Genetic and histological characterization of downy mildew resistance at the cotyledon stage in Raphanus sativus L.

Abstract

Downy mildew (DM; Hyaloperonospora brassicae) is one of the most serious diseases in radish and responsible for high production losses. The objectives of this study were (1) to identify new sources of DM resistance at the cotyledon stage; (2) to determine the inheritance of the resistance; and (3) to clarify the histological and cellular response involved in DM infection. We screened 200 radish accessions for DM resistance at the cotyledon stage. Radish accessions Rd001, Rd004, Rd048 and Rd150 showed a complete resistance response (hypersensitive reaction), and accession Rd193 exhibited a partial resistance with confined sporulation. The highly susceptible accession Rd197 was then crossed with all resistant genotypes for a genetic analysis. F2 segregation analysis suggested that cotyledon resistance in Rd001, Rd004, Rd048 and Rd150 is conferred by a single dominant gene while resistance in Rd193 is polygenic. Genetic data suggests Rd193 resistance might be conferred by two dominant genes with complementary action, althought further experiments are needed to confirm this hypothesis. To conduct a histological study using light and fluorescence microscopy, we collected cotyledons from three genotypes with either differing levels of resistance (Rd004 and Rd193) and high susceptibility (Rd197) at ten different time points, from 3 h to 7 days post inoculation. Tissue necrosis was responsible for the inhibition and reduction of H. brassicae growth in Rd004 and Rd193 within 24 h of inoculation. For susceptible accession Rd197, the first evidence of necrotic cells appeared 48 h post inoculation. Callose deposition associated with intercellular H. brassicae growth occurred 3 h post inoculation for all genotypes and was the lowest in resistant accession Rd004. Cotyledon resistance to DM identified in radish is of high commercial interest and has the potential to be used in breeding programs.