Topic: Carnivorous plants
Categories: Biochemistry, Enzymes, Feeding, Metabolism, Physiology, Plants & Algae
All plants are harmless? Well, not quite - at least not when you're an insect...
Plants are classically thought of as autotrophs - they are photosynthetic and use sunlight as the energy source to build sugars and other compounds from the starting point of carbon dioxide and water. A number of plants have, however, evolved alternative mechanisms of obtaining nutrients. Some, for instance, are parasitic. And others have become carnivorous, trapping insects and other animals with specialised leaves...
Distribution of plant carnivory
Carnivory in plants as a strategy for nutrient acquisition is rampantly convergent, having evolved at least six times in five different orders of angiosperms, dicotyledons as well as monocotyledons. More than 95% of the 600 or so species of carnivorous plant belong to the Lamiales (within which carnivory has evolved more than once) and Caryophyllales, but carnivorous species are also found in the Ericales, Oxalidales and Poales. As Aaron Ellison and Nicholas Gotelli (Journal of Experimental Botany, 2009, vol. 60, p. 19) remark, "in spite of these independent origins, there is a remarkable morphological convergence of carnivorous plant traps and physiological convergence of mechanisms for digesting and assimilating prey".
Energetics of plant carnivory
For a plant to qualify as carnivorous, it must not only trap insects (or other potential animal prey), but also digest them. Such plants therefore secrete enzymes similar to those in our stomach and then absorb the dissolved nutrients, mainly nitrogen and phosphorus. A number of species are considered protocarnivores - they trap insects, but do not produce digestive enzymes or take up the nutrients. Some, therefore, rely on mutualisms with other organisms, such as bacteria or insects, which make the prey nutrients available to them.
These nutrient supplements should be to the advantage of the plant, and, indeed, several benefits have been demonstrated. They include increased growth rates, nutrient storage and reproduction, with more flowers and seeds being formed. There is, however, only weak evidence for higher rates of photosynthesis, and carnivorous plants do not seem to extract carbon from prey (which if they did would allow them to bypass photosynthesis). The overall costs of carnivory are still far from being fully understood, but are likely to include structural (constructing the traps) and energetic (operating the traps) costs. As the traps are modified leaves, it has also been argued that the plants might lose photosynthetic capacity.
So what are the conditions in which plant carnivory has evolved? Carnivorous species typically grow in nutrient-starved or acidic soils and are limited to sunny, waterlogged habitats. Most species are either bog plants or found in humid tropical regions. In such conditions, the benefits of carnivory might exceed the costs, but it has also been suggested that carnivorous plants simply make the best of a very bad situation. This idea has, however, been challenged by recent studies indicating that the structural (and potentially also the energetic) costs of traps might actually be low.
At any rate, one would expect strong selective pressure on maximising prey capture and digestion, and carnivorous plants have adapted their leaves into efficient traps. They employ at least five different trapping mechanisms, ranging from the relatively simple flypaper traps to the highly specialised suction traps.
The use of flypaper traps has emerged at least five times (in the eudicot families Lentibulariaceae, Droseraceae, Roridulaceae, Byblidaceae, and Dioncophyllaceae). Here, plants have sticky leaves, thanks to numerous stalked glands that secrete droplets of mucilage. This gluey substance is well suited to capture small insects, but larger ones are usually able to escape. Trapped prey is then digested by enzymes, such as protease and phosphatase, which are released by sessile glands.
The most famous (and also most advanced) flypaper traps are found in the sundews (Drosera), a large genus with almost 200 species that inhabit every continent except Antarctica but are particularly diverse in Australia and South Africa. Sundew leaves are covered in brightly coloured mucilaginous tentacles, which are very sensitive and able to bend towards a struggling insect, thus entrapping it further. Some sundew species (e.g. the Cape sundew D. capensis) can even curl the whole leaf around the prey. The Australian pygmy sundews are so dependent on nitrogen from insects that they lack nitrate reductase, an enzyme that otherwise allows plants to use soil-borne nitrate.
The Mediterranean dewy pine (Drosophyllum lusitanicum), which is unusual among carnivorous plants in that it grows in dry, alkaline soils, was long assumed to be a close relative of Drosera. Recent molecular analysis has, however, suggested that it is more closely associated with the Dioncophyllaceae. This family contains a liana, Triphyophyllum peltatum, which is characterised by a three-stage life cycle and is carnivorous only in its juvenile stage. Both Drosophyllum and Triphyophyllum do not possess active flypaper traps like Drosera, but passive ones that are incapable of movement.
Other examples of passive flypaper traps are found in the butterworts (Pinguicula), a large genus containing about 80 American, European and Asian species with succulent leaves, as well as in two (or possibly three) other genera. The Australian rainbow plants (Byblis) were long considered to be protocarnivorous. Although they trap insects with mucilage-covered leaves, they were thought to not digest the insects themselves but obtain nitrogen from the excretions of bugs (Setocoris) that feed on these insects. Recently, however, direct digestion of insects by the plant has been demonstrated, meaning rainbow plants swell the ranks of true carnivores. Similarly, the two species in the genus Roridula, which are associated with the capsid bug Pameridea roridulae, are now regarded as fully carnivorous, as R. gorgonias not only shows phosphatase activity, but also takes up prey nutrients. There is still controversy surrounding the status of the Australian triggerplants (Stylidium). This is a speciose genus that if accepted as true carnivores would substantially increase the number of carnivorous plant species. The triggerplants are equipped with glandular hairs that trap insects, and protease secretion has been demonstrated. It is, however, still unclear whether these plants actually assimilate nutrients from trapped insects.
Snap-traps evolved at least 65 million years ago and only once within the family Droseraceae, where they are found in two closely related monotypic genera. Here, the leaf is divided into two lobes and sudden closure quickly engulfs the prey. Operation of the trap is initiated by repeated stimulation of one or more trigger hairs and only takes about 0.3 seconds, making this one of the fastest movements observed in plants (exceeded only by the explosive pollen discharge in mulberry and dogwood). How exactly this is achieved remains unknown, but enthusiasts for convergence will not be surprised to learn that changes in cell turgidity seem to play a role. Due to interlocking "teeth" on the leaf margin, prey cannot escape from the trap, while the tight closure of the trap probably increases the efficiency of digestion and nutrient absorption.
The venus flytrap (Dionaea muscipula), which is only found in wet pine savannahs in southeastern North America, is probably the best-known carnivorous plant and attracted the attention of Charles Darwin, who described it as "one of the most wonderful plants in the world". This plant seems specialised for capturing single, large ground-crawling insects such as ants and beetles. They might walk across the trap accidentally or be attracted by glands on the leaf rim that secrete UV-reflective substances. Small insects often escape from the trap, and it has been suggested that the little energy they would provide might not repay the costs of digestion. It takes the flytrap five to seven days to digest an average-sized insect. The traps of the aquatic waterwheel plant (Aldrovanda vesiculosa) are similar in form but smaller and, accordingly, target small invertebrates. This species has a worldwide distribution, but is rare in its range and declining.
Molecular phylogenetic analysis has confirmed Drosera as a sister taxon of these two genera, implying that snap-traps evolved from flypaper traps. At first sight, the snap-traps seem very different from the sticky traps of the sundews in morphology and action, but in fact they share many similarities. There is only speculation regarding the selective forces that have led to the evolution of snap-traps, but it is generally assumed that they allow the capture of larger prey with higher nutritional value. Other factors could be quicker prey capture and more efficient and complete nutrient assimilation, meaning that fewer nutrients are lost to the environment. So why did snap-traps not evolve more often then? Perhaps it is because they are not only highly intricate, but also stand in competition with pitcher plants. So too the snap-traps are associated with ephemeral habitats, conceivably making it more difficult to establish an evolutionary foothold.
Despite being extremely complex and integrated functional units, pitfall traps have evolved independently at least four times (in the unrelated eudicot families Nepenthaceae, Sarraceniaceae and Cephalotaceae as well as in the monocot family Bromeliaceae). The most famous pitcher plants are the monkey cups (Nepenthes), which comprise about 130 mostly liana-forming species in the Old World tropics, and the ground-dwelling Sarraceniaceae. This New World family contains three genera, Sarracenia (the trumpet pitchers), Darlingtonia (with only a single species, the cobra plant D. californica) and Heliamphora (the marsh pitchers). Less well known are the Australian pitcher plant Cephalotus follicularis and the carnivorous bromeliad Brocchinia reducta (another bromeliad, Catopsis berteroniana, is considered to be a protocarnivore).
These plants employ pitfall traps of different complexity that have all converged on certain features aiding the capture of a wide range of insects. A rolled-up leaf forms a cavity that houses a pool of digestive liquid at the base. To prevent overflowing of the trap, most pitchers are covered by a lid, the so-called operculum. Insects are attracted to the plant by pigmented areas or nectar bribes and then fall into the pitcher, as its rim is slippery. This is either achieved by a waxy covering that negates the otherwise highly efficient adhesive pads of the insects' legs (a strategy that has evolved independently in many other groups of plants) or by the surface being highly wettable, with raindrops quickly forming a thin film of water across which the insect aquaplanes. Intriguingly, certain insects (notably ants) are able to walk on these otherwise lethal surfaces, but how they achieve this feat is not yet fully understood. The sides of the pitcher are usually grooved, making it impossible for trapped victims to climb out, and some plants even release an anaesthetising chemical. If the pitchers are big enough, they cannot only capture insects, but also small reptiles or mammals.
The visual features of pitcher plants, which include a dark centre, stripes or peripheral dots, seem to exploit a sensory bias present in many insects, especially Hymenoptera. Interestingly, very similar patterns are found on insect-pollinated flowers ('floral guides') and stingless bee nest entrances. It has been suggested they might be convergent, having evolved in response to the common function of attracting insects and the requirements of the hymenopteran visual system.
The rootless corkscrew plants in the small genus Genlisea, which are aquatic or found in waterlogged soils in Africa and America, have specialised on capturing small protozoans with so-called lobster-pot traps (also known as eel traps). These traps are modified underground leaves, which once entered are virtually impossible to exit. They have a Y-shape, with a stalk, a vesicle (which serves as the digestive chamber) and a tubular neck divided into two arms that are helically twisted and bear many small openings. Once prey has entered the trap, inward-pointing hairs force it to move down the furrows of the spiralled arm towards the digestive chamber. Although the arms are equipped with glandular hairs that could secrete attractants, experiments suggest that prey might move into the trap accidentally. The trap structure could then be of particular importance, with the openings "mimicking" the spaces between soil particles.
In addition, two pitcher plants, the cobra plant and the parrot pitcher (Sarracenia psittacina), possess features reminiscent of lobster-pot traps, such as sharp, inward-pointing hairs and multiple translucent "false exits".
Close relatives of Genlisea are the bladderworts (Utricularia), which are celebrated for their highly complex suction traps. With about 220 species, Utricularia is the largest and most widely distributed carnivorous plant genus. A few species are epiphytic and some aquatic, but most inhabit terrestrial environments, where they are, however, limited to waterlogged or wet soils. A single terrestrial origin for bladderworts seems likely, while the epiphytic and aquatic habits might have been acquired multiple times. Bladderworts are characterised by a "relaxed" morphology (lacking a clear separation into root, stem and leaves), but they possess extremely specialised bladders (utricles), which trap insects. The trap is set by pumping out water, thus generating a negative pressure inside the sealed bladder. When passing prey (e.g. water fleas or rotifers) trigger one of the hairs at the trap entrance, the bladder suddenly expands and the prey is sucked in at remarkable speed. The bladder then closes and the prey is digested. This is an energy-intensive process, which, interestingly, seems to be supported by changes to the mitochondrial coxI gene, which encodes a subunit of cytochrome c oxidase.
Cite this web page
Map of Life - "Carnivorous plants"
September 27, 2015
(Topic created 1st September 2009) | Last modified: 16th September 2011
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