2.4 Phytochrome: A Light-Activated Switch

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From the course by Tel Aviv University

Understanding Plants - Part I: What a Plant Knows

442 оценки

Tel Aviv University

Understanding Plants - Part I: What a Plant Knows

442 оценки

For centuries we have collectively marveled at plant diversity and form—from Charles Darwin’s early fascination with stems and flowers to Seymour Krelborn’s distorted doting in Little Shop of Horrors. This course intends to present an intriguing and scientifically valid look at how plants themselves experience the world—from the colors they see to the sensations they feel. Highlighting the latest research in genetics and more, we will delve into the inner lives of plants and draw parallels with the human senses to reveal that we have much more in common with sunflowers and oak trees than we may realize. We’ll learn how plants know up from down, how they know when a neighbor has been infested by a group of hungry beetles, and whether they appreciate the music you’ve been playing for them or if they’re just deaf to the sounds around them. We’ll explore definitions of memory and consciousness as they relate to plants in asking whether we can say that plants might even be aware of their surroundings. This highly interdisciplinary course meshes historical studies with cutting edge modern research and will be relevant to all humans who seek their place in nature. This class has three main goals: 1. To introduce you to basic plant biology by exploring plant senses (sight, smell, hearing, touch, taste, balance). 2. To introduce you to biological research and the scientific method. 3. To get the student to question life in general and what defines us as humans. Once you've taken this course, if you are interested in a more in-depth study of plants, check out my follow-up course, Fundamentals of Plant Biology. In order to receive academic credit for this course you must successfully pass the academic exam on campus. For information on how to register for the academic exam – https://tauonline.tau.ac.il/registration Additionally, you can apply to certain degrees using the grades you received on the courses. Read more on this here – https://go.tau.ac.il/b.a/mooc-acceptance Teachers interested in teaching this course in their class rooms are invited to explore our Academic High school program here – https://tauonline.tau.ac.il/online-highschool

From the lesson

What a Plant Sees?

This week we start a systematic review of a plant's sensory systems by starting with plant responses to light. We will cover an overview of human vision, plant responses to light, Darwin's experiments showing plant responses to light, phototropism, phytochrome and flowering, and modern research on phototropism. In other words, this week we get into more advanced concepts in plant sensory biology. The last module is especially advanced, and will be clearer for those of you with a strong biology background. But do not fret, aside from very basic concepts, this module will NOT be included in the exam (you will not be responsible for understanding the intricacies of the experimental methods, etc.). If you have not already, please review the Course Syllabus for general information about this course.

2.1 Introduction5:21

2.2 Darwin's Experiment6:50

2.3 Light and Flowering7:06

2.4 Phytochrome: A Light-Activated Switch7:35

2.5 Arabidopsis Blind Mutant8:42

2.6 What a Plant Sees?5:55

2.7 Molecular Biology of Plant Vision8:53

Meet the Instructors

Professor Daniel Chamovitz, Ph.D.
Dean, George S. Wise Faculty of Life Sciences
Director, Manna Center for Plant Biosciences

So what we actually have here is a type of light activated switch.

Red light's turning it on.

Far red light's turning it off.

If you turn it on with red light and immediately turn it off with the black,

with the far red light, it's as if it was never turned on to begin with.

It's as if you take a, a light switch, and

flick it very quick, the lights don't manage to go on.

Now if you give the red light, then wait, for example,

an hour.

And then give the far red light, then you've done nothing.

The red light is what it remembers.

It's only if you give the red and immediately

after the far red, you switched on and off.

What we have here is actually a molecular switch.

A switch that's working at the level of the protein

within the cell, that lets the plant know when to flower.

The scientists finally isolated this molecule and it's called

phytochrome and what phytochrome is is a light-activated switch.

And I want to go in a little bit to

the biology of the phytochrome because I think it's fascinating.

We can actually understand how light affects a plant at a molecular level.

So we have here is a cartoon of how we imagine phytochrome to look.

And it's actually a protein which has two different components.

One is the protein component, here.

And the other is what we see here on the top, is what's called the chromophore.

This is the part of the protein which absorbs the light.

Now phytochrome can absorb either red or far red light.

And that's because it can be found in one of two confirmations in the cells.

Where it's in the first confirmation which we're going to call Pr, phytochrome red,

it absorbs red light. When it's in a second confirmation, which

we're going to call Pfr, phytochrome far red, it will absorb far red light.

And now here comes the part that gets a little tricky.

Pr absorbs red light, and once it absorbs the

red light, it changes its conformation to become Pfr.

We now know molecularly, actually, there's a slight

change in the way the protein is formed, in

its cons-, in its structure and it becomes the Pfr and that is what makes it active.

I'm going to repeat this a couple a times,

so I know it gets a little complicated.

The Pfr form, it absorbs far red light and when it absorbs the far red light

it changes its structure and goes back to become Pr and now it's inactive.

So again, let's repeat this. We have the first form which is called Pr.

Pr is inactive.

When Pr absorbs the red light it changes it's structure, it now becomes Pfr.

And Pfr is the active form of phytochrome.

This would be the form that would be activating flowering in a long day plant.

When Pfr absorbs the far red light, it changes

its structure back to Pr and it gets turned off.

And so now we can see how this switch works.

The red light turns it on. The forward light turns it off.

If you give the flashes very quickly,

you've gone up down, there's been no effect.

But if you give the red light flash and then wait

awhile before you give the far red, then it's too late.

The signals already being transduced forward.

Now I want to qualify what I just said.

I'm, going to talking about phytochrome in the classic point of view.

And of course the actual biology is a little bit, much more complex.

When Pr absorbs far red, absorbs red light

and becomes Pfr, that is the classically active form.

And when Pfr absorbs far red light and becomes Pr, that's a classically inactive.

Of course, within biology things are much more

complex, and there are examples where Pr can have

some type of activity, and Pfr can have no activity.

But we're going to be talking about, what's the general paradigm.

What we see here is what's called the absorption spectra.

This shows what phytochrome, what color of light

phytochrome actually absorbs when the light hits it.

When it's in the Pr form, we say that

it's absorbing light that's primarily in the red form.

It cannot absorb far red light. The slight change in its confirmation,

means now in the Pfr form, it can absorb the far red light, primarily.

Now, what's important in this from an ecological point of view?

When does a plant see red light? And when does it see far red light?

Now, in the morning as the sun is rising, what we see is that the

sun has a lo-, the, the light waves have a longer path through the atmosphere.

And under these conditions there's more far red light, than there is red light.

But as the sun rises, up in the, to the, to its zenith, the ratio

of red to far red changes, such that there's much more red light, than there is

far red light.

So for the majority of the day, a plant

is seeing primarily red and not far red light.

Now what happens though, at the end of the day as the sun goes down?

As the sun goes down, the ratio again changes such

that the longer and longer wavelengths are reaching the Earth.

We see this in the change of color as the sun as it's setting.

And the ratio of red to far red becomes so low, that the

plant is only seeing at the end of the day, far red light.

And so it's actually in the far red light which

is signalling to the plant that the night has begun.

And when, in the experiment that we showed earlier, when we

turn on the red light, it thinks that the day has begun.

If we do red immediately far red, again it thinks

that the night, the day has ended, and night has begun.

This is how the phytochrome switch works in nature.

Now plants see red and far red, not only according to the time of

the day, but also where they are in your garden or in the forest.

For example, a seedling that's growing under direct light is

being hit by both the red and far red light.

And in the middle of the day it's much more red than it is far red.

But if the same seedling is growing under the canopy, under the shade of

another plant, the type of light that's

reaching it under this leaf is completely different.

And that's because the red light is absorbed by a green leaf.

And it's absorbed for photosynthesis, the energy making machinery of the plant.

The far red light isn't absorbed for photosynthesis.

It goes through the leaf, and it hits the seedling growing under the canopy.

So a seedling growing underneath another leaf is getting

much more far red light than it is red light.

How does this affect the plant?

It lets it know that it's shaded and the plant will respond

by elongating in order to get out of the shade.

So the red-far-red ratio not only lets a plant

know if it's morning, middle of the day, or night.

It lets the plant know if it's in the shade.

And if it's in the shade, the plant will elongate.

You've seen this probably in plants in

your garden, that if they're too shaded, they

become long and spindly, until they reach

the sun, and then they expand their leaves.

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