The bug behind the potato blight
This week, researchers from Germany and the UK identified the pathogen that caused the Irish potato famine which killed a million people in the mid-19th century. Sophien Kamoun heads the Sainsbury Laboratory in Norwich where this work was carried out.
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 identified the strain that caused the disease in 19th century.
Chris - I'm intrigued to think that people kept leaf specimens from affected plants from more than 150 years ago.
Sophien - I know. There's a lot of interesting hidden treasures in all these museums. There's millions of herbarium samples and we stored in museums and 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're able to do something quite cool really to actually look at the genetic makeup of the organism that were in those leaves.
Chris - So, you ground up some of the samples of leaves 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 and 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. So, what was the big question that needed to be answered here, that your researchers enabled us to fill in a mixing gap with?
Sophien - Well first of all, you know it's not a fungus. It's a fungus-like organism, so I've 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 breeding started and took off in the 20th century and scientists starting breeding better potatoes by crossing them to wild derivatives of the potato, probably HERB-1 was at a disadvantage compared to other strains. And 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 the 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 describe this genome as a 2-speed genome. It's composed of two different type 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 virulent genes that are important for the pathogen to infect plants and 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 bits 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.
Chris - Because a similar sort of story is playing out with bananas, isn't it? We know that bananas are cloned plants so they're all genetically identical. So, all it takes is one type of fungus organism to come along which is very good at exploiting one of these plants and then all of a sudden, all banana plants of that particular type are going to be susceptible because they're all cloned.
Sophien - That's an interesting point. This is one of the issues with modern agriculturists, having monocultures in the case of the bananas it's the most extreme case because all those bananas we eat have the exact same genetic makeup. So, these makes these plants very susceptible to pathogens, especially pathogens that are very good at adapting and evolving, and dealing with these crops. So, the challenge really for us as plant breeders and plant biotechnologists, is how can we generate enough variation, enough new varieties to really keep up with this rapidly evolving pathogens?
Chris - So, why did the Irish potato blight happen when it did? Do we have an insight into what caused this perfect storm and why doesn't it happen now?
Sophien - Well, there's two facets to your question. The first one is that we have to keep in mind that in the 1840's, the disease was new to Europe. So, for 3 centuries, potatoes were cultivated in Europe and somehow, the pathogen never made it to the European continent. So, in that case, it was basically a new pathogen being introduced to a region that hasn't even seen it before. It's similar to what we're seeing today with the ash dieback fungus for instance which showed up in the UK recently and essentially has all these susceptible trees to infect. So, that's what happened at that time. it didn't really require particularly aggressive pathogen. The potatoes were susceptible and the pathogen took off like wildfire.
Chris - Arguably, there are lots more potatoes around now that fall into that camp than there were in the 1850s when this happened in Ireland. So, why hasn't it happened now more frequently?
Sophien - It's actually happening now. Potato blight is still a very important disease these days. Potato crop is the third most important food crop in the world and the estimates currently are that the potatoes lost to the blight are enough to feed tens of millions of people. So, it's still an important problem. The difference is now, we can manage it with agricultural practices and with using chemicals and occasionally some resistant varieties. So, the disease is very much around. It hasn't gone.