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So if we want to compare, now, between
plants and human photoreceptors.

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We know that humans have four
photoreceptors.

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And that they can see between the blue to
the red spectrum.

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Plants actually are much more complex than
we are, when it comes to light signaling.

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Plants have upwards of 13 photoreceptors.

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They have one that's called UVR8, which we
didn't go into details of.

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Which allows the plant to detect UV light.

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Light that we're blind to.

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.
Plants can see this and respond.

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Plants have upwards of five photo
receptors that detect blue light.

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They have two, what we call phototropins.

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These are now the name of the photo
receptors in blue light.

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A photo receptor for flowering in blue
light.

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And another photoreceptor

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for letting a plant know about seeding
development called cryptochrome.

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Plants also can detect green light.

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And they have multiple photoreceptors for
red and for far red light.

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Actually Arabidopsis has five different
types of phytochromes.

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So that we can see that, from a

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plant's perspective, humans actually may
be somewhat visually dysfunctional.

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'because plants can

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see much more.

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Now, I have to point out here,

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that, while both animals and plants have
photoreceptors.

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While both animals and plants see blue and
red light.

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The photoreceptors that detect and respond
to the light are completely different.

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Animals don't have phytochrome.

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They have something called erythrolabe,
which allows them to see the red light.

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And they don't have phototropin.

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They have another of the blue cones, which
allow them to see blue light.

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These are not the same photoreceptors.

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There's no evolutionary connection between
them.

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But the ability to see light, and respond
to it, is conserved among all organisms.

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So why would a plant need to be so
sensitive to light.

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Well this goes back to the first lecture
where we talked

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about plants being unmovable.
So what is light for a plant?

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Light is its ability to eat.

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Plants use light, sunlight, to power
photosynthesis.

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A plant needs to know where the light is,
so

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that it could do photosynthesis and make
its own food.

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So a plant needs to know where the
direction of its food is, what the

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intensity of its food is, and when it
should be flowering in order to make

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its next generation.

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So because of the plant can't move, it's

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developed the ability to detect both the
direction,

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and the intensity, and the type of light,

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which is much more complex than what
animals have.

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I want to end with one other thing here.

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I was, I talked about how, plant
signaling, or light

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signaling Is not evolescent,
evolutionarily

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connected between plants and animals.

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And that's correct, except for one
molecule, or one example.

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And that's this molecule here called
cryptochrome.

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Cryptochrome is the blue light receptor,
that allows a plant seedling to

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know how to develop in the light.

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Now the name cryptochrome is actually
quite an oxymoron.

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So while plant scientists had known that
there should be

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a blue light receptor, they didn't know
what it was.

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And Johnny Gressel, Professor Johnny
Gressel

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at the Weizmann Institute named it
cryptochrome.

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Because they didn't know what the receptor
was.

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Today we've cloned that gene, and we know
where this blue light receptor

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is, what it looks like but, the name is,
has remained the same.

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Cryptochrome.

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Now you also have chryptochrome in your
bodies.

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So what was the connection between
cryptochrome and plants?

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And cryptohchrome and animals?

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Well there is one way that we are
influenced by light.

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Which is the same way that plants are
influenced by light.

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And that's our biological clocks.

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If you've ever gone through jet lag, you
know

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that being out in the sun helps reset your
clock.

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And actually through evolution, all
organisms have biological clocks.

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And all organisms have to have the ability

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to have their clocks set by the light
environment.

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And this cryptochrome sets a plants clock,
and also sets an

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animals clock, so at least at this level,

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blue light signaling is conserved between
plants and animals.

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To summarize sight, what sight is for a
plant than what it is for a person.

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We respond to about 400-700 nm Whereas
plants respond to a

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spectrum of 300 to about 750 nanometers, a
wider spectrum of vision.

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For us, light signals are sensed in the

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retina, whereas in plants, it's sensed in
all cells.

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Humans have about

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five photoreceptors.
Plants have 12 or 13 photoreceptors.

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Actually, the number is dependent on which
species we're talking about.

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But in all cases, it's more than in
humans.

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For us, it enables primarily identifying
details.

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Details that we see in pictures.

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Plants don't need to see the details, but
for them it enables them to find food.

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Actually, if you think about it, the
details also enable us to find food.

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Light for us synchronizes the biological
clock.

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And also for plants it synchronizes the
biolgical clock; it enables

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a plant to know what season of the year it
is.