Plant growth responses Sunflower (Helianthemum) © 2016 Paul Billiet ODWS

Plant show behaviour too Plants 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. © 2016 Paul Billiet ODWS

Plant growth substances Hormones Auxin Florigen Gibberellin Abscisic acid (ABA) Cytokinin. © 2016 Paul Billiet ODWS

Coleoptiles The protective sheath round the first leaf of a grass embryo Oat coleoptiles (Avena) © 2016 Paul Billiet ODWS

Charles Darwin 1880 Unidirectional light © 2016 Paul Billiet ODWS

Charles Darwin 1880 Coleoptile tips are positively phototropic Bends behind the tip = zone of cell elongation Tip = the location of the sensor The zone of cell elongation = the effector Covered tips  taller growth and no bending Tip removed  no growth. © 2016 Paul Billiet ODWS

Boyson-Jensen 1910-13 Unidirectional light Gelatin Mica Mica © 2016 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. © 2016 Paul Billiet ODWS

Paal 1914-18 in the DARK Tip cut and displaced Cut filled with gelatin Mica Cut © 2016 Paul Billiet ODWS

Paal 1914-18 In the dark Replicated the responses in the absence of the stimulus Cut stops the movement of the signal Differences in the concentration of the signal on the two sides Result = differences in elongation Coleoptile bends. © 2016 Paul Billiet ODWS

Fritz Went 1926 Decapitated Agar gel Agar placed on one side of a decapitated coleoptile in the dark © 2016 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 © 2016 Paul Billiet ODWS

Indoleacetic acid (IAA) A derivative of the amino acid tryptophan IAA © 2016 Paul Billiet ODWS

The action of auxin at the cellular level Auxin efflux pumps Cells nearer a light source pump auxin out to cells further from the light Membrane efflux pump PIN3 protein Active transport Low R:FR light ratio, typical of shade, stimulates the synthesis of PIN3 © 2016 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 Auxin helps block repressors of growth stimulating genes The growth stimulating genes are free to encourage cell growth. © 2016 Paul Billiet ODWS

The action of auxin at the cellular level Acid growth hypothesis – rapid response Auxin modification of gene expression 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 Plant cell elongates Stem bends towards light. © 2016 Paul Billiet ODWS

Micropropagation Many plants are easy to culture from cuttings Plant hormones control growth and development Cocktails of these hormones in an agar gel are used to clone plants. Propagation of African violet (Saintpaulia) © 2016 Paul Billiet ODWS © P Billiet

Micropropagation Plant cell division occurs in meristems (apex, root tip, or cambium). © 2016 Paul Billiet ODWS Propagating from chichoree (Cichorium intybus) leaf fragments © P Billiet

Micropropagation Dissect out a piece of meristem (explant) Surface sterilise the fragment Check to verify it is virus-free (ELISA test) Generate callus (undifferentiated tissue mass) on agar medium. © 2016 Paul Billiet ODWS

Micropropagation Cut and multiply up callus fragments Eventually transfer to a different medium to encourage differentiation Transfer plantlet to liquid medium or soil for hardening up. © 2016 Paul Billiet ODWS

Applications Producing clones of varieties that are: rare (protected species) difficult to grow (e.g. orchids) products of GM Producing virus-free strains Bulking up new varieties. © 2016 Paul Billiet ODWS