Abstract

Striga parasitism involves a series of events occurring beneath the soil surface. The complex nature of host/parasite interactions and the influence of environmental factors have limited our knowledge of host plant resistant mechanisms and their genetics and slowed the breeding for resistance efforts. However, the development of the new laboratory screening assays in our laboratory, have improved our understanding of host parasite interactions. Using these assays, two sorghum lines, CK32 and KP33, that posses a strong HR response, a potent active host defense response that discourage Striga parasitic establishment, were identified from progenies derived from a wild sorghum line, P47121. The aim of this study was to elucidate the inheritance of the HR response, using the Extended Agar Gel Assay. CK32 and KP33 were crossed to two sorghum cultivars, TX430 and TX2737, which possess no HR response. F₁, F₂, and BC₁ populations were generated from each cross. To evaluate the HR response, parents, F₁, F₂, BC₁ seedlings were infected with Striga and the roots of each seedling were examined for Striga attachment, necrosis, parasitic penetration, or discouragement. CK32, KP33 and their F₁ seedling all developed necrosis with about 80% of attached Striga seedlings discouraged from establishment, whereas TX430 and TX 2737 seedlings had no necrosis around the infection areas and the attached Striga plants were penetrated and fully developed. The F₂ populations from all crosses fit a segregation ratio for 15:1 (HR: no HR) response. Backcross populations segregated 3:1 (HR: no HR). The results of this segregation suggest that the HR to Striga infection in sorghum is conditioned by two nuclear genes with dominant gene action. The gene symbolsHrs1 and Hrs2 (hypersensitive response to Striga) were proposed for this trait.

Key word: Striga, sorghum, parasitism, hypersensitive response, host parasite interactions.