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And one of the things that these
people love to remember and

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reminisce about is weather.

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So we're going to talk about
the long term memory of weather.

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It's a common theme for us, we like to
talk about how hot it was last summer.

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We remember great droughts, extremes
in weather, blizzards, hurricanes or

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just plain cold, very cold winters,
or very hot and very dry summers.

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You may be surprised to learn that
plants also remember weather.

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To talk about this, I'm going to bring in
a guest lecturer who's actually an expert

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in long term memory of plants, my friend
and colleague, Professor Nir Ohad.

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Hi, Nir.

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>> Hi.

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>> Thanks for joining us here in
Coursera and helping us with the class.

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>> Sure, it's a pleasant to be here.

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>> Bringing Nir in actually,
for two reasons.

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One, he's an international expert
who does recently wrote a book

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about plant memory and
plant long term memories.

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And second, he's a very close friend and
colleague, and

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we've been working
together almost 30 years.

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And why am I talking about this is because
that some of you may have the idea

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of science as being a solitary endeavor,
when actually it's very social.

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We did our PhDs in the same lab together.

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We made contact when we were
postdoctoral fellows, and

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we're lucky enough that our labs
are right next to each other.

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So that we interact and collaborate and
share ideas all the time.

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So, Nir, quick question.

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>> Yes.
>> Do plants

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have a long term memory of weather?

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>> Yes indeed, and

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that is exactly what we're going
to talk about in this class today.

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>> So, I'm going to leave him now
here to teach you about actually

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the complex mechanisms that
are involved in plant long term memory.

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>> So the question that we're going
to place in this class today is,

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do plants remember
experiencing a cold period?

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Why is it important to monitor
passing through a cold period?

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Plants growing in regions
where summer is short

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need to exploit as much time available
to complete seed production, and

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therefore starting the process
as early as possible is crucial.

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Moreover, it could be risky for
a plant to grow and flower too early

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as winter could still last,
jeopardizing their offsprings.

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As we can see in our next slide,

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where plants have just flowered
into a blizzard of snow.

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As an example, let us examine the case
of most important crop plant, wheat.

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To flower most strains of wheat
need first to undergo a cold winter

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in the variety of winter wheats.

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Planted in fall in sprouts
they get covered by snow and

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only after that they could flower,
making grain in the spring.

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If no winter, no flowers, no grain.

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A severe problem occurred in the early
1920s of the previous century.

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In the former Soviet Union,
warm winters destroyed wheat yields,

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which led to famine.

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Soviet scientist Trofim Lysenko discovered

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1928 that if seeds were put
into a freezer before planting,

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they would flower and make grain
regardless of being exposed to winter.

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Lysenko called this term
grout jarovization.

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To describe a chilling process he
used to make the seed of winter

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cereals behave like spring cereals.

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This term was translated as Vernalization,
from the latin word,

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[FOREIGN] for western text.

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So, the term was going to use
from now on is vernalization to

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express the transition through
winter allowing plants to flower.

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>> So, Nir what your saying is that
the wheat seeds are first put in

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a refrigerator before planting then
that has the same effect on the wheat

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as when the seedling is
exposed to a cold winter?

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In both cases, whether the seed or

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the seedling is exposed to the cold,
the wheat flowers.

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But what does the cold actually do?

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>> It turns out the plants,
like us, use signs or

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marks to assign for
the cold they experience.

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Just to demonstrate we use
different ways to remember ourself

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things not to forget for
example, we turn our watches or

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we write a note such as do not forget.

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Or we just scribe on our hand

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do not forget to remind us of
the things which was important.

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While using all of the above memos we
did not change of ourselves anything.

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How do plants write their
memos to record cold?

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They do it by placing a chemical sign.

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But where?

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This mechanism of writing signs

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was solved only in recent years
using genetic and molecular tools.

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The marks are placed on the DNA or
protein which anchor the DNA.

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There are proteins of which their function
is to place such marks called writers.

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Before going on,
I would like to review some concept, and

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integrate these new information.

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As we should all know,

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our DNA is composed of four types of
building blocks called nucleotides.

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Which are abbreviated G,A,T and

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C which are found in long strands
millions of nucleotides long.

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In a human cell each of these strands
would reach about two meters long.

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Obviously, a two meter long string
could not fit into a microscopic cell,

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let alone in the even
smaller nucleus of the cell.

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The way that the DNA fits in the nucleus
is that it is highly compact,

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the double helix of the DNA wraps
around proteins called histone

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forming what is known as chromatin.

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This wrapping literally caused the DNA to
be arranged in little beads on a string.

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These beads can twist even more,
just like an overlay twist rubber

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band compacting the DNA and proteins into
highly condensed and packed structures.

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These structures are dynamic.

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Different parts of the chromatin
can unravel or pack up again.

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The importance of the packing
is that when genes are found in

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highly compacted regions,
they are usually turned off.

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The genes are inactive.

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On the other hand when a gene
is in a relatively open,

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less compact region,
then it can be turned on.

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Genes that are turned on,

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those that are transcribed, are found in
areas of the chromatin that are unraveled,

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while genes that are turned off are in
regions that are more condensed.

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The writer proteins, as I've called them,
influence the packing of the chromatin

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by changing the chemical
structure of histones.

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The change is called methylation.

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Histones that are methylated pack tightly,
turning off genes around them.

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While histones that are unmethylated,

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enable the DNA to be less tightly
packed and the genes can be turned on.

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Back to our main theme.

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Plants which experience
cold activate writers.

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These are proteins that place
marks at particular genes

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without changing the DNA sequence.

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This type of mechanism is
designated an epigenetic mechanism,

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meaning above the level of genetics,

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as it affects the read out of the DNA
without changing the DNA itself.

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This marks caused particular genes
to become gradually silent and

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eventually, to stop
exerting their function.

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So which genes are silenced and
what are their function?

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The pioneering work of the British team
lead by Professor Caroline Dean and

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Professor Rick Amasino from the USA,
discovered that flowering is activated

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due to the suppression of particular genes
designated FLC which blocks flowering.

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>> Let me just re-emphasize what you said,
Nir.

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The function of the FLC gene
is to repress flowering.

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In other words until a plant starts
to flower the FLC gene is active.

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That is, it's inhibiting the flowering.

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>> As winter progress, a writer
protein place marks on the repressor

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gene until it is silenced thus
allowing flowering to take place.

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As in math minus times
minus give rise to plus.

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The same in this case, where the cold
leads to suppress the suppressor,

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allowing flowering.

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Let us review this complex process again.

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The specific gene involved is FLC,
which stands for flowering local seed.

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FLC function by inhibiting flowering.

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Once the plant goes through
a period of cold weather,

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the FLC gene is no longer transcribed.

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In other words, the gene is turned off.

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But that doesn't mean that plants
will immediately start to flower.

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It only means that plants can flower
if other conditions such as light and

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temperatures cooperate.

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Professor Dean and
Amasino studies showed us that in fact

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mutant arabidopsis plants lacking
the writer protein which add the methyl

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group on the histone
express FLC continuously,

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even when exposed to cold and
dust, they cannot flower.