Ulrike Bauer: killing on a knife edge

Image Courtesy of Ulrike Bauer

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There’s something unnatural about carnivorous plants. We’re so used to plants being at the bottom of the food chain, that to see them trapping, killing and eating animals seems to go against the laws of nature. But of course, carnivory in plants is very real and has evolved multiple times in response to a lack of nutrients.

One group of carnivorous plants are the pitcher plants; that’s pitcher as in jug, not picture as in photo, and they’re usually found in warm, tropical habitats around the world. Dr Ulrike Bauer studies these plants at the University of Bristol to find out more about how they’re able to successfully trap insects so easily. But before we get trapped in the details, I asked her what does a pitcher plant even look like?

Pitcher perfect

Pitchers come in all different shapes and sizes, as shown by these 16 species. Image courtesy of Ulrike Bauer

Ulrike: A pitcher is a transformed leaf, a very radically transformed leaf, so people often think it's a flower because it's so elaborate, but it is part of the leaf. And it looks a bit like a cup or like a champagne flute. And if you imagine a champagne flute where on the top of the flute where the rim is, you add a little roof over the top, so you have a little stalk at the rear and you have a little sort of flat roof.

Sally: It's like a cocktail umbrella!

Ulrike: It's like a cocktail umbrella. So then you have your water reservoir, and above that you have a bit of a rear section going up to hold a roof that is more or less horizontal, depending on species, that covers the pitcher.

Ulrike: And that has an obvious reason because it prevents it being diluted and all the contents being flushed out.

Sally: So how does a pitcher plant catch an insect? Because this is what your research is on, it's the biomechanics of it all. 'Cause if I just went around holding a champagne flute out in the world, I don't think I'd catch enough to get by.

Ulrike: No, you wouldn't. You wouldn't, we have done that experiment very much. Not quite champagne flutes, but we've put plastic pitchers next to real pitchers actually, to see if insects are attracted to the pitcher, do they also get trapped by our fake pitchers and no they don't. So there's actually a multitude of different mechanisms in pitcher plants, how they trap insects.

Ulrike: And not every species has all mechanisms, but most species will have more than one. So generally there's no moving parts, right, so let's get that clear. So a normal pitcher plant has mainly just slippery surfaces around that champagne flute, both around the rim at the top and on the inner walls of that champagne flute above the champagne level.

Slip ‘n’ slide

The microcrystalline wax surfaces around the opening of a pitcher make it hard for prey to grip, such as this ant who is about to fall to an acidic end. Image courtesy of Ulrike Bauer

Ulrike: But slippery surfaces can work on different principles. So most pitcher plants have some wax coating on the inside, and it's like a crystalline layer of wax. And what that does, it makes the surface really rough on a very fine microscopic scale. So imagine a really fine sandpaper. Really fine grain sandpaper. I don't know if you ever tried, but try taking a piece of sellotape and stick it to a fine grain sandpaper. It will not stick. You can press it down, it will not stick. And the reason being is because sellotape, like an insect foot pad, sticks to things because it makes good contact with the surface. So the better the contact with the surface, the better your sellotape will stick. So on a nice smooth glass plate, your sellotape will stick really strong because a hundred percent of the sellotape area sticks to the glass.

Ulrike: If you have this fine grain sandpaper or this wax layer on the pitcher front, you have microscopic scale surface with little mountains and little valleys. And the only bits that your sellotape will touch are the very tips of the mountains. And we're talking about like tiny, tiny rottenness. So these wax crystals on the inside of the pitcher plant are like little thin platelets that stand on their end. So only the very thin side of the platelet is actually sticking out into the surface. And so most of the surface that you end up with is actually air.

Sally: So it's literally on a knife edge. These insects are on a bed of knife edges.

Ulrike: Yes, they are very much. And that means that maybe only less than 5% of their pad area is actually in contact with a bit of surface.

Sally: Wow. We'll put some pictures in the show notes if you want to have a closer look.

Ulrike: So that's one way of making a surface slippery. The other way that pitcher plants use completely different, has nothing to do with that, but it uses water. So it's basically creating a water slide.

Bed of knife edges

Side on view of the wax crystals forming a microscopically rough surface inside a pitcher plant that makes it tricky for insect feet to stick to it. Image courtesy of Ulrike Bauer

Ulrike: So this pitcher plant surface, and this is the surface that looks like a collar and goes around the rim, like around the opening of the pitcher, right? This is a really unusual surface because it's very wettable. So you place a droplet on one edge and it immediately sucks up and spreads and forms this really thin water film.

Sally: Do all pitcher plants have the same mutation that allows them to have these wettable surfaces?

Knives Out

Top down view of the microcrystalline wax structure that creates a slippery surface. Image courtesy of Ulrike Bauer

Ulrike: There's no one single simple mutation to make this. That would be too simple. So we just started about two years ago to look into the developmental biology underlying this surface. So we had exactly your question, did the plants reinvent the wheel? Did they invent something crazy or are they taking things that are common and widespread, and putting them back together in a new way?

Ulrike: And we know pretty much for sure now that that's the case. They take elements of plant surface development that are common, that we find all over the plants and they recombine them in the new way. So it's a stage wise sequential process of development where different parts of the surface form at different times.

Ulrike: We basically have a stage where papillae form, and papillae are really common in plant.

Sally: Are they bumps?

Ulrike: Yeah, they're like little bumps. Normally cells are flattened. When you have papillae, they grow like little boobs, on the surface, very much. They look like little boobs, they're like little semi circular bumps.

Flowing up: The wax surface on the ridges around the lip of the pitcher plant are so hydrophilic that water will flow against gravity to coat the surface, making it extra slippery. Vide courtesy of Ulrike Bauer

Ulrike: And then these can grow into a hair on a leaf surface, for example. Or in the case of the pitcher plant, they start growing parallel to the surface. They make a structure that looks very much, if you've ever seen a pitcher of shark's teeth, how they're like overlapping inside a shark's mouth?

Sally: Yes.

Ulrike: They look very much like that at some point.

Sally: Almost like roof tiles.

Ulrike: Yeah, like roof tiles, like overlapping tiles. And then in the follow up stage, this surface is then taken and basically stretched. So you end up with still an overlapping structure, but you have the tips of these tees all fused into a ridge. So it's very complex.

Sally: I know you're saying that it's not just one mutation that causes, obviously nothing in life is caused by one mutation, I'm sure, but do you ever see like weird mutant pitchers where something has gone wrong and it just looks odd at the structure level?

Ulrike: We haven't found that yet, no. But we are hoping to find that. We just started looking into the genetics, so we've done two years of doing the microscopy to see what actually happens when this surface forms. So we had a master student cutting open pitchers at different stages and sticking the surfaces into a really high magnification electron microscope to look at it.

Ulrike: And she then characterised all these stages and we're now at the point where like, okay, we have a good idea what happens. So now we're starting to look at the genes. We now just have started extracting RNA, which is the part of the genome that's being read. So we're now starting to look at what is there in the RNA, what sequences are being read and used during different stages of development.

Ulrike: So we focus on like clearly defined stages, like the forming of these little papillae boobs and then the elongation of the cells to stretch the surface.

Little boobs

Surface of little boobs that stretch into overlapping shark’s teeth structures before forming ridges. Image courtesy of Ulrike Bauer

Sally: So you are looking at different points in time and presumably different parts of the plant as well, 'cause the genome is gonna be the same within the individual plant. But you are looking at - "Oh this gene's just suddenly been switched on. And that happens to coincide with all of these grooves being formed. Wonder if that's a gene to do with groove formation?!"

Ulrike: Pretty much we're fortunately not quite as much fishing in the dark as that. So, because as I said, these things like this papillae formation is a very common, widespread thing that happens, and we know which genes are responsible for making papillae in flowers. And so we have some idea of what to look for. But of course it could be a lot more complex. There could be a lot more things, but basically you're right. That's what we're doing.

Ulrike: And that hopefully gives us a set of genes of interest that could be doing something. And then the next step is actually gonna be really exciting because we have a method that potentially could work to switch these genes on and off inside the plants while they're growing. It's kind of a genetic transformation, but it's not the sort of transformation where you transform a plant and then it's a transgenic plant for the rest of its life. But it's a temporary thing where you just, for two or three weeks, you express a gene or you suppress a gene. And then that effect wears off. So we basically give the plants a medication that switches the gene on in a way.

Sally: Wow!

Ridges all the way down

The hydrophilic surface of pitcher plants is formed by big ridges made up of smaller ridges. Image courtesy of Ulrike Bauer

Ulrike: And by doing that, we hopefully can then figure out what these genes do, and we then might hopefully see some surfaces where actually things have gone wrong because we up- or down-regulated a gene that does something in the surface development.

Sally: What is this method called? How does it work?

Ulrike: So it's something that another lab in our building has developed, actually. It uses carbon nanoparticles, so they're called carbon nanodots, and it uses these carbon nanoparticles as a vehicle to get genetic material into the plant. And these nanoparticles are naturally occurring in the world, they're everywhere. So it's not something crazy or toxic or whatever. They are just naturally everywhere. But they have found a way how they can basically modify them, and tag a bit of genetic material on the end of it. And the neat thing about these nanodots are that they are generally taken up with the water and transported with the water stream in the plant.

Just add water: without water, ants are able to walk over the ridged lip of the pitcher without falling in. With water, the microscopic ridges turn into a water slide and the ants fall into the digestive juices at the bottom of the trap. Video courtesy of Ulrike Bauer

Ulrike: So, as you will probably know, you can spray water on a leaf and the plant will take it up. Spray fertiliser works that way, you spray your leaves and the plant takes up the fertiliser. And the water stream in the plant goes generally from the roof to the tip or in the leaf from the base of the leaf to the tip of the leaf. So if you apply your carbon nanodot solution with your genes at the base of a leaf, it moves to the tip of that leaf. It doesn't go the other way. So you can literally transform a single leaf on a single plant by applying your nanodots just in that leaf base.

Sally: So you are looking at completely non-genetically modified plants, and it's only when you add these nanodots is any kind of genetic manipulation or fiddling around happening.

Ulrike: Correct. So we've done a trial at the pitcher plan where we just put a fluorescent protein gene, like a GFP, which you can just visualise that in the microscope.

Sally: It glows green.

Ulrike: Yeah, it glows green. Exactly. So we just put that nanodot solution with GFP tagged on it, put it at a pitcher plant leaf, and then wait a few days and take the pitcher and put it in a fluorescent microscope. And you see green fluorescence in some of the cells.

Sally: So you don't have to even worry about where is it incorporating in the genome or...

Ulrike: No, you don't.

Sally: ... any of the genome, it's literally just once it's inside the cell...

Ulrike: It's being expressed.

Sally: ...the cell will treat it as normal. Yeah.

Ulrike: Yeah. It's just being expressed along other things. 

Sally: Amazing. What is the big goal for the next five years for you? What's the future of pitcher plant research gonna be?

Ulrike: Well, the big ambitious goal is to work out how the plant makes this crazy surface and then see if we can get completely different plants, that are not carnivorous, make the same surface by messing with their genes. So we have a research grant at the moment that's a collaboration, actually with Tanya that you're gonna talk to, to like see if we can use these and express them, let's say in a crop plant.

Ulrike: Wouldn't it be cool if we would have crop plants, especially in the UK where it rains so much, that gets slippery when it's wet. Our average apple that you bite into has seen about 30 to 35 pesticide applications before you bite into it, right? It's been sprayed against insects about 30 times. So if we could have a surface on the fruit, or on the stem that leads to the fruit, that repels insects or even just washes every insect off, every time it rains.

Sally: Oh, I see. On the outside of the fruit.

Ulrike: Then we wouldn't have to spray them that often, and if anything, we would have to just spray them with water when it doesn't rain, which we commonly do anyways, and we would wash off all the pest insects. It's not so harmful for the environment. It's not harmful for the consumer, so it could be a really nice solution how to have better protected crops for the future.