Potato Blight

16 June 2013

Interview with

Sophien Kamoun, Sainsbury Laboratory; Hernano Burbano, Max Planck, Tubingen

An international team of researchers from Germany and the UK have made headlines around the world by identifying the bug that caused the Irish potato famine which killed more than a million people in the mid-19th century.  Sophien Kamoun heads the Sainsbury Laboratory in eLife LogoNorwich and spoke to Chris Smith... 

Sophien -   We knew that the pathogen called Phytophthora infestans, a fungus-like organism was the agent of the potato blight that caused so much havoc in the 19th century and essentially triggered the Irish potato famine.  What we didn't know is which strain caused the disease at that time. So, what we did is we went back to herbarium specimens from museums, extracted DNA from the specimens and we were able - using the latest DNA technology - to sequence the genome of the pathogen and identify the strain that caused the disease in the 19th century.

Chris -   I'm intrigued to think that people kept leaf specimens from affected plants from more than 150 years ago.

Sophien -   There's a lot of interesting hidden treasures in all these museums.  There's millions of herbarium samples and were stored in museums that are studied usually to identify the species by looking for instance at flowers and the morphology of the leaves and so on. But in this case, we were able to do something quite cool with it, but actually look like the genetic makeup of the organism that were in those leaves.

Chris -   So, you ground up some of the samples of leaves and extracted genetic material which would've included both the genetic material of the potato and the genetic material of the blight that killed the plant.

Sophien -   Yes, exactly.  So, we cut small pieces of the leaf and we're able to analyse both the plant and the pathogen. In this case, in this study, we focused on the pathogen, that was the interesting bit.

Chris -   But people have - as you say - known that this was a fungus that was knocking around that did this. What was the big question that needed to be answered here that your research has enabled us to fill in a missing gap with?

Sophien -   Well first of all, it's not a fungus. It's a fungus-like organism. So, I'm corrected you.  Sorry about that, but it's a different type of microbe.  But it does look like a fungus. So often, people refer to it as a fungus. There are many strains of this pathogen. What we discovered was that it's a new strain.  We called it HERB-1 that caused the blight in 19th century and this strain apparently is gone. It's not around anymore.

Chris -   Why do you think that is? Is it that it was so good at devastating potato plants that as a result, people just stopped growing susceptible species and it ran out of plants to infect?

Sophien -   No, we don't think so. What probably happened is that as potato bidding started and took off in the 20th century, scientists starting bidding better potatoes by crossing them to wild relatives of the potato. Probably, HERB-1 was at a disadvantage compared to other strains. We know that in the 20th century, HERB-1 was replaced by another strain we know as US-1. And then later on in the 20th century, US-1 was replaced by additional strains.

Chris -   So, is this sort of model then that you have plants that are susceptible to one of these organisms? The organism becomes more successful at working its way through those plants and then the plants change or new types of plant come along which are more resistant and so, the pathogen changes, and we're just seeing a sort of arms race playing out.

Sophien -   That's certainly part of the equation, but in fact, what's amazing about this pathogen Phytophthora infestans - the potato blight pathogen - is, how adaptable it is. It's very good at adapting to new resistant varieties that breeders are releasing.

Chris -   How does it do that? What makes it so successful?

Sophien -   Well, this is actually work we've been describing in the last few years and we discovered that this pathogen has an amazing genome.  In fact, we described this genome as a 2-speed genome.  It's composed of 2 different types of compartments if you like. One compartment contains the housekeeping genes, the key gene the pathogen needs to be a microbe. The second set of compartments contains all the villains genes that are important for the pathogen to infect plants. That second compartment is evolving and changing much more rapidly than the slow evolving housekeeping compartment if you like.

Chris -   Do you know why those bits of the genome change so fast whereas it's elsewhere in the genome don't? How does the organism do that?

Sophien -   I wish I knew. That's a very interesting topic we're studying.

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