A Dose of Dodder

Strangleweed, Devil's gut, witches shoelaces... all of these colorful nicknames have been given to a genus of plants that have evolved a very different way of survival. Dodder, genus Cuscuta, are a group of roughly 170 species of plants that make their living entirely off of other plants. We have talked about parasitic plants in the past but this group takes it to the extreme.

Dodder begin their lives just like any other plants. Seeds in the soil germinate under a certain set of conditions and begin their trek into the sunlight. However, unlike many other plants who spend a lot of their initial energy setting up root systems, dodder only sends sends out meager baby roots. It says "forget that" and starts searching for a victim. It whips about in a circular motion like a cowboy's lasso. It is looking for the nearest host. If dodder doesn't find a suitable host within 7 to 10 days it will wither and die. How does it find a host? Many theories have been put forth on the subject. From blind luck to changes in light levels, no one could seem to get a firm grasp on exactly how dodder knew where to go. Then, in 2006, a team of researchers discovered that dodder sniffs out its victims.

By honing in on green leaf volatiles, dodder sniffs out its potential prey. Even more interesting is that some species of dodder seem to show preferences. Tomatoes were a big hit with the species that were tested and indeed, many farmers will agree that dodder is a pretty serious agricultural pest. Once a host is located, dodder begins to wind around the stem. Its diminished root system completely dies off. It then uses specialized cells called haustoria to tap into the host's vascular tissue.

This is not so good for the host as it can severely weaken it, leaving it susceptible to viruses and other diseases. To ad insult to injury, these diseases can then be passed to other plants that the dodder has tapped in to. As far as we know, only one species of dodder undergoes any measurable level of photosynthesis. The rest are solely dependent on their host. That being said, there is evidence to support the idea that dodder actually increases plant diversity where it grows. By limiting the strength of dominant plants, dodder allows other, less competitive species to gain a root-hold in that habitat.

So, where does a plant like dodder fit into the evolutionary tree? It is quite a strange plant after all. Originally dodder was placed in its own family but recent genetic work has since changed all that. The genus Cuscuta is now considered to be a member of the morning glory family, Convolvulaceae! Regardless of how you feel about parasites, you really have to respect these plants.

Further Reading:
http://www.npr.org/templates/story/story.php?storyId=6160709

http://www.jstor.org/stable/4043367?seq=1#page_scan_tab_contents

http://www.cabdirect.org/abstracts/19500301574.html;jsessionid=94D7FF07A81A44560C60AFB44AC295B6;jsessionid=528A1AE7148BEA3A563B9F95419BC043;jsessionid=500949B7792FD68E5FA2E32A8E7AD291?freeview=true