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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 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. © 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. © 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. © 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. © 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. © 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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