Chapter 24
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2
Considered plant hormones because of their
ability to cause dramatic changes in growth
and differentiation at low concentrations
Brassica napus
40 kg pollen → 4 mg
Effects only demonstrable
with exogenous application
until Arabidopsis mutants discovered
10-7 M concentrations produces effects
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Polyhydroxylated sterols
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Sterols are terpenoid
Identified in 27 families
Gymnosperm
 Angiosperm
 Pteridophyte -- Equisetum
 Bryophyte -- Marchantia
 Algal -- Hydrodyction
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Brassinosteroids -- wide range of functions
Stimulation of stem elongation
 Pollen tube elongation
 Stimulation of cell division (with auxin and
cytokinins)
 Seed germination
 Leaf morphogenesis
 Apical dominance
 Inhibition of root elongation
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Brassinosteroids -- wide range of functions
Vascular differentiation
 Accelerated senescence
 Programmed cell death
 Responses to biotic and abiotic stress
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Salt
 Drought
 Temperature
 Pathogens
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Most responses also controlled by auxin
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Promotes both cell expansion and cell
division in shoots
Dwarf mutants show smaller and fewer cells
 Brassinosteroid mutants have few microtubules
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Brassinosteroids cause grass leaves to
become horizontal – just like bending of
stems under effect of auxin!
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Cell expansion kinetics between
brassinosteroids and auxins different
Brassinosteroid – elongation begins after 45
min lag – max rate after several hours
 Auxins – elongation begins after 15 minute lag –
max rate within 45 minutes
 Brassinosteroid may involve gene transcription!
 Auxin-related gene expression may be greater
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Operate synergistically
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Implication each hormone requires the other
Brassinosteroids enhance auxin transport,
stimulation of lateral root growth,
differential responses to gravitropism, etc.
Brassinosteroids increase uptake of water
via aquaporins aiding in cell wall loosening
in cell expansion
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Promotes and inhibits root growth
Concentration dependent
 Low – promote root growth (incl. lateral roots)
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Synergy with auxins in lateral root formation
Promotes gravitropic response
Effects independent of both auxin and
gibberellins
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Promote xylem differentiation
Suppress phloem differentiation
Deficient mutants
Fewer vascular bundles
 Irregular spacing
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Required for pollen tube growth
Pollen excellent source of brassinosteroids
 Both the brassinosteroids and the signalling
pathway required
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Promotes seed germination
Interactions with other hormones
 Can cause tobacco seed germination independent
of GA
 Mutants more sensitive to ABA inhibition
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Fiber development in cotton
Increasing crop yield – especially under
stress conditions
Propagation of woody plants
Improved tissue culture
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Auxins – cell elongation, apical dominance &
tropisms
Gibberellins – regulators of plant height and
seed germination
Cytokinins – regulators of cell division
Abscisic Acid – seed maturation and stress
response
Ethylene – only gaseous hormone
Brassinosteroids – cell expansion and
development