Phototropism is the growth of plants in response to light. Most shoots grow towards light (positive phototropism) while roots usually grow away from it (negative phototropism). Early experiments showed that the plant hormone auxin is involved, moving to the shaded side of plants and causing cells there to elongate more, bending the plant towards the light source. Specific genes and proteins like PIN3, phototropins, and NPH1/NPL1 help regulate the asymmetric distribution of auxin in response to light and cause the resulting curvature. There are several proposed models for how exactly auxin distribution is altered by light.

3. CONTENTS
• INTRODUCTION
• HISTORY
• MECHANISM
- Auxin
- Gene action
- Phototropins
• Five models of auxin distribution
• CONCLUSION
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4. INTRODUCTION
• Phototropism is the growth of an organism
which responds to a light stimulus.
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6. • Growth towards a light source is called
positive phototropism, while growth away
from light is called negative phototropism.
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7. • Most plant shoots exhibit positive
phototropism, and rearrange their chloroplasts
in the leaves to maximize photosynthetic energy
and promote growth.
• Roots usually exhibit negative phototropism,
although gravitropism may play a larger role in
root behavior and growth.
• Some vine shoot tips
exhibit negative phototropism
which allows them to grow
towards dark, solid objects
and climb them.
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8. HISTORY OF PHOTOPERIODISM
• The best known early research on phototropism
was by Charles Darwin, who reported his
experiments in a book published in 1880, The
Power of Movement in Plants.
• Darwin studied phototropism in canary grass and
oat coleoptiles.
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9. Darwin conclusions
• The tip of the coleoptile is the most
photosensitive region.
• The middle of the coleoptile is responsible for
most of the bending.
• An influence which causes bending is transmitted
from the top to the middle of the coleoptile.
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10. Boysen-Jensen’s experiment (1913)
• He cut the tips off coleoptiles and placed a thin piece
of mica between the coleoptile and the lower shoot.
• The result was that the shoot did not grow or curve
toward the light.
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11. • When he repeated the experiment using a block of
agar instead, the result was that the shoot grew and
curved towards the light.
Conclusion,
• The chemical signal was a growth stimulant as the
phototropic response involves faster cell elongation
on the shady side than on the illuminated side.
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12. Cholodny-Went model
• The theory that the plant hormone auxin could play
a role in phototropism was first proposed in 1937 by
the Dutch researcher Frits Warmolt Went.
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(IN DARK)

13. • When the agar block was centred on top the
coleoptile grew straight.
• If the agar block was offset, resulting in an uneven
distribution of the chemical on one side, the shoot
would curve as though it was growing towards a light
source.
Conclusions,
• This proved that the response was due to a water
soluble chemical that diffused from the tip of the plant
down the dark / shaded side of the coleoptile causing
it to curve towards the light.
• Went repeated the experiment with agar that had not
been treated, which produced no growth.
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14. MECHANISM
• Several of signaling molecules - determine where
the light source - activates several genes - change
the hormone gradients - allowing the plant to grow
towards the light
• Tip of the plant coleoptile - light sensing occcur
• The middle portion of coleoptile - shoot curvature occurs
• Asymmetric light - AUXIN - move towards the shaded side
and promote elongation of the cells on that side to cause
the plant to curve towards the light source
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AUXIN MECHANISM
• Auxins activate proton pumps.
• Decreases the pH in the cells on the dark side of the
plant
• Activates enzymes known as expansins which break
bonds in the cell wall structure, making the cell walls less
rigid
• Acidic environment causes disruption of hydrogen bonds
in the cellulose that makes up the cell wall
• The decrease in cell wall strength causes cells to swell,
exerting the mechanical pressure that drives phototropic
movement.

17. GENE ACTION
PIN genes
• are auxin transporters - responsible for the
polarization of auxin.
• PIN3 - the primary auxin carrier.
• Receptors receive light and inhibit the activity of
PINOID kinase (PID), which then promotes the
activity of PIN3.
• This activation of PIN3 leads to asymmetric
distribution of auxin, which then leads to asymmetric
elongation of cells in the stem.
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• PIN3 is normally localized
to the surface of hypocotyl
and stem
• pin3 mutants had shorter
hypocotyls and roots than
the wild-type, and the
same phenotype was seen
in plants grown with auxin
efflux inhibitors

19. PHOTOTROPINS
Main phototropins - PHOT1 and PHOT2
• phot1 single mutent - no response in low light.
• phot2 single mutant - as wild type.
• phot1 phot2 double mutants do not show any
phototropic responses.
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• The amounts of PHOT1 and PHOT2 present
depend on the age of the plant and the intensity of
the light.
• AT HIGH LIGHT INTENSITY
-downregulation of PHOT1 transcript
-upregulation of PHOT2 transcript
• Phototropin expression levels change with the
maturation of the leaves (chloroplast rearrangement) –
maximise photosynthesis efficiency.

21. NPH1 and NPL1 gene
• They are both involved in chloroplast
rearrangement.
• The nph1 and npl1 double mutants were
found to have reduced phototropic responses.
• The two genes are both redundant in
determining the curvature of the stem.
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22. Five models of Auxin distribution in phototropism
by Sakai and Haga (2012) on Arabidopsis thaliana
First model
Incoming light deactivates auxin on the light side of
the plant allowing the shaded part to continue growing
and eventually bend the plant over towards the light.
Second model
Light inhibits auxin biosynthesis on the light side of
the plant, thus decreasing the concentration of auxin
relative to the unaffected side.
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23. Third model
There is a horizontal flow of auxin from both the light
and dark side of the plant. Incoming light causes more auxin
to flow from the exposed side to the shaded side, increasing
the concentration of auxin on the shaded side and thus
more growth occurring.
Fourth model
The plant receiving light to inhibit auxin basipetal down
to the exposed side, causing the auxin to only flow down
the shaded side.
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24. Fifth model
Model five encompasses elements of both model 3
and 4.
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deactivates inhibits

25. Effect of Wavelength
• Stem tips exhibit positive phototropic reactions
to blue light, while root tips exhibit negative
phototropic reactions to blue light.
(cryptochromes)
• Both root tips and most stem tips exhibit positive
phototropism to red light (phytochromes )
• Together phytochromes and cryptochromes
inhibit gravitropism in hypocotyls and contribute
to phototropism.
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