This meat-eating plant is only a part-time killer


Most carnivorous plants are full-time predators—the Venus fly trap, for example, lies in wait year-round to snag flies with its jawlike leaves. Now, researchers describe a meat-eating plant that only catches insects when it’s flowering. Overlooked because its sticky hairs are relatively common among plants, the species may be just one of many examples of “cryptic carnivores” yet to be discovered.

“It’s a really interesting finding and a well-designed study,” says Ulrike Bauer, a carnivorous plant expert at the University of Bristol who was not involved with the work.

There are about 800 species of carnivorous plants. Most belong to closely related groups. Many species have a snap trap, like the Venus fly trap, whereas other groups rely on sticky surfaces, and still others lure prey into chambers filled with digestive fluids. The last time a totally new type of carnivorous plant was discovered was in 2012: Researchers found a species in the Brazilian savanna that catches tiny worms with special leaves that grow underground.

The species in the new study, called the western false asphodel (Triantha occidentalis), lives in mountainous bogs and other nutrient-poor locations in western North America. The upper part of its flowering stalk is covered with small red hairs that exude a shimmering, sticky substance. The hairs often trap flies and small beetles in the droplets. But so do many other kinds of plants, which use these hairs as defense against pests rather than as a source of nutrition.

The clue to Triantha’s carnivorous diet emerged from a genomic study of plant evolution. Gregory Ross, a master’s student working in the lab of Sean Graham, a botanist at the University of British Columbia (UBC), Vancouver, noticed T. occidentalis lacks some genes that are also missing in carnivorous plants. (The genes are involved in fine-tuning photosynthesis, for example when plants are exposed to dappled sunlight.)

Qianshi Lin, then a Ph.D. student at UBC, decided to investigate. He prepared a special diet for Triantha: fruit flies that had been fed with an isotope, or form, of nitrogen that is rare in nature, which could reveal whether the plants were absorbing nutrients from the flies. After 150 flies had matured, Lin froze them. Then, he and colleagues visited a bog near Vancouver, where they added fruit flies to 10 individual Triantha plants and, as a control, to a similar-size plant that is not carnivorous.

One to 2 weeks later, the researchers brought the plants back to the lab. They could detect the nitrogen isotope in the stems, leaves, and fruits of Triantha, but not in the noncarnivorous plants. Triantha got more than half of its nitrogen from prey, similar to sundews, a carnivorous plant living nearby, the team reports today in the Proceedings of the National Academy of Sciences.

“That’s the point at which you prove that it’s a carnivorous plant,” Lin says. “I felt quite excited to discover it.” He also showed that the hairs make the same enzyme, phosphatase, that other carnivorous plants use to extract the nutrient phosphorus from prey.

Many carnivorous plants use sticky hairs to snare flies and small beetles, but they locate these traps away from their flowers—it’s no good to eat an insect that’s needed for pollination. The western false asphodel doesn’t do this; it puts these sticky hairs on the main stem bearing its flowers, which grows up to 80 centimeters tall. The authors think the red hairs and shiny droplets attract insects, like in sundews. But the droplets are only sticky enough to trap small insects such as midges and not bees or other pollinators, says co-author Tom Givnish, a botanist at the University of Wisconsin, Madison.

Andreas Fleischmann, a botanist at the State Botanical Collection in Munich, isn’t convinced that Triantha is a true carnivore—for that, he says, it needs to be clearly demonstrated that the plant is luring insects to their deaths. He thinks it’s more likely that the hairs are used to kill insects that might steal pollen or nectar from its flowers without fertilizing them. Triantha is more of a passive killer, Fleischmann argues, not an active one with leaves modified for trapping.

One implication of the finding is that there may be other overlooked examples of carnivorous plants; the researchers found museum specimens of a related species with small insects attached to the flower stalk. Triantha is just a part-time carnivore, Lin says, because it only flowers briefly. Aaron Ellison, an ecologist at Harvard University, notes there’s only one other known example of a part-time carnivorous plant, a vine in West Africa that eats insects only as a juvenile but then outgrows the habit.

There’s something else remarkable about Triantha, Ellison notes: It’s one of just a few examples of carnivorous plants in a large group of plants called the monocots (which includes all grasses and lilies, for example). Why are carnivorous monocots so rare? Lin says it might be because the typical monocot leaf, like a blade of grass, is narrow with parallel veins, which may be less suitable for evolving into complex traps. The presence of carnivory in different ancient lineages is fascinating to study, Fleischmann and others say, because it’s like comparing the evolution of flight in animals as distinct as bats and insects.

“This paper will be an important piece of carnivorous plant biology,” says biophysicist Rainer Hedrich of the Julius Maximilian University of Würzburg., 9 August 2021