Mistletoe: “Berry” tasty or the kiss of death?

Of all the plants used for festive decorations, mistletoe (Viscum album L.) is perhaps the most intriguing. It is an evergreen with a curious parasitic nature, a fascinating history, and a surprising number of medicinal properties.

The characteristic leaves and white fruits of V. album, which contain one seed embedded in very sticky, glutinous pulp. Photo: Llez.

The characteristic leaves and white fruits of V. album, which contain one seed embedded in very sticky, glutinous pulp.
Photo by Llez CC BY-SA 3.0.

Mistletoe belongs to the Santalaceae plant family, which includes 42 genera and over 400 species of shrubs, herbs, and trees in tropical and temperate regions. Indian sandalwood (Santalum album) is the only other economically important member of the family, used extensively in the furniture and perfumery industries. Mistletoe, on the other hand, is well known for its waxy white fruits and is strongly associated with the Christmas period.

One of the most fascinating things about mistletoe is its seed dispersal mechanism, which typically involves attracting birds with these tempting fruits. The white fruits (technically drupes, not berries) contain viscin, a sticky pulp that either clings to the bird’s beak, or passes through its gut unharmed. When the seeds are excreted, or the bird wipes the pulp off on a branch, the viscin hardens and attaches the seed firmly to its new host.

This mutualism gives mistletoe an efficient seed dispersal strategy, and the birds a continuing food supply through winter. In fact, one such mutualistic relationship, between mistletoe and the mistle thrush (Turdus viscivorus), may explain the plant’s common name.

After the seed germinates, a structure known as a haustorium forms. This penetrates the host plant’s tissue and draws water and nutrients from it, and the mistletoe continues its development without a root system in the soil.

Globe-shaped shrubs of mistletoe in parasitised trees, as seen in the wild

Globe-shaped shrubs of mistletoe in parasitised trees, as seen in the wild.
Photo by Philip Halling (CC BY-SA 2.0).

This parasitic behaviour is thought to have a range of effects on its host. Some of these include reductions in the growth of the host, premature mortality, poor wood quality, reduced fruiting of infected trees, and increased susceptibility to other pathogens such as insects or saprophytic fungi 1.

What makes it so interesting is that mistletoe is hemiparasitic; this means the plant is dependent on its host for water and nutrients, but produces its own energy by photosynthesising with its evergreen leaves and stems.

BSBI distribution data of V. album in Britain in 1999

BSBI distribution data of V. album in Britain in 1999
Map copyright Jonathan Briggs2, used with permission.

In the UK, mistletoe grows most commonly in apple (Malus domestica) orchards, and on lime (Tilia spp.), and poplar (Populus spp.) trees. It is locally common in the counties of Gloucestershire, Herefordshire and Worcestershire, but is absent from most of Scotland, Wales, northern England and the extreme southwest2). It is easiest to see growing around this time of year, and its green appearance growing from a lifeless winter tree is, no doubt, one reason why the plant has acquired such mystical significance.

Mistletoe has had a long history of use in folk medicine, from being used as an antiseptic and antispasmodic (suppressing muscle spasms), to treating epilepsy, rheumatism and even certain types of cancer3. Before you get too excited, the species is also extremely poisonous, particularly the leaves and stems. This is due to the presence of proteins called lectins, which have high biological activity; these proteins bind specifically to certain sugars and cause clumping of particular cell types, inhibiting protein synthesis4. Mistletoe poisoning can cause blurred vision, diarrhoea, nausea and vomiting, stomach pains and drowsiness. It comes as no surprise that most cases arise when mistletoe is cut and brought indoors (e.g. at Christmas), particularly with young children and pets involved.

Why then, do we kiss each other under bunches of this toxic, parasitic plant every Christmas? The origin is vague. The tradition may have stemmed from Norse mythology, when a dart of sharpened mistletoe supposedly killed Baldr. The tradition also has possible roots in the festival of Saturnalia in Rome. In pagan times, mistletoe was even considered a holy plant. Interestingly, kissing under mistletoe at Christmas comes with some etiquette, depending on who you ask: normally, the man can only kiss a woman or girl on the cheek and, when he does so, he must remove a drupe from the mistletoe sprig. Once the drupes have finished, the kissing must end too.

A local resident’s Christmas sales of mistletoe in West Bridgford, Nottingham

A local resident’s Christmas sales of mistletoe in West Bridgford, Nottingham.
Photo copyright Waheed Arshad.

While the species has an intriguing past, the future may not be so merry. In light of a changing climate, there are a number of wider factors affecting mistletoe. Agricultural change is causing an ongoing loss of traditional apple orchards, suggesting future reductions in harvestable crops of mistletoe at Christmas. A project launched in 2009 by the National Trust and Natural England aims to reverse the loss of this habitat by restoring traditional orchards. Some have predicted the species may even vanish from Britain entirely if winter temperature rises are greater than summer temperature rises5.

As with many other species, the effects of climate change on mistletoe in the UK could be very dramatic. Ensuring the mistletoe you buy is British and sustainably managed, as well as supporting your local orchards, is one way to help. I am sure you will agree that the toxic parasite with seeds dispersed in bird poop is well worth conserving – on that note, Happy Christmas!

Did you know?

Mistletoe forks branch every year (from the third year after germination) – so, by counting the number of times the branches fork and adding two years, you can easily approximate the age of the plant!

References

1. Knutson, D.M. (1983) “Physiology of mistletoe parasitism and disease responses in the host”. In ‘The Biology of Mistletoes’ (eds. D. M. Calder and P. Bernhardt.). pp. 295-316. Academic Press: Sydney.
2. Briggs, J. (1999) Kissing Goodbye to Mistletoe? BSBI and PlantLife Report.
3. Bussing, A. (2000) Mistletoe: The Genus Viscum (Medicinal and Aromatic Plants – Industrial Profiles). CRC Press.
4. Franz H (1986) Mistletoe lectins and their A and B chains. Oncology 43 (Suppl 1): 23-34.
5. Jeffree, C.E. & Jeffree, E.P. (1996) Redistribution of the potential geographical ranges of mistletoe and Colorado beetle in Europe in response to the temperature component of climate change. Functional Ecology 10: 562–577.

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