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Updated – Transgenic cassava armed with dual disease resistance

George Osodi/Bloomberg via Getty Images

Posted on behalf of Jeff Tollefson.

Long a food staple in Africa, the humble cassava may be poised to grow even more significant as other crops such as maize (corn) wither in the heat and drought of a warming climate. But agricultural scientists know that the hardy tuber has an Achilles Heel — disease — that could curb its future potential.

With that threat in mind, researchers at the Swiss Federal Institute of Technology (ETH) in Zurich this week report the development of a new transgenic cassava variety that is resistant to a pair of viral diseases that are common in different parts of Africa. Published on 25 September in PLoS One, the work is part of a broader effort by the ETH and other institutions to work with local scientists and farmers and then develop disease-resistant strains as well as expertise within African labs.

“If we want to get this going in Africa, we need to have local people on the ground interested in deploying this technology,” says Herve Vanderschuren, lead author on the study and head of a cassava research team at the ETH. In parallel with the his work on disease resistant crops, Vanderschuren has worked with researchers Tanzania, Kenya and South Africa to develop a process that allows for this work to be undertaken locally.

Cassava is an important food source for more than a billion people, from sub-Saharan Africa to Asia and Latin America. It is often planted alongside corn and other staples and acts as a kind of hedge against heat and drought. But farmers and industry have their preferred varieties, and promoting new cultivars is more difficult because the plant is propagated by cuttings, not seeds.

Cassava’s future as a major food staple in Africa and beyond could depend both on scientists’ ability to produce disease-resistant varieties and on farmers’ willingness to adopt them, says Andy Jarvis, a researcher at the International Centre for Tropical Agriculture (CIAT) in Cali, Colombia. In a world of droughts and scorching temperatures, “cassava is the last crop standing,” Jarvis says. “But the big unknown is infective diseases.”

Vanderschuren is working with researchers in multiple countries to standardize a process that will allow this kind of work to proceed independently despite the sub-optimal lab conditions that have long hindered such work inAfrica. Scientists in Kenya and South Africa are already modifying local varieties of plants on a routine basis, he says.

One such scientist is Chrissie Rey at theUniversityofWitwatersrandinJohannesburg,South Africa. Rey says she started trying to modify cassava in an old chemical storage room nicknamed the “broom closet” around 2000. She was fresh off a sabbatical at theUniversityofCalifornia,Riverside, but her experience there didn’t quite translate when she and her students went about the same work inSouth Africa. Working with Vanderschuren, she has been able to establish some basic protocols and is now doing her own transformations. “There’s a slight art to it, but he has made that art a bit easier,” she says.

For their latest work, the Swiss team began with a Nigerian variety that is naturally resistant to mosaic disease, which has caused crop damage throughout sub-Saharan Africa for more than a century. The researchers then inserted viral DNA to engineer resistance to brown streak disease, which has been moving through eastern Africa during the past decade.

Similar work is also under way at the Donald Danforth Plant Science Center in St Louis, Missouri. Plant virologist Claude Fauquet says the center plans to begin field trials on another variety with dual disease resistance next spring. However, he says, the Swiss team is moving in the right direction by targeting a variety that is popular in Nigeria.

His nightmare scenario is that the brown streak virus will cross over the mountains from eastern Africa, spread through central Africa and make it to Nigeria before scientists and farmers are ready. Nigeria is the number one cassava producer in the world, with yields of around 42 million tonnes annually, he says, and a brown streak outbreak would be catastrophic.

“Cassava could be a very good bet for the future, a kind of saviour,” says Fauquet. “But we have to make sure that we don’t get a bunch of diseases coming through to limit that capacity.”

As co-chairman of a research consortium called the Global Cassava Partnership for the 21st Century, Fauquet is pushing for a coordinated global programme to halt or slow brown streak disease. Although Europe was able to bring potato viruses under control with a coordinated international effort after the Second World War, he says, there is no precedent in Africa. “It might be impossible,” he says, “but if we don’t try of course we are going to fail.”

An earlier version of this blog post stated that the Donald Danforth Plant Science Center’s field trials were already under way. In fact, they are expected to begin in the spring.  


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