The mistletoe ( Viscum album ) , also called visco, is a plant with fascinating features that arouses the curiosity of those who observe it, to the point that several myths and legends have formed around it. The Greeks used mistletoe to decorate marriage ceremonies. The Romans, for the Saturnian festivities. In Norse mythology, an arrow or spear smeared with mistletoe extract was what Loki used to make Höðr kill his brother Baldr, who everyone thought was immortal, an act that would be the beginning of a crisis that would end up triggering Ragnarok. And British tradition bestows good fortune on those who kiss under a sprig of mistletoe at Christmas.
A half parasitic plant
Mistletoe is often confused with holly ( Ilex aquifolium ), perhaps because both plants are tied to Christmas. However, they are two very different plants; The leaves of the holly are dark green, very hard and resistant, and generally with a spiny margin, and it bears red fruits. The fruits of the mistletoe are a very striking translucent white , and its leaves, elongated and thin, are smooth and much more delicate and pale.
One of the most peculiar features of mistletoe is that it is a plant that grows on trees, which in botany is called an epiphyte. They are hemiparasitic plants, which although they can and do photosynthesis by themselves, they also feed on the sap of the tree they host, weakening it and sometimes leading to its death.
Three subspecies of mistletoe are distinguished; one of them, V.a. album , parasitizes broadleaf trees and shrubs, such as oaks, ash trees, olive trees, serval trees, birches, poplars, willows, maples, lime trees, hazelnut trees, almond trees, and even fruit trees such as peach, plum, apple and pear trees. The subspecies V. a. austriacum , on the other hand, parasitizes various species of pines, while V. a. abietis parasitizes fir.
Substances with pharmacological action
Mistletoe extract contains a significant amount of active ingredients . They are molecules that the plant produces in its secondary metabolism, with a defensive function, to avoid the attack of herbivores, and when consumed, they have a pharmacological effect. It has been proven that mistletoe extract contains viscotoxins and lectins , active ingredients that are characterized by being inactivators of ribosomes —the organelles that are responsible for translating messenger RNA into protein, and therefore, key players in gene expression and in cell function. This makes them active cytotoxic ingredients.
Furthermore, in vitro, some immunostimulatory activity has been demonstrated; the administration of non-toxic doses of these active ingredients increases the action of a type of cell called “natural killer” (NK) on cytotoxic T lymphocytes . The function of these cells to neutralize and eliminate other cells in poor condition from the body, and in the presence of purified viscotoxins, have shown an increase in their capacity up to four times higher. This immunostimulation of NK cells favors the elimination of cancer cells in insufficient doses to be cytotoxic to the rest of the cells.
Crossing from the laboratory to people, mistletoe extracts alone have not shown sufficient therapeutic action to constitute a treatment . On the one hand, each plant will have a different concentration of active ingredient depending on its life history, and that makes it very difficult to establish the dose. We must not forget that these are toxic species and that too high a concentration will have many more risks than any benefit.
On the other hand, certain pseudotherapies such as homeopathy have attributed multiple properties to extremely diluted mistletoe extract —among which, ironically, cancer is found, violating the supposed homeopathic principle that “like cures like”—. Of course, neither this nor any other homeopathic product has any efficacy superior to placebo .
But, aside from this, extracts whose active ingredients are adequately quantified and dosed have shown some efficacy in improving cancer treatments, used as adjuvants . Especially, a moderate effect in reducing fatigue has been observed in patients receiving cancer treatments. However, the most recent meta-analysis, published in March 2022, still acknowledges that the evidence is limited, as there are not enough double-blind, placebo-controlled clinical trials.
The rarity of its fruit
Mistletoe is an extraordinarily specialized plant with very unusual characteristics among angiosperms. Specifically, the fruit that forms after fertilization is defined as a “pseudoberry containing a naked seed”. But there are several traits that do not fit this definition.
For starters, mistletoe does not have a true seed in the botanical sense of the term. The embryo does not originate from the development of an ovule, as in the rest of the plants, but rather develops through the pollination of embryonic sacs that are formed from cells called sporogens , inside the ovary.
On the other hand, the endosperm —the nutritional tissue that, in most plants, is produced alongside and nourishes the embryo during seed formation— contains chlorophyll , which is very rare. In general, chlorophyll is produced in the first leaves of the seedling, called cotyledons, after seed germination. The absence of testa —the shell of the seed— together with a fruit with a whitish and translucent skin and a mucilaginous and transparent pulp means that the endosperm can carry out photosynthesis on its own even before germinating.
And on the other hand, not one, but several embryos can develop in the same endosperm . And it is that, in reality, each embryo produces its own endosperm, and by not having the cover, later all of them merge into one. So we have a fruit that encloses a false seed, that has no shell, and that in a single endosperm can have several embryos. Quite a rarity of nature .
As far as we know, mistletoe is the only plant that has such a complex fruit. At the moment, it has not been possible to classify successfully in the different types of known fruit. And despite its toxicity, already exposed, there are many animals that can consume its fruits without risk and thereby help its dispersal.
References:
Becker, H. 1986. Botany of European Mistletoe (Viscum album L.). Oncology, 43(1), 2-7. DOI: 10.1159/000226413
Catalan, P. et al. 1997. Viscus. En Flora Ibérica: Vol. VIII. Real Jardín Botánico, CSIC.
Ochocka, J. R. et al. 2002. Biologically active compounds from European mistletoe ( Viscum album L.) 1. Canadian Journal of Plant Pathology, 24(1), 21-28. DOI: 10.1080/07060660109506966
Pelzer, F. et al. 2022. Cancer-related fatigue in patients treated with mistletoe extracts: a systematic review and meta-analysis. Supportive Care in Cancer. DOI: 10.1007/s00520-022-06921-x
Tabiasco, J. et al. 2002. Mistletoe viscotoxins increase natural killer cell-mediated cytotoxicity: Viscotoxins enhance NK cell-mediated killing. European Journal of Biochemistry, 269(10), 2591-2600. DOI: 10.1046/j.1432-1033.2002.02932.x
