Farmers are constantly spraying pesticides on their crops to combat an array of viral, bacterial, and fungal invaders. Scientists have been trying to get around these chemicals for years by genetically engineering hardy plants resilient to the array of diseases caused by microbial beasties. Most attempts so far confer protection against a single disease, but now researchers have developed a rice plant that fights multiple pathogens at once—without loss to the crop yield—by hooking up a tunable amplifier to the plant’s immune system.
“For as long as I have been in this field, people have been scratching their heads about how to activate a defense system where and when it is needed,” says Jonathan Jones, who studies plant defense mechanisms at the Sainsbury Laboratory in Norwich, U.K. “It is among the most promising lines of research in this field that I have seen.”
Plants don’t have a bloodstream to circulate immune cells. Instead, they use receptors on the outsides of their cells to identify molecules that signal a microbial invasion, and respond by releasing a slew of antimicrobial compounds. Theoretically, identifying genes that kick off this immune response and dialing up their activity should yield superstrong plants.
Plant biologist Xinnian Dong at Duke University in Durham, North Carolina, has been studying one of these genes for 20 years—a “master regulator,” she says, of plant defense. The gene, called NPR1 in the commonly studied thale cress plant (Arabidopsis thaliana)—a small and weedy plant topped with white flowers—has been a popular target for scientists trying to boost immune systems of rice, wheat, apples, tomatoes, and more. But turning up NPR1 works too well and “makes the plants miserable, so it is not very useful for agriculture,” Dong says.
To understand why, consider the human immune system. Just as sick people aren’t very productive at work when their fever is high, plants grow poorly when their own immune systems are overloaded. Likewise, keeping the NPR1 gene turned on all the time stunts plant growth so severely there is no harvest for the farmers.
To make NPR1 useful, researchers needed a better control switch—one that would crank up the immune response only when the plant was under attack, but otherwise would turn it down to let the plants grow. Two papers published in Nature this week from Dong’s team at Duke, in collaboration with researchers at Huazhong Agricultural University in Wuhan, China, describe the discovery and application of such a mechanism.
While investigating an immune system-activating protein called TBF1 in Arabidopsis, Dong discovered an intricate system that speedily instigates an immune response. It works by taking ready-to-go messenger RNA molecules that encode TBF1, and quickly translating these molecules into TBF1 proteins, which then kick-start an array of immune defenses. Dong quickly recognized that a segment of DNA, which she calls the “TBF1 cassette,” was acting as a control switch for this plant immune response, so she copied that TBF1 cassette from the Arabidopsis genome and pasted it alongside and in front of the NPR1 gene in rice plants.
The result is a strain of rice that can rapidly and reversibly ramp up its immune system in bursts that are strong enough to fend off offending pathogens but short enough to avoid the stunted growth seen in previously engineered crops.
The researchers demonstrated that their rice was superior compared with regular rice by inoculating their leaves with the bacterial pathogens that cause rice blight (Xanthomonas oryzae pv. oryzae) and leaf streak (X. oryzae pv. oryzicola), as well as the fungus responsible for blast disease (Magnaporthe oryzae). Whereas the infections spread over the leaves of the wild rice plants, the engineered plants readily confined the invaders to a small area. “These plants perform very well in the field, and there is no obvious fitness penalty, especially in the grain number and weight,” Dong says.
The research could be a boon for farmers in developing countries someday, says Jeff Dangl, an expert on plant immunity at the University of North Carolina in Chapel Hill, who was not involved in the study. For instance, rice blast disease, which the plants effectively combatted, causes an estimated 30% loss of the annual rice crop worldwide. “In the developing world, when farmers that can’t afford fungicide get the disease in their fields, they can lose their whole crop,” Dangl says.
Julia Bailey-Serres, a plant biologist at the University of California, Riverside, is excited about the study too. “They haven’t done large trials yet to show how robust it will be, but our back of the envelope calculation shows that this really could have a big impact,” she says. “It could easily be applicable to multiple species of crops,” she says, adding that “it is impressive that it worked across two kingdoms” of fungal and bacterial pathogens.
But all are careful to note that it is still early days for immune-boosted crops. For one, the particular kind of uplift conferred by NPR1 is unlikely to provide protection against plant-munching insects. A second caveat is that the study only tested the rice’s response to microbes that parasitize living host cells; their defense against a different class of pathogens that kill cells for food is still untested. “I would keep the champagne on ice until there are a few more pathogen systems tested in the field,” Jones says.
Still, Jones says he’s hopeful the work—and more like it—could eventually lead to the end of pesticides. “I like to imagine in 50 years’ time my grandchildren will say, ‘Granddad, did people really use chemicals to control disease when they could have used genetics?’ And I’ll say, ‘Yeah, they did.’ That’s where we want to get to.”