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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. [ABSTRACT FROM AUTHOR]

Late blight caused by Phytophthora infestans is one of the most devastating diseases in potato cultivation and is mostly controlled by the application of chemicals. However, introduction of combinations of resistance ( R) genes conferring broad-spectrum resistance from wild Solanum species into cultivated potatoes is considered the most practical and promising approach to achieve durable resistance. This can be realized via classical breeding or genetic modification (GM). Because classical breeding is very time-consuming and is often hampered by linkage drag, a GM approach seems logic in this heterozygous and vegetatively propagated crop. During the last decades, many R genes have been identified in several wild Solanum species. Some have been cloned and more will follow. When these genes are derived from species crossable with cultivated potato (so-called cisgenes), application in resistance breeding using a GM approach is similar to an introgression breeding approach, in that the exploited genes are indigenous to the crop. Pending deregulation or derogation of cisgenesis, the use of cisgenic R genes would be an ideal strategy to accomplish durable resistance in potato. [ABSTRACT FROM AUTHOR]

Identification of resistance ( R) genes to Phytophthora infestans is an essential step in molecular breeding of potato. We identified three specific R genes segregating in a diploid mapping population. One of the R genes is located on chromosome 4 and proved phenotypically indistinguishable from the Solanum demissum-derived R2, although S. demissum is not directly involved in the pedigree of the population. By bulked segregant analysis combined with a resistance assay, a genetic linkage map of the R2-like locus was constructed with 30 coupling and 23 repulsion phase AFLP markers. Two markers flanking the R2-like locus were applied to screen an extended population of 1,586 offspring. About 103 recombinants were selected, and an accurate high-resolution map was constructed. The R2-like resistance was localized in a 0.4 cM interval and was found co-segregating with four AFLP markers, which can be used to isolate the R2-like gene by map-based gene cloning. By analyzing race-specificity and R gene-specific molecular markers, we also found that an R1-like gene and an additional unknown R gene are segregating in the population. [ABSTRACT FROM AUTHOR]

Nine resistance gene homologues (RGHs) were identified in two diploid potato clones (SH and RH), with a specific primer pair based on conserved motifs in the LRR domain of the potato cyst nematode resistance gene Gpa2 and the potato virus X resistance gene Rx1. A modified AFLP method was used to facilitate the genetic mapping of the RGHs in the four haplotypes under investigation. All nine RGHs appeared to be located in the Gpa2/Rx1 cluster on chromosome XII. Construction of a physical map using bacterial artificial chromosome (BAC) clones for both the Solanum tuberosum ssp. tuberosum and the S. tuberosum ssp. andigena haplotype of SH showed that the RGHs are located within a stretch of less than 200 kb. Sequence analysis of the RGHs revealed that they are highly similar (93 to 95%) to Gpa2 and Rx1. The sequence identities among all RGHs range from 85 to 100%. Two pairs of RGHs are identical, or nearly so (100 and 99.9%), with each member located in a different genotype. Southern-blot analysis on genomic DNA revealed no evidence for additional homologues outside the Gpa2/Rx1 cluster on chromosome XII. [ABSTRACT FROM AUTHOR]

Broad-spectrum resistance in potato to the potato cyst nematode (PCN) is commonly regarded as a complex inherited trait. Yet, in this paper we show that, by use of a selected set of PCN test populations, broad-spectrum resistance to the species Globodera pallida can be fully ascribed to the action of two loci: Gpa5 and Gpa6. These loci were readily mapped by means of a strategy based on two steps. Firstly, the chromosomal localization of both loci was assessed by use of an online catalogue of AFLP markers covering a substantial part of the potato genome (http://www.spg.wau.nl/pv/aflp/catalog.htm). Subsequently the chromosomal regions of both loci were identified by means of CAPS markers based on RFLP insert sequences. Locus Gpa5 explains at least 61% of the genetic variation. This locus maps to chromosome 5 on a region which has previously been shown to harbor resistance factors to viral (Nb, Rx2), fungal (R1) and nematodal (Gpa, Grp1) pathogens. The Gpa6 locus exhibits a minor effect on the resistance (24%) and acts additively to Gpa5. Interestingly, the Gpa6 locus maps to a region on chromosome 9 where, in the homoeologous tomato genome, the virus resistance gene Sw-5 resides as part of a resistance gene cluster. In potato, resistance to potato virus X has been reported in the vicinity of this region. The map location of Gpa6 indicates the presence of a resistance gene cluster at the end of the long arm of chromosome 9 of potato. [ABSTRACT FROM AUTHOR]

We describe here novel approaches to high-resolution mapping of repeated and introgressed DNA which may prove generally useful in map-based cloning from complex genomes. These approaches were developed in order to clone the Rx locus in potato. First, we prepared a BAC library from a tetraploid plant carrying Rx in the duplex condition ( Rx, Rx, rx, rx). BAC clones were then isolated with close markers on either side of Rx. However these clones did not extend across Rx: in the cloned DNA the closest markers to Rx were separated by single recombination events on either side of Rx. To bridge the gap we exploited the finding that the BAC clones on the right side of Rx contained resistance-gene homologues. Anticipating that there would be duplicated copies of these resistance gene homologues in the vicinity of Rx, we used low-stringency PCR conditions to identify additional markers. One of these markers was completely linked to Rx in our mapping population and was used to isolate a BAC (BAC77) that had not been previously identified by screening with Rx-flanking markers. Based on two criteria it was concluded that BAC77 spans Rx. There was a chromosomal recombination in one plant of our mapping population that separated the BAC77 right end from Rx. On the other side of Rx it was found that the BAC77 left end was outside the region of DNA carrying Rx that had been introgressed into potato cv Amaryl from a Solanum tuberosum ssp. andigena accession CPC1673. [ABSTRACT FROM AUTHOR]

Broad-spectrum resistance in potato to the potato cyst nematode (PCN) species Globodera rostochiensis and G. pallida is commonly regarded as a polygenically inherited trait. Yet, by use of QTL analysis and a selected set of PCN populations, resistance to both PCN species could be ascribed to the action of locus Grp1. Grp1 confers major resistance to G. rostochiensis line Ro5-22 and G. pallida population Pa2-D383 and partial resistance to G. pallida population Pa3-Rookmaker. Grp1 was mapped on chromosome 5 using previously characterized AFLP markers. Cleaved amplified polymorphic sequence (CAPS) markers available for RFLP loci GP21 and GP179 revealed that Grp1 maps on a genomic region harboring other resistance factors to viral, fungal and nematodal pathogens. The present data indicate that Grp1 is a compound locus which contains multiple genes involved in PCN resistance. [ABSTRACT FROM AUTHOR]