Plant Responses to
Internal & External Signals
CAMPBELL & REECE
CHAPTER 39
Stimuli & a Stationary Life
observation about plants:
 1 part of plant sends signals to another
 sense gravity & direction of light
 responds to environmental stimuli & internal signals
Stimuli & the Stationary Life
 animals have behavioral responses to stimuli
 plants 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 to
natural daylight. The potato
plant begins to resemble a
typical plant with broad green
leaves, short sturdy stems, and
long roots. This transformation
begins with the reception of
light 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
proteins
2. 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 Hormones
 hormone (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 & seeds
Functions:
 stimulates cell elongation
 promotes formation of lateral & adventitious roots
 regulates development of fruit
 enhances apical dominance
 functions in phototropism & gravitropism
 promotes vascular differentiation
Abscisic Acid (ABA)
 can be made in all plant cells, found in all plant





tissues
Functions:
inhibits growth
promotes stomata closure during drought stress
promotes seed dormancy, leaf senescence
promotes desiccation tolerance
ABA
Cytokinins
 mostly made in roots  transported up
 Functions:
 regulate cell division in shoots & roots
 modify apical dominance
 promotes lateral bud growth
 promotes movement of nutrients to sink tissues
 stimulates seed germination
 delays leaf senescence
Gibberellins
 found in meristems of apical buds & roots
 young leaves, & developing seeds are primary sites of
production
 Functions:
 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 division
 promote root growth when low concentrations, when
high retard growth
 promote xylem differentiation
 inhibit phloem differentiation
 promote 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 germination
control of apical dominance
controls mycorrhizal fungi attraction to root
Strigolactones
Ethylene
 gas, produced by most parts of plant
 amt increases as plant ages or during ripening of





fruit or if plant wounded or stressed
Functions:
promotes ripening of most fruits
enhances rate of senescence
promotes root & root hair formation
promotes 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 photoreceptors
2. Phytochromes
Blue-Light Photoreceptors
 Functions:
 phototropism
 opening of stomata
 slowing 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 curvature
3. Zeaxanthin
 with #2  stoma opening
Phytochromes as Photoreceptors
 act like molecular “on/off” switches
 red light turns them on
 far-red light turns them off
 regulate:
 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:
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
1.
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 gravity
 roots show + gravitropism
 shoots show – gravitropism
 stratoliths (starch-filled plastids) enable roots to
detect gravity
Thigmotropism
 growth response to touch
 changes in plant form due to mechanical pertubation
 plants very sensitive to mechanical stress
 ex:
 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 Response
DROUGHT
ABA production, reducing water loss by
closing stomata
FLOODING
Formation of air tubes that help roots
survive O2 deprivation
SALT
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