Parasitic plants steal genes from their hosts

By Jeff Mitton

A photography and camping trip brought us to Sublime Point, a backcountry site on the North Rim of the Grand Canyon, and as the sun sank lower in the west we were walking out to the point to capture some of the breathtaking beauty of the canyon.

Bill Bowman pointed to a small plant and said "Orobanche."

It wasn't much to look at. It was only several inches tall and had no leaves and little color. A single reddish-brown stalk supported three tubular flowers, predominantly cream-colored but tipped in faded purple.

I wrinkled my nose, wondering why he would bother to call this to my attention and trying to think of something charitable to say about this apparently unexceptional little plant.

"It's a parasite," he said; this critical piece of information caught my attention and tweaked my curiosity.

I was familiar with coralroot, a highly specialized parasite that lacks functional chloroplasts and therefore cannot photosynthesize. It forms a reddish-brown flowering stalk 1 to 2 feet tall, bearing eight to 20 white flowers with fuchsia spots. In Boulder County, coralroot is found growing beneath lodgepole or ponderosa pines, but it does not directly parasitize them. It penetrates the ectomycorrhizal fungi attached to the roots of pines, extending and enhancing the tree's root system. Coralroot takes water and nutrients from the fungi, though the sugars and nutrients were photosynthesized in the tree's needles.

The parasitic plant at my feet was broomrape, probably Orobanche ludoviciana, one of about 200 species of Orobanche. It commonly parasitizes Carruth's sagewort, Artemisia carruthii, a short shrub with soft stems and light green, deeply dissected leaves with white hairs.

Both coralroot and broomrape, due to their lack of functional chloroplasts, have effectively lost their leaves, and their stems are reddish-brown or tan. And both lack any visible structures until the flowering stem appears. Broomrape may lie dormant beneath the surface for years until conditions are right for flowering.

The most well-known and common broomrapes are parasitic on crops, and they seriously diminish harvests by draining water and nutrients from sunflowers, peas, fava beans, chickpeas, tomatoes and eggplants.

Recently, as evolutionary biologists have conducted genomic studies of an expanding range of species, they have discovered that parasitic plants are taking more than water and nutrients from their hosts. Parasites are stealing genes from their hosts and incorporating them into their own genomes. In addition, they are moving some of their own genes into their hosts.

Instead of roots, broomrapes and other parasitic plants have haustoria that penetrate the roots of their hosts. The haustoria form the bridge that siphons water, nutrients and genes from the host and delivers genes to the host. The pores of haustoria are larger than those in roots, allowing the parasite to take up sugars, amino acids, proteins and fragments of DNA.

A recent study focused on several parasitic plants in the genus Phelipanche, which is very closely related to the genus Orobanche, and found that Phelipanche had some Orobanche genes. But these parasites do not parasitize one another (honor among parasites?), but only attach to photosynthetically functional plants. However, the parasites are sympatric, meaning they occur in the same place, and they both parasitize the same species, notably sagebrush and Carruth's sagewort.

It appears likely that a broomrape parasitized one of these species and inserted some genes from its chloroplast genome. Then the alien broomrape gene resident in sagebrush or sagewort was slurped up by Phelipanche and incorporated into its genome.

We are familiar with the passage of genes within a species, from one generation to the next. This is called vertical gene transfer, and it is the reason that offspring resemble their parents. In contrast, horizontal gene transfer is the movement of genes from one species to another. We have been aware that this is common in the microbial world, but are just now learning how common it is in plants.

Jeff Mitton (mitton@colorado.edu) is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Camera.

October 2013