CRISPR tackles deadly cassava mosaic disease

Work has begun to potentially develop CRISPR cassava varieties that are resistant to the deadly cassava mosaic disease (CMD) after an international team of researchers was able to identify a gene responsible for the resistance.

The team led by Wilhelm Gruissem, professor of plant biotechnology at the Swiss Federal Institute of Technology Zurich (ETH Zurich), has finally identified the gene responsible for what is known as resistance to cassava mosaic disease 2 (CMD2) in some species cassava varieties. Working with several resistant and susceptible West African cassava cultivars, the team used complex and time-consuming genome analyzes to identify the gene responsible for specific resistance to cassava mosaic virus. The research team includes scientists from the National Crop Resources Research Institute in Uganda, the Donald Danforth Plant Science Center in St. Louis, Missouri, and the University of California, Los Angeles.

The identified gene now serves as a genetic marker for breeders indicating whether resistance is present in their plants and can be used to guide the development of resistant varieties. In an interview with the Science Alliance, Gruissem explained that about 30 years ago, farmers in West Africa found cassava plants in fields damaged by CMD that appeared healthy, indicating they were resistant. Although the trait can be introduced into other cassava varieties using conventional plant breeding processes, such efforts are usually time-consuming and the improved varieties also have undesirable traits.

“Cassava cultivation is a difficult and multi-year effort. Also, because the cassava genome is highly heterozygous, genetic crosses produce progeny with variable phenotypes,” he explained.

A recent statement from ETH Zurich states that “by identifying the gene responsible for resistance, researchers play an important role in increasing food security in tropical and subtropical regions.” Now that the genes have been identified, Gruissem told the Alliance for Science, CRISPR genome editing methods can be explored to develop resistant varieties. He explained that in 2021, two laboratories first reported a new CRISPR-Cas method called “core editing” for targeted mutagenesis in human tissue culture cells and rice. This method allows the introduction of specific single mutations into the DNA.

“The method can be used to introduce the mutations we found that confer resistance to CMD into susceptible cassava varieties. Work is currently underway to test the basic CRISPR-Cas editing method in cassava for this purpose,” he said. “Thus, the basic CRISPR-Cas editing method can be used to introduce resistance to CMD, especially in farmers’ and consumers’ preferred cassava cultivars.” This would help preserve the good qualities of these varieties and by making them resistant to CMD, at the same time help farmers prevent yield and economic losses.”

What is cassava mosaic disease?

Cassava is a staple food for nearly one billion people worldwide. Small farmers in Africa grow it a lot because it is an undemanding crop, does not need fertilizers and grows even in dry areas.

But sap-sucking whiteflies do infect cassava and transmit Geminiviruses, resulting in cassava mosaic disease, which can wipe out entire fields and wipe out yields. The disease causes cassava plants to have twisted leaves, resulting in an overall reduction in both leaf and plant size. The leaves of the plant turn yellow and green. It also causes tuber yield loss as a result of significant storage root yield losses. The disease is a particularly serious problem in Africa and India and is currently spreading in cassava fields in Southeast Asia, hence the urgent need for CMD-resistant cassava varieties. The disease is caused by a virus, which makes it very difficult to treat.

Dr Daniel Dzidzienyo, coordinator of the Anglophone Africa Research Program at the West African Center for Crop Improvement (WACCI), said CMD is a major food security problem on the African continent that needs to be addressed. In Ghana, for example, “CMD is very endemic,” he said. “Recent work done by some researchers has shown that about 96 percent of the farms surveyed in the cassava growing areas of the country have CMD of various levels of severity.”

“CMD carries a risk of about 90 percent yield loss in cassava and the devastating effect of the disease cannot be underestimated. The need for urgent solutions to mitigate the detrimental effect of CMD is very important for both crop productivity and economic objectives,” he noted.

Dzidzienyo said the identification of the resistance gene by Gruissem and his team could have a huge positive impact on efforts to protect cassava from damage and improve food security on the African continent.

“The significance of the occurrence and severity of CMD in Africa as a whole, particularly in Ghana, is greatly affecting yield, leading to economic losses. This discovery of a gene responsible for resistance against CMD is very important for cassava production. especially in Africa and should be studied,” he said in an interview with the Alliance for Science.

“This could potentially mean a huge boost in crop productivity and improve farmers’ livelihoods,” added Dzidzienyo.


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