Lesson video

Hi there.

I'm Mrs. Kemp and welcome to today's lesson all about plant defences against diseases.

This fits into defences against disease, the human immune system and vaccination unit.

So let's get started then.

Our main outcome for today, "I can describe physical, chemical, and mechanical plant defences against pathogens and pests." We'll be using some new key terms today and you can see those on your screen and if you'd like to read those in a bit more detail, please do pause the video.

We've got two learning cycles for today then.

This one is pathogens and pests and plant defences.

We of course, will start with pathogens and pests.

So, communicable diseases are caused by pathogens, okay? Pathogens are microorganisms that can be passed from organism to organism, hence the fact that it is communicable.

A pathogen can be a virus, a bacterium, or any other types of disease.

So things like virus, bacteria, fungi, protist.

Pathogens can infect plants as well as animals.

So we often think about communicable diseases like measles that infects people, but actually plants do also have some diseases.

An example of a plant disease is a tobacco mosaic virus or TMV for short, and we can see on that leaf there of the tobacco plant that it's got that funny mosaic stippling to its its leaves and hence that's why it has its name.

Tobacco plants are closely related to things like tomatoes and cucumbers and therefore they can also have TMV.

We get that discoloration like I mentioned, but also you can see on the edges the leaves are starting to curl up, and that is also something that is caused by the tobacco mosaic virus.

How this affects the plant then is that actually it's got reduced photosynthesis in those areas where there isn't as much chlorophyll.

We're also reducing the surface area of the leaves, and it means that the plant cannot photosynthesize and produce as much glucose as it would normally.

Therefore it will lead to stunted growth of that plant, and so the farmer would get a much decreased yield of the tomatoes, cucumbers, or tobacco.

Another example of a plant disease is one caused by a bacterium, and this is known as crown gall disease.

It's actually really problematic and very, very difficult to get rid of.

The bacteria we find in the soil and we can see a little image there that's been seen through an electron microscope of some of those bacteria.

But we can also see on the other side what happens when a plant gets crown gall disease.

So it actually forms a big mass of cells called a, "Gall" And it is a type of tumour for that plant.

Now, as that tumour develops then, what happens is it actually sort of embeds itself into the vascular system of the plant stem.

So we're talking like the phloem and the xylem.

It then restricts the movement of water and nutrients around that plant and obviously that would then contribute to an overall stunted growth, and eventually it can lead to death of that plant.

Another example of a plant disease is one that's caused by a fungus, and this affects a native species of ash in the UK known as the common ash.

Its real name is Fraxinus excelsior.

So we can see what the common ash looks like there, and what happens when it gets a fungus known as chalara ash dieback is that actually, just as the name suggests, part of that plant actually dies back and we can see that the very top canopy there has actually lost a lot of its leaves.

It is a fungus, and when it infects the plant at first you can see the sort of fruiting bodies of it, and we can see that in the first image there where you've got those like, sort of little mushrooms. And actually later what happens is that brown lesions appear on the trunk of the tree, but also on the leaves as well.

And eventually then we get that ending up of that complete dieback of the shoots.

It actually does kill the tree, and so it is really, really serious and we've got to be very careful not to spread this from our woodland areas in the UK.

Okay, can we match these up then with the type of disease and then the type of pathogen? We've got TMV, which remember is the tobacco mosaic virus, crown gall disease and chalara ash dieback.

I'll give you a moment to think about it, but if you need more time, please pause the video.

Okay, did you get TMV is a virus? Crown gall disease was bacterium and chalara ash dieback was that fungus? Excellent, well done.

So plants not only can get diseases through pathogens, they can also become infested with pests, and those pests can be insects essentially.

And just like pathogens can be spread from a organism to organism, so can the pests.

The pest can also be responsible for actually passing that pathogen from one organism to another, so it will actually contaminate another plant as it's moved from that plant to another plant.

So one example that's very common, you've probably seen these in your garden or may potentially on some plants that you've seen out and about or even in your house, and they are known as greenfly and what they do is that they feed on the sap of young plants such as tomatoes, and you can see them there living happily on that stem.

They are parasites that benefit from taking all the nutrients from another organism.

So that organism is not benefiting whatsoever from that aphid living on the plant, and therefore it is a parasite.

They have really pointy mouth parts that are like, needle-like, and what they do is they poke that into the stem down towards where the phloem is.

You can see that in the top image there.

And then they can suck that sap out, essentially taking all the sugars and the nutrients that are in that sap.

This will reduce the growth rate and can eventually kill the plant if there are enough of them on there.

Now actually what we can see in the bottom image there is a lady bird.

Lady birds happily feed on aphids, and so gardeners will often try to encourage lady birds to move into their gardens in order to keep the number of aphids lower.

Another type of common insect pest then is the mealybug, and there's one there in the image.

and they're a little bit harder to see.

And so you may not have ever noticed one of these before.

I certainly haven't.

They feed on the sap of many common house plants and they can cause stunted growth, so the plant not being able to grow to its full size, and can often lead to the plant being more prone to fungal diseases.

So even if the mealybug hasn't led to the death of the plant, actually it can then get a secondary infection.

They are difficult to get rid of, but gardeners do try washing them off with things just like washing up liquid to be honest, and then sort of spraying on it and hoping that that will remove the mealybug.

Okay, onto a true or false then.

An aphid is an example of a plant pathogen.

Is that true or is that false? I'll give you a moment to think about it.

That of course is false.

Now can you justify your answer for me? A, an aphid causes plant diseases, or B, an aphid is a pest that can spread pathogens.

I'll give you a moment to think about it, but if you need more time, please pause the video.

Okay, did you get B, an aphid? Of course, it's a pest, okay? That can spread pathogens.

It isn't a pathogen itself.

Pathogens are our viruses, bacteria, and fungi.

Okay, onto our first task of the day then.

You can record your answers on your worksheet, so please do get that out now.

Number one.

Explain in your own words the difference between a plant pathogen and a pest.

Number two.

The apple tree shows signs of damage and disease.

Use ideas about pests and pathogens to suggest what could have caused this.

I'll give you a moment to think about it, but if you need more time, please pause the video.

Okay, so should we go through number one first? So pathogens cause plant diseases.

They include bacteria, viruses, fungi and other microorganisms. Pests are insects or other animals that damage plants by feeding on them.

They can also spread pathogens that cause diseases.

For number two, our ideas could be that the damage could have been caused by pests including insects such as aphids and mealybugs.

These pests can also spread pathogens that could have caused the disease in the apple tree.

The disease could be caused by bacterium, virus, fungus, or other microorganism.

There is actually a third part to this task, and this one is all about a plant that is also a parasite.

This plant is called Orobanche reticulata and actually it is extremely rare.

It doesn't actually have any leaves with chlorophyll in like normal plants do, and this species only grows near the woolly thistle.

You can see that purplely flour there is the Orobanche reticulata and the woolly thistle is just next to it.

Now this species will only grow there and the thistle will then become much smaller than usual.

What do you think has the plant has caused the stunted growth? So what do you think has caused the stunted growth of that woolly thistle? I'll give you a moment to think about it, but if you need more time, please pause the video.

Okay, so the Orobanche reticulata is a very rare plant that has no leaves or chlorophyll.

This species only grows near the woolly thistle.

This thistle is much smaller than usual.

What do you think has caused the stunted growth? So, actually the Orobanche reticulata, just like we said in the question, is a parasitic plant and it lives off the woolly thistle.

It is growing on the plant and actually taking its sap.

This means there is less nutrients for the woolly thistle to grow, therefore stunting its growth.

The Orobanche reticulata does not need leaves or chlorophyll as it doesn't need to photosynthesize to make its own food.

So it's a bit like one of those pests that we saw such as the aphid but in a plant version.

It's very, very rare.

We don't get many of these.

It's a really exciting plant.

So, onto the next one then.

This one is plant defences.

So just like humans, plants do need to defend themselves, they have many physical defences against pathogens.

So things like the tree bark, it's actually a dead layer of cells which forms a physical barrier to infection.

I suppose if we were going to compare it to humans, we might compare it to skin for example.

They also have waxy cuticles on the top of leaves.

If you feel a leaf, you can usually tell which bit is the top of the leaf 'cause it feels waxy.

Now, that waxy cuticle helps to prevent infection.

The cell walls around each one of their cells is also difficult to penetrate.

It's made of something called cellulose, which is a long chain of sugars.

Although some pathogens get round this by releasing enzymes and that can help to break down the cellulose and soften that cell wall so that they're then able to get into the cells.

Plants such as thistles and cacti, we can see those in the image there.

They actually have spines that help prevent them from being damaged by pests because obviously it's much more difficult to sort of get hold of and actually eat it if it's prickly like that.

The leaves of a sensitive plant called the mimosa pudica curl up when they are touched, and this helps to protect the leaves from damage.

So if something went along and landed on that leaf, they curl up and the insect would drop to the floor.

We're going to watch that video now to see that happening.

So we saw there that if something landed on that leaf, just like somebody stroking it there, the leaf would curl up and the pest would drop to the floor.

Okay, which physical plant defence can we see in the diagram? A, cellulose cell wall.

B, spines.

Or C, waxy cuticle.

I'll give you a moment to think about it, but if you need more time, please pause the video.

Of course, it's a cellulose cell wall, which we can see all the way around the outside of that cell.

Okay, so as well as physical defences, just like human beings, plants have actually got chemical defences, which you may not have realised before.

So for example, the fox glove and stinging nettles, they actually produce poisons that deter herbivores from eating them, okay? We actually would stay away from a stinging nettle, wouldn't we? Because it would cause irritation to our skin.

So us knowing that mean that we actually leave those populations alone to grow.

Clover and mint produce antimicrobial substances that kill bacteria.

We can use these as antiseptics for humans.

So you can often get things like clover oil, for example, which people often use for teeth infections.

So we have used the fact that the plants produce these chemicals to help ourselves.

Antimicrobial means that it will kill a microbe.

Plants need energy to produce chemicals, so only produce them when they need them.

This is a plant here called maize and is damaged by a herbivore.

Example of a herbivore we've got in the picture there, which is a caterpillar.

And actually what it'll do is it will release compounds into the air that attract wasps.

Now why would it do that, I wonder? Well, actually that's because if it attracts the wasp over, the wasp will end up eating the caterpillar, that herbivore, therefore saving the plant from any further damage.

It's really, really clever that they are able to draw in the help from nature around them.

Which of these plants produces chemicals that are antimicrobial? A, cacti? B maize? or C, mint? I'll give you a moment to think about it, but if you need more time, please pause the video.

Did you realise that it was mint? Excellent, well done.

Okay, onto our final task of the day then.

This one is task B.

You can record your answers on the worksheet, so please do get that out now.

Rainforest leaf cutter ants work together as an army to carve up and transport leaves of the bixa tree underground to feed a fungus.

We can see an image there of the bixa tree and we can also see those leaf cutter plants that have cut up a bit of the tree and they're carrying that back in order to feed a fungus that they have down in the ground.

This is what we call a mutualistic relationship.

And in return, the ant's provided with nutrition from that fungus.

The bixa tree has no direct defence against the ant, but it does produce a fungicide.

A fungicide is something that will kill a fungus in its leaves when damaged.

Explain how this could protect the bixa tree from any further damage.

I'll give you a moment to think about it, but if you need more time, please pause the video.

Okay, so did we think that when the leaf cutter ants carve up the leaves of the bixa tree, it produces a fungicide in response? The ants transport the leaves which carry that fungicide underground to feed the fungus.

The fungicide could weaken or kill the fungus, meaning there may not be enough food for the ants.

The ants may die from a lack of nutrition.

Alternatively, the ants could find another food source of leaves for the fungus, which would mean that the bixa tree is protected against any further damage.

I hope you got all those points down, but if you want to add a little bit more to your answer, please do that now.

Okay, so we're very near the end now, and I just want to go through some of those key learning points with you.

Plants can be infected by pathogens such as viruses, bacteria, and fungi that cause diseases.

Pests, including insects, can damage plants by feeding on them, and can spread pathogens that cause diseases.

Many plants have physical defences against pathogens and pests such as the waxy cuticle on the leaf surface and the cellulose cell walls of a plant cell.

Some plants produce antimicrobial chemicals to kill pathogens, and some plants produce chemicals in response to damage that kill or deter herbivores.

Some plants have mechanical defences such as movement of their leaves.

I've learned a lot today and I hope that you have too, and I look forward to learning with you again.

Thanks very much.