BIOLOGY & MEDICINE_Plant Research Fill up the bowl! Leghold traps, limed rods, pit traps - insectivorous plants have come up with unusual strategies to obtain additional nutrients. Axel Mithöfer at the Max Planck Institute for Chemical Ecology in Jena is investigating how pitcher plants from Southeast Asia entrap and digest their victims. TEXT CATARINA PIETSCHMANN lants that consume animals – there are also purely practical ways of us- That’s why, if you have a carnivorous imagine coming up with an ing them. In Malaysia, for example, the plant at home, you shouldn’t fertilize idea like that! Evolution, traps of the pitcher plant, Nepenthes, are it. “If they can meet their needs for nu- though, doesn’t suffer from filled with sticky rice, vegetables, or trients using their roots, they form qualms. Pitcher plants are ex- meat and eaten. On the island of Bor- fewer traps and instead invest more traordinaryP and have long been a source neo, old pitchers that have lignified are energy in leaves for photosynthesis,” of curiosity and flights of fantasy. used as storage vessels for food and explains Mithöfer. Charles Darwin went as far as to drink or for steaming rice. Of all the insectivorous plants, the dedicate an entire book to these plants, Venus flytrap, Dionaea muscipula, with commonly referred to as carnivorous. FLUID WITH MEDICINAL POWERS its traps reminiscent of leghold traps, is Of the sundew’s trapping mechanism, the most spectacular. It is native to he wrote: “It is surprising how minute The plants are even said to be good for North American bogs and is the best a particle of any substance, ... if placed your health. Indigenous peoples, for ex- known active trapper. The waterwheel in actual contact with the surface of a ample, treat skin inflammations and di- plant, Aldrovanda, indigenous in Ger- gland, suffices to cause the tentacle to gestive disorders with the digestive liq- many but now extinct there, does pos- bend. [...] It is a much more remarkable uid from the pitchers. sess a very similar trapping mechanism fact that when an object, such as a bit Axel Mithöfer also has a practical to the Venus flytrap. Its leaves, howev- of meat or an insect, is placed on the use for insectivorous plants. “In sum- er, are only a few millimeters in size, disc of a leaf, as soon as the surround- mer, I place two or three sundew plants and the plant grows under water, mak- ing tentacles become considerably in- next to the fruit bowl. That deals with ing it relatively unknown. With its ten- flected, their glands pour forth an in- any problems with fruit flies.” tacles that twist around its prey, the creased amount of secretion.” The additional nutrition the plants sundew is also an active trapper. It’s not just our fascination with get from animals helps them to keep The Venus flytrap snaps shut on plant carnivores that have attracted us; up in nutrient-poor environments. its victims at a surprising speed for a Photo: Anna Schroll 60 MaxPlanckResearch 2 | 19 Under UV light, the trap of a pitcher plant resembles a filigree work of art. Insects are attracted by nectar on the rim of the pitcher and slip into the belly of the trap. The lid prevents rain from diluting the digestive fluid in the trap. Photo: Anna Schroll BIOLOGY & MEDICINE_Plant Research Chitin receptors signal the presence of prey to the pitcher plants » – in the true sense of the word, they taste their food. plant. Mithöfer gives me a demonstra- and digestive juices are secreted from of the Venus flytrap, and Mithöfer’s tion of the trapping mechanism in the glands in the trap, bathe the prey and team already possess that of one pitch- greenhouse of the Max Planck Institute dissolve it. The same glands then ab- er plant species of the genus Nepenthes. in Jena. He carefully strokes a fine twig sorb what the hungry plant lacks in its over an open trap, touching the tactile nutrient-poor habitat: nitrogen and NEW TASKS FOR PROTEINS trigger hairs inside it a few times. The phosphate compounds. plant has three of these hairs, barely The feast lasts several days. Then From their genes, it’s clear that carniv- visible to the naked eye, on each half the trap unfolds again and spits out the orous plants have important things in of the trap leaf. “bones” – the indigestible chitin exo- common, even if they developed the skeleton of the fly. In the wild, the next ability differently. They all use the en- STRUGGLING IS DEADLY rain shower washes out the remains of zymes that evolution passed them down the banquet. from their ancestors. “Plants have been The trap snaps shut in a tenth of a sec- The sundew does things entirely dif- exposed to herbivores and pathogens, ond. “If the fly were smart, it would ferently. If an insect lands on its leaves, such as fungi, for 400 million years. stay motionless for two hours. The trap which are covered with fine tentacles, And that’s the reason why they devel- would then open, and it could fly off,” it gets trapped by countless droplets of oped enzymes like chitinases that can explains Mithöfer. But staying standing a sticky liquid. After this, the tentacles break down the cell walls of fungi,” said still isn’t in the nature of flies. Under- at the edge of the leaf first maneuver Axel Mithöfer. standing why isn’t hard. Caught be- the animal into the middle of the leaf, It’s a fortunate coincidence that tween the trap leaves, it panics and where many of the glands that produce the carapace of insects is also made of struggles wildly to free itself. digestive juice are located. The leaf then chitin. Carnivorous plants take advan- Each time it touches a trigger hair, slowly curls over the “meal”, and the tage of this fact and simply re-deploy it generates a tiny electrical impulse, prey is digested. their existing tools, turning defensive just like that in a nerve cell. “The plant There have been multiple indepen- enzymes into digestive secretions. sums the impulses to determine if it dent instances of plants developing They can now use chitinases to has actually caught living prey, or if ways to consume animals. Depending break down the chitin shells of insects only a drop of rain has landed on it,” on the expert you ask, there have been and get at the nutrients within. The Mithöfer explains. “Two, three con- four to nine separate lines of develop- plants get the nitrogen and phosphate tacts are okay. But nine or ten means ment. The genomes of some species from the bodies of the insects using it’s curtains for the fly!” A plant that such as of a bladderwort, Utricularia proteins and enzymes that digest nu- can actually count! gibba, and the Australian Albany pitch- cleic acids. While the trap leaves squeeze to- er plant, Cephalotus, have already been Mithöfer’s particular field of exper- gether tightly and crush the insect, the deciphered. In Wuerzburg, researchers tise is pitcher plants. There are just over flytrap literally starts to salivate. Acid are currently working on the genome 120 species of the genus Nepenthes in Photos: Anna Schroll (3) 62 MaxPlanckResearch 2 | 19 Above left The Venus flytrap has a sophisticated capture mechanism. The leaves have been repurposed as folding traps and contain trigger hairs, which send signals to the plant when an insect is trapped. Within fractions of a second, the two leaf halves snap shut and capture the prey. Above right The various species of sundew – here the Cape sundew – in contrast rely on sticky traps. Looking like sweet nectar on red flowers glittering in the sunlight, the traps are actually covered with a sticky liquid designed to attract and hold insects. Below Alberto Dávila-Lara is a doctoral student in Axel Mithöfer’s research group. The photograph shows him collecting nectar from the rim of a pitcher. Photos: Anna Schroll (3) 2 | 19 MaxPlanckResearch 63 the humid tropical rain forests of At its rim, the pitcher secretes nectar, “We do that to prevent flies from escap- Southeast Asia, most of them, climbing which becomes exceptionally slippery at ing or something else from falling into plants. Four species of Nepenthes grow high humidity. There’s no way of grip- the pitcher.” in the greenhouse of Mithöfer’s Insti- ping, even using the fine adhesive pads And how do the pitchers sense that tute in Jena. of insects feet. Incautious insects fall and prey is near? Unlike the Venus flytrap, Pitcher plants primarily prey on soil drown in the fluid of the pitcher. Nepenthes produces chemical rather insects such as ants and termites. After Axel Mithöfer investigates how di- than electrical signals. The plant’s traps a pitcher has matured and filled to a gestive secretions are generated by feed- probably have receptors for chitin – the good quarter with fluid, the lid opens ing the pitchers with fruit flies and substance that forms the carapace of and the “hunting season” commences. then placing nylon socks over them. insects. While other plants use recep- Above Using a sterile syringe, Alberto Dávila-Lara extracts digestive fluid from an unopened pitcher for further analysis. Below left A fluorescence microscope reveals the glands in the pitcher wall that produce the digestive fluid. Below right The glands lie in depressions of the epidermis and are covered by protruding hoods (The bar corresponds to one-twentieth of a millimeter). Photos: Anna Schroll (above), MPI for Chemical Ecology (below, 2) 64 MaxPlanckResearch 2 | 19 BIOLOGY & MEDICINE_Plant Research Simple leaf Nepenthes leaf Leaf base Leaf base = Petiole/stalk = Lamina basal lamina-shaped structure tendril Petiole Lamina = pitcher Metamorphosis from leaf to trap: over the course of evolution, the leaf base of an ordinary leaf in Nepenthes has developed into a basal structure and the petiole into the tendril of the pitcher.