Unravelling the molecular genetic basis of Striga resistance in cereals: integrating Quantitative Trait Loci (QTL) and genomic approaches

Rice and sorghum are two of the major staple foods for millions of people in sub Saharan Africa (SSA) and the semi-arid tropics yet a major constraint to crop production and yield improvement is the parasitic weed Striga. This parasite attaches to the roots of the host plant causing severe stunting and loss of grain yield. Currently Striga species infest over 40% of the cereal producing areas of SSA; their effects are greatest on infertile soils and those most severely affected are the poorest subsistence farmers upon whom the weed exerts major impacts on poverty and health. At present the most commonly used strategies for alleviating the impact of this parasite include hand weeding, improving soil fertility and the use of 'tolerant' cultivars (which produce some grain even when infected), however success has been limited. While combining several control measures is likely to be necessary for control of Striga, crop losses due to the parasite could be reduced significantly through introducing host resistance genes into the most commonly used cultivars. However, the use of resistant cultivars is limited by a lack of resistant material and by a lack of understanding of the molecular genetic nature of host resistance to Striga. Over the last 5 years we have carried out an extensive screening programme in rice and have identified cultivars that show good post-attachment resistance to S. hermonthica. The discovery of resistance in rice to Striga is of great significance as it is currently the best 'model' cereal for molecular genetic studies as the genome of this crop plant has been sequenced. In this programme we propose to take an integrative approach to identify and investigate the molecular genetic nature of resistance to Striga in rice by combining our extensive knowledge of Striga-host interactions, our novel plant growth systems and modern genomic and comparative genomic techniques. We will then use information gained from rice to determine the extent to which similar resistance occurs in sorghum. Not only will the project enhance our fundamental understanding of the molecular genetic nature of resistance to Striga but it will contribute to more efficient breeding methods for Striga resistance that could be used in regional and national breeding programmes in Africa for rice and sorghum improvement. The improvement of varieties which stabilize yields and their adoption by subsistence farmers is critical for enhanced food security and poverty reduction in West and sub Saharan Africa in the long term.

Striga species are angiosperm parasites that cause devastating losses in crop yield throughout sub Saharan Africa. The use of Striga-resistant cultivars would represent a cost effective control measure, however, such a strategy is limited by a lack of resistant germplasm and by a lack of understanding of the molecular genetic basis of host resistance to Striga. Over the last 5 years we have carried out an extensive screening programme in rice and have identified cultivars that show good post-attachment resistance to S. hermonthica. Using a mapping population of rice we have identified some Quantitative Trait Loci (QTL) underlying the resistance. In this project we propose to take an integrative approach to identify and investigate the molecular genetic basis of resistance to Striga in rice by combining our extensive knowledge of Striga-host interactions, our novel plant growth systems and modern genomic and comparative genomic techniques for the improvement of both the rice and sorghum crop in Africa. Our specific objectives are: (1) To screen selected African rice cultivars for resistance to different ecotypes of S. hermonthica, S. asiatica and S. aspera and to determine the phenotype of the resistance. (2) To identify QTL underlying resistance in rice to these different ecotypes and species of Striga using two different mapping populations of rice in order to select the most genetically stable QTL for use in Marker Assisted Breeding Programmes (MAB). (3) To identify genes that are up and down regulated in rice roots undergoing a resistance reaction and, by integrating the results with the QTL data, to identify candidate resistance genes and (4) to utilize our knowledge of the molecular genetic basis of resistance in rice to Striga species to take a comparative genomic approach to identify and confirm the existence of homologous QTL and resistance genes in sorghum.