"Long before it's in the papers"
July 30, 2015


Plant wars: vampire weeds use chemical “radar” to stalk victims

July 30, 2015
Courtesy of 
and World Science staff

Cer­tain par­a­sit­ic plants pa­tiently rely a form of chem­i­cal “radar” to stalk oth­er plants, ac­cord­ing to new re­search.

Par­a­sit­ic plants bas­ic­ally “eat oth­er plants,” said Un­ivers­ity of Geor­gia ge­net­i­cist Da­vid Nel­son, co-author of a pa­per on the find­ings. Among the re­sults: bil­lions of dol­lars in lost mon­ey for farm­ers, many of them poor.

Witchweed (Striga asiatica) at­tacks crops in­clud­ing corn, sorg­hum, su­gar cane and rice. (Image cour­tesy USDA)

How do these plants find their vic­tims?

“The seeds of some par­a­sit­ic plants, like witch­weed for ex­am­ple, can lie dor­mant in soil for more than a dec­ade, wait­ing to grow un­til they de­tect the pres­ence of a host,” Nel­son ex­plained. They then siphon nu­tri­ents off the host’s roots.

“We wanted to un­der­stand how the par­a­sites know oth­er plants are near­by so we could de­vel­op new ways of com­bat­ing them,” he added. This pro­cess “has been a mys­tery in our field for more than 50 years.”

There are thou­sands of par­a­sit­ic plant spe­cies, but the for hu­mans the most an­noy­ing are those that in­fil­trate farm­land and crops, of­ten in de­vel­oping coun­tries where her­bi­cides and oth­er con­trol meth­ods are less avail­a­ble. 

As plant roots grow, they send hor­mones called strigo­lac­tones in­to the soil, Nel­son ex­plained—a sig­nal that nor­mally helps at­tract fun­gi that are help­ful for a ben­e­fi­cial, nutrient-trading rela­t­ion­ship. 

But par­a­sit­ic plants’ seeds can al­so sense strigo­lac­tones. These prompt them to grow, latch on­to the host root and suck off nu­tri­ents. “It’s kind of like root radar,” said Nel­son. 

“The in­cred­i­ble thing,” he added, “is that this strigo­lac­tone de­tection sys­tem seems to have evolved from plant genes that nor­mally con­trol a seed’s abil­ity to de­tect fire.”

When a for­est burns, com­pounds in the smoke and ash leach in­to the soil. Many plants have evolved the abil­ity to de­tect these com­pounds, which sig­nal that their competition—large shady trees or dense ground cover—has been de­stroyed and it might be an op­por­tune time to grow.

Nel­son and his col­leagues found that dur­ing the ev­o­lu­tion of par­a­sit­ic plants, the smoke de­tector gene du­pli­cat­ed and some cop­ies switched to be­come strigo­lac­tone de­tectors. This crit­i­cal switch is what al­lows the par­a­sites to rec­og­nize and at­tack near­by hosts.

Nel­son and col­leagues’ find­ings are pub­lished in the jour­nal Sci­ence.

“Now that we un­der­stand the fun­da­men­tal ge­net­ics that give par­a­sit­ic plants this abil­ity, we can de­vel­op new ways to help farm­ers fight them,” Nel­son said.

Re­search­ers might, for ex­am­ple, de­vel­op chemi­cals that dis­rupt the re­cep­tors, or chem­i­cal gate­ways, that par­a­sit­ic plants use to sense strigo­lac­tones, he re­marked. Or, some­one might cre­ate com­pounds that mim­ic strigo­lac­tones, to trick par­a­sit­ic plants in­to com­mit­ting su­i­cide by try­ing to grow where no hosts live.

“This could open the doors to a lot of use­ful new tech­nolo­gies to help those in great­est need,” he said.

* * *

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Certain parasitic plants patiently rely a form of chemical “radar” to stalk other plants, according to new research. Parasitic plants basically “eat other plants,” said University of Georgia geneticist David Nelson, co-author of a paper on the findings. Among the results: billions of dollars in lost money for farmers, many of them poor. How do these plants find their victims? “The seeds of some parasitic plants, like witchweed for example, can lie dormant in soil for more than a decade, waiting to grow until they detect the presence of a host,” Nelson explained. “We wanted to understand how the parasites know other plants are nearby so we could develop new ways of combating them.” “The process that parasitic plants use to sense their hosts has been a mystery in our field for more than 50 years,” he added. There are thousands of parasitic plant species, but the for humans the most annoying are those that infiltrate farmland and crops, often in developing countries where herbicides and other control methods are less available. As plant roots grow, they send hormones called strigolactones into the soil, Nelson explained—a signal that normally helps attract fungi that are helpful for a beneficial, nutrient-trading relationship. But parasitic plants’ seeds can also sense strigolactones. These prompt them to grow, latch onto the host root and suck off nutrients. “It’s kind of like root radar,” said Nelson. “But the incredible thing is that this strigolactone detection system seems to have evolved from plant genes that normally control a seed’s ability to detect fire.” When a forest burns, compounds in the smoke and ash leach into the soil. Many plants have evolved the ability to detect these compounds, which signal that their competition--large shady trees or dense ground cover--has been destroyed and it might be an opportune time to grow. Nelson and his colleagues found that during the evolution of parasitic plants, the smoke detector gene duplicated and some copies switched to become strigolactone detectors. This critical switch is what allows the parasites to recognize and attack nearby hosts. Nelson and colleagues’ findings are published in the journal Science. “Now that we understand the fundamental genetics that give parasitic plants this ability, we can develop new ways to help farmers fight them,” Nelson said. Researchers may, for example, develop synthetic compounds that interfere with the receptors, or chemical gateways, that parasitic plants use to sense strigolactones, he remarked. Or, someone might create chemicals that mimic strigolactones, to trick parasitic plants into committing suicide by trying to grow where no hosts live. “This could open the doors to a lot of useful new technologies to help those in greatest need,” he said.