Plant growth responses
Image Credit: Sunflower (Helianthemum)
Plant show behaviour too
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Usually they respond on a different time scale to
animals
They show growth responses
Tropisms
Phototropism
Geotropism
Hydro- or chemotropism
Thigmotropism
Positive tropism = growth towards the stimulus
Negative tropism = growth away from the
stimulus.
© 2010 Paul Billiet ODWS
Plant growth substances
Hormones
 Florigen
Gibberellin
Abscisic acid (ABA)
Cytokinin.
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© 2010 Paul Billiet ODWS
Coleoptiles
The protective sheath
round the first leaf of a
grass embryo.
Image Credit: Oat coleoptiles (Avena)
© 2010 Paul Billiet ODWS
Charles Darwin 1880
Unidirectional light
© 2010 Paul Billiet ODWS
Charles Darwin 1880
Coleoptile tips are positively
phototropic
 Bends behind the tip = zone of cell
elongation
 Tip is the location of the sensor
 The zone of cell elongation is the
effector
 Covered tips = taller growth
 Tip removed = no growth.
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© 2010 Paul Billiet ODWS
Boyson-Jensen 1910-13
Unidirectional light
Mica
© 2010 Paul Billiet ODWS
Gelatin
Boyson-Jensen 1910-13
Signal passes down the coleoptile
 Signal is water soluble
 Dark or shaded side elongates
 Signal passes down the DARK/SHADED
side
 The substance is a growth PROMOTOR.
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© 2010 Paul Billiet ODWS
Paal 1914-18 in the DARK
Tip cut and
displaced
Gelatin
Mica
© 2010 Paul Billiet ODWS
Cut filled
with gelatin
Cut
Paal 1914-18
In the dark
 Replicated the responses in the absence of
the stimulus
 Cut blocks the movement of the signal
 Differences in the concentration of the
signal on the two sides
 Result in differences of elongation
 Results in bending.
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© 2010 Paul Billiet ODWS
Fritz Went 1926 in the DARK
Decapitated
Agar gel
© 2010 Paul Billiet ODWS
Agar placed on one side of a
decapitated coleoptile in the dark
Fritz Went 1926
Grew faster on the side with the agar
block
 Degree of bending proportional to signal
strength
 Called the signal AUXIN
 Auxins are a group of chemicals
 Indoleacetic acid (IAA) purified
 Found naturally in plants in very small
quantities (and urine)
 It moves slowly through the tissues
1cm h-1.
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© 2010 Paul Billiet ODWS
Indoleacetic acid (IAA)
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A derivative of the
amino acid
tryptophan.
IAA
© 2010 Paul Billiet ODWS
The action of auxin at the cellular level
Acid growth hypothesis – rapid response
 Causes rapid pumping of H+ out of the
plasma membrane
 Acidifies cell wall
 Hydrolyses bonds between cellulose fibrils
 Loosens cell wall
 Cell swells under turgor pressure.
© 2010 Paul Billiet ODWS
The action of auxin at the cellular level
The regulation of gene expression – slow
response
 Auxin affects about 10 genes that
determine cell growth
 Acts at the transcription of these genes.
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© 2010 Paul Billiet ODWS