Credit: Adapted from C.G. Pereira et al., PNAS Early Edition (9 January 2012)
Patches of white sand dot the Campos Rupestres savanna in Brazil's central highlands. One of the strangest plants that thrives in these tracts of nutrient-poor soil is a spiny, purple-flowered genus called Philcoxia, which inexplicably grows with its leaves buried underground. Researchers have now discovered why: The leaves are a snare for tiny worms that the plant absorbs and eats.
The sand patches, each about 300 meters wide, used to occur in many more areas of Brazil's mountain savannah, says plant ecologist Rafael Oliveira of the University of Campinas in São Paulo, Brazil. But they are very vulnerable to environmental change, and "real biological treasures might be disappearing," he says. Those treasures include Philcoxia, whose 1-millimeter-wide underground leaves are still able to photosynthesize despite being covered by soil. But Oliveira and colleagues were mystified about where Philcoxia got its raw materials. The plant has only one taproot for taking up water rather than a root network that would be useful for absorbing nutrients from the soil.
Previously, researchers had discovered that Philcoxia's leaves contained structures that resembled the sticky glands seen in many carnivorous plants. When Oliveira and colleagues looked at the leaves under an electron microscope, they noticed tiny roundworms called nematodes sitting on the leaves (inset image). To figure out what the worms were doing there, the researchers created a miniature food chain. They grew bacteria in a culture containing a variant—or isotope—of nitrogen that is heavier than the normal element. They then fed these bacteria to nematodes, who took up the heavy isotope into their own tissues. Finally, the scientists placed these nematodes near the leaves of a few Philcoxia plants and waited.
The next day, the worms had crawled onto the leaves, and the team was able to detect the heavy nitrogen isotope from the worm in the plant's tissues. Within 48 hours, 15% of the heavy nitrogen in the worms had been taken up by the leaves, the researchers report online today in the Proceedings of the National Academy of Sciences. This result suggests that not only had the plant eaten the worms, but they constituted a very large part of its diet.
"When I first saw the results, I couldn't believe those underground leaves were actually eating nematodes," Oliveira says. He and his colleagues didn't find any fungi or other organisms associated with the leaves, but Oliveira says they "can't throw out the idea that [Philcoxia] could digest other really small creatures," and he plans to study this further. The researchers also want to know how the plant attracts nematodes to its sticky death trap in the first place.
Although Philcoxia is not the first worm-eating plant to be discovered—plants such as bladderworts catch worms and other critters with the trapdoor in their bubblelike traps—nematodes appear to be especially important to the plant's diet, which is novel, says Aaron Ellison, an ecologist at Harvard Forest in Petersham, Massachusetts. "It's a really good study," he says, not the least because "everybody loves carnivorous plants." Even Charles Darwin, writing in 1875, called them "the most wonderful plants in the world."
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