Face of the Enemy: Adult Western Corn Rootworm chewing on the silks of a corn ear. Picture from wikimedia, apparently in the public domain
This post discusses the paper Degenhardt, J et al. (2009) “Restoring a maize root signal that attracts insect-killing nematodes to control a major pest”
The Western Corn Rootworm (which as you can see from the picture to the right are not actually worms) is estimated to cost farmers in the US alone one billion dollars a year in lost yield and pest control measures. The newly hatched larva begin feeding on root hairs and as they get bigger start attacking the main roots of a corn plant. The damage to the roots from the feeding itself is exacerbated by the open wounds becoming infected. The loss of roots both stresses the plant and reduces yield by decreasing the plant’s supply of nutrients and water, but also makes it much more vulnerable to lodging (getting blown down by a gust of wind). Oh, and did I mention the adults like to feed on the corn plant’s reproductive tissues, decreasing yield even further?
Rootworms are one of the pests controlled by plants genetically engineered to express BT a protein taken from organic agriculture. Without it, the 1 billion dollar price tag for rootworm damage and control would be even higher. But this isn’t an article about bt, it’s an article about how some corn already knows how to call for help when rootworms attack.
Rootworm larva may feast on the roots of maize, but they are in turn eaten by some species of nematodes.* And it turns out some kinds of corn know how to attract nematodes, and when they’re under attack by rootworms they do just that. The nematodes get a delicious meal of rootworms and the corn plant gets to keep more of its roots intact.
How do corn plants attract their, unintentional, nematode defenders? Those corn plants can produce a chemical called (E)-B-caryophyllene. Previous research has shown higher densities of beneficial nematodes around corn that produces this chemical. All corn contains the gene which encodes the protein that makes (E)-B-caryophyllene, however most of the varieties grown in North America don’t ever seem to express the gene.** (The book on how to attract nematodes is still in the genome’s library, but nobody is checking it out.)
The problem in comparing two different lines of corn is that there are lots of differences that could be causing any observed effect. Remember that unrelated lines of corn can have as many genetic differences as are found between chimpanzees and humans. There are several ways to approach tying a specific effect to a specific genetic difference.*** The fastest and most definitive when researchers already have a candidate gene in mind is the one used in this paper. The authors took a line of North American corn that doesn’t normally produce the nematode attractant, and transformed it with a functional copy of the gene, driven by a constituative promoter (one that tells the plant to transcribe the gene in all its tissues).
Now with corn plants that are truly identical except for producing the namatode attracting chemical, it can be shown:
- Without nematodes, there was no difference in the success of western corn rootworms between the plants producing (E)-B-caryophyllene and the plants that don’t. <– The chemical itself doesn’t effect the rootworms
- In the presence of large numbers of nematodes, fewer western corn rootworm adults emerged from plants producing the nematode attracting chemical ((E)-B-caryophyllene) than the plants that don’t. <– The nematodes have a significant effect on rootworm survival underground
The technical name of this is a tritrophic interaction (sorry if that is spelled wrong, I’ve only ever heard it spoken aloud). Rootworms interact with corn by eating it. Corn interacts with nematodes by catching their attention with (E)-B-caryophyllene. Nematodes interact with rootworms by eating them. Tritropic interactions seem to be quite the hot topic in ecology (judging from the three weeks of a chemical ecology course I sat in on a couple of years ago), and this paper demonstrates they may be agriculturally significant as well.
Before we declare nematode attracting chemicals the next BT, tests will have to show how much damage nematodes actually prevent (this study focused on the number of surviving rootworm adults per plant, which is great for determining there IS an effect, but not so good for quantifying it) and also if the effect is still significant in environments without artificially augmented nematode populations (the researchers added millions of nematodes to the field).
I’m also curious why most North American corn contains a non-expressed copy of this gene. The four possibilities I can think of are, from best to worst:
- It’s pure chance. That can happen, especially with the successive genetic bottlenecks domesticated crops go through.
- The functional nematode attraction gene is in negative linkage with another gene important for yield or stress resistance.****
- Some other north american beastie with less beneficial intentions than the nematodes also uses the this chemical to home in on corn plants.
- Producing (E)-B-caryophyllene to attract nematodes takes a lot of energy, and yields are higher when corn plants let rootworms do their worst and focus their energy on producing food for people. <– I really doubt it is this one
It’s a really cool article, and a reminder of the vast genetic variation contained just within the corn species. Special thanks to kim_of_cornell for pointing out this paper to me.
*Nematodes, at least the ones I’m interested in as a plant biologist, are tiny worms that live in the soil. Some are parasites that latch onto the roots of plants and suck out nutrients. Those were the ones discussed in this post on nematode resistance in soybeans. Others are carnivores, hunting down equally microscopic prey like rootworm larva.
**Which makes me want to look up the maize genome’s version of the gene in CoGe and see if it’s missing promoter sequence relative to sorghum.
***Large scale analysis over many different maize lines (for example Ed Buckler’s diversity panel) is one possibility. The downside is they’d have to grow a LOT of to get enough replicates, and some of the more old school geneticists are going to question data based purely on association studies anyway.
Another option is to create near-isogenic lines by introgressing functional and non-functional copies of the gene into the same genetic background, essentially create plants that are genetically identical except for that one gene. (I talked in more detail about introgression in the context of breeding blight resistant American Chestnuts). Introgression takes time. For scientific work, they’d probably need at least six generations (98.5% identical genomes (1-.5^6)) which would take three years in an academic lab that gets two growing seasons one during the summer and another over the winter in the tropics. On one extreme, seed companies can do four generations a year using growth chambers and culturing immature embryos, on the other, some academic maize labs are giving up their winter nursuries as too expensive, limiting themselves to one generation per year. And even after six generations the 1.5% of non-identical DNA represents hundreds of genes, although if the researcher has done multiple independent introgressions they shouldn’t be the same hundreds of genes (unless they’re genes that are physically close on the chromosome to the gene being introgressed, which makes them much more likely to be inherited together).
****In other words the version of some other gene breeders selected for happened to be near a non-functionaly copy of the gene needed to produce (E)-B-caryophyllene, and when we selected for that gene the broken copy of this one got pulled along for the ride. It would make things a little more complicated, but nothing plant breeders don’t have to deal with all the time. And with marker assisted breeding, if they need to screen 10,000 plants for the single one where the chromosomes recombined in just the right way to put the good versions of both genes together, well they’re set up to do that now.