An unknown chemical triggers the death. Whatever the chemical’s identity, when there’s enough of it, it gives the gene the nod to carry out the plot.

Within hours, the plant’s cells are dead.

This isn’t your routine hit. When a plant bumps off its own cells this way, it’s part of a resistance response—the plant battling a fungus or other microscopic pathogen. We’ve all seen the evidence on leaves—tiny flecks or holes scientists call “lesions.”

Scientists aren’t sure why the plant cells die, says Jeff Dangl, professor of biology. Could be, that by killing its own cells, the plant deprives the pathogen of nutrients—starves it to death. Or maybe the plant kills the pathogen some other way, and the dead plant cells are just innocent bystanders.

Either way, Dangl’s team of scientists aren’t all that worried about the cell death itself. But they’re on its case because they think it’ll lead them to their real target—the mechanism that kills the invader.

A plant can’t fight just any invading pathogen, only those it’s genetically programmed to recognize. Once it detects the invader, a plant launches a defense that somehow leads to the death of the pathogen. Scientists call this the “disease-resistance pathway.”

If Dangl’s team can understand this complex pathway, they can learn how to help make plants more resistant to disease in general. They want to find out what genes control the process, what chemicals signal those genes to set it off and to stop it.

They’re asking those questions of a plant called Arabidopsis, or thale cress. They’ve chosen thale cress because it grows quickly and contains less DNA than other plants, making it easy to reproduce and investigate. In particular, the scientists test thale cress mutants—plants in which gene defects cause cell death to somehow go awry.

“We’re pretty hard-core geneticists,” Dangl says. “The approach is to find the mutant, study it, and infer from it what the normal gene does.”

Before the researchers study a mutant plant, they make sure that its cell death is indeed part of the resistance pathway—a kind of death called “hypersensitive response.”

“Sometimes, a cell just gets screwed up, and it dies,” Dangl says. And a cell can also die slowly as the result of disease. “But with this particular triggered kind of cell death, the cells explode, and that’s it,” Dangl says. “Boom, game over.”

Several sources tip the scientists off if a case of cell death is part of a resistance response. For one, the plant will make autofluorescent compunds that appear to glow when viewed under certain wavelengths of light. There are also certain genes that get “turned on” during a resistance response, so the scientists look for the presence of the RNA from those genes.

Once they’ve found a mutant plant that shows these signs, the scientists begin a thorough interrogation. One mutant in particular they’re well acquainted with—Lsd-1. Dangl’s lab discovered it. The trouble with the Lsd-1 mutant is that its Lsd-1 gene is deleted, as Bob Dietrich, senior postdoctoral fellow, discovered when he cloned the gene. Somehow, this deletion makes cell death easy to trigger, and, once it starts, it continues, out of control, until the whole leaf is dead.

In the Lsd-1 mutant, cell death can be triggered by treating the plant with chemicals, shifting its light exposure, or infecting it with pathogens—even ones that the plant isn’t genetically programmed to recognize. That would make this mutation ideal for fighting off disease, Dangl says, except that “unfortunately, there’s this very nasty side effect that the whole leaf dies.” In a normal resistance response, the cell death is confined to a limited area.

The scientists think that, in a normal plant, the Lsd-1 gene acts as a gatekeeper, or “negative regulator.” That is, it keeps cell death turned off until it receives the right signal.

“This is a common concept in all of cell biology,” Dangl says. “Proteins are ready to go, but they’re held down somehow. They’re just waiting for the signal.” In this case, the signal could be any number of chemicals produced by the plant in response to the pathogen. “There’s probably several different ways into the cell-death pathway,” Dangl says.

One known player in the disease-resistance pathway is superoxide, alias O2. The scientists have found that superoxide is always around when cell death occurs. But superoxide itself doesn’t kill the cells—when researchers inject a normal plant with a substance that produces superoxide, no cell death occurs. Possibly, when a pathogen is present, superoxide signals another chemical to kill the cells.

In a normal plant, a chemical signal may also tell the Lsd-1 gene when to stop the cell death. “Why kill only three cells, not thirty?” Dangl says. “There must be something out here at a certain point that says, `Stop killing cells, you’ve killed enough.’”

That something could be salicylic acid, a close cousin of acetosalicylic acid, or aspirin. “When a normal plant is infected with a pathogen, the plant’s production of salicylic acid goes up a hundred fold at the infection site,” Dangl says. “And then as you go in concentric circles out from the site, the level of salicylic acid goes down quite dramatically.” Maybe, in the areas where there’s more salicylic acid, the Lsd-1 gene lets cells die. But when salicylic acid gets down to a certain level, the Lsd-1 gene stops the death.

But salicylic acid is just one suspect. There could be any number of chemicals that trigger the Lsd-1 gene to start and stop the resistance pathway.

The scientists continue to search for these perpetrators and for other genes and chemicals that play a role in the cell-death pathway. Once they solve the case of thale cress, they’ll have clues as to how resistance works in other crops. Mutants that show inappropriate cell death have been known in corn, rice, and tomatoes for decades, Dangl says, and these plants probably use similar defense strategies.

But Dangl’s team has their work cut out for them. “In our little weed, thale cress, there are probably about two hundred different resistance genes,” Dangl says. “If there’s two hundred in the weed, you can imagine how many there are in corn, which has a genome twenty times larger.” With all these unknowns, scientists will be investigating this case for years to come.
  

Behind the Scenes

To determine how plants fight off fungi and other invaders, scientists in Jeff Dangl’s lab study the inner workings of thale cress. Daniel Aviv, a doctoral student in genetics and molecular biology, tries to determine the function of a particular gene, Lsd-1, that plays a role in disease resistance. He makes specific changes in different parts of the gene’s DNA and inserts it back into the plant. Then he notes how the altered DNA changes the plant’s reaction to invaders or chemicals.

In addition to Lsd-1, there are many other genes that play a role in disease resistance. J.B. Morel, a doctoral student from the University of Orsay in Paris, France, finds clues to the function of these genes by breeding different mutant plants together and observing the effect. For example, in plants in which the Lsd-1 gene is mutated, the plant cell death caused by the immune response goes out of control, causing the whole leaf to die. If Morel crosses this plant with another mutant, and the offspring behave normally, then the second mutant has suppressed the Lsd-1 mutation. This indicates that the gene that’s mutated in the second plant may play an important role in disease resistance and should be studied further.

A third doctoral student, Mike Richberg, mutates or “knocks out” additional genes in the Lsd-1 mutant plant. If, when the second mutation is added, the Lsd-1 mutant plant no longer shows out-of-control cell death, then Richberg knows he’s found another gene that plays an important role in the cell-death pathway.