or control 1 or more specific physiological processes in a plant
Plant Hormones
control every aspect of plant growth & development to some degree
Phototropism
Tropism: any growth response that results in plant organs curving toward or away from stimuli
Phototropism: growth towards light (+ phototropism) or away from light (- phototropism)
Phototropism
Phototropism & Auxin
Auxin
Auxin
produced in shoot apical meristems & young leaves
high levels found in developing fruits & seedsFunctions:stimulates cell elongationpromotes formation of lateral & adventitious
rootsregulates development of fruitenhances apical dominancefunctions in phototropism & gravitropismpromotes vascular differentiation
Abscisic Acid (ABA)
can be made in all plant cells, found in all plant tissues
Functions:inhibits growthpromotes stomata closure during drought
stresspromotes seed dormancy, leaf senescencepromotes desiccation tolerance
ABA
Cytokinins
mostly made in roots transported upFunctions:regulate cell division in shoots & rootsmodify apical dominancepromotes lateral bud growthpromotes movement of nutrients to sink
tissuesstimulates seed germinationdelays leaf senescence
Gibberellins
found in meristems of apical buds & rootsyoung leaves, & developing seeds are
primary sites of productionFunctions:stimulate stem elongation, pollen
development, pollen tube growth, fruit growth, & seed development & germination
regulate sex determination & transition from juvenile to adult phases
Gibberellins
Brassinosteroids
found in all plant tissues, several types, act locally
Functions:in shoots promote cell divisionpromote root growth when low
concentrations, when high retard growthpromote xylem differentiationinhibit phloem differentiationpromote seed germination & pollen tube
formation
Brassinosteroids
Strigolactones
carotenoid-derived made in roots in response to low phosphate levels or high auxin flow from shoots
Functions:promote seed germinationcontrol of apical dominancecontrols mycorrhizal fungi attraction to root
Strigolactones
Ethylene
gas, produced by most parts of plantamt increases as plant ages or during
ripening of fruit or if plant wounded or stressed
Functions:promotes ripening of most fruitsenhances rate of senescencepromotes root & root hair formationpromotes flowering in pineapple family
Hormones that Affect Seed Germination
Plant Responses to Light
Photomorphogenesis: the effects of light on plant morphology
effects of light on plants includes: photosynthesis triggers key events in plant growth &
development allows plants to measure the passage of
days & seasons
Plant Responses to Light
plants detect the presence or absence intensity direction wavelength (color) of light
Action Spectrum
depicts the relative effectiveness of different wavelengths of radiation in driving a particular process
useful in studying any process that depends on light (phototropism included)
by comparing action spectra with absorption spectra of pigments close correspondence for a given pigment suggests the pigment is the photoreceptor mediating the response
2 Major Classes of Light Receptors
action spectra reveal that red & blue light most important colors in regulating a plant’s photomorphogenesis
Major Classes of light receptors:1. Blue-light photoreceptors2. Phytochromes
Blue-Light Photoreceptors
Functions:phototropismopening of stomataslowing of hypocotyl elongation that occurs
when seedling breaks ground
Blue-Light Photoreceptors
plants use 3 or more photoreceptors to detect blue light
1. Cryptochrome similar to DNA repair enzymes inhibits stem elongation
2. Phototropin protein kinase mediates phototrophic
curvature3. Zeaxanthin
with #2 stoma opening
Phytochromes as Photoreceptors
act like molecular “on/off” switchesred light turns them onfar-red light turns them offregulate:
shade avoidance germination of many seeds
Photoreversible States of Phytochrome
Phytochromes & Shade Avoidance
provides plant with information about quality of light
during daylight hrs amt of red & far-red light ~= plants use ratio of the 2 to determine
quality of light ex: tree under canopy getting more far-red
than red light uses its resources to grow taller/ tree getting mostly red light will use resources to grow bushier
Circadian Rhythms
cycles with ~ 24 frequencies not affected by any known environmental variables
Photoperiodism
regulates time of flowering in many species:1. Short-Day Plants
require a night longer than some critical value to flower
ex: mums, poinsiettias, some varieties of tobacco, soy beans
2. Long-Day Plants need night length shorter than some
critical value to flower ex: spinach, radish, lettuce, irises, many
cereals
Photoperiodism
3. Day-Neutral Plants unaffected by photoperiod
flower when reach certain maturity ex: tomatoes, rice, dandelions
Photoperiodism
some plants require 1 single exposure to photoperiod required to flower
others need several successive days of required times
& others only respond to photoperiod if previously exposed to some environmental stimulus (period of cold or warm weather)
Vernalization: period of cold b/4 flowering
Vernalization
Florigen
flowering signal, probably a protein, made in leaves under certain conditions
travels shoot apical meristems inducing them to switch from vegetative reproductive growth
Gravitropism
bending of an organ in response to gravityroots show + gravitropismshoots show – gravitropismstratoliths (starch-filled plastids) enable roots
to detect gravity
Thigmotropism
growth response to touch changes in plant form due to mechanical
pertubationplants very sensitive to mechanical stressex:
measuring leaf length affects its future growth
Thigmotropism
rapid leaf movements involve transmission of electrical impulses called action potentials resemble nervous system action potentials but thousands times slower
Environmental Stresses
Environmental Stress Major ResponseDROUGHT ABA production, reducing water
loss by closing stomataFLOODING Formation of air tubes that help
roots survive O2 deprivationSALT Avoiding osmotic water loss by
producing solutes tolerated @ high concentrations
HEAT Synthesis of heat-shock proteins , which reduce protein denaturation @ high temperatures
COLD Adjusting membrane fluidity, avoiding osmotic water loss, producing antifreeze proteins
Responses to Attacks by Herbivores
plants release chemicals that are distasteful or toxic
ex: canavanine an unusual a.a., similar to
arginine insect that eats plant incorporates this a.a.
in place of arginine which adversely affects protein shape alters functions insect dies
jackbean
Defenses Against Herbivores
some plants able to attract predatory animals that help defend plant against herbivores
Ex: parasitoid wasps inject their eggs into caterpillars eating
plant eggs hatch inside & larvae eat their way
out
Defenses Against Herbivores
chemicals released in response to herbivores can also function as early warning system for nearby plants of same species
in response neighboring plants release biochemical responses making them less vulnerable to attack
ex: strawberries, lima beans
Defenses Against Pathogens
Hypersensitive Response: seals off infection & destroys both pathogen & infected host cells in region is localized & specific infected area release antimicrobial
molecules use methlysalicyclic acid as signaling
chemical to rest of plant rest of plant then activates systemic
acquired resisitance
System Acquired Resistance
nonspecific protection against diverse pathogens