A Plant that Can Smell

So there is this plant called dodder that parasitizes other plants, but until recently it was not known how it found the other plants. Recent research suggests that it does so by a form of smell.

Dodder is in fact a plant, but when it generates seedlings they will actually wave around towards other plants. If they find an appropriate one they will latch on to them and burrow roots into them to steal nutrients. It does this because while it is a plant and does have chlorophyll, it does not produce enough energy from photosynthesis to survive. Here is the skinny on dodder:

Dodder parasitizes various kinds of wild and cultivated plants, and is especially destructive to alfalfa, lespedeza, flax, clover and potatoes. Ornamentals attacked included chrysanthemum, dahlia, helenium, Virginia-creeper, trumpet-vine, English ivy and petunias. Dodder is particularly troublesome where alfalfa, clover and onion are grown for seed because dodder seed is difficult to remove from the desired seed crop and can be spread with infested seed. Its water, minerals and carbohydrates are absorbed from the host through haustoria that penetrate the host's tissue. In dodder the haustoria are modified adventitious roots.

Dodder is said (Wilson, et al.) to contain some chlorophyll in the buds, fruits and stems, but the amount of food manufactured in this tissue is of little significance to the survival of the plant.

The flowers are numerous, white, pink or yellowish, small (2 to 4 mm long depending on species), and can be borne in tight balls or in a loose cluster (again depending on species). Flowers normally appear from early June to the end of the growing season. The fruit is about 1/8 th inch in diameter, with thin papery walls and contain 1 to 4 seeds. The seeds are yellow to brown or black, nearly round and have a fine rough surface with one round and two flat sides.

Dodder produces seed that drops to the ground and germinate the next growing season if a suitable host is present. If no suitable host is present, the seed may remain dormant for five years.

Dodder seedlings must attach to a suitable host within a few days of germinating or they die. The young seedling is sensitive to touch and yellowish stem gropes in the air until it makes contact with a plant. The contact is made firm by one or more coils about the stem. If this plant happens to contain foods suitable to the dodder then a secondary stimulus is aroused which causes root-like branches (haustoria) to form and penetrate the stem. The basal part of the parasite soon shrivels away so that no soil connection exists.

Here is a video of the dodder wiggling around trying to find a host:

Researchers at Penn State conducted experiments
to determine how the plant does this. It turns out that it does so by smelling for appropriate plants, and it has preferences of some plants over others.

The question of how dodder finds a host plant has puzzled researchers. Many thought it simply grew in a random direction, with discovery of a plant to attack being a chance encounter.

But the researchers led by Consuelo M. De Moraes found that if they placed tomato plants near a germinating dodder, the parasite headed for the tomato 80 percent of the time.

And when they put scent chemicals from a tomato on rubber, 73 percent of the dodder seedlings headed that way.

"It opens a new avenue" for understanding parasitic plants, lead researcher Consuelo De Moraes said in a telephone interview.

Co-author Mark C. Mescher added, "One of the interesting things we found was that the plants make choices."

When they gave the dodder seedlings a choice between a tomato plant and a wheat plant, they preferred the tomato.

Dodder will infect wheat if there is no choice, he said, but they discovered that one of the volatile chemicals given off by wheat repels dodder, so it will choose the tomato if allowed to pick.

So, finding one compound that tends to be repellant could lead to ways to either treat crops to resist dodder or even engineer them to produce the compound themselves, Mescher said.

While they were attracted to tomato plants and the chemicals they release, the dodders showed no particular interest when offered a fake tomato plant, a pot of moist soil or vials or red or green colored water.

The plants don't have a nose, of course, so it's not clear how they sense the chemicals given off by potential host plants. When the seedlings start out, they tend to rotate in various directions, and they somehow sense the direction where the chemicals are strongest and then grow toward them, Mescher said.

He noted that many plants respond to light by tilting to one direction or another. And, Mescher added, some research has indicated that plants may be able to sense chemicals emitted by other plants that are attacked by insects.

Pretty cool, huh. I always thought that the convergent evolution that allowed insect and mammalian eyes to operate with relatively similar principles was pretty crazy, but this is crossing between animals and plants.

Maybe given another 2 billion years of evolution, dodder will stalk its prey and have little plants societies -- or turn into Swamp Thing.


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