New genetic source for black shank resistance and genomic loci associated with its resistance
Black shank causes severe yield loss of tobacco in Japan because of root and leaf infection by Phytophthora nicotianae, the causal agent of this disease. Several cultivated varieties in Japan have partial resistance derived from Florida 301, which is known as the most popular resistance source for black shank, but these varieties do not exhibit adequate resistance. This study was conducted to find new genetic source with high resistance to black shank and to identify the genomic loci associated with its resistance. We first evaluated the levels of black shank resistance of several varieties. As a result of root and leaf inoculation tests, a tobacco variety Black Shank Resistant (BSR) exhibited high levels of stable resistance. Further experiments were therefore conducted using BSR. A doubled haploid (DH) population was generated from a cross between BSR and Bright Yellow 4 (susceptible to black shank). Then a linkage map was constructed from genotyping data by next generation sequencer. Subsequently, resistant and susceptible DH lines were selected by inoculation tests. A few genomic loci were determined from associations between genotype and phenotype in the resistant and susceptible DH lines. We excluded the loci that seemed to be already introgressed into the several commercial varieties from Florida 301 and investigated the effects of the remaining loci on black shank resistance. An F3 population from a cross between a resistant DH line and a breeding line was generated. The F3 individuals that possessed the loci exhibited high resistance to black shank. Our results suggest that the genomic loci we found in BSR can improve black shank resistance of the current commercial varieties.