Striga hermonthica: When plants turn nasty…

By Caroline Wood, University of Sheffield

Climate change, drought, and insect pests – surely these are the greatest threats to food security? It may surprise you, but in certain regions it is actually other plants which are the farmer’s worst fear. These aren’t your usual weeds that can be treated by a good dose of Roundup: I’m talking about parasitic plants that infect and feed off their host. One of these, Striga hermonthica, is particularly notorious for devastating harvests across Sub-Saharan Africa. For my PhD project, I hope to uncover why these parasites are so effective at gaining entry to their victims; knowledge that could help us protect our future harvests.

A surprisingly large number of plants – over 4000 in fact – are parasitic, including the Christmas favourite mistletoe (a stem parasite of conifers). For the most part, these remain mere botanical curiosities – until they infect our key staple crops. Striga hermonthica is particularly deadly as it infects almost every major cereal crop, including rice, sorghum and maize. It has also evolved a cunning lifecycle strategy that makes it particularly difficult to control. Firstly, each plant can produce up to 100,000 seeds which can remain dormant in the soil for decades. The seeds only germinate in response to specific chemicals produced by the roots of suitable host plants, which stops the young plants from dying before they can make an attachment. After attaching, the parasite produces an absorptive organ (the haustorium) which forces its way through the host’s root until it reaches the central bundle of vascular tissues. Once connected, the parasite can then freely plunder the host’s resources.

As a hemiparasite, Striga mainly obtains water from the host (other parasite species can be holoparasites, which withdraw both water and carbon sugars). But Striga also causes a bizarre shrinking effect on the host, dwarfing the height of the mature crop (see image 1). It is unknown why exactly this happens, but the effect is so marked that the common name for Striga is ‘witchweed’, as many farmers believe their crops had been bewitched.

Image 1: Striga-infected sorghum. Note the shrunken and withered appearance of the infected crops (foreground) compared with the healthy crops (background). Image credit Joel Ransom.

Because germination, attachment and infection all take place underground, the farmer doesn’t even know his crop is infected until the parasite shoots emerge above ground (which would be quite pretty if they didn’t spell such bad news). At this point, it is too late to save the crop. Some soil treatments can have an effect, but these are typically too expensive for subsistence farmers, who often resort to pulling the shoots out by hand. As a result, it is thought that Striga causes crop losses crop losses of approximately US $10 billion each year each year [1].

However, some crop cultivars and their wild cousins have natural resistance against Striga – which is where my lab comes in. Here at the University of Sheffield, our lab group, headed by Professor Julie Scholes, is researching the basis of this resistance, with the eventual aim of introducing it into key crop plants. So that we can observe the infection process, we grow our plants in rhizotrons (root observation chambers): these are basically large square petri dishes which we wrap in foil to simulate darkness (Image 2).

 

CW-pic3
Image 2: Arabidopsis plants growing in rhizotrons (root observation chambers). This allows us to observe the process of Striga infection and attachment non-invasively. Once the foil layer is removed, the lid of the petri dish can simply be lifted off to access the root system.

Most of the lab group works on rice or maize to identify cultivars with broad-spectrum resistance which can be used to map Striga-resistance genes. My project, however, focuses on the more fundamental aspects of infection. I am particularly interested in how the parasite manages to overcome the immune system of the host so successfully. Potentially, Striga may deliberately trigger certain defence signalling pathways to overwhelm the host: a strategy employed by some fungal root pathogens. This would be almost impossible to investigate in a crop plant, as these are typically have gigantic genomes and often duplicated sets of chromosomes. So my model organism is Arabidopsis thaliana, the workhorse of the plant science world, whose genome has been fully sequenced and annotated. Arabidopsis isn’t a natural host for Striga hermonthica, but is susceptible to the related species, Striga gesnerioides, which normally infects cowpea.

 

CW-pic4Image 3: A host Arabidopsis plant growing in a rhizotron. I applied Striga gesnerioides seed applied to the roots three weeks ago. You can see that the parasites which have attached successfully have now swollen to form an absorptive haustorium.

It’s certainly enough to keep me occupied for the next two years of my PhD! You can keep up with my progress by following my blog and my posts on Twitter (@sciencedestiny). I also write blogs on the challenges of balancing the PhD life for DigitalScience.

Now if you’ll excuse me, I have some plants to infect…

The best things about my PhD:

  • Working on a project that could have a real impact on future food security
  • Planning experiments to try and uncover a real mystery in the world of parasitic plants
  • Being able to raise awareness of how important plant science is by talking at public engagement events; taking part in school outreach activities and running demonstrations at science festivals

The not so good parts:

  • The long hours spent infecting my Arabidopsis with Striga seeds – a laborious process, which involves literally applying the seeds with a paintbrush directly on to the host root. It doesn’t help that the roots are pale yellow against a white background, which sends my eyes funny!
  • Manually counting the number of attached parasites … surely someone could write a computer program to do this automatically?!

See the Global Food Security website for more information about the work of the Scholes lab.

References:

[1] Westwood, J. H., J. I. Yoder, et al. (2010). “The evolution of parasitism in plants.” Trends in plant science 15(4): 227-235.

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