New research reveals mutation responsible for cassava disease resistance

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Groundbreaking research conducted by Rebecca Bart, PhD, Associate Member, and Nigel Taylor, PhD, Associate Member and Distinguished Researcher of Dorothy King, Donald Danforth Plant Science Center, and their collaborators at ETH Zurich, University of California at Los Angeles and the National Crops Resources Research Institute (NaCRRI) in Uganda, identified a genetic mutation that confers resistance to cassava mosaic disease (CMD). Their findings have important implications for improving cassava yield and sustaining farmer incomes in the face of widespread disease, and their finding could also shed light on disease resistance of other major crops.

Cassava, a carbohydrate-rich starchy root, is one of the world’s most important staple foods; it feeds almost a billion people, mainly in the tropics. Cassava is considered a staple in many developing countries, especially among smallholder farmers, due to its drought tolerance and ability to grow in poor soils. However, farmers in Africa, India and Southeast Asia – some of the largest cassava growing regions in the world – all too often suffer huge yield losses from CMD. Plants infected with CMD are stunted and do not fully develop storage roots that are used for food.

Cassava mosaic is caused by a family of closely related viruses. The virus hijacks the DNA replication system of cassava, compromising its development and thus suppressing yield. “Improving the breeding of CMD-resistant cassava varieties has important implications for securing the livelihoods of smallholder farmers,” says Taylor. CMD-resistant cassava varieties do exist and are believed to have been initially identified and bred by farmers decades ago. Many of these varieties are still widely cultivated and used by breeders as sources of resistance to create new and improved cassava varieties. “Cassava remains mysterious in many ways,” Taylor says, “compared to corn or soybeans, where hundreds of research labs have been working on these crops for decades.” Several years ago, Taylor’s team unexpectedly discovered plants in the field that had lost their resistance to CMD, thus triggering the present investigation into the genetic basis of resistance to the virus. “If the CMD resistance hadn’t mysteriously been lost,” Bart describes, “we might never have pursued this project.” The researchers knew immediately that they had discovered something important. With initial support from the Institute for International Crop Improvement at the Danforth Center, the Danforth team initiated experiments to uncover the genetic basis of CMD resistance. The collaboration has expanded to include partners from NaCRRI, UCLA and ETH-Zurich and has been supported by additional funding from the Bill & Melinda Gates Foundation.

By identifying the exact genetic region that confers resistance to CMD – in fact, down to a single nucleotide – their work sheds light on how resistance was acquired and how to maintain resistance to ensure yields in the future. Additionally, similar viruses “attack many other crops, including cotton and tomato,” Bart says. “This finding could also provide disease resistance strategies in these crops.” Studying cassava in the field is crucial to the researchers’ findings. For two years, the NaCRRI team cultivated cassava and documented the prevalence of the disease in the field in Uganda, a recognized “hot spot” for CMD. The Danforth Center and NaCRRI, which is based near Kampala, have built a long-standing relationship, allowing for levels of collaboration and resource sharing between institutions. These new findings are the result of the combined expertise of researchers around the world – and key contributions from Danforth Center postdoctoral fellow and co-first author Dr. Ben Mansfeld, research associate Kerrigan Gilbert and other key co-authors. “The other collaborators,” Taylor adds, “were the African farmers who identified CMD-resistant plants and nurtured them over generations. We have now discovered the gene that caused the resistance.”

The discovery of a single mutation that confers resistance to CMD has opened the door to more questions and opportunities. First and foremost, explains Bart, “we need to understand the mechanism of How? ‘Or’ What the mutation confers resistance. Understanding the mechanism of resistance will likely shed light on the stability of resistance over time. “There’s a lot more to learn,” Taylor concludes. “How many more secrets exist about cassava yet to be uncovered?” The research team will continue their collaborative, global cassava studies and in doing so, highlight the importance of focusing research on lesser-known crops.

This work was supported by a grant from the Bill & Melinda Gates Foundation (Investment INV-008213), funding from ETH Zurich and the Institute for International Crop Improvement at the Donald Danforth Plant Science Center.

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Material provided by Donald Danforth Plant Science Center. Note: Content may be edited for style and length.

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