Hi, I'm Sheng Yang He. I'm a professor at Michigan State University. And I'm an Investigator of the Howard Hughes Medical Institute. Today, I'm going to tell you about the fascinating world of plant-pathogen interactions. Now, why do we care about plant-pathogen interactions? Some of you may know a disease called potato late blight. This disease devastate... devastated the potato crop in the 1840s in Ireland. That event basically killed about a million people, and another million emigrated... forced emigration... out of Ireland. Many of them actually ended up in the United States. So, this just illustrates how a plant disease can have a profound effect on human survival and emigration. There are many such diseases, not to this scale, but they are major threats to our global food securities. So, one of the other diseases, you know, like rice blast, I grew up with in China. I lived in a small village, and it was very severe when I was growing up. But, you know, now I go back 40 years later. It's still a very severe disease. In fact, this is the number one disease in rice production across the globe. There also are new diseases like kiwi bacterial canker, which is caused by a bacterial pathogen that I'm very familiar with called Pseudomonas syringae, sweeping across New Zealand and some European countries right now. So, you see there are old and new diseases. They really pose a great threat to agriculture. And so the... many researchers are, you know, involved in trying very hard to understand the molecular basis of these diseases. And the goal is to... hopefully, to come up with very innovative solutions to solve all these diseases. It's been very challenging. But I think, you know, this is an area that we have to do, because the crop production has to increase to meet the demand of the rising population in the next century, actually. So these diseases are one of the obstacles to increase the yield production. And also quality... you know, disease affects quality as well. And so, in today's... this part of my talk, I'm going to introduce some of the very general concepts dealing with host-pathogen interactions. On the one side, I'm going to talk to you about plant immunity. Yes, plants do have immune responses like a human. But I also want to talk about pathogen virulence factors and so-called effectors. That's Part 1. In Part 2, I'm gonna illustrate these concepts using the model Arabidopsis-Pseudomonas syringae system that we and many others are actually working on. So, I hope you're able to watch both parts, because if you only see one you may not get enough information from this talk. So, what is effector-triggered immunity in plants? There's an older name for this. This is called, actually, gene-for-gene resistance. This is describing a phenomenon, probably noticed by farmers or by, you know, other people, over many years, thousands of years, probably. You know, if you go into the wheat field where there's different cultivars that are planted, some cultivars will be severely diseased in some years. And at the same time, some cultivars will be green and, you know, yielding really well. What's the molecular basis of that? What's the genetic basis of it? And that's been, you know, puzzling for many people for a long time, until this scientist named H. H. Flor. He's a plant breeder and a plant pathologist. He studied a disease called the flax rust disease. It is caused by a fungus. He was very careful. He studied many strains of fungal pathogens, but also many cultivars of the flax plant. And he studied the genetics of the interaction, and came up with this very interesting hypothesis called the gene-for-gene hypothesis. What he thought is that maybe the pathogen has so-called avirulence genes, or Avr genes, some strains. And some cultivars that are resistant contain so-called resistant genes, or R genes. Okay? So, this is the diagram he would use to describe these interactions. If he'd taken a pathogen without any avirulence genes, it's going to infect the plants that either have the R genes or no R genes, right? Because it's virulent, okay? But, if when the pathogen has Avr genes within it, it's going to only infect the plants with no corresponding R genes, okay?, which is depicted right here. If the plant has R genes that can recognize genetically this Avr gene, then the plant will be resistant. So, you needed both the R genes in the plant but also Avr genes in the pathogen for a plant to be resistant. So, this has... you know, was a hypothesis only, okay? But about 10 or, you know, 15 years later, there's actually molecular proof for the existence of these interactions. So, scientists started to clone these so-called Avr genes from different kind of pathogens. The initial few Avr genes were actually cloned from bacteria. And this was done by Brian Staskawicz at UC Berkeley and the late Noel Keen at UC Riverside. And then about ten years later, a number of R genes have been cloned from plants, from different plant species. Okay? So, there were some original predictions of how the Avr proteins and R protein would work, actually, right? So, the idea was really inspired by an animal receptor signaling kind of model. It says that, you know, this Avr protein may be made in the pathogen but is secreted outside of the bacteria. Okay? And the R proteins may be receptors. They may be in the membrane of the plant cell. So, it indicated this classical ligand-receptor kind of interaction. When the Avr genes and R genes are cloned, you know, we'll see whether this model actually holds, right? So, as I said, many R genes have been cloned from different species against different kinds of pathogens. So, we have N gene cloned from tobacco against a viral pathogen. A Cf9 gene... you know, the names... it's not important... but this particular gene is against a fungal disease called leaf mold. There's also, you know, genes... R genes that are against bacterial diseases, in this case, from Arabidopsis. And also some R genes actually against worms, like, nematodes. So, it's very different kinds of pathogens. Initially, we were thinking that maybe there's different kind of R genes, you know, molecularly. But it turns out many of these genes actually share the same kind of motif, including the so-called leucine-rich repeat, or LRR. And this is very exciting because if you line up a sequence against a database, some of the genes that come up are actually involved in animal immune... immunity, so immune receptors, for instance Nod1 is the bottom one diagrammed here. It contains the leucine-rich repeat like the plant receptors here. It also contains so-called NB domains, or nucleotide binding domains. So, here's a very interesting parallel between the animal immune system and the plant immune system. They are based on the same kind of protein to defend against different kinds of pathogens. So, remember this model that I showed you just a few minutes earlier, that indicated that these Avr proteins may be secreted from the pathogen and the R proteins are probably localized to the plasma membrane in the host cell. When you look at the Avr protein sequence, however, you actually don't see this classical signal peptide that indicates the protein will be secreted through the conventional secretion system in the bacteria. So, this model is probably not correct in terms of this particular step. Actually, it turns out most R proteins are also not localized to the plant plasma membrane as originally predicted. Most of them actually localize inside of the cytosol. So, what's going on? Now, this is really Puzzle #1 for a lot of people. It doesn't really make sense. Until we discovered that, actually, most of these Avr proteins from bacteria actually are directly injected into the plant cell through the type III secretion system. And this is actually a very conserved system in bacterial pathogens of plants and animals, again. So, you can see that type III secretion system. You can see it under the electron microscope like a syringe-like thing. The injection system allows bacteria, in this case, to penetrate through the plant cell wall. So, the plant cell has a cell wall, unlike the animal cell. And injecting through the plasma membrane into the cytosol. So, that explains why Avr proteins could be potentially recognized by R proteins located inside the plant cell. And this translocation system actually is very common for other types of plant pathogen: fungus and even, you know, nematodes. They inject these proteins into the plant cell as a very common mechanism during infection. So, gene-for-gene resistance, you know, became effector-triggered immunity, the common term today. This is another way of depicting it. So, you can see that bacteria are injecting these red colored effectors into the plant cell. And they're being recognized by these immune receptors, either containing the coiled-coil domain, CC domain, or the TIR domain, and they are LRR proteins. Okay? So, it's called effector-triggered immunity. So, when the plant genome was sequenced in early 2000, first from Arabidopsis, people were interested to see how many R proteins are there in plants, right? In humans, we know we have these antibodies. You know, it's this endless combination of antibodies that can recognize all kinds of microbes, right?, 10^14 specificity. So, we wanted to know how many R proteins are encoded from the plant genome. There was a puzzle, actually. When you see this, there's only hundreds of these genes. How can hundreds of genes, immune receptors, recognize thousands of microbes? So, that's really a big puzzle. And that was the puzzle based on this directed recognition, so, saying that one Avr protein from a pathogen can be recognized by a particular R protein in the plant. So, it can't do this more than a hundred times, right? This puzzle was partially solved by this realization that there's a lot of so-called indirect recognition by R proteins of these Avr proteins. So, this is actually happening in many diseases. So, this is a one example. Imagine that this light blue colored circle is a plant protein called RIN4 in Arabidopsis. This protein is actually attacked by two avirulence proteins, AvrB and AvrRpm1 from Pseudomonas syringae. What they do is that these two Avr proteins, well, they attack a RIN4 protein, in this case inducing the phosphorylation of RIN4, of the plant protein. This phosphorylation event induced by two different Avr proteins is recognized by the Rpm1 R protein. Okay, so in this case one R protein recognized two Avr proteins through this common modification of another plant protein. It's called indirect recognition. There's actually another Avr protein called AvrRpt2, which modifies RIN4 differently. It actually cleaves the RIN4 because it's a protease. That is being recognized by another R protein called Rps2. So, you can see there's a lot of variations of so-called indirect recognition that could potentially explain why a limited set of R proteins could potentially recognize many different Avr proteins from different pathogens, because they could induce modification of another plant protein and that modification, then, is sensed by the pathogen to say, this is not normal; it's not my normal thing, okay? So... so then there's another puzzle, okay? I've being telling you these avirulence proteins from pathogens... indicating... when you have these Avr proteins, then the pathogen is avirulent. Okay? Why would a pathogen send avirulence proteins into the plant cell to become avirulent? That... no... no... that makes no sense, okay? And so that's Puzzle #3. Why would the pathogen send avirulence proteins into the plant to be recognized by R proteins? What is the original function of these proteins? Okay? So, I'll remind you of this again. So, we have been talking about this effector-triggered immunity because these particular cells contain R proteins. The plants are resistant against pathogens, okay? In this case, the effector proteins, or avirulence proteins, are basically not good for pathogens. They're being recognized. Actually, in most plants without resistant proteins, these effector proteins or avirulence proteins are doing something else. They're actually suppressing another branch of immune response called pattern-triggered immunity. So, this is depicted on the left. So, pattern-triggered immunity is distinct from effector-triggered immunity. They use different signaling pathways. But they are normally suppressed by these effector proteins to induce disease, okay? So, that's why you want to send these Avr proteins into the plant cells, because the R protein is rare. So, what is pattern-triggered immunity? This branch of immunity is not triggered by effectors of the pathogen, but it's triggered by common patterns from microbes. There can be pathogens. It could be non-pathogens, okay? And so, they've evolved to recognize all kinds of microbes. They are probably more ancient then effector-triggered immunity. They are probably more related to the animal system of the immune system. So, one example of these patterns from bacteria is called bacterial flagellin. This is obviously very common because most bacteria have to swim, so they have to have these traits. And that common trait is now recognized by pattern-triggered immunity. So, one example you can see here... you know, flagellin subunits make up the flagella. It's like about 10,000 copies of this to make a viable flagella. Flagellin has a conserved domain at the N-terminus and the C-terminus, a variable region in the middle of the protein, and there's a peptide called flg22. This is a 22- amino acid peptide, which is now used very commonly in the study of pattern-triggered immunity, called flg22. People have identified the receptor in Arabidopsis for flg22 and flagellin. This is done by Thomas Boller's group, very nice work. This receptor looks like a traditional membrane-bound receptor. You have a leucine-rich repeat domain, which recognizes the flagellin or flg22 peptide, but then you have a kinase domain inside the plant cell that transduces the signal to do phosphorylation. So, it's very similar to the animal signal/receptor system. A critical question is, is this receptor important for disease resistance, right? Okay. So, this is done by Cyril Zipfel in Thomas Boller's group, many years ago now. They created this receptor mutant in Arabidopsis. So, this mutant will fail to recognize flagellin of bacteria, including Pseudomonas syringae. On the left, you have a wild type plant containing the full, functional fls2 receptor. On the right is the receptor mutant. And you can see... you see more disease after infection with Pseudomonas in the receptor mutant compared to the wild type, indicating the receptor is very important. The importance of the receptor is actually most obvious if the infection is done by putting bacteria onto the leaf surface, okay? For bacteria to infect the plants, bacteria have to actually go into the leaves. And one of the routes is through stomata. So, these are microscopic pores on plant leaves that allow plants to uptake CO2 to do photosynthesis. But the stomata pores are big enough for bacteria to go in there, so for a long time people thought this is a passive process. The bacteria takes advantage of the open pores to get into the plant tissue. But I just told you... so, the fls2 receptor mutant phenotype is most obvious when you inoculate bacteria onto the surface because they have to go through the stomata to infect. If you inject bacteria directly into the leaf, bypassing the stomata, there's not much difference between the wild type plants and the immune receptor mutant plants. Okay, so, why? It turns out... actually, my group figured out... that this is because... these are stomata cells that... each stomata is actually made up of two guard cells. They actually can recognize flagellin as a molecular pattern and then they close the pore. It's the first line of defense against bacterial infection. So, this is a kind of interesting immune output, very unique to plants. They're recognizing the molecular pattern and do this stomata closure as the first line of defense. So, to summarize this part of the talk, there are two branches of plant innate immune systems. One is involving pattern-triggered immunity, probably very ancient. It evolved to recognize all kinds of pathogens or non-pathogens so the plants won't be eaten by these microbes, then, because plants are really rich in sugars and other nutrients. But then, the pathogen has evolved effectors to shut down the pattern-triggered immunity as a mechanism of pathogenesis. And this is a called effector-trigger susceptibility. But then plants are smart. They evolved this effector-triggered immunity to recognize individual effectors, which used to be called avirulence proteins, to activate the second branch of immunity to fight against these pathogens. So, this... if you go into the wheat field right now, you have this continuation of evolution. Sometimes the pathogen wins; sometimes the plants win. What we want to do is to identify a way to speed up the evolution so that we can fight against plant... emergence of new diseases before they become a problem. So, now I want to acknowledge colleagues who actually gave me some slides for this talk, so, including the slides I had, Cyril Zipfel provided a few interesting slides for this part of my talk. Thank you very much.
