"Cassava's importance in Africa and South America can not be overstated. As a drought-resistant crop that does well in poor soils, cassava is a starch-heavy lifesaver for low-income areas."

—Eric Goldschein, "The 10 Most Important Food Crops in the World," Business Insider, September 20, 2011

The challenge    

Cassava is a major source of calories in for 500 million people, mainly in Africa, and is ranked as the fifth most important food crop in the world. Because it is highly productive even in poor soils, it is an important component of food security for subsistence farmers. However, it is vulnerable to a number of diseases. Two important diseases that can cause high losses to subsistence farmers are bacterial blight (Xanthomonas axonopodis pv. manihotis) and cassava brown streak disease, caused by two related viruses. 

 

cassava

Cassava harvest in Busia, Kenya

The strategy

2Blades is testing genetic strategies that eliminate vulnerabilities in the cassava plant that are exploited by pathogens. Some genes in the host plant are manipulated by pathogens to subvert defenses or aid its replication.  These ‘susceptibility’ genes can be altered or eliminated with great precision by scientists to allow the host plant to resist infection.  Recent advances in gene editing technology, such as 2Blades TAL Code,  have this powerful approach for engineering disease resistance feasible.  

The science

Bacterial Blight

Bacterial blight of cassava is caused by Xanthomonas axonopodis pv. manihotis. Like many xanthomonas pathogens, it makes use of  proteins called transcriptional activator like effectors (TALEs) to target susceptibility genes. The bacteria make a syringe-like apparatus that delivers these effectors directly into the plant cell.  Here they activate expression of particular plant genes to create a better environment for bacterial growth. Work in the Staskawicz lab has defined important TALE target genes in cassava, and the TALE binding sites in these targets are being altered by gene editing to test if we can block X. axonopodis infection to generate cassava plants that are more resistant to blight.

Cassava Brown Streak Disease

Two related viruses, cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV), cause brown streak disease. The viruses are spread by whiteflies and also by propagation of cassava plants via cuttings of infected plants. Like all viruses, CBSV and UCBSV rely on their host’s cellular machinery to replicate themselves.  Work in the Staskawicz lab focuses on plant proteins used by CBSV and UCBSV, to test if removing them by gene editing can produce a resistant plant.

Program impacts

Cohn M, Morbitzer R, Lahaye T, and Staskawicz B (2015). Comparison of gene activation by two TAL effectors from Xanthomonas axonopodis pv. manihotis reveals candidate host susceptibility genes in cassava. Molecular Plant Pathology, in press. doi 10.111.mpp12337.

Cohn M, Bart R, Shybut M, Dahlbeck D, Gomez M, Hou B, Frommer W, Lahaye T, and Staskawicz B (2014). Xanthomonas axonopodis virulence is promoted by a transcription activator like effector mediated induction of a sweet sugar transporter in cassava. Molecular Plant-Microbe Interactions 27: 1186-1198.  doi: 10.1094/MPMI-06-14-0161-R.