Tanya Renner: Creating carnivorous ketchup

Image Courtesy of Tanya Renner

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And finally we’re moving away from pitcher plants and looking at another group of carnivorous plants that trap their prey in a completely different way, and that’s the sundews. I had a chat with Dr Tanya Renner from Pennsylvania State University whose interested not only in how these plants evolved but also whether we can add carnivorous genes into non carnivorous plants…

Sally: What kind of carnivorous plants are you interested in? 'Cause there are lots in the world.

Tanya: Yeah, we mostly study the sticky ones, so those that have trichomes on them, sticky trichomes, which are plant hairs.

Sticky hairs

Cape sundew showing the red trichomes with sticky, digestive juices at the tips

Sally: Are trichomes only found on carnivorous plants?

Tanya: No many plants have trichomes. Your basic tomato plant has sticky trichomes as well.

Tanya: So this is a way that plants usually use to protect themselves against herbivores that would be crawling on the leaf surface or on the stem surface.

Sally: What are the sorts of carnivorous plants that use stickiness?

Tanya: Two basic ones that folks might be familiar with are sundews. Then those have lots of sticky hairs on them. And then there's the butterworts, and those also have sticky hairs.

Tanya: They're different morphologically, so how they look and the internal structure of them. But generally they look like a little hair that has stickiness at the very end of them, a droplet of a dew.

Sally: We know about pitcher plants that insect falls in and gets digested, and then you can absorb it through the liquid. We're all very familiar with Venus flytraps, it gets trapped and then the plant can absorb the nutrition that way. How does a sundew go about trapping its insect and eating it?

Tanya: Right, so sundews are pretty exciting and interesting, because they actually can curl with their stalked glands over a trapped piece of a prey, so an insect.

Tanya: And so, they basically try to get as much surface area on the insect as possible, and the dew on the ends of those trichomes smother the insect and they're not able to get, at least very easily.

Sally: It's like a fly stuck on fly paper. Once you've got the insect stuck, how does it eat it?

Tanya: Right. So those glands have multiple roles.

Tanya: They have the sticky role, and then they have the other role where they can excrete digestive enzymes from those same glands, and then also absorb nutrients from the digested prey through that gland. So I guess in that terms, they have three different roles: sticking, digesting, and absorbing.

Sally: How on earth does that evolve? Where did these digestive enzymes come from? What were they being used for before?

Tanya: So we think that the ancestor of carnivorous plants likely had these sticky glands that were involved in defence. And then over time they evolved special roles to be able to move enzymes out of them and to absorb nutrients into them. And some other unusual things about some of the carnivorous glands is that they can have conductive tissue associated with them. And so...

Sally: When you say conductive tissue... it's conducting what? Conducting electricity?

Tanya: No. So conducting fluid, that would be it.

Sally: Okay.

Tanya: So for example, in the sundews they have what is called xylems and we know xylem moves water and other dissolved nutrients throughout the plant.

Tanya: And so this is kind of a weird thing for a trichome to have. And so there's some ideas that possibly the stalked glands of sundew might actually be a modified leaf, like a little leaflet.

Sally: So you think a early carnivorous plant would've had these hairs or leaflets that were exuding these digestive, or these sticky substances we should say. How does it then absorb the nutrition from that? Because I don't think of a plant as having like a gut. So why would it be producing digestive enzymes?

Tanya: So all plants make defensive enzymes, and what's really interesting about those enzymes that we find in the fluid of carnivorous plants is that many of those proteins that are doing the digesting belong to the same protein families as these defensive genes in non-carnivorous plants.

Tanya: So we think that defensive genes that underlie these proteins, that they were co-opted or taken for a new function, that being used for digestion. And so, by having some conductive tissue, this may have pushed for some of these proteins to actually be exported to the outside of the plant body. And because of those conductive tissues, at least in some carnivores, these glands have those, that may also help with absorbing the digested nutrients on the outside of the plant and bringing them to the inside of the plant.

Sally: So the same tubes that are delivering these chemicals can also suck them back in again.

Boggy beauty

Most sundews are tiny plants that can be hard to spot mixed in amongst sphagnum mosses. Image courtesy of Tanya Renner

Tanya: Yes, that's the idea. Again, we're still learning a lot about this.

Sally: This is a theme it seems among carnivorous plant researchers is that noone really knows.

Tanya: We don't know!

Sally: Yeah, and I suppose the bit that I couldn't quite work out is that they're not, I shouldn't think of them as digestive enzymes. They're first and foremost defense enzymes. And it just so happens that a plant's way of defending itself against these herbivores, happens to also involve digesting them.

Tanya: Right. So a lot of these activities of these defensive genes, could be acting against these insect herbivores. So when an insect is biting on a plant and digesting that tissue, they could be uptaking these defensive proteins and then they go into the stomach of the insect and can be digesting the insect from their stomach outwards!

Tanya: And that will make the insect really sick and keep them from wanting to eat more of that plant.

Sally: No kidding! That is incredible. Now you've been looking at digestive enzymes and defensive enzymes in carnivorous plants, but now you are taking them out of carnivorous plants and putting them into crop plants! Tell me more about that.

Tanya: Alright, so one of the things that we're really interested in is whether or not we can take these genes that are used in carnivory and pop them into crop plants. And so we've been working with a model system, that being tobacco, to try and get these carnivorous genes into tobacco. And so what we want to see is that if we put some of these digestive enzymes in tobacco, does that actually help tobacco be more resistant to herbivorous insects than if it was just using its own defensive protein?

Sally: And I have to ask, why tobacco? Because it's a plant that I see, hopefully we won't be growing too much of in the future.

Tanya: Yeah, so it is considered a genetic model because it's really good for understanding protein functions. And so we are started there, and then we're interested in moving to other plants that are related to tobacco like tomato. And so that would be a next, second plant to try this in.

Sally: So how do you go about working out which genes are the ones that you need? Because it's never gonna be, you need just one gene.

Tanya: And there are lots of genes!

Sally: That'll be like the gene make a protein, and the gene to move the protein around. And then the gene to turn on the gene that makes the protein. So how do you work out which ones are necessary and then how do you go about actually putting that not only in the tobacco genome, but that it gets expressed in the right place.

Tanya: Right. So this is a conundrum because there are thousands of genes, right? And so one of the things that we're working with Ulrike, and then also another collaborator at the University of Würzburg, Kenji Fukushima, is to figure out which types of enzymes are shared across distantly related carnivorous plants. So we're kind of whittling it down to groups of proteins that have actually convergently evolved to be present in the digestive fluid of distantly related carnivorous plants.

Tanya: And the way to get it into tobacco is we make a construct that has that carnivorous plant gene in it, and then we actually put it into Agrobacterium. And use Agrobacterium to infect these carnivorous plants that we have in the lab. And Agrobacterium-mediated transformation is what we use to try and get these genes into the tobacco.

Sally: They work kind of like viruses, don't they? You're hijacking a natural bacterial "I'm just gonna insert my own DNA in so you can replicate it" system.

Tanya: Right, exactly. Yeah. So now, it's not actually going into the genome of tobacco, but on that construct there is a promoter on there that helps turn on that gene.

Tanya: And so we're hoping that it gets into the plant's body and that we can confirm these things, and we're just starting that right now! We don't have our carnivorous tobacco yet, but we're getting close.

Sally: And why, other than the fact that it gives you a great opportunity to learn more about carnivorous plants, why might you want to make non-carnivorous plants carnivorous?

Tanya: So one of the big things that we're really interested in is preserving our crops and feeding lots and lots of people. And so we wanna figure out if there are natural proteins that exist in plants that are here today, you know, on earth, if we can use those to help protect our food. And so one idea is that once we can confirm this happens in a model, we would want to try maybe tomato, for example, and other types of crops.

Living fly paper

As the beetle continues to struggle, it’ll touch more of the trichomes, covering itself even more in sticky digestive fluid.

Tanya: And so we're trying to develop a way to understand and to protect plants from insects.

Sally: So 10 years down the line, are we going to have greenhouses of tomatoes that not only stop insects in their tracks that are trying to eat them so we don't have to spray pesticides, but also then eat the insects and get more nutrition so we don't have to apply fertilisers! Is that gonna happen?

Tanya: That would be fantastic. And that is the ultimate goal. But that would be great, right? To be able to have this dual function enzyme that is involved in protecting, but also bringing in essential nutrients to the plant.

Sally: And tomatoes would be your first choice of like commercial crop because it is so closely related as a species to tobacco.

Sally: Tomatoes are also in the same family as potatoes, right?

Tanya: Yes, Solanaceae. 

Sally: Yeah. So does that mean that we can also have our carnivorous chips with our carnivorous ketchup?

Tanya: Potentially. So again, we're using model plants that have these sticky glands, and so the idea is that if we focus on those plants for now, since our carnivorous plants also have sticky glands, that perhaps those sticky glands will allow these crop plants to be able to export these digestive enzymes to the outside of those plants.