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Auxin is arguably the most important

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hormone in plant development, and it
influences

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all stages of plant development from
germination

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through to making fruit and and seeds.

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Now, in this course, we're not going to go
into the myriad functions of our of

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auxin, how it functions just going to give
you a quick list of what it's involved in.

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It's involved in growth, it's involved in
phototropism and gravitropism.

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We'll talk a little

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bit more about that soon.

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It's involved in branching of the plant,
it's involved in the

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development of the seed it's involved in
the development of the meristems.

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but I want to talk for a second about two
of it's main roles.

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One is, an initiation of roots.

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Auxin is the hormone which causes roots to
form.

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And auxin is the hormone which stimulates
cells

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to elongate.

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So let's talk about this role that one of

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the major roles of auxin is to stimulate
cell elongation.

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I want to tell you one other thing.

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Is that one of the major sites where auxin
is

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produced is at the tip of this, of the
shoot, which

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is called the shoot marrow stem, those are
the embryonic

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cells of the green part of the plant at
the top.

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So, auxin is

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made in the shoot marrow stem at the tip
of the seedling.

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And it's, one of its major roles is to
stimulate cell elongation.

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But can you see a problem here visa vi
photo tropism?

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Now, I said that auxin is produced at the
tip, but what we

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saw from all the experiments, was that

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the elongation happened further down the
stalk.

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So how can we

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solve this conundrum, that while auxin is
needed

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for elongation, we don't get elongation at
the tip?

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Well the answer is, that the effect of a
hormone is dependent on several factors.

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And this is not only true for
phytohormones,

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it's also true for hormones in our own
body.

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The action of a hormone is dependent on
where it's functioning.

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The same hormone can have different
functions in different parts of the body.

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When it's functioning, at what
developmental stage,

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and also, the concentration of the
hormone.

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A single hormone can have different

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functions at high, and at low
concentrations.

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And that's actually what's going on here.

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So for example, we know that at

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very high concentrations auxin actually
inhibits cell elongation.

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It keeps cells small.

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But as the,

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the concentration is diluted, as it gets
slightly

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lower, then it inhibits, then it induces
cell elongation.

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Until then, it's so diluted that it stops
causing cell elongation.

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So at the top, we're having a very, at the
tip of the seedling, we have a very high

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concentration of auxin, there's no cell
elongation, and apparently,

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as it goes down the stalk, you're getting
a lessening

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of the, a reduction in its concentration

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and there you're getting the cell
elongation.

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So what we're assuming here, is that we
have a higher concentration of

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auxin on the dark side of the plant than
on the light side.

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And this is exactly what's been shown in
subsequent studies.

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When the light is shone onto the seedling,

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the auxin accumulates on the dark side,
and

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there you're getting more elongation, such
that

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the seedling is then bending towards the
light.