In 1768, William Young, the royal botanist, imported living plants of the Venus fly-trap to England. They were shown to John Ellis, a member of the Royal Society, who recognised the Venus as a carnivorous plant. He wrote a letter and sent it with a dried plant to the Swedish scientist, Carl von Linne. Among others Ellis wrote: “Nature may have some views towards its nourishment in forming the upper joint of its leaf like a machine to catch food: upon the middle of this lies the bait for the unhappy insect that becomes its prey … the two lobes rise up, grasp it fast, lock the rows of spines together, and squeeze it to death … the small erect spines are fixed near the middle of each lobe, over the glands, that effectually put an end to all its struggles”. Linne gave this species the name Dionaea muscipula Ellis. This name comes from the Greek word Dionaia, the goddess of love.
The very restricted natural occurrence of this unique species led to the investigation of the methods of its cultivation and propagation. Moreover, extracts of D. muscipula are used against malignant diseases.
Distribution and General Morphology
The carnivorous plant Dionaea muscipula Ellis (the Venus flytrap) is a monotypic genus belonging to the Droseraceae family. This native and endemic plant species is restricted to the coast of south-east North Carolina and the coastal corner of South Carolina in the USA, where it favours damp soil which is predominantly sand with a small proportion of peat.
Dionaea muscipula Ellis is a rhizomatous and perennial plant. The vegetative shoot of the Venus flytrap is a rosette of specialised leaves radiating from an elongated and fresh rhizome. The leaves have broad, winged petioles ending in a blade which is a trap. The modified blade traps insects. It is kidney-shaped to circular, the inner part with reddish granular dots, variously coloured and patterned in shades of green, yellow and red, is hinged along the centre and fringed with up to 8-mm marginal bristles (); each half of the blade bears three sensitive hairs on the upper surface which, when touched in succession by an insect, cause the two halves to snap rapidly together, thus trapping the prey.
The flowers are radially symmetric and they form a cluster on leafless stalks 15-45 cm in height. Five sepals, five petals, white with green veins, spatulate 1.1-1.3 cm, stamens 15-20 cm. Ovary superior, one-celled; one style. Fruit an ovoid capsule 3-4 mm, which splits irregularly. Seeds are numerous, black.
D. muscipula has one of the most sophisticated systems for catching and digesting insects, and hence it is probably the most fascinating carnivorous plant. The leaf-lobes shut rapidly like a pair of jaws at a rate similar to that of a mammalian. Two successive bends of the trigger hairs in a time interval shorter than 20 s are sufficient for a quick closure of the trap within 0.5 s. On the physiological and biochemical level, the stimuli of the trigger hairs evoke action potentials, which spread over the trap tissues with a velocity of ca 10 cm/s, whereas changes in the concentrations of cytosolic-free Ca2+ ions are considered to play a key role in this mechanism. However, in spite of intensive studies, the trapping mechanism of Dionaea is still not fully understood (Trebacz, pers. comm.). Then, the prey is digested by enzymes (proteases) present in the fluid excreted by the numerous digestive glands located on the inner faces of the lobes. It seems that naphthoquinone – plumbagin also plays a role in the digestion process due to the inhibition of bacterial growth, thus eliminating competition from bacteria, as well as predigestion by oxidative modification of proteins of the prey.
Dionaea muscipula, a legally protected carnivorous species, arouses increasing interest. This plant has been cultivated for 200 years in collections of various botanical gardens, where it is conventionally propagated. Recently, the Venus flytrap has attracted the attention of numerous hobby gardeners, and professional gardeners are absorbed in its commercial production. Also, many researchers have carried out comprehensive physiological and biochemical studies of this species since it produces digestive enzymes.
Because of the above-mentioned trends, the possibility of a quick and large-scale propagation of this plant is considered necessary.
Dionaea muscipula also has medicinal importance since it is considered to be an anticancer drug with oncolytic, anti-proliferative and immunomodulatory effects. The concept of such an application was raised in the 1970s by Keller who based it on his own theory that carnivorous plants, which can break down primitive epithelia, might be able to reduce malignant tissue. The commercial preparation containing the expressed juice of the fresh plants cultivated in a greenhouse, named Carnivora, has been produced on a large scale in Germany since 1981. Repeated clinical trials in a number of patients showed limited or no remission and in consequence, the efficacy of the Dionaea preparation in the treatment of human cancer was not considered to be proved scientifically. In vitro investigations showed moderate antiproliferative activity of Carnivora on five tumour cell lines (sensitive- and drug-resistant) at high concentrations of 100 μg/ml and higher under continuous long-term exposure. Currently, the preparation is indicated for unconventional anticancer treatment. Extract of Dionaea was found to be cy to toxic at concentrations of 1-0.1 mg/ml and more recent investigations have also indicated such effects of Dionaea extracts at the high dose level of 100 μg/ml. This activity was related to the major 1,4-naphthoquinones present in the species, plumbagin and hydroplumbagin 4-O-glucoside. The former compound, otherwise well known from its occurrence in several plant families like Droseraceae, Nepenthaceae, Plumbaginaceae, Dioncophyllaceae and Ebenaceae, as well as a broad pharmacological potency, showed in vitro immunosuppressive cytotoxicity at concentrations 1-0.01 mg/ml and a reversed effect (immunostimulating) at very low concentrations. The dual effect of plumbagin was observed on human granulocytes in in vitro studies, where the compound exerted cytotoxic effects at high concentrations (100 μg – 100 ng/ml) and stimulation of phagocytosis at low concentrations (100 pg – 10 fg/ml) and no effect at dosages of 2.5ng-25pg/ml. Similar dose-dependent activity was observed in a parallel study for several well-known cytotoxic or cytostatic agents (e.g. vincristine). It should be noted that plumbagin was found to be toxic in mice; LD50 = 28mg/kg i.p.. Furthermore, hydroplumbagin glucoside showed immunostimulating activity as it exerted a high phagocytosis coefficient K = 1.6 in an in vivo carbon-clearance assay in mice at a dose of 0.1 mg/kg i.p., as well as enhanced in vitro proliferation of T-lymphocytes in very low concentrations of 100 pg – 1 pg/ml. Minor naphthoquinones of the species, droserone and chloroplumbagin, were inactive. Since the effect of active naphthoquinones was observed with unusually small doses, it was, therefore, concluded that the antitumour effect of the drug Carnivora depended on the induction of the immune system and was due to the low content of plumbagin and hydroplumbagin glucoside in the daily dosage of the drug
Conventional Practices for Dionaea Propagation
Conventional cultivation of the Venus flytrap is difficult. Encke and Slack described its propagation from seeds and leaf cuttings which can be slow and not very effective. The seeds sown before winter germinate in spring. Seedlings initially grow very slowly. They take from 5 to 7 years to reach maturity. In late spring, fully expanded leaves of the Venus flytrap are excised in order to obtain leaf cuttings. After a few weeks, at the base of each petiole, a bud is formed which develops into a shoot and plantlet. Enke also propagated the Venus flytrap from shoots with pieces of rhizomes. Dionaea is cultivated and propagated on medium consisting of sphagnum peat moss and sand (fine gravel) under glass or in a cold greenhouse.
Dionaea muscipula Ellis (Venus Flytrap): Conclusions and Prospects
Dionaea muscipula Ellis is a carnivorous plant, the natural occurrence of which is restricted to a small area in North America. The plant catches its prey in snap traps by a rapid movement at a rate similar to that of animals. Hence, it is considered as a model for a biomimetic material with built-in sensors and actuations.
Dionaea muscipula can effectively be mass-propagated in vitro. Seeds disinfected by surface sterilisation germinated in vitro produce numerous seedlings, which after a few months developed into plantlets. For mass propagation, excised shoots with pieces of rhizomes or excised leaves are used. The researchers have been unanimous in their opinion that the propagation rate is high enough to allow the production of numerous plants in a short time.
One shoot produces 20-30 lateral shoots in 3 months, producing ca. 380,000 plants per annum. One excised leaf, which undergoes two multiplication cycles per annum, can produce 4000 shoots and plantlets per annum.
Propagation of plants on a half-strength MS medium and RM medium with or without such growth regulators as cytokinins (2iP and kinetin) and auxins (NAA and IBA) at low concentrations gives satisfactory results. The optimum conditions are: temperature 20-26°C, light 100 nEm-2s-1, photoperiod 16h.
Because no plant variations have been reported, it is possible to develop a potential procedure of revitalising continuously grown in vitro cultures of Dionaea muscipula. A drop in temperature to 5°C allows the long-term storage of cultures and stimulates rooting.
The plant is considered to have anticancer properties and the expressed juice of the fresh plants provides the commercially available drug designed for unconventional treatment. Both extracts of Dionaea, as well as its constituent, plumbagin, were shown to have dual in vitro cytotoxic and immunostimulating activity, like some known anticancer agents. It is interesting that another carnivorous plant with anticancer properties, Sarracenia flava (golden trumpet, Sarraceniaceae), containes no naphthoquinones and the active principles appeared to be terpenes.
The plants obtained by micropropagation on either Reinert-Mohr or diluted MS medium produce such phenolic compounds like naphthoquinones, phenolic (ellagic and gallic) acids derivatives and flavonoids. Many of them are present in Drosera species obtained by in vitro micropropagation. It should be emphasised that ellagic acid is well known for its chemopreventive, anticancer properties. Pharmacologists conclude that further pharmacological and clinical investigations to evaluate the usefulness of preparations from Dionaea muscipula in cancer therapy are desirable. Our suggestion is that further possibilities in this area should be searched for within ellagic acid derivatives and maybe other, so far unknown, constituents of Dionaea like non-phenolic compounds, in analogy with the case of Sarracenia ().
Selections from the book: “Medicinal and Aromatic Plants XII” (2002).