No abstract

Black shank, caused by Phytophthora nicotianae, is typically the most important disease affecting tobacco (Nicotiana tabacum L.) production in the United States. Pedigree information suggests that most black shank resistance was derived from the cigar tobacco cultivar Florida 301. This resistance is thought to be polygenic in nature. The objectives of the current experiment were to (i) evaluate lines from a recombinant inbred line population derived from a cross between Florida 301 and the black shank‐susceptible cultivar Hicks for partial resistance using replicated field and greenhouse testing, (ii) genotype the population and use quantitative trait loci (QTL) analyses to identify Florida 301 genomic regions associated with resistance, and (iii) compare results with those obtained from a previous QTL analysis of a population derived from a cross involving ‘Beinhart 1000’. A total of 11 QTL affecting area under the disease progress curve were identified in both the field and greenhouse experiments. The QTL with the largest effect explained 16.9 and 18.6% of the phenotypic variation in the field and greenhouse experiments, respectively. This QTL was also found to have the largest effect on resistance in a Beinhart 1000 × Hicks doubled haploid mapping population. A major QTL found to affect resistance on linkage group 15 in the latter population, however, was not found to be important in the current population. Quantitative trait loci identification using greenhouse data was comparable to, if not superior to, use of field data.

Tobacco (Nicotiana tabacum) is an important cash crop and an ideal experimental system for studies on plant-pathogen interaction. The sequenced tobacco genome provides an opportunity for examining resistance gene homologs (RGHs) in the tobacco genome. Thirty nucleotide-binding site-type RGHs were annotated from genomic data, and another 281 putative RGHs were identified via PCR amplification from wild and cultivated tobacco. The newly identified RGHs are similar to other known RGHs, and some were categorized into new groups or branches that are different from known Nicotiana R genes or RGHs. Of the 281RGHs, 146 were identified from a single tobacco genome. We did not find any polymorphism at the RGHs in cultivated accessions, implying that strong domestication selection and/or demographic effects might have caused a sharp reduction in nucleotide diversity. Three positive selection sites were found in several RGH groups, while purifying selection is pervasive in the RGH family.Our results provide a primary RGH pool and several positively selected sites for the further functional validation of resistance genes in tobacco.

Brown spot (BS) caused by Alternaria alternata is one of the most destructive foliar diseases affecting tobacco (Nicotiana tabacum L.) production and quality in China. Breeding of BS-resistant cultivars is difficult because the resistance has proved to be quantitatively inherited. To facilitate marker-assisted selection, we carried out a study of mapping quantitative trait loci (QTLs) for BS resistance. We developed an F 2 population consisting of 213 individuals from a cross between a BS-susceptible cultivar ÔChangbohuangÕ (CBH) and a BSresistant cultivar ÔJinyehuangÕ (JYH) and constructed a genetic map consisting of 196 simple sequence repeat (SSR) markers based on this population. Using disease index (DI) as the indicator of BS resistance, we detected three QTLs located between SSR markers TM20534 and TM10737, TM10589 and TM10216, and TM10443 and PT60669, respectively. The resistant alleles of the three QTLs were all from the resistant parent JYH. The three QTLs together could explain $86% of the DI difference between the two parents in total, with $61% explained by their additive effects. Therefore, the three QTLs will be useful for BS-resistance breeding.