Some do not like it hot — including a fungus that rots the flesh of chili peppers, data from lab experiments and field studies show. The new research provides convincing evidence that the bitter and sometimes toxic chemicals found in many fruits serve to prevent microbial attack — a popular idea that has been difficult to demonstrate.
From a plant’s point of view, a fruit is any product of plant sex. A fruit’s job is to bear seeds, and move them away from the parent plant (maple helicopters, poppy heads and milkweed pods are all fruits). Plants that make fleshy fruits, such as peppers and berries, are usually dispersed by birds and mammals, versus wind and water. But fruit succulence comes at a cost — it may attract critters that will damage rather than disperse the precious seeds.
“It’s an evolutionary paradox,” says Josh Tewksbury, who led the new study, to appear in Proceedings of the National Academy of Sciences.
“You use fats and sugars to attract birds and large mammals, but the problem is, everyone likes fats and sugars,” says Tewksbury, of the University of Washington in Seattle. “So you end up attracting not only birds and mammals, but insects, small chewing wasps, microbes or a fungus — these are equally attracted to fruit but they harm it and never disperse the seeds.”
The best defense being a good offense, the noxious compounds, such as the cyanide in apple seeds, in fruits or parts of them are often cited as being a pre-emptive strike against microbial attack.
“The question is, what function does a given spice serve within a living plant in its natural setting, before domestication or harvesting?” comments Rob Raguso of CornellUniversity. “Many people love hot food but have no idea why on earth a plant would be spicy. It’s almost a religious question. Let’s not forget why Columbus sailed from Spain in the first place; spices were big business before refrigeration.”
But it has been difficult to cleanly demonstrate that less spice means more microbes, Tewskbury says. The discovery that the amount of spiciness varies from plant to plant in particular species of chili pepper provided a nice testing ground for the microbial defense hypothesis, he adds.
Led by Tewksbury, the researchers investigated wild Capsicum chacoense chili plants along a 300-kilometer stretch of southeastern Bolivia. In the southern region of the area, almost all of the plants are super spicy — due to chemicals known as capsaicinoids. But in the area’s north and east, there are more and more bland plants and among the northernmost populations, fewer than 30 percent of the plants make pungent capsaicinoids.
The researchers speculated that the non-pungent plants would be infested with Fusarium, a black fungus that is delivered to the plant via the mouthparts of a tiny sap-sucking bug. Sure enough, seed infection rates in blander chili peppers were twice as high as infection rates in the super spicy chilis. The researchers also found that in regions that had a lot of the fungus-delivering bugs, populations of Capsicum chacoense had more spicy plants. Lab experiments on cell culture spiked with capsaicinoids verified Fusarium’s distaste for the spice.
“Much of the evidence for this paper — and the inferences that led the authors to their conclusions — accumulated over years of study,” Raguso notes. “So it’s a nice example of a satisfying ending that emerged after several lines of evidence converged.”