Developing a natural variation platform for pest-resistant cassava breeding

Lead Research Organisation: National Inst of Agricultural Botany
Department Name: Genetics and Breeding

Abstract

Cassava (Manihot esculenta) is the third most important source of calories in the tropics and a key staple for millions in Africa, Asia and Latin America. Predictions of rising temperatures linked to climate change favour the productivity of cassava, a crop that performs well under stressful growing conditions linked to drought and high temperatures. However, cassava is susceptible to a number of pests and diseases, and projections of movement of cassava pests linked to climate change predict substantial yield losses. As such, cassava breeders are prioritising traits linked to pest and disease resistance within their breeding programmes. Within Latin America, the largest pest and disease threats to cassava production are whitefly, cassava frogskin disease and bacterial blight. Bacterial blight is also prevalent throughout cassava growing regions in Africa and Asia. These can cause yield losses of 76-92%.

We can explore cassava's wild relatives to introduce novel disease resistance traits through conventional or molecular breeding techniques. Crop wild relatives have been used in wheat, rice, tomato, potato, sunflower and other crops to provide sources of novel resistance to pests and diseases. The Genetic Resources Program (GRP, genebank) at the International Center for Tropical Agriculture (CIAT) in Colombia currently houses ~4,900 accessions of cassava and its wild relatives. The mission of genebanks includes the conservation of materials, but also aims to make the material available for use. However having limited information for the vast majority of accessions makes it difficult for users, or even genebank managers, to select materials that may contain useful properties for breeding.

Through sequencing the genomes of cassava wild relatives we will reveal the genetic diversity of their resistance genes. In addition we will check the performance of these species against whitefly, cassava frogskin disease and bacterial blight. We will also develop novel tools to explore these datasets, making it easier for breeders to choose which wild materials to include in their programs. All the information generated through this project will be made publicly available for breeders and researchers to access via the GRP portal.

Breeders tend to be cautious about including wild materials into their breeding programs as wild species bring a mixture of desirable and undesirable characteristics into the offspring. Breeding is a slow process, and removing the undesirable traits can take many generations. Therefore, having more information about the physical and genetic characteristics of these wild species will help breeders to make the most appropriate choices of plants to use and encourage the inclusion of wild relatives to provide novel sources of pest and disease resistance into cassava.

CIAT has centres in Colombia and Vietnam, with researchers working in 53 countries worldwide. Having novel sources of pest resistance will benefit cassava breeders and growers in many countries, however we will target breeders in Colombia, Brazil, Vietnam, Cambodia and Lao People's Democratic Republic. Through the reduction of losses due to pests and diseases, this project will contribute to sustainable agriculture and increasing the resilience of food systems to climatic variability.

Technical Summary

We will explore wild relatives of cassava (Manihot esculenta) to identify novel sources of disease resistance. The Genetic Resources Program (GRP, genebank) at the International Center for Tropical Agriculture (CIAT) in Colombia currently houses ~4,900 accessions of cassava and its wild relatives. We will phenotype 100 accessions from 30 wild Manihot species for resistance to whitefly (Aleurotrachelus socialis), cassava frogskin disease (a complex of virus and phytoplasma infections) and bacterial blight (Xanthomonas axonopodis pv. Manihotis). These can cause yield losses of 76-92%.

The existing cassava reference genome remains fragmented, as such we will sequence the same accession (AM560-2) using PacBio (30x). We will assemble these long reads and integrate with the existing reference and genetic map data. We will transfer the existing annotation and reannotate the R-genes. In addition we will sequence the 100 accessions of wild Manihot (Illumina 20x) and call variants with respect to the improved reference. Unaligned reads will be assembled to produce species-specific contigs. We will also develop novel tools and visualisations, making it easier for breeders to explore these datasets and choose which wild materials to include in their programs. All the information generated through this project will be made available publicly for breeders and researchers to access via the GRP portal.

CIAT has centres in Colombia and Vietnam, with researchers working in 53 countries worldwide. Having novel sources of pest resistance will benefit cassava breeders and growers in many countries, however we will promote our new resources to breeders in Colombia, Brazil, Vietnam, Cambodia and Lao People's Democratic Republic, using CIAT's existing extension networks.

Planned Impact

Cassava (Manihot esculenta) is a globally important crop and a key staple for millions in Africa, Asia and Latin America; it is often grown under harsh conditions and in poor soil where other crops would fail. Cassava's roots and leaves are consumed as food, and cassava also has important uses for bioenergy and the production of industrial starch. Improving the disease resistance of cassava varieties will reduce yield losses globally, improving the welfare of communities dependent on this staple crop.

The direct beneficiaries from this project will be cassava researchers and (pre)breeders, as they will be able to make genomically and phenotypically informed wide crosses using cassava wild relatives. Cassava pre-breeders will have genomic information for 30 Manihot species and their phenotypes in response to three pests/diseases (whitefly, cassava frogskin disease and bacterial blight) to help make informed breeding decisions about the inclusion of wild materials. Breeders will be able to take advantage of novel sources of pest and disease resistance for their breeding programmes, and improved varieties will benefit farmers and consumers through a more predictable supply. CIAT's genebank will also benefit from the inclusion of additional genomic and phenotypic information, and novel query tools and visualisations, encouraging the use of their materials for breeding projects.

Indirect beneficiaries include researchers in other crops, as this approach may serve as an exemplar. Also genebanks for other species could benefit from the adoption of the query tools and visualisations which we will develop.

In the long term, cassava-growing regions will benefit as they face fewer losses associated with pests/disease, in particular in Latin America and South East Asia (e.g. Colombia, Brazil, Vietnam, Cambodia and Lao People's Democratic Republic). Increased yield of cassava will improve food security and public health in the face of climate change. Reduced dependency on pesticides will also provide environmental benefits.

This programme will generate new opportunities for collaborative work between NIAB and CIAT. The proposed work will directly impact the local UK community with the generation of new jobs in the field of computational biology. This collaboration will contribute to reinforce the UK's leadership in bioinformatics and genomics, translating this expertise into the breeding of a tropical crop and having a positive impact on the global economy.

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