In a study published in Science in 2001, Andre Kessler and Ian Baldwin quantified and characterised the volatiles emitted by Nicotiana attenuata when attacked by herbivores and mimicked their release to see if they reduced herbivory by attracting egg predators inhibiting herbivore oviposition. Their results suggested that release of volatiles could reduce presence of herbivores by over 90%. Fifteen years after the paper was published I spoke to Andre Kessler about his motivation to do this study, memories of field and lab work, and what we have learnt since about the role of plant volatiles in protecting plants from herbivory.
Citation: Kessler, A., & Baldwin, I. T. (2001). Defensive function of herbivore-induced plant volatile emissions in nature. Science, 291(5511), 2141-2144.
Date of interview: 13 December 2016 (via Skype)
Hari Sridhar: What was the motivation for this particular piece of work was in relation to the rest of your PhD?
Andre Kessler: In a way this was the focus of my PhD to begin with, because it was started from a question that was generally asked in that time.You have to consider that, in the 90s primarily, there was increasingly mounting evidence that indirect defense was a very common thing, and particularly mediated through volatile compounds that are induced by herbivores. However, most of the studies, until I brought my study out, were done on agricultural systems in the lab. There was not even a field agricultural system that was intensively studied, except for one exception: Jennifer Taylor’s work on tomato. That was also an agricultural system. And so, there was this question of whether herbivore-induced volatile emission actually functions as an indirect defense in natural systems. Agricultural systems, as well as experimental laboratory or greenhouse systems, tend to use genetically identical plants. Usually, they were inbred lines of plants or the same genotype of plants. And that, from the perspective of approaching predators and parasitoids provides a very clear signal. The volatile emission produced in response to herbivores is very similar in every single plant in that population. But it was known already, at that point, that different genotypes of plants may do different things in response to the same herbivore. Therefore, in nature, where you have a variation of plant genotypes, there may be a total confusion of the information that’s transferred chemically from the plant to the searching insect,such as a parasitoid or predator. And that’s why I decided to ask that question, is there indirect resistance mediated by herbivore-induced plant volatiles in a natural system, with this wild tobacco plant growing in Utah. And then, if it was true, which is what was shown in that Science paper in 2001, that there is attraction of predators, does that attraction have some significant effect on herbivore loads on the plants? This is pretty much what we did in this experiment. We first measured volatiles under field conditions and showed that there was herbivory-inducible volatile emission. We found that there was a predator species responding to that signal, numerically, and dramatically reduced herbivore loads on these plants in the field. Finally, we also manipulated volatile emissions. There were a couple of volatile compounds that were identified as commonly emitted by all genotypes of plants in response to herbivory. When we perfumed the plants with those volatile compounds,we were able to attract predators to the plants,which then had dramatic effects on the herbivore load.The second important part of that paper, which was also novel at that time, was we found that the adult moths use the same information that predators used to find their prey. The adults use that same information to avoid already damaged plans for oviposition. We had calculated that that combined effect of attracting predators and repelling adult moths for oviposition reduced the herbivore load by about 90%. This is a very dramatic effect. It is more than what you usually accomplish with pesticide spray or with any direct resistance trait.It was the combination of the two that made that happen. And, given the high predation pressure and pretty targeted search of the predators for volatiles that come from damaged plants, the adult moths must, very likely, have avoided ovipositing on damaged plants because they would put their offspring directly in danger.
HS: Stepping back a bit, how did you get interested in plants, and you know, specifically this topic of herbivory and plant-animal interactions?
AK: My Master’s thesis was at University of Würzburg in Germany on insect biodiversity patterns on trees. We tried to understand how insect communities are structured and what external factors are driving the composition of insect communities in tree crowns. We did experiments, both, in Europe as well as in Borneo in Malaysia. Already, at that time, there was quite a bit of evidence in the literature that plant chemistry is mediating interactions with insect herbivores or with any kind of herbivore. But the question was, in how far can plant chemistry determine or influence insect community composition.This is pretty much what started that question. Back then, I was very naive, because I didn’t know how to analyze chemistry really well. And so, I took an internship at the, then, newly founded Max Planck Institute for Chemical Ecology, with Ian Baldwin, who later became my PhD advisor. I got there and I remember, very naively, standing and asking, I want to learn how to analyze plant chemistry, to then go back and analyze the tree chemistry, and then try to use that as a correlate for how arthropod communities were composed. Of course, this was very naive, but I learned that very quickly while I was there, and then started my PhD on this wild tobacco plant, which was, at that point, beginning to become a model system for studying plant-insect interactions in a wild species. There were a couple of agricultural model systems, but, as I said in the beginning, there is always this question, what do studies on domesticated species tell us about what’s happening in nature? Is that actually how nature works or is it an artifact of our very well-enclosed systems? Even if you work in an open agricultural field, it’s still a very close system, in a way. It’s one habitat that has, usually, one plant species and a certain set of insect species interacting with them.But this is not necessarily how it works in nature.There are some more complex interactions. And the question back then was, is it at least comparable? And by now, I think we know it is quite comparable, even though there are some big differences.
HS: Why is it that, around this time, people began to think that tobacco was a useful model system?
AK: That’s a very good question. There are two things to be said about that. There was already domesticated tobacco – Nicotiana tabacum – as well as another species, Nicotiana benthamiana, an Australian tobacco plant that was a model system in genetic transformation research. These species were, already, molecular biology genetics model systems. Therefore, another wild tobacco model has the benefit of being able to use all the molecular and genetic tools that were already developed for the other two species. You could readily do molecular biology and genetics on that plant. But, it could have been any wild tobacco plant. It just turned out that the person who started that, my former advisor, Ian Baldwin, had worked with this plant for his PhD thesis. And he continued working on his PhD model system, and at the Max Planck Institute for Chemical Ecology, had this opportunity to take a wild species and turn it into a molecular and ecological and even physiological model system. Resources were more readily available at Max Planck, at least back then, and this allowed him and his group to make this giant leap, and establish a model for many molecular biology and genetics studies on other wild species to follow. This has kind of revolutionized the field because we, now, can really ask evolutionary questions in chemical ecology, in addition to the mechanistic questions that gave birth to the field of chemical ecology. And so, it was a good model system because there was something known about its ecology, to begin with,and it had relatives that were already models in molecular biology and genetics.
HS: How did you choose to work in this field site in Utah?
AK: The wild tobacco plant grows in habitats of the Great Basin Desert, of Utah, Nevada, and Colorado. And this particular site, Lytle Preserve, is owned by Brigham Young University. They had a little station, to which you could go back every evening after work. And because this was already established, pretty much in the middle of the habitat where wild tobacco plants grow, this was a straightforward choice. You have to consider that wild tobacco plants grow only when the late succession vegetation, which is a sagebrush-dominated community of plants, is burnt down. Tobacco is one of the pioneer plants that comes after a wildfire destroys the original vegetation. And so, you’re dealing with a situation where you have to find a new population every year. Every time you go out for field research, you have to find that one burnt area where wild tobacco plants are growing. And so, you need some sort of field base from where you can go to all the places that potentially have wild tobaccos.
HS: In the paper, you say that this particular 2.72 square kilometer area where you worked burned for seven days after a lightning strike on 17 June 1999. Did you decide to work in this particular 2.72 kilometer area after you discovered that it had burned?
AK: Yes. In 2000, we found that burnt site with tobacco plants in it, and, retrospectively, wrote in the paper about the history of that burn. It was important to make sure this site was in the first year after the burn. Tobacco plants can grow up to two years after an area has burned, but in the second year, other plants come in. In the first year, there are very few plant species other than wild tobacco. That’s why this information was important, that it was the first year after the burn. Tobacco will not germinate immediately after a burn in June because there is no water. The entire desert is fed by water that comes from precipitation during the winter – snowpack as well as some spring rain. There are some June-July thunderstorms, which probably caused this burn, too, but they will not provide a lot of water. And so, there’s, usually, no germination after May. There’s simply not enough water. And that’s why the plants emerge only the year after the burn.
HS: You mention an ID for the fire – Fire Number W246. Tell us about that.
AK: That’s a very good question. This is typical for public land. In the western United States, there are large areas, especially in the desert, that are considered public land. This public land is administered by the Bureau of Public Land Management. They keep track of everything. For example, when they lease land to farmers that have cattle there, they write that down – particular areas that they close off for them and things like that. And then, when a burn happens, they are also responsible to either let it burn and record it or extinguish the burn. If it becomes too large, they start fighting the fire. And as soon as the firefighters step in, that burn gets a name. They have to refer to it, somehow, in the documents, as well as, when they coordinate the firefighting.
HS: Was 2000 the first time you visited this site, and the first time you saw a wild tobacco plant?
AK: No, I was there in 1999. That was my first year, actually, in the field. That was back in 99. I wasn’t an official PhD candidate yet. So, it was, pretty much just, learning the system.
HS: Was this field site as already being used for a long time by Ian Baldwin?
AK: Not the particular field site, but the larger area. The benefit of working on public land is you don’t need additional research permits that you would need on private lands. We just need a general research permit from the Bureau of Land Management which we had. And then you have a very large area where you can work, which, in this particular system, is very important because the burns are in different places every year, across a very large area.
HS: Was all the work that went into this paper done in 2000?
AK: Let me think about that. I’m just trying to think if there were some small experiments that we didn’t do then. Yes, I think I did all the experiments, as well as collecting the adult moth oviposition data, in that same season.
HS: Could you give us a sense of what this work involved? What was your daily routine?
AK: This is a very good question. 1999 and 2000 were the first two years in which we went into the field in Utah from Max Planck. Ian Baldwin had been there before, obviously, for his PhD thesis,and also when he was an assistant professor at University of Buffalo. This was the first time that he sent students out into the field. In1999 , I was with another PhD student, and we were camping in tents in that preserve. From there, we drove to the field sites every day. We had a jeep that could go off road because it’s a very rough terrain. In the second year, there were two more students,in addition to me.We still had tents, but we’d also rented a small trailer that we could haul around and park in that station. We did our field trips from that station. In 2001, Ian Baldwin’s lab in Max Planck, started to put more trailers there, so that more students could be there. And we also had one horse trailer, which is really funny. The trailers that transport horses are a little bit larger, and so, we turned one of those horse trailers into our scientific trailer, which had a lab in there. This was in 2000. When we saw how important plant volatile emission is, Ian Baldwin bought a gas chromatograph mass spectrometer that we put into that trailer. And then we put, I think,two four square meter solar panels up, on which we ran the GCMS. It was really impressive. There’s a lot of sun light, since it is the desert, and we were able to run a GCMS off of solar panels. It was really impressive and it is still there. It grew more and more, and, now, there’s almost a little village of little trailers that facilitate the work of the researchers there.
HS: How did you do the GCMS work during your experiments? You say, “Analysis was performed at the field sites with a Shimadzu (Model 5000) quadrupole GC-MS”.
AK: That means, actually, that we didn’t collect all the data in that one year. I was just thinking of that. Some volatile analyses were actually done in the field. So, in the first year, which is part of the volatile data, I collected the volatiles onto charcoal traps that I then sent home to Jena in Germany, where they were extracted and then analyzed on a GCMS. And that’s why the data actually appear in the paper. I finished the paper actually in the field. In early 2001, we already had the Shimadzu GCMS in the in the field, and so I collected another set of volatiles, to see simply if the volatile emission was different from the ones that we sent home. This was just to make sure, for the emissions that we had collected the year before, and then sent home, whether or not the shipping had changed something on the traps. It hadn’t. That’s why in that paper, it is another volatile set that includes the other two species of herbivores. This is an important point. The question was, if it’s a generalist predator that we had in the system, would that predator respond to any kind of damage,and not only to the damage of the caterpillars that were the focus of our study? There are two other herbivore species that are in the volatile data – a Chrysomelid beetle and a Mirid bug. These data were collected a year after and then analyzed on that Shimadzu GCMS.
HS: What were the names of the students who were working alongside you?
AK: In 1999,it was Cathy Preston. And in the other year it was Grit Glawe and Claudia Voelckel. They were all students from Max Planck and went on to do great things.
HS: Did Ian Baldwin visit you during field work?
AK: He’s out in the field more often these days. Back then, he would come for a week or so, every season, just to see how things are going and whether we’re still alive. But we had weekly phone calls, I think. Back then, we didn’t have internet in the in the field station. Now they actually have a satellite internet connection that we didn’t have back then. So we had weekly to bi-weekly trips to the town and went to the library to get an internet connection.
HS: What was the name of the town?
AK: St. George.
HS: PhD student – supervisor interaction span a wide range, from supervisors who are completely hands-on to those that let the students do whatever they want. Where would you place your relationship with Ian Baldwin along this spectrum?
AK: I had a lot of freedom. This, simply, has to do with the logistics of the system. When we started, in the first year, we had only these big battery, car phones, It was such a remote place that we would have to drive to the top of the mountain to get any phone reception. And it was very expensive to call Germany on a cell phone. That’s why we had only weekly connections, either through the internet when we drove down to town and went to the library, or talking on the phone. This meant that, in the field, we had to do our own stuff. Of course, we had discussed most of the experiments that we were going to do before we actually went to the field. But then, as you may know, when you’re in the field, things are always different. You have to adjust, and have to add an experiment, and have to do a different experiment, and alter the experimental procedure or things like that. All this, we had to do on our own. This generated some very interesting stuff, being allowed to be that free and open and start random experiments,which did not, necessarily, have to do with the specific target experiment that we wanted to do.It opened up all these other new studies that we ended up doing. I’ll give you an example.I measured volatiles from plants that were damaged by Mirid bugs. So, we saw them already as one of the major herbivores in these studies. But in that year, when I did the other study, we had another plant population, and I did some research on how the adult moths are actually choosing the plants for oviposition. The major herbivores were tomato hornworms – Manduca quinquemaculata – and, as I had found out in that first study, they would avoid plants that were previously damaged, for oviposition. This is, very likely, because they avoid high predation pressure, but also bad food quality, because the plants very easily and very strongly induceresistance after damage. I had different field sites that I constantly visited to see if there are differences in oviposition rates of these moths. Some of the populations had very high oviposition rates of the Manduca moths, but then almost none of the larvae survive to adulthood. It was very low survival rate. And it turned out – because I did these surveys that were not planned before – that this was associated with the presence of this Mirid bug, this very small bug, that turned out to actually vaccinate the plants against the more damaging caterpillars. And it was simply because the generalist predator would attack, not only the caterpillars but also the Mirid bugs, which meant that if the Mirid bug is on the plant, the predator will search for food, but then would get the caterpillars, which are much easier prey. And this became a very nice study in The Plant Journal that came out in 2004, and that generated many more studies that came afterwards. And it’s also one of my most cited papers, and this was totally born out of initially unintended experimentation that I just did in addition to what we had planned. And I think this is where, Ian Baldwin, to me at least, was a very open advisor, and very open to me doing whatever I felt was important.
HS: In the paper you refer to unpublished results in two places. First is where you say, “Previous laboratory experiments with N. attenuata revealed that whole-plant VOC emissions were quantitatively related to the amount of herbivore-specific leaf damage.” And then, you also refer to it in relation to an extensive study of mortality factors of both Manduca species.” Did you publish these results in another paper subsequently?
AK: Yes. The Manduca data were published in an Ecology paper in 2002. The data on amount of damage ended up in two places. One is in the 2004 paper in The Plant Journal. Well, part of those data were already in a paper by Rayko Halitschke in 2000 in Oecologia.
HS: In one place you mention a continuous seven-hour sampling. Could you say a little more about that?
AK: Yes, this, kind of, revolutionized the field. It’s a very cheap method. We had to come up with something that was portable. Until I did these experiments, inducible volatile emission was measured in closed systems, usually with glass chambers that were supplied with purified air. So you overload with purified air, and then you pull the air coming off the plant out of those chambers, strapping them onto some adsorbent tube. You can have charcoal-based adsorbent tubes, you can use something called Tenax or Super-Q, which are synthetic adsorbents. These are all adsorbent compounds that hold on to organic compounds, and then you elute the compounds that you trap on those adsorbents with some solvent like dichloromethane or hexane, and analyze it in GCMS. We, obviously, could not have glass jars out there, for two reasons. One is, it would have been too expensive to transport them around, and even trap from them, because then you need some electricity or something like that, because the pumps are larger. And, more importantly, you cannot do ecological experiments with it. We had a system set up at Max Planck, that was in a growth chamber, that had only two of these glass chambers. So you could only trap from a control and one treatment at a time. And then, you had to repeat that experiment several times to get replicates. In the field, you cannot do that.In the field, you want to have all your samples, ideally, collected at the same time. You need a system that’s, first, portable – you also need portable electricity to run the pump -and that allows you to trap from many plants at the same time. So, I developed a system, which is very, very simple and straightforward using plastic cups. The type that you get in a coffee shop when you order an iced coffee – clear plastic cups. We put that over a leaf or a plant and supplied it with a little connector that fit the adsorbent trap. In our case, it was charcoal traps in most cases. And then, we connected those traps to a manifold that went from just one pump. What that did was we could trap up to 16 samples from one pump, at a time; if you had two pumps, you could do 32 samples. And we could run the pumps from 12 volt car batteries. And this kind of revolutionized everything. We don’t go for precision. Because the problem is, you suck in ambient air, so you get all the volatiles that surround the plant as well. But for that we had empty chamber controls, where we said, everything that comes into that empty chamber control, is not necessarily coming from the plant,and we can subtract that from the other chromatograms. We traded precision in the chemical analysisfor being able to do an ecological experiment with multiple replicates. This is,really, what my lab now goes on to do. We, now, do even evolutionary studies where we have up to 400 samples.We have big representations of populations that we trap volatiles from. When we started that, we thought, maybe we’ll have huge problems with that background we are trapping. Under the Utah sun, there’s high UV radiation. When you have very strong sunlight, UV radiation breaks down organic compounds very quickly. Everything that’s emitted off the plant is almost immediately broken down. This means that the air that we sucked in was very purified air. So this system, with these cups,works very well in the field, but it does not work at all in greenhouses.In the greenhouse, all plants in there emit something, and this will all end up as your background.You can still do that when you have very few plants in the growth chamber or the greenhouse. But in the greenhouse or growth chamber, it’s better to have a very close system, where you supply purified air. Coming back to your question about the seven hour sampling – this is an important point, too – because we don’t have purified air, you have to elevate what’s coming from the plant above the background. And for that, you need cumulative trapping. If you have this, very expensive system that I was talking about, with the glass chambers, you can trap for just one hour and you get sufficient amount of compounds emitted to analyze them. But you will not see the difference between the, open chambers in the field, when you do that with ambient air. That’s why we had to trap for six to eight hours.
HS: You say, “In another experiment in the same population of plants, in which distances between similarly treated plants were only 3 to 5 m of each other, predation rates were 13-fold higher (Fig. 2B; data for MeJA and control treatments shown), probably because of the lack of independence of replicates within a treatment.” Did you first try this clumped treatment, realize the problem, and therefore do the transect treatment?
AK: No. The question was, if you have a patch of damaged plants, is that more attractive to a predator, than if you have only a single plant within a large ocean, pretty much, of non-damaged plants. It was, actually, planned from the very beginning that way. We wanted to understand, does a predator have the ability to find its prey,even if there’s only one plant in the entire field? And is that targeted search by the predator still more efficient than just randomly running around? The treatments were actually done so that every plant was supplying food for the predator. The control plants also had eggs on them. So, if there had been just random movement, independent of the volatile emission, they should have gotten all of it equally well. Yeah. I had to have the clumped treatment to address that question, but it was almost my safeguard, in case it would not work with individual plants. It was more of that! The independence of replicates in these lines is not referring to statistical independence but rather to ecological independence. And, this is, pretty much, what the data shows. If you have more than one plant there, it’s actually easier to find. This was an important question, because there was a lot of discussion about whether that can ever be used in agriculture? Would the predators find them, even if you have relatively small population sizes? In agriculture, it’s important to prevent the growth of pest populations. If the predators are not efficient in finding them, as soon as they appear, it will not work. This was, kind of, the point of this study, at least for this part of the study.
HS: Do you still continue to work in this study site?
AK: No. We did for a while. In 2011,we had the last paper that involved tobacco. I continued when I started as a professor here, at Cornell. I continued to work on tobacco quite intensively still, especially on that part with Mirid bugs. I had talked with Ian Baldwin about that, so that we don’t overlap much. But then, my group got more excited about other study systems that were closer to home. But conceptually, I’m still working on similar things.
HS: When was the last time you visited that site?
AK: That’s a good question. Actually, in 2014, I went there with my brother. I chaired the Gordon Conference on plant volatiles in 2014, which was in California. After the conference was over, and I was happy that it was over being the chair, I thought I deserved a little vacation. So, I drove for three days with my brother to Utah, who was also working, actually, in Ian Baldwin’s lab. It is funny that you ask, because we actually went to the site where I did these experiments.
HS: How has it changed from the time you did these experiments?
AK: This is also an interesting question. The last time that I was actively working there was 2005.By that time, it had grown over completely again. But when we went back in 2014, it had, actually burned a second time, just a year before. It was a huge burn, far more than it was in 1999-2000. So, it looked actually very similar, at that point.
HS: What about the larger preserve within which this site is located? Has that changed in anyway?
AK: There have been a lot of changes. Max Planck invested a lot into building infrastructure. They now have two or three laboratory trailers and, I think, four living. They have another set of large solar panels. They also have field sites that are in the station, where they can do common garden experiments. Now they also have all these phenotypically-modified plants that I had started working with in 2003. My second Science paper together with Rayko Halitschke and Ian Baldwin, in 2004, was on that, the first release of a wild plant that was genetically transformed to not induce resistance. So that’s when we started having common garden field sites that were in the preserve. This has grown dramatically today, with really big fields, now, for common garden experiments. It has changed quite a bit.
HS: Is your brother an ecologist too?
AK: Yes. Danny started as a technical assistant and in Ian Baldwin’s lab, but published very extensively and very successfully, and then finally did his PhD a few years ago. He just started heading the greenhouse facilities at Max Planck, but still doing some research.
HS: How long did the writing of the paper take, and when and where did you do most of the writing?
AK: That’s a good question. Back then, I have to tell you, I did most of my writing on the train, for whatever reasons.In Germany, you use the train and public transportation a lot, and I did a lot of writing on the train. I, definitely, did most of the writing back in Germany; not in the field. This is, mostly, because most of the chemical analyses were done in Jena. Even though we ran the samples in Utah, detailed analyses were done on computers in Jena. I’m not very good in writing in the field. I’m too distracted.
HS: Do you remember how long it took?
AK: It was very quick. It was, pretty much, as soon as I came back. We did the experiments in 2000, I came back to Germany in August that year, and the paper was published in April 2001, I think. Do you have the dates?
HS: It was submitted in October 2000 and accepted in February 2001.
AK: Yes, so, I wrote it immediately when I came home. I definitely had a lot of the data analyzed by the time I came home, because, I remember, I gave a presentation the day after I came home. So, I probably had written the material and methods and had analyzed all the data except the volatiles. I wrote it very quickly after that.
HS: Was Ian Baldwin involved in the writing?
AK: I wrote the first draft, but he was definitely involved. This was the first paper that I wrote as a PhD student. Science has a very unusual format. It’s a much more storytelling format, not a typical scientific way of writing. Back then, it was even more so. The Material and Methods section was a footnote. The style of writing is very different,and that’s why he was, quite a bit, involved.
HS: Can we go over the names of people you acknowledge, to get a sense of who these people were and how they helped?
AK: Sure.
HS: P. Feeny
AK: This is very interesting. Paul Feeny was a Cornell professor. And, in a way, I got his position. He retired a few years ago. He was the chemical ecologist in the Ecology and Evolution Department at Cornell here. I think we sent him the paper for some comments. Back then, Science had that rule that, if you wanted to submit a paper, you had to already have comments from, at least, two important people in the field. You needed an endorsement by two important people in the field, before it went out to review. It was, almost, like you had to do your own review cycle. Paul Feeny was one of them. He’s a giant in chemical ecology; one of the pioneers in chemical ecology. He’s contributed a lot to the theory of chemical defenses in plants. He’s still around and I see him regularly here at Cornell.
HS: J. Gershenzon
AK: Jonathan Gershenzon was the other one reading it. He’s also one of the directors at Max Planck for Chemical Ecology. Ian Baldwin is one of the directors. He leads one department and Jonathan Gershenzon leads another department. He is one of the absolute top plant physiologists in the world, and he has done a lot to understand plant metabolism, in particular, volatile biosynthesis or terpenoid biosynthesis. And that’s why it was logical to send that paper to him for pre-review. He would have had a conflict of interest, anyway, as a reviewer, right, because he was also at Max Planck.
HS: M. Hilker
AM: Monika Hilker was the third pre-reviewer! She’s a chemical ecologist from the Freie University in Berlin. She is a specialist in studying induced responses to oviposition, probably the world authority on studying oviposition responses. She studies the elicited compounds that come out of oviposition fluids from insects on the surface of the egg that then induce metabolic responses in the plant. She was a logical person to review the aspect on oviposition by Manduca moths in our paper.
HS: J. McNeil
AK: Oh, he’s another one. Might have been four! Jeremy McNeil is a professor in Canada. He’s in London, Ontario. He’s very high up in academia in Canada and is one the leading entomologists in the world. He studies a lot of these complex multi-trophic level interactions in insects.
HS: F. Roces
AK: Oh, this is funny. Flavio Roces also read the paper and gave some comments. He was one of my advisors when I was an undergrad student at the University of Würzburg. He commented on the volatile chemistry because he had studied bee behavior in response to volatiles.
HS: A. Roda
AK: Amy Roda was a postdoc, at the time, at Max Planck, in Ian Baldwin’s lab. She also commented on the manuscript.
HS: E. Wheeler
AK: Emily Wheeler was Ian Baldwin’s wife. She’s a linguist. She proofread the paper for language. This is a really impressive team! I didn’t remember that. I should write to them all.
HS: B. Crock
AK: Bernd Crock. He ran the chemical analytical facility at Max Planck. I, pretty much, learned how to analyze the chemistry data from him.
HS: H. Thomas. You thank him and B. Crock for purifying the cis–α-bergamotene.
AK: Oh, that’s right. There was no commercially available bergamotene. Bergamotene was one of the compounds that was strongest induced in the experiments. I wanted to use the ones that were strongest induced, which explained the difference between damaged and non-damaged plants, I wanted to use those compounds in the field experiment, to perfume the plants with them, right. I don’t know H. Thomas very well, but I think he was an intern in the lab at that time, an assistant of Bernd Crock. They were both chemists and they purified the bergamotene from opoponax oil, which is an essential oil that has a relatively high content of bergamotene. It was a lot of labor. You have to distill that oil, and then fractionate it to get the bergamotene separately.
HS: R. Baumann, for assistance with species determinations
AK: He is a professor at Brigham Young University, and the curator of the insect collection in the Natural History Museum there. He helped us to identify, especially, the flea beetles that we had in the experiment. We were able to identify all the other insects in our own, but the flea beetles were something that we gave to him. This was, actually, from more than just this experiment. It was funny, I kept collecting these beetles, and sending them to him, and they found one beetle that was new to Utah, and one beetle that was entirely new to science, in that survey.
HS: Would you know if the insect community has changed in this study site, especially in terms of the three dominant herbivores and the predator species that you found?
AK: They are all very flexible. The burns on which these tobacco plants grow are very ephemeral habitats. The insects have to find them. In some cases, they don’t even find the plant population. So,you might not have any herbivores on a particular population. This is perfect for an experiment because you can then introduce insects. Because of this stochasticity, who becomes dominant is usually a question of who arrives first. So, there could be populations that have only Mirids, or populations that have only Manduca, and so on.
HS: Did this paper have a relatively smooth ride through peer review? Was Science the first place you submitted this to?
AK: Let me think about that. It was actually very straightforward. It was probably one of my easiest papers ever accepted. There was only minor revision, actually. And then it went.
HS: Was Science was the first place you submitted this to?
AK: Yes.
HS: At the time when the paper came out, do you remember how it was received? Did it attract a lot of attention?
AK: Yes, this is still the paper that has attracted most attention. I thought it would be the Science paper on the genetically transformed plant. This was in 2004, and so I thought there would be a lot of interest because of the genetically transformed organisms we used. I thought we will be washed away by the public attention to that 2004 paper.But that didn’t actually happen. There was relatively minor attention to that 2004 paper. Actually, scientifically, there was a lot of attention.It is now my third most cited paper. But the interest in it was nothing in comparison to the 2001 paper. The 2001 paper attracted a lot of attention. For about a month, I had at least one interview every day. It was even covered in Wall Street Journal and, interestingly, in the big business newspapers in Germany, as well. The Financial Times covered it. I don’t know why it drew that kind of attention. I remember, I went on vacation three weeks after it came out, and I had this interview with a German radio station from vacation. And I remember people thinking that because it works in nature, this must be something that can be used in agriculture very efficiently. It was, kind of, viewed as, may be, the future of organic agriculture. And, one question kept coming up: Is there, now, some sort of a spray that you can put on the plants to attract predators? This was referring to the experiment that I did where I perfumed plants and was able to direct the predators. The thing in this case is, because the volatile emission is information, it will always be associated with what that information stands for. That means, if a predator gets the information without getting a reward – without finding prey – they will quickly associate that information with no prey, and it will no longer work. In other words, if you spray it randomly into the fields, it will never work. And people have found that. This is weird, because an entire branch of research started off, where people did exactly that. They sprayed compounds like methyl salicylates and terpenoids into fields, found increased numbers of predators in the fields, but almost never found higher predation rates. In other words, they spread the information everywhere, but it becomes non-information. On a larger scale, it is attracting the predators because it’s associated with damaged plants. But once they come there, they cannot find the prey because the information is everywhere. It’s almost like you’re in a room and everybody’s screaming and you’re trying to find that one particular person. This doesn’t work. It almost disappoints me sometimes that people didn’t get that point; at least some people don’t get it.There’s still a whole branch in agricultural research where people try to use sprayed volatile compounds to attract predators.
HS: Did this paper also attract attention within ecology?
AK: Yeah, I think so. At the time when the paper came out, there was nothing controversial about it anymore. There was never a critical discussion, because, at that point, it was almost expected. It was just very nice that, finally, there was a natural system that demonstrated it. And, because of that, it is cited very well.
HS: Did you, sort of, anticipate this kind of impact, when you were doing the work? Or,did it come as a complete surprise?
AK: I didn’t anticipate that the impact would be so big. I mean, I knew it was important, because, if you go through the earlier papers on this topic, there was always this question: Is that actually important in nature,or is it simply an artifact of what we do in an agricultural field, or, even more importantly, in a greenhouse? You have a genetically homogeneous plant population that all do the same thing in response to damage, and you have a very enclosed system. So, the predators and parasitoids don’t move somewhere else. Was that all an artifact or is it something that is really important in nature? That’s why it needed that natural system kind of approval. It was important to have that paper, but I had not expected it to be so well-received, or so important to be cited so much. As I said, at that point, it was not a controversy, anymore. It was clear it was happening, it was simply not clear how important it really was, as a mechanism for a predator to find its prey in natural systems. I think, what made it important was the promise, almost, that, because it works that way in nature, there’s a good chance that you can make it work in an agricultural system and in organic pest control. The fact that there was a natural predator that was responding to it that way, meant, also, that you could tap into the natural arthropod communities. This way of control works very well in greenhouses. But at the greenhouse, you have to supply the predator or the parasitoid. You have to release additional ones in there, so that you can overwhelm the system with the predators by continuously supplying them. This study gave hope that you could do that type of control in the field, using the naturally available communities of predators and parasitoids.
HS: Do you have a sense of what the paper, mostly, gets cited for?
AK: That’s a good question. I don’t know. When people mention that there’s indirect defense,people cite it. I think the more recent once, increasingly, cite it for the fact that the adult moths avoid the plants for oviposition. Sometimes, it is cited for the method, too. I actually cited it for the method of trapping volatiles in the field.
HS: Soon after this paper, you published a paper in Annual Review of Plant Biology with Ian Baldwin, which is also highly-cited. What kind of an impact did these two papers have on your career?
AK: Well, it gave me a professorship in Cornell right after my PhD. I think this is, probably, the major impact. I was relatively young, especially in German terms, for getting a professorship. I think this paper and the Annual Review paper with Baldwin were fundamental to that.
HS: Did you join Cornell immediately after your PhD?
AK: Because of this research, I got an award – the Otto Hahn medal – from the Max Planck Society. It is for excellence during your PhD time; something like that. Back then, this award meant that I was paid a postdoc stipend for one year, to do whatever I wanted. By then I had already applied for the position at Cornell, and got that position, I negotiated to start my position at Cornell one year later. I used that postdoc time to prepare myself, more or less. The Otto Hahn medal was also, I guess, a result of those two papers, primarily. There were four more, officially, in my thesis – I had a total of six papers in the thesis – but these two papers obviously made the cut.
HS: Would you say that the main conclusions of this paper still hold true,more or less?
AK: Yeah, definitely. There are a couple of studies that recently came out, including one on which I’m a co-author. Rayko Halitschke is the main author. We did an experiment with genetically modified plants that do not emit volatiles, and it, pretty much, confirmed what I did with these perfuming experiments with genetically modified plants. It was an Ecology Letters, paper, in 2013, I believe. And then there are two more recent papers, both authored by a current junior researcher at Max Planck, Meredith Schuman, who has continued work along those lines in that same system. One caution, though: I wrote a review paper with Martin Heil in Functional Ecology in 2011, in which we highlighted the importance of an evolutionary perspective on indirect resistance, in general. In that paper, we kind of collected all the evidence that is out there on indirect resistance mediated by volatile emission, as well as other. In the excitement of that study in tobacco,there was, in general – not even our doing – hype that this is something very important in nature, right.I still think it is very important in wild tobacco. The fact that the plants can emit these volatiles can reduce the variable load by 90%., which is more than what you can accomplish with insecticide spraying. This is true in this system, but,these days, I think, it’s not necessarily that dramatic in all systems, because certain things have to be all in place for it to happen. And we describe those things in this Functional Ecology paper in 2011. First, you have to have a herbivore that is damaging the plant to an extent that reduces fitness. This is not trivial. It sounds very trivial, because, from an agriculture perspective, this is what we care about. But there are many insect species out there, whose herbivory is not actually reducing fitness, in the sense of reduced seed production or offspring in the next generation. Then you have to have a predator or parasitoid that is responding to the volatile emission. For that, we have a lot of evidence, these days. There are hundreds of species that are now known to do that; hundreds of species that are known to be attracted to those volatile compounds. So, there’s evidence that some herbivores damage the plants, to certain extent, that reduces fitness of the plant. Then, there is a lot of evidence that predators and parasitoids can use the volatile inflammation. Now, the third point – and this is the important part – the predators and parasitoids attracted to the plant have to reduce the herbivore load to such an extent that it causes a positive fitness effect. Evidence for that is very bleak. Even my paper in 2001 didn’t actually address that, because we didn’t measure fitness of the plants. If you take the 2004 paper that I was talking about earlier, with the Mirid bugs, in addition to that paper, you can make a case for it. But it turns out that this is very rare. So, the tobacco system is among a handful of systems where the effect is strong enough to consider it as some major mechanism through which plants control their herbivore load. In the 2011 Functional Ecology paper, we are not saying it’s not possible, but we think it’s relatively rare that it as efficient as in tobacco. But that allows you to figure out the circumstances under which it works very well. And then, you can adjust agricultural systems to make them work that same way. I have learned, over the years, and through other systems that inducible volatile emission is very important, but not necessarily for third trophic level interactions. We published a paper last week in Functional Ecology on plant-plant communication in a system where predation and parasitism of the major herbivore does not play any role, but inducible volatile emission is the most important factor controlling herbivory, but by means of transmitting information from plan to the herbivore insect and from plant to plant, And so, I guess what I’m saying is that the conclusions are still totally right, but I don’t think you can universalize it easily to any other system.
HS: If you were to redo these experiments today, would you change anything given the advances in technology and in theory that’s happened since?
AK: In a way, we have already done a little bit of that. I would probably use genetically modified plants that are not able to induce. And we have done that, in that Ecology Letters paper. This is about the only thing I would do differently, but I start doing an additional experiment to measure natural selection on the trait [now published in Current Biology]. I would look for natural variation in the ability to induce volatile emission, and then measure natural selection in the presence and absence of predators. This is the missing piece. And this is where, I think, most of chemical ecology is now shifting to. Chemical Ecology is changing from being very mechanistic, concentrating on identifying that particular compound or groups of compounds that do something to pretty studying the evolution of those traits. This is very important.Now, we’re looking into the evolution of inducibility, per se, of volatile emission, in particular.
HS: Have you ever read the paper after it was published?
AK: That’s a good question.I think so,but not recently. I sometimes pull it out for students, to explain something on the fingers. So it’s not really reading, in that sense
HS: Would you count this as one of your favorites?
AK: It could be one of my favorites, but it’s not the favorite.
HS: What do you like about it?
AK: That’s a very good question. I think it’s the conceptual contribution that it makes, the overall conclusion. I think this is more important: what makes it special is the finding that the inducible volatile emission was, at the same time, attracting the predators as it was repelling the adult moth for oviposition. This is conceptually very important because this illustrates that this inducible volatile emission is simply information. It cannot stand alone. There has to be induced resistance associated with it, and there has to be a response that’s coming from the environment – the attraction of a predator – that makes it work as such. That implication, I think, makes it a very important paper.
HS: What would you say to a student who is about to read this paper today? Would you guide their reading in some way? Would you add any caveats to their reading?
AK: That’s a good one. Probably, the one caveat that I would add is, the one that I mentioned, that we published in the 2011 review. This paper shows what is possible and can be very important in certain systems, but it may not be the major factor driving plant resistance, as it is in this particular case. As a caution, I would tell the students to view it as one possibility, but keep their minds open about the alternative hypotheses for the function of inducible volatile emission in a particular system. It can be one of the possibilities, one of the major factors, but it may not be at all. Otherwise, I don’t have regrets in any way. There are some good things about the studies we have done since then,that confirmed a lot of what came out of that and what was, kind of, suggested by the data, right. And, this is, of course, a very nice situation when it happens.
0 Comments