On a recent trip to Big Bend Ranch State Park, I was amazed by the amount of dodder growing by the side of the road. Tangled, yellow filaments—looking like a Silly String party gone wild—weighed down the branches of the shrubs.
Dodder is a stem parasite with a fascinating life history. It’s a plant that can see, smell, and even sweet-talk its host.
Dodder is typically an annual that starts life a little late, giving its unsuspecting host a head start. Once emerged, the seedling immediately begins to move, sweeping the area in a counterclockwise direction, looking for the perfect host. It can’t be just any plant. It has to be the right plant.
Dodders use phytochromes (pigments that plants use to detect light) and volatile cues to “see” and “smell” the plants around them. They avoid potential hosts that are too young or too sick leading one researcher to describe the search for a host as an “intelligent choice and intention.”
The dodder seedling must work quickly—its root organ will wither away within a few days leaving the dodder reliant on its host plant for almost all of its nutrients.
Once the dodder finds the perfect host (“welcome, come on in”), it wraps itself up the stem, producing haustoria that invade the host-plant cells. The hyphae or filaments of the haustoria hijack the host’s nutrient transport system, transferring food back to the dodder.
The host plant isn’t entirely passive during this process. They can, and often do, fight back with barrier tissues that block the dodder’s advancing hyphae or inhibitors that retard hyphae growth.
Research published in Science this week shows that dodder and its host plant “talk” to each through the exchange of messenger RNA molecules. So what are the plants talking about? Researchers aren’t in on the conversation yet, but one suggestion is that the dodder is engaged in some “sweet talk,” sending messages that instruct the host plant to lower its defenses and allow the dodder in.
A few species of dodder (who really are serious agricultural pests) have given the whole group a bad reputation. But Dr. Mihai Costea, a botanist at the Wilfrid Laurier University in Canada, says we shouldn’t be so quick to dismiss dodder.
According to Dr. Costea, dodder should be respected as a keystone species in some ecosystems. Many host plants can be bullies. They grow fast and aggressively, crowding out the shy, retiring types. But dodder, through its ability to reduce the host plant’s biomass and alter the way it uses resources, can actually modify the structure of the plant community, keeping the bullies in check and allowing other plants to flourish.
I’m not sure which species of dodder I saw the other day (there are six that grow in the Trans-Pecos region of Texas), but I sure am glad I stopped to take a look at this fascinating little plant.
Costea, M. 2007 through present. Digital Atlas of Cuscuta (Convolulaceae). Wilfrid Laurier University Herbarium, Ontario, Canada.
Costea, M., and F.J. Tardif. 2006. The biology of Canadian weeds. 133. Cuscuta campestris Yuncker, C. gronovii Willd. ex Schult., C. umbrosa Beyr. Ex Hook., C. epithymum (L.) L. and C. epilinum Weihe. Canadian Journal of Plant Science 86:293-316.
Gunjune, K., M.L. LeBlanc, E.K. Wafula, C.W. dePamphilis, and J.H. Westwood. 2014. Genomic-scale exchange of mRNA between a parasitic plant and its hosts. Science 15 August 2014:345 (6198): 808-811.
Virginia Tech. 2014. Plants may use newly discovered molecular language to communicate. ScienceDaily 14 August 2014. www.sciencedaily.com/releases/2014/08/140814191939.htm.