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The R10 late blight differential of potato, 3681ad1, exhibits good field resistance. Progeny from the cross between 3681ad1 and the susceptible cultivar ‘Katahdin’ were assessed for late blight resistance to three Phytophthora infestans isolates, using a detached leaf assay. Progeny differed in response to the three isolates. Resistance to isolates IPO-0 and 99018 was controlled by quantitative trait loci (QTL), whereas resistance to isolate 89148-9 was inherited as a dominant R gene, designated as R10 in this study. Statistical analysis revealed that one of the resistance QTLs to isolates IPO-0 and 99018 is linked to the R10 gene, which maps to chromosome 11 in a region where a complex late blight resistance locus has been reported previously. A high-resolution map of R10 was constructed using a large segregating population, and the gene was delimited to a genetic interval of 0.26 cM. The clustering of the qualitative gene R10 with resistance QTLs could explain the field resistance observed with 3681ad1.

Similar to SCAR, an extended random primer amplified region (ERPAR) marker is a PCR amplified genomic DNA fragment at a single genetically defined locus. However, ERPAR uses specific primer pairs derived from RAPD primers by adding bases sequentially to their 3′-ends. As an example, an ERPAR marker was derived from a RAPD marker (OT11900) linked to a dominant male sterility gene in cabbage (Brassica oleracea var. capitata). After two cycles of base adding and primer pair screening, a primer pair (5′-TTCCCCGCGACT-3′and 5′-TTCCCCGCGAGA-3′) amplified a single intense band with the same size as OT11900. The identity of the new marker and OT11900 was verified by segregation analysis. The new marker amplified by this extended primer pair was named as EPT11900. The development of ERPAR exploits the importance of 3′-end bases of primers in PCR ERPAR shares advantages of SCAR, but eliminates the need for cloning and sequencing. It is a fast and universal way of converting RAPD markers into stable markers.

Massive resistance (R) gene stacking is considered to be one of the most promising approaches to provide durable resistance to potato late blight for both conventional and genetically modified breeding strategies. The R3 complex locus on chromosome XI in potato is an example of natural R gene stacking, because it contains two closely linked R genes (R3a and R3b) with distinct resistance specificities to Phytophthora infestans. Here, we report about the positional cloning of R3b. Both transient and stable transformations of susceptible tobacco and potato plants showed that R3b conferred full resistance to incompatible P. infestans isolates. R3b encodes a coiled-coil nucleotide-binding site leucine-rich repeat protein and exhibits 82% nucleotide identity with R3a located in the same R3 cluster. The R3b gene specifically recognizes Avr3b, a newly identified avirulence factor from P. infestans. R3b does not recognize Avr3a, the corresponding avirulence gene for R3a, showing that, despite their high sequence similarity, R3b and R3a have clearly distinct recognition specificities. In addition to the Rpi-mcd1/Rpi-blb3 locus on chromosome IV, the R3 locus on chromosome XI is the second example of an R-gene cluster with multiple genes recognizing different races of P. infestans.

Bacterial wilt (BW) caused by Ralstonia solanacearum (Rs) is an important quarantine disease that spreads worldwide and infects hundreds of plant species. The BW defense response of potato is a complicated continuous process, which involves transcription of a battery of genes. The molecular mechanisms of potato-Rs interactions are poorly understood. In this study, we combined suppression subtractive hybridization and macroarray hybridization to identify genes that are differentially expressed during the incompatible interaction between Rs and potato. In total, 302 differentially expressed genes were identified and classified into 12 groups according to their putative biological functions. Of 302 genes, 81 were considered as Rs resistance-related genes based on the homology to genes of known function, and they have putative roles in pathogen recognition, signal transduction, transcription factor functioning, hypersensitive response, systemic acquired resistance, and cell rescue and protection. Additionally, 50 out of 302 genes had no match or low similarity in the NCBI databases, and they may represent novel genes. Of seven interesting genes analyzed via RNA gel blot and semi-quantitative RT-PCR, six were induced, one was suppressed, and all had different transcription patterns. The results demonstrate that the response of potato against Rs is rapid and involves the induction of numerous various genes. The genes identified in this study add to our knowledge of potato resistance to Rs.

The genome of tomato (Solanum lycopersicum L.) is being sequenced by an international consortium of 10 countries (Korea, China, the United Kingdom, India, the Netherlands, France, Japan, Spain, Italy, and the United States) as part of the larger “International Solanaceae Genome Project (SOL): Systems Approach to Diversity and Adaptation” initiative. The tomato genome sequencing project uses an ordered bacterial artificial chromosome (BAC) approach to generate a high-quality tomato euchromatic genome sequence for use as a reference genome for the Solanaceae and euasterids. Sequence is deposited at GenBank and at the SOL Genomics Network (SGN). Currently, there are around 1000 BACs finished or in progress, representing more than a third of the projected euchromatic portion of the genome. An annotation effort is also underway by the International Tomato Annotation Group. The expected number of genes in the euchromatin is ∼40,000, based on an estimate from a preliminary annotation of 11% of finished sequence. Here, we present this first snapshot of the emerging tomato genome and its annotation, a short comparison with potato (Solanum tuberosum L.) sequence data, and the tools available for the researchers to exploit this new resource are also presented. In the future, whole-genome shotgun techniques will be combined with the BAC-by-BAC approach to cover the entire tomato genome. The high-quality reference euchromatic tomato sequence is expected to be near completion by 2010.