Lesson video

Hello and welcome to this lesson from the unit Plant growth and development.

The title of today's lesson is Auxins, phototropism, and gravitropism, and we're gonna be looking at plant hormones and how they allow plants to respond by growth to the stimuli of light and gravity.

My name's Mrs. Barnard, and I'm going to be taking you through today's lesson.

So by the end of today's lesson, you should be able to explain how hormones called auxins cause plant shoots and roots to respond to light and gravity.

And we've got some keywords in today's lesson.

And our keywords are: hormone, auxins, tropism, phototropism, and gravitropism.

So if you wanna write those down, then you can pause the video now.

Otherwise, we'll go through them as we go through today's lesson.

So our lesson today's in three parts: auxins, so we're looking at those plant hormones, what they are and where they're made; phototropism, so how plants respond in growth to light; and gravitropism, how plants respond in growth to the direction of gravity.

So let's get started with the first part of today's lesson, which is auxins.

So a little bit of a recap for you to start with.

So plants are living organisms and therefore they carry out the same common processes as all other living organisms. Now this is for you to sort of recall and think back where you might have learned about this before.

So can you recall those processes? So to give you a clue, there's seven of them.

So talk to the person next to you and see how many you can come up with.

Okay, how did you do? So did you remember the mnemonic MRSGREN? Some of you might have known that.

So MRSGREN.

So we've got movement and respiration and sensitivity, growth and reproduction, excretion and nutrition.

So those are our common processes.

So what we're looking at with plants today is nutrition and how they get their nutrition.

So they get their nutrition through photosynthesis.

So this is where they take carbon dioxide from the air and water from the ground through their roots, and they react those together with the energy from sunlight.

And in that process they make glucose.

Now that is their nutrition because glucose can then be used in respiration to release the energy the plants need, and the waste product of that process is oxygen.

So plants need to grow in a way that increases their chance of taking in enough water and light for photosynthesis.

So these responses are called tropisms, and they can be positive or they can be negative.

So a positive tropism is a growth towards a stimulus.

So for example, a plant growing towards the light.

And a negative tropism is growth away from a stimulus.

So for example, a shoot growing away from the direction of gravity.

So gravity acts towards the centre of the Earth, so therefore a shoot has to grow away from the centre of the Earth.

So therefore it shows negative tropism.

So we've got a time-lapse here of a plant near a window.

You can see it's on a table there near a window.

And we've timed it so you can see the timer going round and you'll be able to see what looks like the plant is moving towards the light when in fact it's actually growing towards the light.

It's pretty cool to see it sped up.

So I'll play that for you now.

So the growth of plants towards or away from a stimulus is controlled by a group of hormones called auxins.

Now, these are produced in the meristem of shoots and roots.

And the meristem is where the stem cells are and they allow differentiation to take place.

So all different cells come from the meristem.

So in this image here down a microscope, you can see the cells at the bottom, that's the root cap.

So that's the bit that pushes through the soil.

But behind them is a set of cells.

So those are the cells that will divide by mitosis or they will differentiate into other types of cell.

So auxins will diffuse from this meristem tissue to other parts of the plant, so other cells.

And therefore in these cells they will regulate cell division, mitosis, and cell elongation.

So that is cells themselves getting longer rather than dividing.

So let's have a look at what that looks like.

So auxins can stimulate mitosis, but only in meristem tissue.

So you can see in this image here, we've got mitosis of those undifferentiated cells at the bottom and then they will divide to make more.

And as they divide to make more, you're going to get growth of your plant.

But in non-meristem tissue, what those auxins can do is that they can cause the cells to elongate.

So again, you can see in this example, these non-meristem cells will elongate, they'll get longer, and that gives you growth.

So there's two different ways of getting growth that's stimulated by auxins, dependent on whether you are in meristem tissue or non-meristem tissue.

Shoots and roots actually respond differently to auxins.

So in shoots, cells are stimulated by auxins and so will elongate.

So we can see as the auxin diffuses down, so we've done it there with pink arrows, so as it diffuses down, the cells will elongate.

But in roots, auxins actually inhibit elongation.

So you can see as auxin is coming down there, it's preventing the cells from elongating, and that's important as we'll look at later in the lesson with gravitropism.

So plant hormones can be used artificially in agriculture and horticulture.

So agriculture is where we grow things for food, farming, and horticulture is more gardens.

They could be commercial gardens as well as your own garden.

So some examples of where we might use those hormones are, auxins can be used as a weed killer because in very, very high concentrations they can actually disrupt plant growth, but they can also be used to allow you to stimulate roots to grow.

So if you're taking cuttings like they do with tea plants, so a lot of the tea plantations, they're made from the same plant and they take cuttings of it.

And what they do is they dip the bottom of the stem in rooting powder, and that encourages the roots to grow.

And auxins can also be used in growth medium for tissue culture.

So that's when you just take a few cells of a plant and you grow it in a Petri dish and then you make little plantlets, which then go on to form your little plants.

So time for a quick check.

So true or false? Auxins that are made in the root tip diffuse and stimulate cells to elongate.

So once you've decided whether it's true or false, what I would like you to choose is which of the statements below best justifies your choice.

So pause the Video while you do that and then we'll see how you've got on.

Okay, so auxins made in the root tip diffuse and stimulate cells to elongate.

This is false, and the reason is because auxins actually inhibit the elongation of cells in the roots.

So if you've got that right, then well done.

And here is your practise task.

So here's Sam, and she has observed that these cress seeds will grow upwards within a week from germinating seeds.

So you can see that in these images.

And she says, "The plants have grown upwards due to cell division, mitosis." So Sam is only partly correct.

So what I'd like you to do is to describe how the cress seedlings have grown due to the action of auxin and explain why this is important.

So pause the video while you do this, and then I will give you a model answer after.

Okay, let's see how we got on with that task then.

So Sam is only partly correct when she says the plants have grown upwards due to cell division, mitosis, only.

So we are explaining how they have grown up, thinking about the action of auxin.

So here's a model answer for you.

Sam is partly correct, as the cress seedlings have grown due to cell division, mitosis, but she's also incorrect as they have also grown due to cell elongation.

Shoot cells are stimulated by auxins and so will elongate.

This causes a plant to grow upwards, which is typically where they would find light, and the seedlings require light for photosynthesis.

So if you've got that in your answer, well done.

If there's anything you want to add in, now is the time to do it before we move on to the second part of our lesson.

So the second part of our lesson today is phototropism.

So phototropism, so photo meaning light, and tropism, as we've already talked about, is the response to stimuli, is a plant's response to the stimulus of light.

So light is detected by cells in the shoot tip, and when that light is detected, they release auxin.

And we've got the auxin here as these little pink blobs that are moving down the stem.

So auxin that is released will move by diffusion.

So this is the process of substances moving from an area of high concentration to an area of low concentration.

So you can see there that they're diffusing down.

And shoots will therefore grow upwards, which would be positive phototropism, whereas roots would grow away from the light, so that would be negative phototropism.

So when the light is detected by the cells in the shoot tip, that auxin is released and the auxin then diffuses down the tip evenly.

So you can see that that's shown by these pink arrows, that auxin diffusing down.

And then those cells will elongate and the shoot will grow upwards towards the light and therefore it can get more light for photosynthesis, which seems to make lots of sense.

However, what if the light is coming from one direction? Same thing happens, that the cells in the tip will be stimulated by the light and they detect it and they'll release auxin.

But this time, instead of the auxin diffusing evenly down the cells, it actually diffuses down the shaded side of the stem.

So you can see it's going down that side.

So we say that's an uneven diffusion or an uneven distribution of auxin.

And because those cells on the shaded side will elongate more, you can see what happens.

It means that the shoot then grows towards the light.

So if you look carefully, can you see those cells on the shaded side have got longer, where the cells on the light side still have remained the same, and then that causes it to bend towards the light, so grow towards the light.

So let's have a look at the investigation that they did in order to work this out.

It's quite a famous one.

So an investigation to prove the effect of light on the direction of growth.

So we've got a control, so we've got the light here coming from a direction, you can see it there.

And then we've got a control.

And in that control plant, you can see it is growing towards the light.

Then we've got this shoot in the middle and this is being covered, so it's blocking out the light.

So that means that that cannot detect the light.

And then we've got this final one here with the shoot tip removed.

So in the first one, the shoot grows in the direction of the light.

In the second one, the shoot grows upwards because the shoot cannot detect the light, so therefore it just grows upwards.

And finally, the shoot doesn't have a tip, so therefore it doesn't release auxin.

So therefore we don't have any growth in that one.

So a quick check for you now.

Select the images that show phototropism.

Now look very closely before you select them.

Okay, pause the video, then we'll come back and see how you've got on.

Okay, so the correct answer was A.

So we can see that that cell elongation as the light is coming from above.

So we would have an equal distribution of auxin.

And then the second one is C.

So again, we can see the light is coming from a direction, but it is diffusing down the shaded side, causing elongation down the shaded side.

So not that middle one because the elongation is in the wrong place.

It shouldn't be on the light side.

So if you've got those right, then well done.

So time for a practise task.

So it's your turn now to interpret an investigation.

So we've got an experiment here where we've got our control, the same as we just looked at in that model investigation.

And then we've got a shoot tip that is covered with a transparent material.

And then we've got the shoot tip covered with an opaque material.

And then finally we've got the shoot tip that's been removed and then a block of gel has been put on the end and then the shoot tip has been put back into place again.

So what I would like you to try to do is using your knowledge, talking about auxins, talking about elongation, to be able to explain the results of this experiment for each of these shoots.

So it'll take you a little bit of a time to write this because it's an extended task.

So pause the video while you do that, and then we'll come back and we'll look at some model answers.

Okay, so let's see how you got on with that then.

So we're explaining the results of these four different shoots that have been grown in slightly different conditions.

So for the first one, that's our control.

So auxin will diffuse down the shaded side and that will cause elongation on the cells on that shaded side and therefore the shoot will grow towards the light.

But number two, the cells in the tip of the shoot, because it's a transparent material, can still sense the direction of the light and so therefore they will grow in the same way as the control.

So the auxin will diffuse down the shaded side, causing elongation.

And then number three, the cells in the tip cannot sense the direction of the light, and so therefore the auxin will diffuse down the shoot equally, so therefore the cells will elongate equally and the shoot would grow upwards.

And finally, the cells in the tip of the shoot again sense the direction of light and auxin can diffuse through the gel.

This is because auxin is a chemical so it can move through the gel.

This is one of the investigations that they did to prove that auxin, or whatever it was that they were looking for that controls growth, was a chemical, and this is how they did it.

So again, in this case, the cells in the tip of the shoot sense the direction of the light, and auxin can diffuse through the gel and so therefore this shoot grows in the same way as the control, with the cells on the shaded side elongating.

So if you've got those right or words to that effect, then well done.

If you need to make any corrections, then please do so before we move on to the next part of our lesson, which is gravitropism.

So gravitropism is a plant's response to gravity.

So we can see here we've got a plant, and gravity acts downwards towards the centre of the Earth.

So therefore roots show positive gravitropism because they're growing in the direction of gravity, whereas shoots show negative gravitropism because they're growing against gravity.

So we've got negative gravitropism there and positive gravitropism there.

So if a stem is placed horizontally, so if you get a plant pot and we turn it on its side, the auxin is still made in the cells of the shoot tip.

What happens is that the auxin builds up on the lower side of the stem, so it responds to gravity by moving down to the lower side of the stem.

So what that means is that in the shoot, because auxin stimulates elongation, the cells will elongate on that lower side of the stem and then the shoot would grow upwards against gravity.

So you might see this, and if you haven't, maybe you could look out for it.

So maybe in plants around school or at home or out in the street.

You can see sometimes that when plants are on their side, that then eventually they will grow upwards against gravity.

So let's talk about roots now.

So the lateral roots, lateral means like sideways.

The lateral roots will grow out horizontal from that main taproot that grows down the middle of the plant.

And I sometimes got confused about this focus of auxin at this point because I wasn't concentrating on the lateral roots, which are the ones that come out sideways.

I was thinking about that long taproot that goes down the middle.

So we're focusing on the lateral roots, and they grow out horizontally and auxin builds up on the lower side of the root.

Now in roots it's slightly different because auxin inhibits the elongation of cells on the lower side.

So we can see that buildup of auxin there on the lower side, but what it's going to do is it's gonna inhibit those cells from elongating.

So if you inhibit the underside cells from elongating, it means that the cells on the upper side will still elongate, and as they do, you can see it causes the lateral root to grow downwards into the soil in the same direction as gravity.

Now that's important because it enables those roots to get the water that they need for photosynthesis.

So this is why it's a really important response in plants that they can do this, so that the roots can reach water.

So the effect of gravity can be seen when you place a plant on its side.

So you've got a plant growing normally, and again, if we're focusing on that taproot that goes down the middle and usually grows directly downwards, if we have a little look at that and we place the plant on its side, that auxin will build up on the lower side, which causes the cells on the upper side to elongate, whereas the ones on the lower side are inhibited from elongating, which causes that root to grow downwards in the direction of gravity.

Again, it's responding because mostly you would find water lower down in the soil.

So that's its plant response to find water.

So time for a quick check.

Tick the correct statements that end the following sentence.

So the sentence starts with auxin dot dot dot.

So tick the statements that could finish that sentence.

So pause the video while you do that and we'll come back and see if you've got it right.

Okay, let's see how you got on with that then.

So how do we finish this sentence? Auxin inhibits cell elongation in roots.

Yes.

Auxin stimulates cell elongation in shoots.

So if you've got those two right, then well done.

So now it's time for a practise task.

So what I would like you to do is use your knowledge of the role of auxin in roots and label and explain this image and then explain why this process is important to plants.

So you can see we've taken all the labels off it, so you might want to label this on your worksheets or you might wanna draw it yourselves.

But either way, if you could have a big picture and label around the outside what's going on and why it is important to plants.

So pause the video while you do that and then I'll give you some feedback after.

Okay, so let's have a look at your labelled image.

So it should look something like this.

So we've got gravity acting downwards.

We've got elongation of our cells there on the upper surface.

We've got inhibition on that lower surface.

You might have put some extra labels on there, and that's fine, those are our two main ones.

And then our explanation now.

So auxin builds up on the lower side of the root and inhibits the elongation of cells.

Those on the upper side elongate as normal, which causes the root to grow downwards and in the direction of gravity.

This response allows a root to reach water that it can use in photosynthesis in order to make glucose.

So again, if you've got those words or words to that affect in your answer, then well done.

And if you need to make any corrections, then now's the time to do it.

So now comes the end of our lesson.

So let's read our summary for today, which is Auxins, phototropism, and gravitropism.

So hormones control growth and development in plants and enable plants to respond to some environmental factors in order to survive.

Auxins are plant hormones that control tropisms, which are growth responses in plants and that provide a survival advantage.

Positive tropism is when a plant grows towards a stimulus, and a negative tropism is when it grows away from a stimulus.

Phototropism in a shoot: auxins build up on the shaded side and cause cell elongation, so the shoot bends or grows towards the light.

In gravitropism in a shoot: the auxins build up on the lower side and cause cell elongation, so it bends upwards.

And gravitropism in a root: auxins build up on the lower side, but this stops cell elongation, so it bends or grows downwards.

So well done for your work in today's lesson and we'll see you soon.