Stacking disease resistance and mineral biofortification in cassava varieties to enhance yields and consumer health

 

Cassava mosaic disease (CMD) and Cassava brown streak virus (CBSV) are two viruses that cause the most amount of damage to cassava cultivation in Africa where this root is the main source of calories for several countries including Tanzania, Uganda, and Kenia. Both viruses are transmitted by the whitefly Bemisia tabaci and disseminated when farmers plant contaminated stems. Plants affected by CBSD have inedible dark necrotic roots, and there are no resistant varieties available to farmers. CMD decreases photosynthesis levels resulting in lower root yields. These diseases originated in east Africa but are now spreading to the west threatening more countries like Nigeria, the world’s largest producer of Cassava.

Considering the micronutrient malnourishment existing in large African populations, studies appoint biofortification as a good cost-benefit strategy to improve staple crops. Cassava only provides around 10% iron and 15% zinc of daily recommended doses and there are ongoing biotechnology efforts to increase iron and zinc concentrations because traditional breeding is limited by insufficient genetic variation for these minerals.

Regarding disease resistance transgenic expression of a near full-length coat protein (CP) sequence of the CBSV virus provided high-level resistance to CBSD in the greenhouse and in many field trials in Uganda and Kenya, this strategy was combined with already developed resistance to CMD and modified ferritin and iron transporters. Results showed that storage roots of transgenic plants were found to have accumulated 8-13 times higher iron and 2-7 times higher zinc concentrations compared to the same tissues in non-transgenic controls. Therefore, mineral biofortification of cassava roots can be stacked with resistance to CBSD and CMD and the functionality of all traits were demonstrated in four cultivars obtaining healthy and stable plants.

After many years of research, however, the benefits of crop biotechnology have had little impact on farmers’ fields in Africa. Regulatory aspects are a long process but there is progress and CBSV resistant cassava has been recently approved for national performance trials in Kenya.

By Rosana Segatto

Reference:

Narayanan, N., Beyene, G., Chauhan, R. D., Grusak, M. A. and Taylor, N. J. (2021) Stacking disease resistance and mineral biofortification in cassava varieties to enhance yields and consumer health. Plant Biotechnol J, https://doi.org/10.1111/pbi.13511

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