Plant responses to hormones & environmental stimuli Responses include – Developmental transitions Dormancy Germination Flowering – Growth
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Plant responses to hormones & environmental stimuli
Responses include– Developmental transitions
Dormancy Germination Flowering
– Growth
Hormones & environmental signals involve signal
transduction pathways
Internal and external signals
Hormones influence gene expression
Gene expression regulated by– microRNAs– transcription factors
Plant hormones & growth
(abscisic acid)
Hormones interact to promote/inhibit development
Auxins
tryptophan
Responses involving auxin
Phototropism Gravitropism Cellular elongation Initiation of leaf primordia Apical dominance Root development Fruit development
Tropisms Permanent, directional growth in
response to an external stimulus
– Positive tropisms– Negative tropisms
Phototropism Stems are positively phototropic
How can plants grow towards light?
Auxin and cell elongation
Acidified cell walls have increased elasticity
Phototropism researchPhototropin (NPH1)
and phototropism-initiates a signal transduction pathway-nph1 mutants non-phototropic
Gravitropism
Gravitropism
How can it be demonstrated that auxin is involved in gravitropism?
Gravitropism and root cap amyloplasts
Gravity regulated auxin transport, Ottenschlaumlger, Iris et al. (2003) Proc. Natl. Acad. Sci. USA 100, 2987-2991
Auxin and initiation of leaf primordia
Pin mutant link
Responses involving auxin
Apical dominance
Responses involving auxin
Formation of adventitious roots
Auxin produced by seeds promotes ovary tissue growth
Plant hormones1. Are proteins encoded for by
genes2. Act individually to bring about
changes in plant development3. Function as receptors for
environmental signals4. Both 1 and 35. None of these
Auxin1. Prevents apical dominance2. Is produced in shoot apical
meristems3. Promotes seed development
inside fruit4. All of these5. None of these
Phototropin 1. Is a type of auxin2. Promotes apical dominance3. Is involved in stem growth
towards light4. Is produced by seeds5. All of these
Cytokinins – cell division and differentiation
Cytokinin & tissue culture
From callus to somatic embryos
Gibberellins
Promotes:• Germination• Stem elongation• Flowering• Fruit development
Gibberellins Breaking dormancy
– Seed germination
Gibberellins
Promotes cell division & elongation
Gibberellins Promotes
bolting in biennials
Gibberellins
Promotes:• Germination• Stem elongation• Flowering• Fruit development
Gibberellin is part of a complex signal transduction pathway
(see supplemental reading, for related information)
Della proteins restrain growth– GA and GID2 degrade Della proteins
Gibberellins and germination
Gibberellin promotes vegetative growth
Abscisic acid Inhibits growth Promotes dormancy Closes stomata
Abscisic acid – inhibits germination
– Promotes dormancy– Leached by imbibition
ABA and stomatal closure
ABA delays flowering
FCA – an RNA binding protein
FY – an mRNA processing factor
Flowering Locus C – a flowering repressor
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)
Thigmomorphogenesis
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
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)
Figure 13.12 (p.290)
Figure 13.12 (p.290)
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)
Responses to environmental stimuli: light
Phototropism Stomata opening Stem elongation Photodormancy (photoblastism) Photoperiodism
Phytochrome Phytochromes are proteins with a
light absorbing pigment attached (chromophore) – Mediates stem elongation, seed
germination, timing of flowering
Phytochrome structure
Two forms of phytochrome
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
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
Lettuce is positively photoblastic
30-60% lettuce seed germinate in dark 85-95% lettuce seed germinate in light
Phytochrome, photoperiodism & flowering
Manipulation of photoperiod
Poinsettia industry Chrysanthemums
Why won’t my Christmas cactus bloom?
Brassinosteroids1. Promote seed germination in
response to light2. Promotes flowering in response to
day length3. Are proteins with an attached light
absorbing chromophore4. Regulate transcription factors
involved in growth5. All of these
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
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
Circadian rhythms – sleep movements (nyctinasty)
Nyctinasty
Solar tracking (heliotropism)
Response to mechanical stimuli (seismonasty)
Seismonasty – Mimosa pudica
Seismonasty
Seismonasty Venus flytrap
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)
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
Response to environmental stimuli:
Induced resistance Pathogens & the hypersensitive
response (HR)
HR response & systemic acquired resistance (SAR)
SAR responses Lignification of cell walls Antimicrobial molecules
– PR-proteins (pathogen related proteins)
– Chitinases– Phytoalexins (inhibit protein
synthesis
SAR model
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