Chapter 39: Plant responses to internal &
external signals
Plants=
sessile
must respond to environmental changes/cues by adjusting
patterns of growth & development

Signal transduction pathways in plants
(ex. De- etiolation/greening)
Environmental
Ex.
signal conformational change in protein
Phytochrome (photoreceptor) in cytoplasm detects light
Second
messengers amplify signal
Ex. Phytochrome activation increases levels of cGMP
Ca2+ changes in ion channels activating protein
&
kinasesactivate other enzymes
Regulation
usually
of one or more cell activities
by activating enzymes or stimulating gene transcription
Ex. Secondary messengers activate transcription factors
stimulate photosynthesis enzymes, chlorophyll production,
or affect hormones that regulate growth
Plant hormones & their responses
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Hormones=chemical signals that
coordinate different parts of an organism
Types:
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Auxin (AA)
Cytokinins
Gibberellins
Brassinosteroids
Abscisic acid (ABA)
Ethylene
Auxin (AA)/Indoleacetic acid (IAA)

Functions:
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Stimulate elongation of cells in young shoot
Stimulates proton pumps action potential
activates expansin enzymesallow cell membrane
to expandmore H2O enters
 Increases gene expression of growth proteins
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Induces cell division in vascular cambium &
differentiation of secondary xylem
Regulates fruit development
Produced in:
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Embryo of seed
Apical meristems
Cytokinins
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Functions:
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Stimulates cytokinesis
Influences differentiation
Stimulates germination
keeps plant green longer
Works in conjunction with auxin
Most common cytokinin=zeatin
Produced in:
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Actively growing tissues, particularly roots, embryos,
& fruit
Gibberellins
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Functions:
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Elongate stems (stimulates expansin enzymes)
Stimulate leaf growth
Stimulate fruit growth
Stimulate seed germination (break dormancy)
Produced in:
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Roots
Young leaves
Embryos of seed
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Functions:
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Brassinosteroids
Induce cell elongation & division in stem segments &
seedlings
Slow leaf abscission
Promote xylem differentiation
Inhibit root growth
Chemically similar to cholesterol & animal sex
hormones
Produced in:
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Seeds
Fruit
Shoots
Leaves
Floral buds
Abscisic Acid (ABA)
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Functions:
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Produced in:
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Inhibits growth
Closes stomata during H2O stress (alters Ca2+ levels
which effects K+)
Promotes seed dormancy
Leaves
Stems
Roots
Green fruit
Slows growth
Inactivated by large amounts of H2O, light or
prolonged cold exposure
ABA to giberellin ratio often determines whether
seed germinates or remains dormant
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Functions:
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Promote fruit ripening (attracts animals to spread
seeds)
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Causes enzymes to break down cell walls to soften fruit
Causes enzymes to convert starches to sugars for taste
Activates scent & color to display ripeness
May promote or inhibit development of roots, leaves,
& flowers (dependant on species)
Opposes auxin affects
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Ethylene
Activates enzymes to break down chemical components of
cells leading to leaf abscission or plant death after flowering
positive feedback mechanism
Produced in:
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Tissues or ripening fruit
Nodes of stems
Aging leaves & flowers
Photomorphogenesis
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effects of light on plant morphology
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Light effects plant growth & development
Plants detect light presence, direction, intensity, &
wavelength (color)
Blue-light photoreceptors
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Initiates responses in plants including
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Phototropism (phototropin blue pigment)
Light induced stomata opening (carotenoid based zaxanthin)
Light induced retardation of hypocotyl elongation upon
emerging from soil (cryptochrome pigment)
Phytochromes photoreceptors
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Photoreceptor switches between Pr & Pfr forms
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When Pr active- growth inhibited; Pfr active- growth
stimulated
Responses include:
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Stimulating seed germination
Aid in tracking seasons
Shade avoidance
Biological clocks & circadian
rhythms
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Physiological processes of plant including
production of photosynthesis enzymes, stomata
opening/closing, raising/lowering of leaves
follow a 24 hour pattern even under controlled
conditions
Circadian rhythms
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Cycle in 24 hour frequency
Not necessarily connected to an environmental
variable
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Without environmental signal rhythm occurs in a 21-27 hour
frequency
Light places rhythms on a 24 hour cycle
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In plants a result of blue light photoreceptors &
phytochrome
Light causes phytochrome to switch between Pr & Pfr
Allow plants to adjust to seasonal differences in day &
night
Photoperiodism
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Physiological response of plant to photoperiod
(relative length of day or night)
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Seasonal responses of plant
Short day plants (long night)
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Long day plants (short night)
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Flower only if light period is longer than critical length
Flower in late spring or summer
Day neutral plants
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Flower only if light period is shorter than critical length
Flower in late summer, fall, or winter
Flowering unaffected by photoperiod
***Plants really respond to night length not day
length as names suggest***
Leaves detect photoperiod
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signal buds to develop flowers through signal florigen
Florigen activates organ identity genes
Other environmental influences on
plant growth: abiotic
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Gravity
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Shoot grows upward (negative gravitropism)
Roots grow downward (positive gravitropism)
Statoliths (starch dense plastids) settle at lower
portions of cell triggering redistribution of Ca2+ &
thereby affecting auxin
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Help determine up vs. down in plant
Mechanical stimuli
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Thigmomorphogenesis- plant changes from due to
mechanical perturbances
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Viny plants grasp & grow around supports it encounters to
stabalize itself
Touch sensitive plants can send action potentials similar to
animal nerve impulses to neighboring organs
Other environmental influences on
plant growth: abiotic
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Drought
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Flooding
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Low H2O in leaf- guard cells lose turgor- stomata close
H2O deficit increases abscisic acid in leaf to keep stomata closed
Cell expansion inhibited by lack of H2O; young leaves do not
grow- decreases surface area for H2O loss
Conserves H2O but minimizes photosynthesis thereby decreasing
crop yield
Too much H2O suffocates roots- no O2 for respiration
Excess H2O- increased ethylene – root cortex undergoes
apoptosis creating air tubes for O2 to reach submerged roots
Salt stress
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Too much salt lowers H2O potential of soil causing roots to lose
H2O to soil
Plant responds by producing organic compound solutes that
keep Y of cells lower than soil
Other environmental influences on
plant growth: abiotic
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Heat stress
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High heat denatures enzymes of plant, damaging
plant metabolism
Transpiration lowers leaf temps through evaporative
cooling
Heat shock proteins produced which protect enzymes
from denaturing
Cold stress
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Cold causes problems with membrane fluidity &
transport
Plants increase unsaturated fatty acids & membrane
solutes to prevent dehydration & keep membrane
fluid
Other environmental influences on
plant growth: biotic
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Defenses against herbivores
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Thorns
Chemical defenses
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Distasteful or toxic compound production
Chemicals that attract defensive predators
Defenses against pathogens
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First line: epidermis & periderm
Second line: chemicals released to kill pathogen &
prevent its spread from infection site
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R genes produce proteins that bind to pathogen protein
Elicitors produced
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Oligosaccharins- released from damaged cell wall to
stimulate production of antimicrobial phytoalexins
PR proteins activated that directly attack pathogen
Stimulate strengthening of cell wall to prevent spread