Parasitic plant gets more than a meal

Host molecules might be the best defense for crop plants

The parasitic vine known as dodder really sucks. It pierces the tissue of other plants — some of which are important crops — extracting water and nutrients needed for its own growth. But it also consumes molecules that scientists could manipulate to bring on the parasite’s demise.

TIGHTLY WOUND The parasitic dodder vine (genus Cuscuta) steals food and water by piercing its host plant with fang-like haustoria, which can be seen developing on the vine above. Scientists have learned that other molecules, such as RNA, are also transferred to the parasite. Click on the image for story. Rakefet David-Schwartz

CONQUERED Like many parasitic plants, dodder isn’t green — it doesn’t bother with photosynthesis. Instead it gets nutrients from an unwitting host plant. Colin Purrington

Some RNA molecules siphoned from the host plant remain stable in dodder, traveling several centimeters within the parasite, researchers report in an upcoming New Phytologist. That finding hints at a means for parasitic plants to coordinate growth with their hosts, prolonging the parasite’s survival. And the result suggests that scientists could design “attack RNA” for a host plant, which would interfere with dodder’s growth, debilitating the parasite.

“This is very exciting from the point of view of controlling parasitic plants,” comments James Westwood of Virginia Tech in Blacksburg, who reported last year that some RNA molecules from pumpkin and tomato were indeed transferred to dodder. “You could use the RNA interference strategy to really elegantly control dodder on some crops.”

Resembling orange cheese sprayed in threads from a can, dodder looks a bit like the modern art version of parasitic plants: messy, perhaps vulgar and with a decidedly non-plant color. (It also looks like something a two-year-old could make.) But the odd plants have their fans.

“I think parasitic plants are wild things,” says Neelima Sinha of the University of California, Davis, who led the new work. “They are just so interesting.”

For the most part, the 100 to 170 dodder species have abandoned both photosynthesis and normal roots, the standard means that plants use to acquire food. If tiny dodder plantlets don’t find a host plant to glom onto they’ll die within a week. The parasites actually sniff the hosts out, homing in on airborne host chemicals, researchers reported in Science last year.

When dodder reaches a host plant — crops and ornamental hosts include alfalfa, tomato, flax, potatoes, chrysanthemum and petunias — it drills itself into the unwitting host’s flesh using structures known as haustoria. Evolutionarily, haustoria are modified roots, and dodder’s grow with enough strength to pierce tinfoil. The parasite penetrates the host’s plumbing, accessing its water and nutrients.

Scientists have demonstrated that viruses can be transmitted from a host plant to dodder and out to another host on the other side. But only recently have they learned that host RNA also made it into dodder. Now Sinha and colleagues have found that some tomato RNA was still detectable in a shoot growing 30 centimeters from where the tomato-dodder tissue connected.

“From a purely biological perspective, this suggests a way that a parasite can get in sync with its host,” says Sinha. Many plants die soon after procreating — the flowers yield fruit and the seeds of the next generation. It might be very useful for a parasite to know its host may be dying, says Sinha. The parasite might be getting all kinds of information from its host, in addition to free eats.

Scientists also might be able to design tomato RNA molecules that would interfere with dodder’s growth. This strategy could be applied to other parasitic plants as well, says Sinha.

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