Plant growth responses Image Credit: Sunflower (Helianthemum) Sunflower (Helianthemum)

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.


ColeoptilesThe protective sheath round the first leaf of a grass embryo.

Image Credit: Oat coleoptiles (Avena)


Charles Darwin 1880

Unidirectional light


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.


Boyson-Jensen 1910-13

Unidirectional light

Mica

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.


Paal 1914-18 in the DARK

Cut

Cut filled with gelatinGelatin

Mica

Tip cut and displaced


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.


Fritz Went 1926 in the DARK

Agar gel

Decapitated

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.© 2010 Paul Billiet ODWS

Indoleacetic acid (IAA) A derivative of the

amino acid tryptophan.

IAA


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.


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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Plant growth responses Image Credit: Sunflower (Helianthemum) Sunflower (Helianthemum)