











Phosphate Wars
Other researchers are tweaking the agribiome to help deliver crucial nutrients to plants. Farmers have, of course, recognized for thousands of years that soybeans, peanuts and other legumes have an almost magical power to fertilize the soil. Further, scientists have known for more than a century that it is not, in fact, the plants that manage the trick of pulling nitrogen out of the air, it is the rhizobial bacteria living in nodules on their roots.

Plants also require phosphate, which is exceptionally low in the soils of many tropical nations. Farmers in developing countries often depend entirely on the international market for phosphate fertilizer. In 2007 and 2008 prices for phosphate and other fertilizers spiked, contributing to food riots from Mexico to Bangladesh. In some countries, farmers now just skip phosphate fertilizer altogether and take their chances with starvation.

Yet researchers have known for decades of a possible remedy. Soil microbes called arbuscular mycorrhizal fungi form spores and filaments inside and around a plant's roots and help them acquire phosphate. There has never been a good way to mass-produce and deliver the stuff. Soil containing the spores that form new fungi can, in fact, be shipped from one country to another, but the environmental impact of introducing foreign fungi as exotic species remains uncertain. And the spores of the fungi are so thinly concentrated that a farmer planting a crop like cassava needs to apply the enriched soil at a rate of a metric ton per hectare.

With the help of new technologies, a few companies can now mass-produce the fungi in culture and market it in a highly concentrated gel. A farmer can carry enough to cover a hectare in a soda bottle. Research teams can collect local strains of the fungi, test which ones look most promising, then deliver them to a manufacturer for production. Ian R. Sanders of the University of Lausanne in Switzerland and Alia Rodriguez of the National University of Colombia began field studies last year focusing on cassava, a root crop that is a staple food for much of the developing world.

In the field, a farmer dilutes the gel in a bucket of water and dips a mesh sack of cassava stems in the bucket for a few seconds before planting them. In the first season of testing, that treatment cut phosphate use in half and boosted yields by 20 percent. Sanders and Rodriguez are now crossbreeding multiple fungus strains on the three or four common cassava varieties. They are also testing the strains in Africa and, if successful, will expand this program to half a dozen countries there, thus enabling the technology to benefit subsistence farmers.


Another promising path to agricultural symbiosis involves studying the chemical signals that microbes use to communicate with one another. Researchers monitor this everyday chitchat to identify which bacteria may be suited to the task of supplying plant nutrients or to find weaknesses in pathogens. This strategy has given rise to a potential weapon against Xylella fastidiosa, the bacterium that causes Pierce's disease, which is killing off vast swaths of California grapes. The bacterium is quiescent until its insect host (the glassy-winged sharpshooter) feeds on a grape plant. It wakes up inside the plant but later becomes quiescent again when it is time to be acquired by another insect.

“Basically, the lifestyle that it takes on to be transmitted by an insect is incompatible with its ability to move in the plant,” says Steve Lindow of the University of California, Berkeley. Lindow took genes that the pathogen uses to signal quiescence and spliced them into the grape genome. When the pathogen arrives, the plant's transgenes tell it to behave as if it were about to be acquired by an insect, rendering it harmless.

Unmet Promises
In the past new microbial methods in agriculture have often failed to yield the promised results in the field, in part because of a lack of funding to translate basic research into practical applications. Molecular biologists also often lack the inclination to transfer their know-how to farmers. “It's a tale of two worlds,” says Ken Giller of Wageningen University in the Netherlands, who works in Africa on improving use of rhizobial bacteria for nitrogen-fixing legumes. The molecular work on the genetics of nitrogen fixation has been “an absolutely fascinating story,” he remarks. Meanwhile farmers continue to treat their plants with bacterial strains first isolated 30 years ago. “And it's largely because the scientists doing this are hell-bent on finding the next finer detail,” Giller says. “A lot of interesting discoveries aren't being picked up and taken through to the point of application.”

Many products that do make it into the field are ineffective because they have not been adequately tested or because they are manufactured carelessly, perhaps fraudulently. The International Institute of Tropical Agriculture (IITA) in Nigeria has tested 106 different farm products, most of them microbial. All but five failed because they did not contain the active ingredient on the label, they did not have enough of it, or they were not effective in greenhouse and field trials.

Many of the flawed products come from Europe, the U.S. and Japan. Rather than taking on the manufacturers, IITA is training regulators in the target countries to do their own quality testing. The institute is also developing a seal of approval to let buyers know when a product meets reasonable standards. The program aims to help farmers understand not just which microbial products work but where and under what conditions.

Getting farmers to understand the new rules of the agribiome is “going to be incredibly complicated,” says Ann Reid, director of the American Academy of Microbiology, but it will also be “very cool.” It means convincing farmers that their work is not a simple business of inputs and outputs—some water here, some pesticides there. Instead it means waking up to what farming has always been—a collaboration with the vast community of microbes. If farmers and scientists together can get that right, we will have come a step closer to feeding a hungry world.