Researchers Find Gene for Breeding Virus-Resistant Cassava

AFRICA - Scientists have discovered the gene that confers resistance to the deadly cassava mosaic disease in some cassava cultivars. The discovery is a critical first step in developing virus-resistant cassava varieties.

Manioc, also referred to as cassava, is a key source of raw materials and a staple food for close to one billion people. Smallholder farmers benefit financially, particularly those in Africa. Cassava is a low-maintenance crop that can even grow in dry areas without fertilizer.

Cassava Mosaic Disease

But many diseases and pests have an impact on cassava farming. Crop damage from diseases like cassava mosaic disease (CMD) is common. CMD is caused by DNA geminiviruses, which can wipe out entire fields and drastically reduce yields. These viruses are spread to plants by sap-sucking whiteflies (Bemisia tabaci).

In Africa and India, cassava mosaic disease is a serious issue that is now also spreading to Southeast Asia.

The research team under the direction of Wilhelm Gruissem, a professor of plant biotechnology at ETH Zurich, employed genome analyses to pinpoint the gene in charge of a particular resistance to the cassava mosaic virus. They worked with several resistant and susceptible West African cassava cultivars.

Virus Resistance

Farmers in West Africa first noticed the resistance when they noticed that while the majority of their field's cassava plants had perished due to the viral infection, a few plants had survived. Researchers became interested in this and started looking for the root of this resistance.

The team demonstrates, as described in Nature Communications, that the resistance is brought on by a single gene that serves as the framework for a DNA polymerase, which is an enzyme in charge of replicating DNA inside of a cell. However, the DNA polymerase does more than just replicate DNA; it also engages in "proofreading" to fix any errors that might develop in the order of the DNA building blocks as the process progresses. And the geminiviruses specifically need this enzyme to replicate their DNA and subsequently procreate.

Cassava has two copies of each gene because it has two sets of chromosomes. One copy of the DNA polymerase gene must be altered for the virus infection to end. However, neither copy of the DNA polymerase gene carries the mutation that results in CMD resistance in plants that are prone to the disease.

Gruissem says that They still don't fully understand how the resistance mechanism operates. But this is something that needs to be looked into in more extensive research. He surmises that the mutations have an impact on an area of the enzyme involved in DNA replication error correction. These modifications may have an impact on DNA polymerase's functionality, making it less effective at catching errors in viral DNA that is replicating. In the end, these mistakes prevent the virus from multiplying and spreading throughout the plant.

To improve food security in tropical as well as subtropical areas, the researchers are making a significant contribution by identifying the gene that causes the phenomenon known as CMD2 resistance. Breeders can now use the gene that the team has discovered in thier study as a genetic marker to determine whether or not the resistance is prevalent in their plants.

Export Roadblocks

For field propagation, it is not practical to export stems from West African cassava plants that are virus-resistant to Asia for economic and agronomic reasons. Asiatic breeders must therefore come up with an alternative method of introducing the resistance into their plants. The precise editing of the DNA polymerase gene and activation of disease resistance using contemporary CRISPR-Cas technology is one option, according to Gruissem, Futurity reports.