"This difference in salicylic acid levels raises several questions," Salt said. "If you modify other plants so that the level of salicylic acid is always high, those plants are not happy. They look sickly. With salicylic acid continuously elevated, a plant thinks it's under some massive attack by a pathogen. It's expressing all its pathogen response proteins, and at such a high level, they can have a deleterious effect on the plant."
Metal hyperaccumulators like Thlaspi, however, don't show any negative effects from their constant exposure to high levels of salicylic acid.
"These plants have tons of salicylic acid, but for some reason that salicylic acid is not initiating the pathogen response. That tells us some part of the pathway doesn't sense salicylic acid - that the signal is blocked," he said. "It's like yelling into the phone louder and louder, but no one can hear it."
Salt and his colleagues also show in the current study that salicylic acid induces production of a molecule called glutathione, a potent antioxidant that protects plants from metal. Because the production of glutathione is tied to the production of salicylic acid, most plants normally have fairly low glutathione levels and, consequently, can't tolerate metals.
Thlaspi, on the other hand, is brimming with glutathione, thanks to its elevated salicylic acid levels. When grown in nickel-enriched soil, Thlaspi takes up 3 percent of its body weight in the metal. Salt and his colleagues have shown that this metal content is what makes the plants resistant to pathogens.
Salt proposes a scenario in which at some point in evolutionary history some plants acquired a mutation that disrupts the salicylic acid signaling pathway, leaving them unable to fight off pathogens.
"In most settings, those plants would be toast - they'd be immediately selected out of the population," he said. "This whole system raises the questio