Development of Herbicide-Tolerant Cassava Using Random Mutagenesis

Nigeria is the largest producer of cassava (Manihot esculenta) in the world, with cassava grown primarily for food by subsistence farmers and also produced for industrial purposes. Weed management is a significant problem for smallholder farming families. The majority of weed control is performed by women and children through hand weeding, which is time consuming, physically difficult manual labor and often ineffective, resulting in yield losses estimated between 20-80%. Introduction of integrated weed management systems is likely to have a significant economic and social benefit for Nigerian cassava farming families. To make herbicide-tolerant cassava freely available to Nigerian farmers in the shortest timeframe possible, we are using random mutagenesis in Nigerian farmer-preferred varieties to produce acetolactate synthase (ALS) herbicide-tolerant cassava mutants. Production of herbicide-tolerant crops using chemical mutagenesis has already been achieved in several commercial crop species. Since cassava does not produce large volumes of seed, we are using a tissue culture-based platform for high-throughput chemical mutagenesis of friable embryogenic callus. We are performing dosage curves for several mutagens to determine mutation spectrum and frequency using Sequence-Based Genotyping (SBG®), and for several ALS herbicide classes to determine appropriate concentrations for selection of ALS-tolerant regenerants in tissue culture. Spectrum and efficacy of ALS herbicide tolerance in regenerated mutant plants will be quantified in vitro and in glasshouse assays. The specific ALS mutation responsible for tolerance will be identified by KeyPoint® MB sequencing. ALS herbicide-tolerant cassava plants will be made available to the International Institute of Tropical Agriculture (IITA) in Nigeria for multiplication and dissemination to smallholder farmers and for use in cassava breeding programs. The Bill and Melinda Gates Foundation is gratefully acknowledged for financial support of the project.