No kissing here
For the white spruce tree (Picea glauca), mistletoe is the kiss of death.
When Barry Logan, an associate professor of biology at Bowdoin College, began studying the interaction between white spruce and the parasitic eastern dwarf mistletoe (Arceuthobium pusillum) 10 years ago, he figured the mistletoe sapped the spruce of food and water until the tree could no longer sustain the drain. “When I entered into this, I mostly thought about the parasite as just a sink for resources,” he says. He soon discovered the system was much more complex. Ten years on, he’s drawing some surprising conclusions about how the petite flowering plant brings down the stately spruce.
Seeking out mistletoe-infected spruce in coastal Maine forests isn’t much of a challenge, Logan says. The parasite causes trees to grow twisted, tangled branches called witches’ brooms. These misshapen branches are dead giveaways that mistletoe is wreaking havoc on the tree’s growth and development.
Dwarf mistletoe isn’t always lethal. Forty-two species (including the eastern variety) exist, and they impart varying degrees of harm on their favorite host trees. In Washington and Oregon, western hemlocks show evidence of having weathered infections of hemlock dwarf mistletoe for 80 years or more, Logan says. White spruce, on the other hand, succumb to eastern dwarf mistletoe in 15 or 20 years.
Maine’s coastal forests may be uniquely vulnerable, Logan says. A century ago, farmers felled the native mixed-hardwood forests along the coast to make room for sheep farming. These pastures were quickly abandoned, and white spruce (which thrives in full sun and enthusiastically colonizes old fields) moved in to the vacant spaces. Today these forests contain dense, monospecific stands of very mature spruce trees—a perfect recipe for damaging mistletoe infections. As the aging coastal white spruce succumb to the infection, Bill Ostrofsky, a forest pathologist at the Maine Forest Service, says he expects that the composition of these forests will change—a return, perhaps, to the oaks and other hardwoods that disappeared from the coastal forests during colonial times.
But historical symmetry offers little consolation for the white spruce. In addition to growing tangled witches’ brooms, the individual needles on infected trees are much smaller than normal (Plant Biol, 4:740–45, 2002). The parasite disturbs the tree’s water balance and stunts tree growth overall. But it’s a certain counterintuitive habit that makes mistletoe infections so deadly for the white spruce, and so intriguing to the scientists.
Trees typically respond to parasites and pathogens by shedding infected branches and sending resources to unaffected limbs. Other tree species respond to mistletoe in this manner, and white spruce, too, react this way when attacked by other pathogens. When plagued with dwarf mistletoe, however, white spruce ship water, nutrients, and sugars to the infected branches, at the expense of the uninfected boughs. “Something about the mistletoe is overriding the white spruce control mechanisms,” says James Lewis, a plant ecologist at Fordham University, who has been collaborating with Logan for the last 4 years.
The mistletoe, it seems, may be meddling with the tree’s hormones. In a not-yet-published study, Logan and his team discovered that needles on infected white spruce branches have twice the concentration of cytokinins as do uninfected branches. These growth-promoting hormones trigger branching and direct the movement of resources into the branch. 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. “All of this comes together nicely,” Logan says, to explain how witches’ brooms form and thrive.
Exactly how the mistletoe is manipulating the tree’s hormones remains to be seen. “I think it’s possible that the mistletoe is moving hormones into the host and not just withdrawing resources,” he says. He hopes to undertake more hormonal analysis to get to the bottom of the interaction between parasite and host.
Understanding this parasitic interaction—and the spruce’s maladaptive response—could go a long way toward helping scientists better understand plant ecology in general, the researchers say. “The way [mistletoe] is killing the tree is so different from what we usually see. I think that’s one of the reasons why this is a nice system for addressing broader issues of how plants interact with other organisms,” Lewis says.
“There’s an interplay between [host and parasite] which I didn’t appreciate when I first tackled this project,” Logan adds. “It makes it more interesting, and more complicated.”