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

PhototropismGeotropismHydro- or chemotropismThigmotropism

Positive tropism = growth towards the stimulus Negative tropism = growth away from the

stimulus.

© 2016 Paul Billiet ODWS

Plant growth substances Hormones Auxin

FlorigenGibberellinAbscisic acid (ABA)Cytokinin.

© 2016 Paul Billiet ODWS

ColeoptilesThe protective sheath round the first leaf of a grass embryo

Oat coleoptiles (Avena)© 2016 Paul Billiet ODWS

Charles Darwin 1880Unidirectional 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 1910-13Unidirectional light

Mica

Gelatin

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

Cut

Cut filled with gelatinGelatin

Mica

Tip cut and displaced

© 2016 Paul Billiet ODWS

Paal 1914-18In 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

Agar gel

Decapitated

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 levelAuxin 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 levelThe 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 levelAcid 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.

© P BillietPropagation of African violet (Saintpaulia)© 2016 Paul Billiet ODWS

MicropropagationPlant cell division occurs in meristems (apex, root tip, or cambium).

© P BillietPropagating from chichoree (Cichorium intybus) leaf fragments

© 2016 Paul Billiet ODWS

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