Tuesday, September 1, 2015

New paper: plant external chemical defenses!

When I came to grad school, I was convinced I'd be working on plant-caterpillar-parasitoid relationships, with a focus on plant chemistry or biocontrol. I wrote my NSF-GRFP on the artichoke plume moth and several of its parasitoids. I spent a few months looking for plume moth caterpillars on thistles (a scratchy job) with relatively little success, though not for lack of trying. My focus then shifted to a cute little butterfly, Brephidium exilis, with strange population dynamics and then parasitoid sex ratios. All of these failed (either through logistical problems or me half-assing them because they just weren't that interesting to me).

And then I happened onto Blitum (=Chenopodium) californicum at Bodega and my research took an unexpected turn. As I describe here, I was fascinated by the little fluid-filled pockets on leaf surfaces. I ran a number of small tests and found a defensive function of the bladders (probably one of many, many functions) and wrote it up and it was quickly published in Oecologia, a good journal. This being the very beginning of my second year of grad school (fall 2013), I was pretty jazzed. My committee, however, thought that I should be working on "the bigger picture". And so the ideas for this new paper on external plant defenses came about.

Writing this paper was WAY harder than I thought it was going to be. Instead of a formulaic paper, here's why I did the study (intro), here's how I did it (m&m's), here's what I found (results) and here's why its important (discussion), I was faced with a blank slate. I could write this however I wanted and that was a bit daunting. Primary and secondary school taught me how to write a coherent 3-5 paragraph essay, secondary school and college taught me how to write a term paper and college and grad school have taught me how to write a scientific paper, but no one taught me how to write a synthesis/idea/review paper. I'm glad I did it, though I think it will be a few years before I start on another paper like this.

This caterpillar (an unidentified pterophorid) lives on a plant (Hemizonia congesta) with lots of glandular trichomes, the factories of many external defensive chemicals. It blends in nicely with its "glandular trichomes". 
Taking Rick's lab motto, a Buckminster Fuller quote - "dare to be naive" - to heart, I started by thinking of what ecological differences would occur if a defensive plant chemical was situated on the plant surface instead of inside plant tissues. I came up with five basic differences between chemicals on the surface of plants (external chemical defenses: ECDs) and those inside plant tissues (internal chemical defenses: ICDs):

(1) they are in direct contact with the abiotic environment;
(2) they are not in direct contact with plant tissues apart from the cuticle;
(3) they are first contacted by the vast majority of interacting organisms;
(4) they may contact more than just the feeding and digestive parts of interacting organisms;
(5) they are secreted from specialized structures or cells (or derived from a secretion thereof).

As discussed in a prior post, glandular exudates are often sticky and can have cool tritrophic effects. Here is a mayfly (Ephemeroptera) stuck on serpentine columbine (Aquilegia eximia). 
I then took this list and delved into the literature, reading hundreds of papers on plant chemical defenses over a several month period (I cited 180 in the final paper, but probably skimmed or read abstracts of  twice that number). While external chemical defenses had not been formalized as a class, many wonderful studies had investigated plants with ECDs and I was able to find many examples both in terrestrial systems and in marine alga. I wrote up a massive tome - over 18,000 words - with carefully detailed natural history of many of the studied systems. Of course, this was not publishable, though I was proud of it (I like nothing more than to put cool natural history into an ecology/evolution framework). I worked and worked on cutting it down to its basics. In the process, I found more references and presented it at ESA last year, getting some more feedback. The process dragged on and I got more and more interested in doing experiments and less and less interested in this mammoth synthesis paper. I submitted it a couple times in various stages of cutting and was basically told it was too long. So after this past field season, I sat down for a couple weeks with no other distractions and made it into a far more focused paper, which I submitted to Biological Reviews, as it was still a bit long for most other journals. Fortunately, it was accepted with helpful reviews and after tossing a few minor points back and forth with the editor, it is now out for you to read!

Without getting into the specifics (you can read them in the paper, if you so choose), I found that many chemicals are on plant surfaces, many of these chemicals are defensive, and these may be systematically different from internal chemical defenses in the ways I hypothesized. This paper is important for three reasons: 1) hundreds of papers are published on plant chemistry and plant chemical ecology each year, but it is ecologically important where certain chemicals are located; 2) we have a rich body of theory on plant chemical defenses, but some parts of it are rather untested, and ECDs may allow some tests of certain theories (e.g. optimal defense theory) and; 3) many important crop plants have external defenses, which are easily manipulable in many cases, and it may be useful to think about them in this way to come up with better pest management schemes.

I'm really curious about how this paper and this new classification scheme is received. Am I just cluttering the literature with new terms, or are these ecological differences informative and useful? We will see!

Castilleja minor, a species of paintbrush and a hemiparasite, has really cool oily exudates. The pictured caterpillar, possibly an Autographa (?) species, seems undeterred, though it does mostly eat the insides of the flowers and fruit, which may avoid the exudates. 

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