This Month's Sci-Fi Worthy Parasite: Arceuthobium spp.

Ok, I kind of covered this parasite earlier this year. In that post, I explained that Mistletoe is a hemi-parasite (only partially dependent on its host for survival) and that the family represents one of the nine times that parasitism evolved in the plant kingdom. I also mentioned that unlike most parasites, parasitic plants are keystone species, where their ecological impact is disproportionate relative to their abundance. But I didn't talk about the many varieties of mistletoe, and this time, I'd like to focus on one: dwarf misteletoe, of the genus Arceuthobium.

Recently, the dwarf mistletoes have become newsworthy thanks to some novel research coming out of Yellowstone National Park. Within the park and its surrounding forests, Dwarf mistletoes grow on a variety of trees, but they can be particularly damaging to the beautiful white spruce. While some trees handle infection quite well, as is the case with western hemlocks which show evidence of having mistletoe infections for 80 years or more, white spruces succumb to Arceuthobium in less than two decades. In that time, their branches twist and mangle into tell-tale "witches brooms."

Recently, scientists have found that the little parasitic plant has an even greater effect than just killing its hosts. When biologists Ken Cullings and Julie Hanley from the NASA Ames Research Center at Moffett Field, CA, measured the amounts of carbon dioxide emitted from the soil surrounding trees infested with Arceuthobium americanum, they found that the average rate of carbon dioxide emission was more than 70 percent higher than the rate emitted from the soil around trees nearby that were free of the parasitic plant.

Why, exactly, this effect is seen isn't entirely known, but researchers suspect that the emitted CO₂ is the result of a cascade of downstream effects of the mistletoe's parasitism. Because the mistletoe takes sugars from its host tree, fewer of these sugars reach the tree's roots where soil fungi tap in for extra nutrients. Without those sugars, the root fungi must get their nutrition elsewhere, and do so by breaking down organic matter in the soil, a process which happens to produce carbon dioxide.

Scientists are still looking into how much of an effect this little plant has on our climate, but it seems to be a lot more than we'd expect. Even the leeching of carbon dioxide from the soil isn't the extent of its impacts, as infected trees tend to be smaller with less leaves and smaller ones, leading to less carbon-absorbing ability.

But that's not all that mistletoe does, at least according to some fresh research by Barry Logan, an associate professor of biology at Bowdoin College. His research into the interaction between dwarf mistletoe and its hosts has found some surprising results: white spruce actually feed their parasites. Most trees respond to mistletoe infection by shedding infected branches and sending resources to unaffected limbs, and indeed, white spruce reacts this way to many other pathogens. But when infected with Arceuthobium pusillum, a spruce-specializing eastern Dwarf mistletoe, the spruce actually does the opposite. The trees ship water, nutrients, and sugars to the infected branches at the expense of the uninfected ones. In a soon-to-be-published paper, Logan and his team found that needles on infected white spruce branches have twice the concentration of cytokinins, a class of hormones which promote growth, as uninfected branches. Infected branches also have significantly reduced concentrations of abscisic acid, a stress-related hormone that some studies have linked to the shedding of old branches. This hormone manipulation explains both the characteristic "witch's brooms" as well as the spruce's poor resistance to infection. Scientists have yet to uncover how the mistletoe is manipulating the treeâs hormones, but Logan thinks that the parasite may actually be injecting them into the host.

It may seem like a small change, but some studies suggest that dwarf mistletoe infests trees in as much as 70 percent of the 48,000 km² ecosystem that includes Yellowstone Park. That's a lot of mistletoe!

But don't get all Grinch-y about the mistletoe over your doorway - these effects, both climatic and hormonal, only apply to dwarf mistletoe, which isn't the species encouraging those holiday smooches. So feel free to kiss whomever you like under that mistletoe without worrying about all of this. Happy Holidays to you all!