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More Food from Fungi? Crop-Enhancing Microbes Challenge Genetic Engineering

Researchers investigate how fungi and other symbiotic microbes could improve plants

To feed an exploding global population, scientists have called for a doubling of food production over the next 40 years. Genetic manipulation might seem the best way to quickly boost characteristics essential to plant growth and crop yields. New findings from different laboratories, however, suggest that fungi, bacteria and viruses could be an exciting alternative to increase agricultural productivity.

Scientists have long known that microbes can work symbiotically with plants. For instance, mycorrhizal fungi, which are associated with 90 percent of land plants, extend from roots to bring in moisture and minerals in exchange for plant carbohydrates. But microbes have recently been found among plant cells themselves and seem to confer benefits, such as more efficient photosynthesis and increased ability to fix nitrogen from the air. In fact, Mary E. Lucero, a biologist at the U.S. Department of Agriculture’s Jornada Experimental Range in Las Cruces, N.M., believes that plants actively recruit these microbes rather than simply being passive hosts for them.

In the lab, Lucero has given this recruitment a hand by transferring fungi from four-wing saltbush to grama grass, which is important for grazing cattle. The fungi-infused grass grew larger and produced more seed, probably by improving nutrient uptake and water usage, she speculates. Lucero also points out that harnessing microbial help for capturing nitrogen could reduce the need for chemical fertilizers. “It is far easier, more efficient and less expensive to inoculate a plant with a beneficial fungi than to come up with a genetically modified species,” she remarks.

Rusty Rodriguez, a microbiologist with the U.S. Geological Survey’s Biological Resources Division in Seattle, is trying to tackle another agricultural demon: excessive heat. In experiments to improve the ability of tomato plants to resist high temperatures, he inoculated them with fungi taken from plants near hot springs in Yellowstone National Park. The result: tomatoes that can grow at 148 degrees Fahrenheit. “That’s about the internal temperature of a medium cooked prime rib,” Rodriguez notes.

Furthermore, by isolating a virus in the fungus, he discovered a three-way symbiosis that was required for thermal tolerance. “Without the virus the plants could handle only about 100 degrees F,” Rodriguez says. The fungus and virus also conveyed heat tolerance to rice and wheat, a process that could not only boost yields but also help crops fend off the effects of climate change.

Analyzing plants from beaches, deserts and polluted areas, Rodriguez has also isolated microbes that help plants resist salt, drought and heavy metals. Curiously, the same fungi taken from plants living in unstressed areas did not confer tolerance. “It has to be the right microbe from the right habitat,” Rodriguez says. Choosing microbes from heat-stressed areas could boost rice production, which drops 10 percent for every 1.8 degrees F of warming. Once acquired, however, stress-tolerant microbes can be passed in seed coatings to the plant’s progeny.

Christopher L. Schardl, a plant pathologist at the University of Kentucky who studies certain species of tall fescue grass, observes that the mutualism between microbes and plants has agricultural drawbacks, too. Many microbes in plants produce biologically active alkaloids, which repel insects, birds and herbivores. In fact, in the early 1950s grazing livestock picked up a disease related to alkaloids in grass known as fescue toxicosis. It can induce tremor and stupor, as well as an aversion to further grazing. “It costs the livestock industry about $1 billion a year,” says Schardl, adding that producers raising grass-fed cattle are now sowing cultivars with nontoxic fungi.

Identifying plant microbes is not easy, because microbial cells are embedded in plant tissue. Lucero uses scanning electron microscopy and new pyrosequencing techniques to identify the DNA of microbes in plant tissue.

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