Tue. Feb 18th, 2020

Yellow Dragon: Without weapons against the plague that is annihilating the orange trees of the planet | Science

A plague knocks on the doors of Spain and threatens to ravage the crops of oranges, lemons and tangerines. And it's not an exaggeration. The yellow dragon or huanglongbing, caused by a bacterium of Asian origin, arrived in Florida (USA) in 2005 and three years later already colonized the entire state. The microbe deforms the fruits, bitter their flavor, atrophies their seeds and yellows the trees until they die. There is no treatment beyond starting the plants and burning them. In the last decade, the production of oranges for juice in the US has fallen 72%.

“It is a matter of time before the bacteria arrive in Spain. And we have absolutely no treatment against it, ”laments the biologist Manuel Talón, director of the Genomics Center of the Valencian Institute of Agricultural Research. In 2014, despite precautionary measures, the African citrus psyllid – the sucking insect that carries the yellow dragon's microbe – was detected in the Galician regions of A Barbanza and O Salnés, although the bacteria has not yet appeared. But it will appear, Talón warns: "The huanglongbing has managed to jump all the barriers."

"It's a matter of time before the bacteria arrive in Spain," warns biologist Manuel Talón

The plague solution could be found in the least thoughtful place: the laboratory of Sharon long. The biologist, from Stanford University (USA), has spent 40 years of her life studying the bacteria that collaborate in the growth of alfalfa. You have never seen a tree infected by the yellow dragon. However, Long and his colleague Melanie Barnett They have decided to face the plague that is killing orange and lemon trees in half the world.

“It is very difficult to study this disease, because the bacteria that cause it do not survive in laboratory conditions. They need to live inside the insect or a citrus tree, ”says Barnett. The strategy of the Americans is ingenious. The scientists have taken the genes responsible for the virulence of the yellow dragon bacteria – the Liberibacter asiaticus– and have introduced them to a relative of yours, the Sinorhizobium meliloti, which lives in symbiosis with alfalfa. Through genetic engineering, Long and Barnett have made bacteria with activated virulence glow green with a certain light.

The biologist Sharon Long, from Stanford University (USA).

The biologist Sharon Long, from Stanford University (USA).

Thanks to this triquiñuela, the team was able to test 120,000 chemical compounds in genetically modified alfalfa bacteria in the laboratory. About 130 of them inhibited the green glow without affecting other beneficial microbes. Some of these substances could be the cure for huanglongbing. “The next step is to further investigate the compounds we have identified. None of them have yet been tested on diseased plants. We cannot do it, because the US regulation does not allow us to own or study diseased plants in our university facilities, ”explains Barnett, a Californian who has been raised among orange, lemon and mandarin trees.

Both she and Sharon Long take the war against huanglongbing as a personal matter. Both show old photos of their great-grandparents and other relatives linked to citrus cultivation for a century. The scientists have donated their new system to the public domain, so that any researcher in the world can use it to find the antidote against citrus plague. Your results are published today. in the specialized magazine PNAS. Long, he says, paid the first months of work in his pocket.

Spain is "seriously threatened", according to the Ministry of Agriculture

“We not only enjoy the fruits, but also the trees. Especially in December, around Christmas holidays, when citrus fruits are so beautiful, with their colorful lemons, oranges and tangerines, ”says Barnett. “I imagine the same thing happens in Spain, which is so famous for its oranges. Spanish missionaries planted the first citrus in California, ”recalls the biologist.

The Spanish Ministry of Agriculture approved in 2016 a National Contingency Plan Against the plague “Spain, the sixth largest citrus producing country, and the disease-free citrus regions (Mediterranean basin, Uruguay, Chile, Australia and New Zealand) are seriously threatened,” the document warns. The introduction of plants from outside the EU is prohibited. And, within European borders, citrus fruits must travel with a phytosanitary passport that guarantees that they come from plague-free areas.

The Generalitat Valenciana has come to proclaim that "any person or entity that knows or has indications of an illegal introduction must communicate it immediately" to the authorities. "In the face of huanglongbing, the irresponsibility of a few should not put in check the good work of the great majority because (…) if this disease comes we would all regret it" affirms the Generalitat. The 2017-2018 campaign was closed in the Valencian Community with a production of almost 3.2 million tons of citrus fruits, especially sweet oranges and tangerines.

Long and Barnett "offer a methodology to detect compounds that can be used against huanglongbing," says Manuel Talón, who has not participated in the new research. “They are honest, they don't throw the bells on the fly. This is a long term war. It is a problem of which we do not know practically everything. If you arrive in Spain, the same will happen in Florida, ”says Talón.

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