Two against one

Bacteria and fungal cells join forces to cause rice seedling blight.

Chris - Fungi are common plant pathogens, and they often use toxins to break open plant cells to liberate raw materials that they can scavenge for their own use, but the fungus doesn't necessarily have to make the toxin itself. Christian Hertweck has discovered that the rice blight fungus contains a bacterium that lives inside the fungus, and in return for board and lodging, produces a potent chemical weapon that the fungus uses to attack the plant.

Christian - For a long time, it has been known that a particular fungus can cause a plant disease named rice seedling blight. The rice seedlings swell and then they decay, that is because the toxin that is secreted by the fungus. The toxin is known to inhibit the cell division of the rice seedling.

Chris - Does the fungus make the toxin or does it get it from somewhere else?

Christian - Well, actually, that was a big surprise. When we wanted to find out how the fungus makes it, we found out that it is actually not producing these toxins itself but it harbours the bacteria that live within the cells of the fungus, and these bacteria have all what is needed to make the toxin.

Chris - So, this is a sort of symbiotic relationship. The bacteria live with fungus and they give rise to this toxin, which the fungus then deploys at its victim, the rice plant, presumably, so both can access nutrients by breaking down the plant cells.

Christian - Right. So, these bacteria belong to a genus called Burkholderia, and these bacteria are known to inhabit various habitats, so they have already been known to live in symbiosis with various organisms but not with fungi so much.

Chris - Two questions are rising then, I mean, one, how did this happen? and two, how did the bacteria form this alliance with the fungus? How did they get recruited? How did they get in? and then how do they get deployed into making this toxin?

Christian - In fact, it is a surprise that the bacteria can live within the fungal cell, and of course, the question is how has it ever got inside? We now recently found out how it does it? So, because that's not a trivial thing, the fungus is covered by a thick cell wall, which is typically a barrier to everything that comes from outside, so the bacteria found a way to dissolve a particular spot in this barrier. So, just enough to slip inside. 

Chris - How did you do this? Did you literally watch fungal hyphae, the little protrusions under a microscope, so you could see the bugs getting in and out?

Christian - It's actually not so easy to see them. In fact, it has been overlooked for decades. So, we were quite surprised to first detect the bacteria inside by a technique that is called, confocal laser scanning microscopy, and using this technique, we could first, localize the bacteria within the fungal hyphae, but the question of course was how can we catch these bacteria in action? and that was just recently achieved, taking some snap shots of the bacteria on the way inside the fungal cell.

Chris - You have these microbes. They have the ability to produce a cell wall dissolving chemical that gets them into the fungus. How does the hole get healed afterward, and how do the bugs get out again to then spread and penetrate other fungi or multiply?

Christian - In fact, we don't yet know how the hole is closed again, but we know that the enzyme that is secreted to dissolve the cell wall is actually only produced in a small quantity. So, that probably means that only where the bacterium meets the fungus, the concentration may be high enough to just to make a small hole where the bacteria can slip through. This symbiosis persists by just cracking the spores, and without the bacteria, this particular fungus cannot sporulate anymore. So, the trick is, that in the presence of the bacteria, spores are formed and in each spore, you can find the bacterium. So, when a spore lands somewhere and the fungus germinates, the bacteria are already present.

Chris - How do you think this interaction got started? In the first place, if you look at the relationship and you ask those sorts of questions, genetically, how do you think this happened?

Christian - A plausible scenario would be that initially, these bacteria took fungi as a prey, and typically, fungi are also sensitive to the toxin that is produced by the bacteria but the fungi that were now chosen as a host, they are resistant to the toxin. So, what we think is, that there has been a so-called, parasitism-mutualism shift. That means, initially, the bacteria was an enemy of the fungus but then it has become something like a friend, or that's at least what we think that it is now because they don't harm each other.