Posting Date: January 25, 2010
Slugfests in the Soil
True confession time: I was once mad for Mad magazine. (You can consider this part of my misspent youth.)
But if you experienced the same weakness, you know whereof I speak. Who could resist those hilarious fold-out (or was it "fold-in") pictures? The movie parodies? And of course, there was always "Spy versus Spy."
Mad was such a part of my growing-up that to this day, whenever I hear the phrase "X versus Y" (fill in the components of your choosing), I have to stifle a grin; I guess the word "versus" is ruined for me forever!
Maybe more than a few of the scientists at the Agricultural Research Service (ARS) also read "Spy versus Spy" in their youth, because that's a technique that pops up again and again — to great effect — as they look for non-chemical ways to combat the natural nasties that conspire to ruin our food crops.
A strain of Pseudomonas bacteria defends itself from an attacking amoeba by releasing a lipopeptide that blows the amoeba apart.
There's a whole complex of pathogenic culprits behind apple replant disease, from Rhizoctonia fungi to the water mold Pythium oomcetes and the parasitic nematode Pratylenchus penetrans. Yes, chemically fumigating the soil can help keep them in check, but that can be costly, and do we really want to soak our soil with those chemicals?
The ARS scientists have a better idea: letting the natural denizens of the soil slug it out, to our benefit. One of the "good guys" in the soil is a type of bacteria called Pseudomonads — and if these bacteria are living around the roots of the apple trees, they'll help keep the troublemaker pathogens at bay.
There's just one problem: Certain single-celled amoebas in the soil love to hunt down and devour bacteria. But when those predatory amoebas go after the Pseudomonads, the good-guy bacteria fight back by banding together and oozing a biochemical defense in the form of proteins called cyclic lipopeptides (CLPs). You can click on the video link in the image above to watch the Pseudomonads' CLPs stop the amoebas in their tracks — or, to be more precise, blow them apart!
In experiments, the ARS scientists noticed that when the amoeba Naegleria americana came up against Pseudomonas strains capable of producing CLPs, the amoeba steered well clear of them. But mutant Pseudomonas strains that didn't produce CLPs rapidly became the amoeba's "blue plate special." This insight helps the ARS scientists pinpoint just which Pseudomonads could prove useful in protecting our precious apple crop out West, without resorting to chemical controls.
Another "spy versus spy" scenario could help keep the hummus bowl full on our party trays. There's a fungus called Ascochyta rabiei that's a worldwide threat to the chickpea crop. But ARS scientists say this mold could be about to meet its match in another fungus called Aureobasidium pullulans.
Farmers have to resort to treating chickpea seeds with fungicides, planting resistant varieties, plowing crop fields before planting, and rotating chickpeas with other crops to try to control Ascochyta blight, which forms dark lesions on the plant's stems, leaves and pods.
The bad fungus survives the winter on chickpea stubble in the fields and forms asexual spores called ascospores, which can infect chickpeas planted in the following spring.
When the ARS scientists evaluated 28 different fungi for ones that could take down A. rabiei, A. pullulans scored highest because of its ability to stop A. rabiei from forming those troublesome spores in the first place.
Does it really work? In small-scale field trials, the scientists inoculated the previous chickpea crop's stubble with A. pullulans spores called conidia (talk about fighting fire with fire!), and cut Ascochyta blight in the next spring's chickpea crop by 38 percent. The scientists say they can boost that score even higher by using substances to help the good spores stick to the stubble, all while sticking to the rules of biological control.
Maybe Mad could come up with a new feature: "Spore versus Spore"!
The Agricultural Research Service is the chief in-house scientific
research agency of the U.S. Department of Agriculture. You can read
more about ARS discoveries at http://www.ars.usda.gov/news/.
About the author
"Everybody's
Science" is written by Sandy Miller Hays, Director of Information for
the Agricultural Research Service, the chief scientific research agency
of the U.S. Department of Agriculture. Hays is a native of Fort Smith,
Ark. From the late 1970s until early 1988, Hays was a reporter, editor
and columnist at the Arkansas Democrat (now the Arkansas Democrat-Gazette),
a Little Rock-based daily newspaper. She joined the ARS Information
Staff in 1988, and became Director of Information in April 1998.