The Biology of Mistletoe

Some plants are so entwined with tradition that it’s impossible to think of one without the other. Mistletoe is such a plant. But set aside the kissing custom and you’ll find a hundred and one reasons to appreciate the berry-bearing parasite for its very own sake.

David Watson certainly does. So enamored is the mistletoe researcher that his home in Australia brims with mistletoe-themed items including wood carvings, ceramics and antique French tiles that decorate the bathroom and his pizza oven.

And plant evolution expert Daniel Nickrent does, too: He has spent much of his life studying parasitic plants and, at his Illinois residence, has inoculated several maples in his yard — and his neighbor’s — with mistletoes.

But the plants that entrance these and other mistletoe aficionados go far beyond the few species that are pressed into service around the holidays: usually the European Viscum album and a couple of Phoradendron species in North America, with their familiar oval green leaves and small white berries. Worldwide, there are more than a thousand mistletoe species. They grow on every continent except Antarctica — in deserts and tropical rain forests, on coastal heathlands and oceanic islands. And researchers are still learning about how they evolved and the tricks they use to set up shop in plants from ferns and grasses to pine and eucalyptus.

All of the species are parasites. Mistletoes glom on to the branches of their plant “hosts,” siphoning off water and nutrients to survive. They accomplish this thievery via a specialized structure that infiltrates host tissues. The familiar holiday species often infest stately trees such as oaks or poplar: In winter, when these trees are leafless, the parasites’ green, Truffula-like clumps are easy to spot dotting their host tree’s branches.

Yet despite their parasitism, mistletoes may well be the Robin Hoods of plants. They provide food, shelter and hunting grounds for animals from birds to butterflies to mammals — even the occasional fish. Fallen mistletoe leaves release nutrients into the forest floor that would otherwise remain locked within trees, and this generosity ripples through the food chain.

“Yes, ecologically, they are cheats,” says Watson, a community ecologist at Charles Sturt University at Albury-Wodonga (Australia is home to nearly 100 mistletoe species). “People hear the ‘P’ word, they think parasite and they think they are all necessarily bad. But it’s an extremely loaded term.”

The bottom line: Mistletoes share their wealth. “They grab onto these nutrients, and then they drop them,” Watson says. “They’re like, ‘I’ve got all this good stuff, and now you can have it.’”

They’re wily, versatile and ingenious

To pilfer all those nutrients, mistletoes must infect a host, and researchers are still busy figuring out the fine details. The process goes something like this: The seed lands on a host plant (often delivered by a bird — more on that later) and penetrates the bark with a structure that, for a standard plant, would develop into a root. Secreted digestive enzymes may help it insinuate itself into the tree, says Carol Wilson, a botanist at the University and Jepson Herbaria at the University of California, Berkeley. Once in, the mistletoe squeezes its way around the host cells toward the tree’s plumbing, the xylem. Then comes a fateful kiss: Mistletoe cells connect with the host’s plumbing or nearby cells, and the pilfering of water and nutrients begins.

Mistletoes and other parasitic plants make a well-defined structure for invading their hosts — it’s called a haustorium, from the Latin word haustor, meaning drinker or drawer of water. Mistletoe haustoria are quite diverse, Wilson says, reflecting the varied ways they approach their parasitic habit.

Some species, for example, form a sort of flattened pad that encircles a host branch. A wedge then develops on the pad’s underside and penetrates the host. Other species coax the host’s own wood to develop a frilly-edged mass; these are called “wood roses” and people carve them into intricate figures such as birds and fish. (Wilson, whose workplace houses perhaps the largest collection of mistletoe haustoria in the world, has some carved wood roses at home, including a lizard, a monkey and a chicken.)

Other mistletoes send out slender vine-like extensions called epicortical runners — they have multiple haustoria for more widespread invasions. This approach allows the mistletoe to crawl along the host tree and capture choice sunlit territory among upper branches, but also crawl back toward the tree’s water-laden trunk. And some mistletoes create stringy “bark strands” that spread beneath the host tree’s bark and develop lots of tiny (initially microscopic) haustoria, called sinkers. These tap into host plumbing.

The haustorium has been termed “the essence of parasitism” by renowned Canadian botanist Job Kuijt. And for the essence of this essence, look no further than Western Australia’s Nuytsia floribunda, in a group of plants known as the “showy mistletoe” family, the Loranthaceae. Technically speaking, Nuytsia is not a true mistletoe, says Nickrent, a researcher and professor emeritus at Southern Illinois University, Carbondale, because it infests roots instead of branches. But it’s an extremely close relative and requires a mention, if only because its haustorium houses a sickle-like blade used to slice into host plant roots that is sharp enough to draw blood. Neither buried electrical cables nor telephone lines are immune to Nuytsia’s cutting device.

They exploit animals — and nourish them too

Mistletoes don’t just exploit plants, they depend critically on animals to get around — a varied selection, it turns out. Their name derives from Anglo-Saxon words meaning “dung-on-a-twig” — typically the dung of birds, which eat the seeds and disperse them to new host plants. (There are exceptions: Some mistletoe genera make explosive fruits that hurl their seeds toward nearby trees, reaching distances of 10 meters or more.) Most mistletoe fruits are berries containing a single seed that’s surrounded by a sticky layer of goo called viscin, which cements the seed to a new host tree.

The berries are rich in minerals and glucose, and some contain all 10 essential amino acids. Roughly 90 bird species are considered mistletoe specialists. Some swallow the fruits whole, others peel the fruit then eat the seed and gluey viscin; still others dine on the viscin alone.

Australia’s mistletoebird (Dicaeum hirundinaceum) is one such devotee. It devours the berries whole, excreting the seed in record time, thanks to a modified gut that rapidly absorbs glucose from the viscin surrounding the seed. “Within a few minutes, the bird passes the entire seed through the digestive tract,” Nickrent says. The mistletoebird also has evolved specific behaviors that seem to aid the plant. “It wiggles its little behind, attaching the mistletoe seed to the branch of the tree,” Nickrent says.

Birds do double duty: They also serve as pollinators — many mistletoe flowers are rich in nectar. Though the holiday-associated species have rather drab little flowers, others sport outlandishly showy blooms. “If you’re into floral diversity, mistletoes are a crowning glory,” Watson says. Many of the bird-pollinated species have gaudy, lipstick-red flowers, including South America’s Tristerix corymbosus, whose slender blooms are festooned with bright yellow stamens.

And certain mistletoes have evolved elaborate mechanisms that aid in bird pollination: The petals of some of these species are fused together; when a nectar-seeking bird pries open the flower, the pollen explosively sprays the bird’s head.

They take and give

Mammals too, are known to dine on mistletoe berries. Ditto errant fish: There’s a record of the dusky narrow hatchetfish eating fruits during floods. But the largest dietary contribution of mistletoe may be their leaves. Around the world, animals including deer, porcupines, rhinoceroses and possums feed on mistletoes, as do the caterpillars of numerous butterfly species.

Lizards and birds will hunt for insects in dense leafy, clumps of mistletoe, which also can serve as dwellings. Squirrels and other mammals are known to make their homes in mistletoe, as are birds of all sizes, from hummingbirds to waterfowl: A survey in Australia found that 245 bird species nest in mistletoes. And a variety of raptor species — hawks and their kin — use fresh mistletoe sprigs to line their nests.

When mistletoe leaves fall to the ground, their decaying leaves feed plants, insects, fungi and more. “Mistletoe leaves generate a steady stream of nutrients to the forest floor,” Watson says. “It’s like a dripping tap of fertilizer.”

Back in 2001, Watson made the case in an article in the Annual Review of Ecology and Systematics that mistletoes are keystone species on which much of the life surrounding them depends. Evidence for this role has since grown. A study of mistletoes in the savannah of Zimbabwe, for example, found that mistletoe leaf litter pumps additional nitrogen, phosphorus and other nutrients into the soil, influencing the diversity of grasses.

And in one experiment, Watson and colleagues removed some 5,000 mistletoe plants from 20 sites in the Billabong Creek watershed in Australia. Those areas ended up with 25 percent fewer birds, compared with 20 neighboring areas whose mistletoes were left intact.

They evolved over and over and over

Mistletoes are a motley group of plants defined more by their lifestyle than their lineage. They all have three features in common: They are parasitic, woody and aerial (meaning they infect above-ground plant parts, rather than roots). The lifestyle they enjoy evolved at least five separate times in five different plant families.

Yet mistletoes do cluster in one main area of the vast plant family tree, and they all have ancestors that were parasites not on branches, but on roots. Mistletoes made the leap to branches — an event that happened many times over.

“All of the mistletoe lineages represent independent evolutionary events, independent historic moments when something happened and this under-story, root-parasitic, nondescript shrubby thing switched to being an aerial, parasitic shrubby thing,” Watson says.

Moving up the tree helped to solve a problem that all plants grapple with: competing for water and sunlight. (Despite their parasitic nature, most mistletoes still photosynthesize, and so need healthy access to light.) And they never looked back. Orchids, cacti and more: Today’s varieties invade plants of all stripes, including themselves — a number of species have been documented parasitizing other mistletoes. They’ve even been spotted going three layers deep: a mistletoe on a mistletoe on a mistletoe.

And while a handful of species inflict damage on horticulturally important trees, including members of the pine family that are valued for their timber, most mistletoes don’t infect economically important crops, Watson says.

Dig into mistletoe biology and you uncover layer upon layer of complexity — and wily ingenuity. It’s easy to see why they hook in curious minds. “They are sneaky; they’ve worked out these little loopholes,” Watson says.

So maybe there’s poetic sense in stealing kisses under thieving mistletoes.

Rachel Ehrenberg is associate editor at Knowable and a big fan of parasitic plants. Follow her on twitter @Rachelwrit.