phytohormones and their functions
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Chapter 11 – Plant responses to hormones & environmental
stimuli
Responses include– Developmental transitions
Dormancy Germination Flowering
– Growth
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Hormones & environmental signals involve signal
transduction pathways
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Internal and external signals
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Hormones influence gene expression
Gene expression regulated by– microRNAs– transcription factors
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Plant hormones & growth
(abscisic acid)
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Hormones interact to promote/inhibit development
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Auxins
tryptophan
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Responses involving auxin
Phototropism Gravitropism Cellular elongation Initiation of leaf primordia Apical dominance Root development Fruit development
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Tropisms Permanent, directional growth in
response to an external stimulus
– Positive tropisms– Negative tropisms
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Phototropism Stems are positively phototropic
How can plants grow towards light?
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Auxin and cell elongation
Acidified cell walls have increased elasticity
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Phototropism research
Phototropin (NPH1) and phototropism-initiates a signal transduction pathway-nph1 mutants non-phototropic
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Gravitropism
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Gravitropism
How can it be demonstrated that auxin is involved in gravitropism?
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Gravitropism and root cap amyloplasts
Gravity regulated auxin transport, Ottenschlaumlger, Iris et al. (2003) Proc. Natl. Acad. Sci. USA 100, 2987-2991
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Auxin and initiation of leaf primordia
Pin mutant link
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Responses involving auxin
Apical dominance
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Responses involving auxin
Formation of adventitious roots
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Auxin produced by seeds promotes ovary tissue growth
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Plant hormones
1. Are proteins encoded for by genes
2. Act individually to bring about changes in plant development
3. Function as receptors for environmental signals
4. Both 1 and 35. None of these
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Auxin
1. Prevents apical dominance2. Is produced in shoot apical
meristems3. Promotes seed development
inside fruit4. All of these5. None of these
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Phototropin
1. Is a type of auxin2. Promotes apical dominance3. Is involved in stem growth
towards light4. Is produced by seeds5. All of these
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Cytokinins – cell division and differentiation
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Cytokinin & tissue culture
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From callus to somatic embryos
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Gibberellins
Promotes:• Germination• Stem elongation• Flowering• Fruit development
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Gibberellins
Breaking dormancy– Seed germination
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Gibberellins
Promotes cell division & elongation
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Gibberellins
Promotes bolting in biennials
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Gibberellins
Promotes:• Germination• Stem elongation• Flowering• Fruit development
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Gibberellin is part of a complex signal transduction pathway
(see supplemental reading, for related information)
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Della proteins restrain growth– GA and GID2 degrade Della proteins
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Gibberellins and germination
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Gibberellin promotes vegetative growth
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Abscisic acid Inhibits growth Promotes dormancy Closes stomata
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Abscisic acid – inhibits germination
– Promotes dormancy– Leached by imbibition
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ABA and stomatal closure
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ABA delays flowering
FCA – an RNA binding protein
FY – an mRNA processing factor
Flowering Locus C – a flowering repressor
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Ethylene (CH2=CH2)
Fruit ripening (promotes) Flowering (inhibits) Abscission (promotes) Sex expression in monoecious
species (ratio of ♀ to ♂) Thigmomorphogenesis (reduced
stem elongation in some environments)
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Thigmomorphogenesis
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Brassinosteroids (BRs)
60 types, brassinolide most common Stimulates cell elongation, leaf expansion BR mutants – extreme dwarfs, small
crinkled leaves– Dark grown BR mutants – de-etiolated
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Plant Genes on Steroids Science, Vol 307, Issue 5715, 1569-1570 , 11 March 2005
BIN2 catalyzes breakdown of BES1 & BZR2 proteins (phosphorylation)BR regulates activity of key growth transcription factors
-BES1(activator)-BZR1(repressor)
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Figure 13.12 (p.290)
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Figure 13.12 (p.290)
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Plant Genes on Steroids Science, Vol 307, Issue 5715, 1569-1570 , 11 March 2005
BIN2 catalyzes breakdown of BES1 & BZR2 proteins (phosphorylation)BR regulates activity of key growth transcription factors
-BES1(activator)-BZR1(repressor)
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Responses to environmental stimuli: light
Phototropism Stomata opening Stem elongation Photodormancy (photoblastism) Photoperiodism
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Phytochrome Phytochromes are proteins with a
light absorbing pigment attached (chromophore) – Mediates stem elongation, seed
germination, timing of flowering
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Phytochrome structure
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Two forms of phytochrome
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Phytochrome & stem growth
•Etiolation occurs in low light or dark …why?
•Does Pfr inhibit or promote stem elongation?
Phytochrome and hormonal control of stem elongation
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Phytochrome and seed germination
Photodormancy & photoblastic seeds– Germination activated by light
Some plants, by red light Some plants, by far-red light
Negative photoblastism (tomato), Pfr inhibits germinationPositive photoblastism (lettuce), Pfr promotes germination
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Lettuce is positively photoblastic
30-60% lettuce seed germinate in dark 85-95% lettuce seed germinate in light
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Phytochrome, photoperiodism & flowering
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Manipulation of photoperiod
Poinsettia industry Chrysanthemums
Why won’t my Christmas cactus bloom?
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Brassinosteroids
1. Promote seed germination in response to light
2. Promotes flowering in response to day length
3. Are proteins with an attached light absorbing chromophore
4. Regulate transcription factors involved in growth
5. All of these
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Which of the following is true of phytochrome?
1. Pfr absorbs red light and Pr absorbs far red light
2. Pr is the active form of phytochrome and Pfr is the inactive form of phytochrome
3. Pfr promotes germination in seeds requiring light
4. All of these 5. None of these
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Photoperiodism1. Determines seed
dormancy/germination in response to light/dark
2. Determines flowering in response to day length
3. Is a protein with an attached light absorbing chromophore
4. Controls stem growth in response to light/dark
5. All of these
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Circadian rhythms – sleep movements (nyctinasty)
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Nyctinasty
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Solar tracking (heliotropism)
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Response to mechanical stimuli (seismonasty)
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Seismonasty – Mimosa pudica
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Seismonasty
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Seismonasty
Venus flytrap
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Response to environmental stimuli:
Induced resistance Herbivore attack, systemin (18aa
polypeptide hormone) & jasmonic acid (1-alpha, 2-beta-3-oxo-2-(cis-2-pentenyl)-cyclopentane
acetic acid)
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Figure 1 Model for the activation of defense genes in tomato in response to wounding and insect attack. After wounding, systemin is released from its precursor prosystemin by proteolytic processing. Systemin subsequently binds a membrane-bound receptor to initiate an intracellular signaling cascade, including the activities of a MAP kinase, a phospholipase, a calcium dependent protein kinase, an extracellular alkalinization, and the release of linlenic acid from membranes. Linolenic acid is converted to jasmonic acid, a messenger for early defense gene activation. Catalytic activity of polygalaturonase, an early gene, leads to generation of hydrogen peroxide acting as a second messenger for late gene activation. R, receptor; MAPK, MAP kinase; Ca2+PK; calcium dependent protein kinase; PLA2, phospholipase A2; LA, linolenic acid; JA, jasmonic acid; pm, plasma membrane.
H2O2 prevents cell wall digestion by fungal pectinases
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Response to environmental stimuli:
Induced resistance
Pathogens & the hypersensitive response (HR)
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HR response & systemic acquired resistance (SAR)
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SAR responses Lignification of cell walls Antimicrobial molecules
– PR-proteins (pathogen related proteins)
– Chitinases– Phytoalexins (inhibit protein
synthesis
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SAR model