The Dodder – The Thinking Plant That Challenges Classifications

CN's picture

The definition of life is a very ambiguous one. It means different things to different people, to different cultures, and to different areas of study. One would think that a biologist’s definition of life would be succinct, to the point, and all encompassing. However, after a number of thorough class discussions and a number of laboratory sessions and experiments, it has become obvious that no such definition exists or could exist. This is because what is true for one organism might not be true for another, thus, the working definition used is purposely broad to try and to account for all walks of life. While most organisms fit nicely into the definition, there are always a few organisms that are drastically different from the rest. The dodder plant is one such organism.

The dodder plant, from the cuscuta genus of the plantae kingdom, is a group of 100-170 species of parasitic plants classified as having thin stems and no leaves (2). Unlike other plants, the dodder plant has very little chlorophyll, does not have the ability to photosynthesize effectively, and has no roots (7). Because of this, the dodder depends on other plants for survival. If, in five to then days after germination, the young dodder plant does not find a suitable plant to latch onto, it dies. These other plants, also known as host plants, are essentially robbed of their nutrients by the dodder and usually have their “agricultural productivity” reduced by as much as 90% (5), thus showing how dependant the dodder is on its host organism.

Although this is most unfortunate for the host plant, the actions of the dodder plant are not particularly controversial, unheard of, or remarkable. Organisms are constantly living off of each other. What is remarkable, though, is how the dodder actually chooses what plant it will feed off of. Originally, it was thought that the dodder vine might have been attracted to other plants by its water vapor or the reflection of light off of the plant (4). However, new evidence has come to light that has refuted these earlier claims and presented even more interesting, and controversial, possibility. Recently, a series of studies and experiments conducted by the Department of Entomology at Pennsylvania State University has suggested that the dodder plant follows the scent, or chemical volatile, of the host plant. If this theory is found to be true, it also strengthens the notion that plants have the ability to have airborne, plant to plant, interactions.

The team of researchers came to this interesting conclusion after a number of observations, experiments which prompted new observations, and more experiments were performed. To begin with, the team placed a dodder plant in a vial of water and then placed 20-day-old tomato seedlings a certain distance away and waited four days. 80% of the time, the dodder plant grew in the direction of the potential host tomato plant. In another experiment, the team again placed the tomato seedlings a certain distance away in one direction, yet this time; they also placed fake plants the same distance away in the opposite direction and waited four days. Again, about 77% of the time the dodder plant grew toward the real tomato plant (6). When the same experiment was tried with a pot of moist soil, a vial of red water, and a vial of green water, the dodder plant showed no interest in any of these (1).

 

 

Although thus far, a number of potential plant related cues for attracting the dodder had bee ruled out, another experiments were performed and this time without any real plants. Instead, just the scent, or chemical volatile, was sprayed on an artificial plant and placed a certain distance away. The dodder was once again placed in the middle. In the opposite direction and equidistance away from the dodder, a simple solvent was sprayed on a separate artificial plant. After four days, the dodder plant grew toward the rubber septa with the tomato volatile 73% of the time. When this same experiment was repeated with different types of chemical volatiles such as tomato, impatient flower, and wheat, the dodder grew toward the tomato volatile more often, possibly showing the ability of the dodder to choose its host plant (6).

One of the authors of the study, Mark C. Mescher, said that, “One of the interesting things we found was that the plants make choices.” Even more fascinating is the choice of their host plant made by the dodder is essential to their survival. A majority of the time, when given the choice, the dodder will choose a tomato plant over a wheat plant as its preferred host. This is most interesting because wheat plants are a poor host because they lack a plentiful variety of nutrients. It is harder for a dodder plant that is attached to a wheat plant to survive than it is for a dodder attached to a nutrient-rich tomato plant (5). So not only does a dodder plant have the ability to choose between a number of potential hosts, the majority of the time it will also choose the plant that will better its chances for survival.

 

 

The implications of these experiments are great and far-reaching. To begin with, it strengthens the idea that plants have their own chemical language or method of communication, which is a notion that has been passionately debated in the scientific community for the past two decades (5). Before, the idea of airborne plant to plant interactions was just that, an idea. Now, however, there is legitimate observational research to back such a claim. But to take it a step further, if plants do have their own method of communicating with each other, one would think that this also changes what they are capable of in general and could change the way in which they are classified.

Looking to the way in which life in general, but more specifically plant life, has been categorized in our class; the main problem soon presents itself. The idea that life can be classified according to whether or not the object is inanimate, animate, or conscious was a great topic of debate in numerous class and laboratory sessions. Many people agreed that plants were animate, but not conscious, organisms. Yet this new discovery about the ability of the dodder plant to sense the “scent” of another plant and to make a decision concerning what type of plant it wants as its host completely changes our fragile classification system. According to this study, plants are more active and engaged then previously thought. Richard Karban, a community ecologist at the University of California, Davis, said that “This is a pretty cool example of plants behaving in a way most people think only animals behave” (5).

Even our basic understanding of what constitutes as a plant does not fully include the dodder. While in our laboratory sessions, the groups had to first define what a plant was. Many of the groups had criteria that included “have a base/root”, “produce its own food”, “predominantly stationary and receives the bulk of its nutrient intake through the soil”, and is “composed of different substances which interact within itself to perpetuate its own existence” (3). Yet when the dodder plant is taken into consideration, it actually fits into none of these categories. It has no roots and does not have the ability to produce its own food. In fact, after the dodder attaches to its host plant, the basal part of the parasite soon shrivels away so that no soil connection exists (7). How is the dodder a plant if it does not even connect to the soil? According to the various requirements complied in our numerous classes, the dodder is not.

 

 

These new findings, while answering some questions, also raise a lot more. Is the dodder a true plant, or a parasite or fungi of some sort? Is the way in which we classify plants correct? Is the way in which we classify life correct? Are animate, inanimate, and conscious good indicators of life? Most importantly, however, it is just another example of the “seriously loopy” scientific method at work. There is an idea, or summary of observations. As more and more observations are made and new implications are formed, often times new ideas are formed, more questions raised, and more observations are needed. This need for continuous revision makes biology a living science with the ability to adapt according to the information as it is presented. The dodder plant and its experiment is just one example.

Works Cited Page

1) CNN. 28 Sept. 2006 <http://www.cnn.com/2006/TECH/science/09/28/sniffing.plant.ap/index.html>.

2) "Cuscuta." Wikipedia. 29 Sept. 2006 <http:llen.wikipedia.org/wiki/Cuscuta>.

3) "Darwin's Voyage Revisited Revisited." Serindip. Sept. 2006. Sept. 2006 <http://serendip.brynmawr.edu/forum/viewforum.php?forum_id=418&end=20394>.

4) Malakoff, David. "Devious Dodder Vine Sniffs Out Its Victims." NPR, Health and Science. 29 Sept. 2006. NPR. 29 Sept. 2006 <http://www.npr.org/templates/story/story.php?storyId=6160709>.

5) Pennisi, Elizabeth. "Parasitic Weed Uses Chemical Cues to Find Host Plant." Science Magazine 29 Sept. 2006. 29 Sept. 2006 <http://www.sciencemag.org/cgi/content/full/313/5795/1867a?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=dodder&searchid=1&FIRSTINDEX=0&resourcetype=HWCIT>.

6) Runyon, Justin B., Mark C. Mescher, and Consuelo M. De Moraes. "Volatile Chemical Cues Guide Host Location and Host Selection by Parasitic Plants." Science Magazine 29 Sept. 2006. 29 Sept. 2006 <http://www.sciencemag.org/cgi/content/full/sci;313/5795/1964>.

7) Swift, Curtis E. "Cuscuta and Grammica Species - Dodder, a Plant Parasite." Colorado State University Cooperative Extension. Colorado State University. 29 Sept. 2006 <http://www.coopext.colostate.edu/TRA/dodder.html>.

Comments

Jessica Orr's picture

Neat post! I have to say

Neat post! I have to say though, I don't think theres really a question as to dodder's place in the Angiosperms. Phylogenticially, it's a plant. A very comprehensive 2007 study placed the genus in the Morning Glory family using chloroplast and nuclear DNA sequence comparisons. It's funny that they were actually able to obtain enough cpDNA from the dodder species, they have so little! I can't imagine how frustrating it must have been for the researchers on that project.

Anyway, dodder is a really good example of why we should rely solely on morphological/life history data when deciding how to group species. If we only looked at dodder's life history, we might think it's a fungus, but in terms of it's evolutionary history, it's very much a plant.

Dan's picture

I think dodder still fits

I think dodder still fits the definition of a plant. It had flowers, seeds, (some) chloroplasts, and cell walls.

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