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

2. 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

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

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

5. Charles Darwin 1880
Unidirectional light
© 2016 Paul Billiet ODWS

6. 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

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

8. 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

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

10. Paal
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

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

12. 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

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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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