Plant growth responses
Image Credit: Sunflower (Helianthemum)
Plant show behaviour too Usually they respond on a different time scale to
animals They show growth responses Tropisms
PhototropismGeotropismHydro- or chemotropismThigmotropism
Positive tropism = growth towards the stimulus Negative tropism = growth away from the
stimulus.
© 2010 Paul Billiet ODWS
Plant growth substances Hormones Florigen
GibberellinAbscisic acid (ABA)Cytokinin.
© 2010 Paul Billiet ODWS
ColeoptilesThe 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.
© 2010 Paul Billiet ODWS
Boyson-Jensen 1910-13
Unidirectional light
Mica
Gelatin
© 2010 Paul Billiet ODWS
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.
© 2010 Paul Billiet ODWS
Paal 1914-18 in the DARK
Cut
Cut filled with gelatinGelatin
Mica
Tip cut and displaced
© 2010 Paul Billiet ODWS
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.
© 2010 Paul Billiet ODWS
Fritz Went 1926 in the DARK
Agar gel
Decapitated
Agar placed on one side of a decapitated coleoptile in the dark
© 2010 Paul Billiet ODWS
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.© 2010 Paul Billiet ODWS
Indoleacetic acid (IAA) 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.
© 2010 Paul Billiet ODWS
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