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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
P
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
traordinary 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-
Photo: Anna Schroll
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
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.
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
Photos: Anna Schroll (3)
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
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.
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. The leaf lamina eventually developed into the actual trap.
tors to detect predators, pitcher plants on the body of the pitchers are not only as long as possible. If too many insects
use theirs to sense the presence of prey. sweet, they also glow violet under black are caught, the plant can no longer pre-
So, in the true sense of the word, they light in the laboratory. Chemical anal- vent bacteria from proliferating in the
taste their food. ysis of the nectar revealed that this is pitcher fluid. When this happens, the
probably due to a flavonoid. “Maybe trap is broken down and discarded.
CHITIN PROMOTES DIGESTION it’s this glow that attracts the insects,”
says Mithöfer. LEAVES CONVERTED INTO TRAPS
“As soon as the receptors detect the To prove their hypothesis, Mithöfer
chitin of an insect’s carapace, a signal and his team intend to test the behav- Even if the pitchers are usually striking-
cascade is triggered that activates the ior of ants by comparing how they be- ly colored and patterned, they’re not
production of digestive enzymes,” says have presented with natural Nepenthes the flowers, but elongated, widened,
Mithöfer. One of the cascade’s most im- nectar and then with an artificial nec- and specialized leaves – a beautiful ex-
portant signaling substances is the phy- tar lacking the flavonoid. To perform ample of how plant leaves can perform
tohormone jasmonic acid, which is also the experiment, they plan to fly an en- a variety of tasks. They hang from the
used to defend against predators. The di- tire ant colony, including the queen, plants in all stages of development: un-
gestive enzymes originate from glands from Indonesia to Jena. ripe and closed, active with the lid open,
in the lower third of the pitcher. These The pitcher fluid does already con- and shriveled and dead. If a pitcher is
glands have two functions. “They both tain small amounts of enzymes before incapable of consuming food, it is usu-
exude secretions and also transport the the first prey lands in it. But it can take ally only discarded when the plant has
nutrients from the prey into the plant.” between two and three days for enough extracted as many nutrients as possible
Typically, it’s in their flowers that digestive enzymes to form. “Nepenthes from it. A pitcher plant, after all, doesn’t
plants secrete nectar; Nepenthes, on the can take its time; the prey isn’t going have resources to spare.
other hand, generates it at the rims of anywhere. It’s a real gourmet!” Not all carnivorous plants catch
its pitchers. And the nectar isn’t used to Mithöfer’s team extracted the fluid their own food. Some have a “delivery
attract pollinators, but, instead, to from still young, unopened pitchers service” and reward the couriers with
Graphic: MPI for Chemical Ecology
catch prey. Unlike flower nectar, which and discovered potassium chloride, sweet rewards. There are also Nepen-
can contain aromatic substances, pitch- trace elements, digestive enzymes, and thes species that provide shelter for
er nectar is entirely odorless. Even so, naphthoquinones. As soon as the trap ants. They live sheltered in cavities of
ants and beetles are tremendously at- has caught something, it produces addi- the pitcher plant and, in return, de-
tracted to it. tional digestive enzymes and acid to ac- fend their landlady against predators.
Mithöfer’s doctoral student Alberto tivates the enzymes. The naphthoqui- They themselves are unaffected by the
Dávila-Lara discovered that the rim of nones have an antimicrobial effect and digestive juices, which they simply
pitchers and the dot-like nectar glands probably keep the juice free of germs for swim through. >
2 | 19 MaxPlanckResearch 65
BIOLOGY & MEDICINE_Plant Research
Axel Mithöfer is investigating
the signal cascades used by
plants to protect themselves
from predators and pathogens.
Insectivorous plants have
modified some of the
molecules in the cascades to
turn themselves into preda-
tors. A bushy angelica grows
in the tea room of Mithöfer’s
research group, but ... it
wouldn’t harm a fly.
The plant also benefits from extracting
the valuable nutrients from the waste
material of the ants. In some instances, SUMMARY
this type of nutrition can take on biz- l T here have been multiple instances of plant species independently evolving
zare forms. “On the island of Borneo, as carnivores to consume insects.
tree shrews and rats sit on the rim of l Few plants species have evolved the ability to eat insects as new inventions;
particularly large pitchers of certain instead, they have tended to repurpose existing tools.
Nepenthes species, lick sweet nectar l Pitcher plants do not attract insects with the smell of the nectar they secrete
from the inner surface of the lid – and on the rim of their pitchers. Instead, the nectar reflects ultraviolet light, and
simultaneously release their droppings this is probably what attracts insects.
into the pitcher. Another Nepenthes
species provides daytime shelter for
bats, whose excrement they then uti-
GLOSSARY
lize,” Mithöfer explains.
Besides carnivores and excrement Pitcher plants: At present about 120 Nepenthes species are known, and researchers are
still discovering new species. Their range extends from Madagascar to New Caledonia
feeders, pitcher plants can even be her- and from China to Northern Australia. The leaves consist of a leaf base, a tendril, and
bivores. Nepenthes ampullaria with its the actual pitcher trap. Some species have traps up to 50 centimeters in size. Many
large pitcher openings, for example, Nepenthes species are threatened with extinction due to loss of habitat and illegal trade.
waits for leaves to fall into its pitcher. Venus flytrap: The traps of these carnivorous plants capture insects by snapping shut
A plant that, during the course of on them, after which they begin by exposing them to hydrochloric acid from gland
evolution, consumes first light, then cells. After a delay of a few hours, the glands also release digestive enzymes. The bro-
Photo: Anna Schroll
ken down nutrients are then absorbed by the same glands. These glands, are therefore
animals, and finally other plants –
similar in function to roots. This explains why only those genes that are typical for both
could there be any better way of leaves and roots are active in the trap cells.
demonstrating nature’s almost infinite
versatility?
66 MaxPlanckResearch 2 | 19“Insight must
precede application.”
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