What is it that plant pathogens are attacking?

Ben Goldacre’s book title I Think You’ll Find it’s a Bit More Complicated Than That, could easily be taken as a motto for botany. One example I’ve seen today is an opinion piece in Molecular Plant Pathology that looks at what plant pathogen effector proteins are doing.

Nick Snelders and colleagues ask if we should look at plant pathogen effector proteins as manipulators of host microbiomes, and along the way if we should be taking a different view on what effectors are.

If you’re wondering what an effector is Snelders et al. say: “According to the initial, narrowest, definitions, effectors are small, cysteine-rich proteins that function through the manipulation of plant immune responses.” What happens is a microbe wants to invade a plant, but there’s not much point doing that if it’s guaranteed to get killed in the process. So what it does is secrete molecules to interfere with plant immunity.

If you know more about plant pathology than me (nearly everyone), then you’ll not be happy with this definition. Snelders and his co-authors note that these molecules are doing a lot more than just effecting the immunity, and that there are other kinds of molecules pathogens use to fight the plant. So they note instead that: “effectors should be defined as microbially secreted molecules that contribute to niche colonization.”

They don’t mention plants in this new definition, and that’s no accident.

It’s easy to focus on the plant-pathogen interaction, that’s probably why we’re interested in the microbe in the first place, but that’s not the only interaction the pathogen has. There’s likely to be quite a microbial community around the roots of a plant, and that’s a competition for resources. Instead of looking for plant immunity effectors, Snelders et al. argue we should be looking for three types of effector. The first is obviously the effectors targeted at plants. There’s a second, aimed at plants and microbes. The example they give is the Zt6 effector from the wheat pathogen Zymoseptoria tritici.

The final category they have are effectors that a pathogen aims at other microbes. They point out that weakening a plant by removing some of its microbial support could indirectly aid colonisation of the new plant.

I can see that this is a perfectly reasonable idea, but it also looks much more difficult to investigate. It means that looking at the plant is no longer enough and there’s a lot of diversity to tackle to examine microbe-microbe interactions. What makes the paper more than merely interesting is that the authors acknowledge this does look like a problem, but they also show how you might go about tackling it.

“Subsequently, functional screens aimed to determine their direct effect on other microbes should reveal whether or not the effector candidates have potential microbiota-manipulating abilities. An initial (and potentially overlooked) medium- to high-throughput screen might be to first test whether candidate proteins can be expressed in either prokaryotic or eukaryotic recombinant expression systems. Our recent discovery of the multifunctional Zt6 effector from Z. tritici initially came from our inability to express full-length recombinant protein in either Escherichia coli or Pichia pastoris expression systems, potentially due to toxicity (Kettles et al., 2017).”

Studying this means we getting a better idea of how pathogens are attacking plants, but Snelders and colleagues conclude with one more idea. These effectors are toxic to microbes, so how to the pathogens – which are microbes themselves – survive them? If you can understand the biochemistry around that, then you can start making novel advances in pathogen control.

Both papers below are Open Access, so worth visiting if pathogens are your thing. The MPP paper is a bit too new for the DOI to work, so once that’s live I’ll update the post and remove this sentence. In the meantime there’s a direct link to the paper.