Since the dawn of invertebrates, plants have had to defend themselves against hordes of nibblers. On at least a half-dozen occasions, however, plants turned the tables and became predators: the sundew with its sticky tentacles, pitcher plants with their beckoning pools of enzymes, and the flytrap with its swift clamp of death. These plants’ aggressive feeding habits help them survive in poor soil by giving them a new source of nitrogen and other nutrients. Many biologists suspect this predatory behavior evolved when ancestors of today’s carnivorous plants turned mechanisms that normally detect and defend against insect pests into offensive weapons.
Now, this hypothesis has gained support from a detailed genetic study of Venus flytraps (Dionaea muscipula) as they snared crickets and began to digest them alive. Led by biophysicist Rainer Hedrich and bioinformaticist Jörg Schultz of the Julius Maximilian University of Würzburg in Germany, a team tracked the genes expressed as the plants sensed and then digested their prey. The research, published online before print in Genome Research, provides the most detailed view so far of the molecular action during prey capture. “This is a great study,” says plant geneticist Victor Albert of the University at Buffalo. “It’s much richer” than previous studies of the process.
To catch an invertebrate that has blundered into its snare, the flytrap relies on an ancient alarm system. It starts ringing when the victim jostles trigger hairs. The hairs in turn generate electrical impulses that somehow stimulate glands in the trap to produce jasmonic acid—the same signal that noncarnivorous plants use to initiate defensive action against herbivores. Patterns of gene expression in the two kinds of plants confirm the similarity, Hedrich says.
The consequences of the alarm, however, are quite different. In noncarnivorous plants, jasmonic acid triggers the synthesis of self-defense toxins and molecules that inhibit hydrolases, enzymes that herbivores secrete to break down the plant’s proteins. As part of their counterattack, plants also produce their own hydrolases, which can destroy chitin and other components of insects or microbes. In the flytrap, in contrast, jasmonic acid triggers a voracious response: Tens of thousands of tiny glands make and secrete hydrolases. The trapped invertebrate is drenched in the same digestive enzymes that another plant might use in smaller quantities to ward off an enemy. “It’s just a change in emphasis,” says Edward Farmer, a plant physiologist at the University of Lausanne in Switzerland.
After a few hours, the glands inside the trap turn on another set of genes that helps the plant absorb nutrients from its meal. Experiments showed that many of these genes are the same ones expressed in the roots of other plants. “We looked at each other and said, ‘Yes, it’s a root,’” Hedrich says. “It made immediate sense,” because the flytrap draws its nutrition not from soil, but from its prey.
“This is the way evolution works,” says Andrej Pavlovic, a plant physiologist at Palacký University, Olomouc, in the Czech Republic, who compares the flytrap’s innovations to the modification of a bat wing or whale fin from the limb of their terrestrial ancestors. The molecular repurposing that allows carnivorous plants to harvest their nutrients from the air is no less inspiring.