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Cyst nematodes are considered a dominant threat to yield for a wide range of major food crops. Current control strategies are mainly dependent on crop rotation and the use of resistant cultivars. Various crops exhibit single dominant resistance (R) genes that are able to activate effective host-specific resistance to certain cyst nematode species and/or populations. An example is the potato R gene Gpa2, which confers resistance against the potato cyst nematode (PCN), Globodera pallida population D383. Activation of Gpa2 results in a delayed resistance response, which is characterized by a layer of necrotic cells formed around the developing nematode feeding structure. However, knowledge about the Gpa2-induced defense pathways is still lacking. Here, we uncover the transcriptional changes and gene expression network induced upon Gpa2 activation in potato roots infected with G. pallida. To this end, in vitro-grown Gpa2-resistant potato roots were infected with the avirulent population D383 and virulent population Rookmaker. Infected root segments were harvested at 3 and 6 dpi and sent for RNA sequencing. Comparative transcriptomics revealed a total of 1,743 differentially expressed genes (DEGs) upon nematode infection, of which 559 DEGs were specifically regulated in response to D383 infection. D383-specific DEGs associated with Gpa2-mediated defense mainly relates to calcium-binding activity, salicylic acid (SA) biosynthesis, and systemic acquired resistance (SAR). These data reveal that cyst nematode resistance in potato roots depends on conserved downstream signaling pathways involved in plant immunity, which are also known to contribute to R genes-mediated resistance against other pathogens with different lifestyles.

Verticillium wilt caused by Verticillium dahliae results in severe losses in cotton, and is economically the most destructive disease of this crop. Improving genetic resistance is the cleanest and least expensive option to manage Verticillium wilt. Previously, we identified the island cotton NBS-LRR-encoding gene GbaNA1 that confers resistance to the highly virulent V. dahliae isolate Vd991. In this study, we expressed cotton GbaNA1 in the heterologous system of Arabidopsis thaliana and investigated the defense response mediated by GbaNA1 following inoculations with V. dahliae. Heterologous expression of GbaNA1 conferred Verticillium wilt resistance in A. thaliana. Moreover, overexpression of GbaNA1 enabled recovery of the resistance phenotype of A. thaliana mutants that had lost the function of GbaNA1 ortholog gene. Investigations of the defense response in A. thaliana showed that the reactive oxygen species (ROS) production and the expression of genes associated with the ethylene signaling pathway were enhanced significantly following overexpression of GbaNA1. Intriguingly, overexpression of the GbaNA1 ortholog from Gossypium hirsutum (GhNA1) in A. thaliana did not induce the defense response of ROS production due to the premature termination of GhNA1, which lacks the encoded NB-ARC and LRR motifs. GbaNA1 therefore confers Verticillium wilt resistance in A. thaliana by the activation of ROS production and ethylene signaling. These results demonstrate the functional conservation of the NBS-LRR-encoding GbaNA1 in a heterologous system, and the mechanism of this resistance, both of which may prove valuable in incorporating GbaNA1-mediated resistance into other plant species.

Late blight, caused by the oomycete pathogen Phytophthora infestans, is the most devastating disease in potato. For sustainable management of this economically important disease, resistance breeding relies on the availability of resistance (R) genes. Such R genes against P. infestans have evolved in wild tuber-bearing Solanum species from North, Central and South America, upon co-evolution with cognate avirulence (Avr) genes. Here, we report how effectoromics screens with Avr2 of P. infestans revealed defense responses in diverse Solanum species that are native to Mexico and Peru. We found that the response to AVR2 in the Mexican Solanum species is mediated by R genes of the R2 family that resides on a major late blight locus on chromosome IV. In contrast, the response to AVR2 in Peruvian Solanum species is mediated by Rpi-mcq1, which resides on chromosome IX and does not belong to the R2 family. The data indicate that AVR2 recognition has evolved independently on two genetic loci in Mexican and Peruvian Solanum species, respectively. Detached leaf tests on potato cultivar ‘Désirée’ transformed with R genes from either the R2 or the Rpi-mcq1 locus revealed an overlapping, but distinct resistance profile to a panel of 18 diverse P. infestans isolates. The achieved insights in the molecular R – Avr gene interaction can lead to more educated exploitation of R genes and maximize the potential of generating more broad-spectrum, and potentially more durable control of the late blight disease in potato.

Following the often short-lived protection that major nucleotide binding, leucine-rich-repeat (NB-LRR) resistance genes offer against the potato pathogen Phytophthora infestans, field resistance was thought to provide a more durable alternative to prevent late blight disease. We previously identified the QTL dPI09c on potato chromosome 9 as a more durable field resistance source against late blight. Here, the resistance QTL was fine-mapped to a 186 kb region. The interval corresponds to a larger, 389 kb, genomic region in the potato reference genome of Solanum tuberosum Group Phureja doubled monoploid clone DM1-3 (DM) and from which functional NB-LRRs R8, R9a, Rpi-moc1, and Rpi-vnt1 have arisen independently in wild species. dRenSeq analysis of parental clones alongside resistant and susceptible bulks of the segregating population B3C1HP showed full sequence representation of R8. This was independently validated using long-range PCR and screening of a bespoke bacterial artificial chromosome library. The latter enabled a comparative analysis of the sequence variation in this locus in diverse Solanaceae. We reveal for the first time that broad spectrum and durable field resistance against P. infestans is conferred by the NB-LRR gene R8, which is thought to provide narrow spectrum race-specific resistance.

Candidate effectors from lettuce downy mildew (Bremia lactucae) enable high-throughput germplasm screening for the presence of resistance (R) genes. The nonhost species Lactuca saligna comprises a source of B. lactucae R genes that has hardly been exploited in lettuce breeding. Its cross-compatibility with the host species L. sativa enables the study of inheritance of nonhost resistance (NHR). We performed transient expression of candidate RXLR effector genes from B. lactucae in a diverse Lactuca germplasm set. Responses to two candidate effectors (BLR31 and BLN08) were genetically mapped and tested for co-segregation with disease resistance. BLN08 induced a hypersensitive response (HR) in 55% of the L. saligna accessions, but responsiveness did not co-segregate with resistance to Bl:24. BLR31 triggered an HR in 5% of the L. saligna accessions, and revealed a novel R gene providing complete B. lactucae race Bl:24 resistance. Resistant hybrid plants that were BLR31 nonresponsive indicated other unlinked R genes and/or nonhost QTLs. We have identified a candidate avirulence effector of B. lactucae (BLR31) and its cognate R gene in L. saligna. Concurrently, our results suggest that R genes are not required for NHR of L. saligna.

Candidate effectors from lettuce downy mildew (Bremia lactucae) enable high-throughput germplasm screening for the presence of resistance (R) genes. The nonhost species Lactuca saligna comprises a source of B. lactucae R genes that has hardly been exploited in lettuce breeding. Its cross-compatibility with the host species L. sativa enables the study of inheritance of nonhost resistance (NHR). We performed transient expression of candidate RXLR effector genes from B. lactucae in a diverse Lactuca germplasm set. Responses to two candidate effectors (BLR31 and BLN08) were genetically mapped and tested for co-segregation with disease resistance. BLN08 induced a hypersensitive response (HR) in 55% of the L. saligna accessions, but responsiveness did not co-segregate with resistance to Bl:24. BLR31 triggered an HR in 5% of the L. saligna accessions, and revealed a novel R gene providing complete B. lactucae race Bl:24 resistance. Resistant hybrid plants that were BLR31 nonresponsive indicated other unlinked R genes and/or nonhost QTLs. We have identified a candidate avirulence effector of B. lactucae (BLR31) and its cognate R gene in L. saligna. Concurrently, our results suggest that R genes are not required for NHR of L. saligna.

[EN] Potato (Solanum tuberosum L.) is nowadays the most important non-cereal food crop in the world. It is prone to huge annual losses due to late blight, the disease caused by the oomycete pathogen Phytophthora infestans. Late blight can be controlled by weekly application of enormous amounts of chemicals. However, there are strategies to control late blight more environmentally safe as for instance by the introduction of durable Rpi-genes. Consequently, in order to breed for more durable late blight resistance, virulence knowledge about the current P. infestans population is a prerequisite for selection of the most durable or best combination of R-genes. In the present study a phenotypic characterization of Agrico Research collected P. infestans isolates was carried out based on Agrico Research differential set which include part of Black differential set and most of the available new Rpi-genes. Furthermore, a protocol to obtain sterile P. infestans in order to make a molecular characterization was carried out. High diversity of virulence against important Rpi-genes has been observed on part of the P. infestans in Agrico Research collection. Moreover, some of the R-genes tested have shown the desirable properties to be considered for future breeding programs. Finally, a methodology both, to obtain sterile P. infestans and to extract DNA was developed. In this way the present study has achieved enough data to develop a new and informative differential set in order to perform an efficient DLA., [ES] La patata (Solanum tuberosum L.) es actualmente el cultivo no cereal más importante del mundo. Anualmente el cultivo de la patata sufre grandes pérdidas económicas debidas principalmente al tizón tardío, enfermedad causada por el oomiceto Phytophthora infestans. Pese a que la enfermedad puede ser controlada por la aplicación semanal de grandes cantidades de fungicidas, el precio de los agroquímicos y la contaminación de los suelos han hecho que se opte por la búsqueda e introducción de nuevos genes de resistencia como estrategia más factible y respetuosa con el medio ambiente. El conocimiento del espectro de virulencia de los aislados que afectan a una región es esencial para la selección de los genotipos con genes de resistencia o combinaciones de estos, de manera que proporcionen una resistencia más durable. En el presente trabajo se ha llevado a cabo la caracterización biológica de la colección de aislados de P. infestans de la empresa Agrico Research en base a un conjunto de genotipos diferenciales, incluyendo parte del conjunto de diferenciales de Black, que contienen la mayoría de genes de resistencia (genes Rpi) disponibles. Simultáneamente, se ha desarrollado un protocolo para la esterilización de los aislados de P. infestans de la colección procedentes del campo. Además se ha determinado la metodología idónea para la extracción de ADN a partir de micelio puro de P. infestans. Se ha observado una gran diversidad de virulencia dentro de la población de P. infestans analizada. También, se ha visto que algunos de los genes R analizados presentan buena resistencia y pueden ser considerados para futuros programas de mejora. Se ha conseguido aislar P. infestans estéril a partir de aislados procedentes de campo mediante medios de cultivos diferenciales y se ha determinado la metodología idónea para la extracción de ADN a partir del micelio cultivado. Como resultado del trabajo realizado se han conseguido establecer el conjunto de genotipos diferenciales necesa

Resistance genes (R genes) in plants mediate a highly specific response to microbial pathogens, often culminating in localized cell death. Such resistance is generally pathogen race specific and believed to be the result of evolutionary selection pressure. Where a host and pathogen do not share an evolutionary history, specific resistance is expected to be absent or rare. Puccinia psidii, the causal agent of myrtle rust, was recently introduced to Australia, a continent rich in myrtaceous taxa. Responses within species to this new pathogen range from full susceptibility to resistance. Using the myrtle rust case study, we examine models to account for the presence of resistance to new encounter pathogens, such as the retention of ancient R genes through prolonged 'trench warfare', pairing of resistance gene products and the guarding of host integrity.

Blackleg disease, caused by the ascomycete fungal pathogen Leptosphaeria maculans, is a devastating disease of canola (Brassica napus) in Australia, Canada and Europe. The pathogen is considered a global invasive species and poses a threat to canola production in China, where only the weakly aggressive strain L. biglobosa is present. In Canada, breakdown of blackleg resistance has been shown. In order to develop a more effective disease management strategy, there is a need to elucidate host resistance and defense mechanisms underlying the B. napus-L. maculans pathosystem. This is the very first study to investigate major resistance genes (R genes) and adult plant resistance (APR) in Canadian canola germplasm. This study also analyzed the avirulence allele frequency in L. maculans populations in western Canada. R genes were detected in the majority of these B. napus germplasm, with the Rlm3 gene being predominant. The frequency of AvrLm3 allele in field fungal populations was extremely low. APR was identified in more than 50% of the germplasm. This indicated the breakdown of Rlm3 resistance, which could be due to the widespread use of this single resistance gene in Canadian B. napus germplasm and varieties. To address concerns of introducing L. maculans from Canada into China, this study further characterized R genes and APR to L. maculans in a collection of Chinese B. napus germplasm. R genes were detected in more than 40% of the germplasm tested, with Rlm3 and Rlm4 being predominant. A large portion of the Chinese germplasm showed high to moderate APR in field trials at three locations in MB, SK and AB in western Canada. This study highlighted the availability of fair to good resistance in the Chinese B. napus germplasm against blackleg disease and was the first study to investigate a large number of Chinese germplasm against Canadian fungal populations in different environments. RNA sequencing of resistant and susceptible host tissues and a streamlined bioinformat

[EN] Potato (Solanum tuberosum L.) is nowadays the most important non-cereal food crop in the world. It is prone to huge annual losses due to late blight, the disease caused by the oomycete pathogen Phytophthora infestans. Late blight can be controlled by weekly application of enormous amounts of chemicals. However, there are strategies to control late blight more environmentally safe as for instance by the introduction of durable Rpi-genes. Consequently, in order to breed for more durable late blight resistance, virulence knowledge about the current P. infestans population is a prerequisite for selection of the most durable or best combination of R-genes. In the present study a phenotypic characterization of Agrico Research collected P. infestans isolates was carried out based on Agrico Research differential set which include part of Black differential set and most of the available new Rpi-genes. Furthermore, a protocol to obtain sterile P. infestans in order to make a molecular characterization was carried out. High diversity of virulence against important Rpi-genes has been observed on part of the P. infestans in Agrico Research collection. Moreover, some of the R-genes tested have shown the desirable properties to be considered for future breeding programs. Finally, a methodology both, to obtain sterile P. infestans and to extract DNA was developed. In this way the present study has achieved enough data to develop a new and informative differential set in order to perform an efficient DLA., [ES] La patata (Solanum tuberosum L.) es actualmente el cultivo no cereal más importante del mundo. Anualmente el cultivo de la patata sufre grandes pérdidas económicas debidas principalmente al tizón tardío, enfermedad causada por el oomiceto Phytophthora infestans. Pese a que la enfermedad puede ser controlada por la aplicación semanal de grandes cantidades de fungicidas, el precio de los agroquímicos y la contaminación de los suelos han hecho que se opte por la búsqueda e introducción de nuevos genes de resistencia como estrategia más factible y respetuosa con el medio ambiente. El conocimiento del espectro de virulencia de los aislados que afectan a una región es esencial para la selección de los genotipos con genes de resistencia o combinaciones de estos, de manera que proporcionen una resistencia más durable. En el presente trabajo se ha llevado a cabo la caracterización biológica de la colección de aislados de P. infestans de la empresa Agrico Research en base a un conjunto de genotipos diferenciales, incluyendo parte del conjunto de diferenciales de Black, que contienen la mayoría de genes de resistencia (genes Rpi) disponibles. Simultáneamente, se ha desarrollado un protocolo para la esterilización de los aislados de P. infestans de la colección procedentes del campo. Además se ha determinado la metodología idónea para la extracción de ADN a partir de micelio puro de P. infestans. Se ha observado una gran diversidad de virulencia dentro de la población de P. infestans analizada. También, se ha visto que algunos de los genes R analizados presentan buena resistencia y pueden ser considerados para futuros programas de mejora. Se ha conseguido aislar P. infestans estéril a partir de aislados procedentes de campo mediante medios de cultivos diferenciales y se ha determinado la metodología idónea para la extracción de ADN a partir del micelio cultivado. Como resultado del trabajo realizado se han conseguido establecer el conjunto de genotipos diferenciales necesa

[EN] Potato (Solanum tuberosum L.) is nowadays the most important non-cereal food crop in the world. It is prone to huge annual losses due to late blight, the disease caused by the oomycete pathogen Phytophthora infestans. Late blight can be controlled by weekly application of enormous amounts of chemicals. However, there are strategies to control late blight more environmentally safe as for instance by the introduction of durable Rpi-genes. Consequently, in order to breed for more durable late blight resistance, virulence knowledge about the current P. infestans population is a prerequisite for selection of the most durable or best combination of R-genes. In the present study a phenotypic characterization of Agrico Research collected P. infestans isolates was carried out based on Agrico Research differential set which include part of Black differential set and most of the available new Rpi-genes. Furthermore, a protocol to obtain sterile P. infestans in order to make a molecular characterization was carried out. High diversity of virulence against important Rpi-genes has been observed on part of the P. infestans in Agrico Research collection. Moreover, some of the R-genes tested have shown the desirable properties to be considered for future breeding programs. Finally, a methodology both, to obtain sterile P. infestans and to extract DNA was developed. In this way the present study has achieved enough data to develop a new and informative differential set in order to perform an efficient DLA., [ES] La patata (Solanum tuberosum L.) es actualmente el cultivo no cereal más importante del mundo. Anualmente el cultivo de la patata sufre grandes pérdidas económicas debidas principalmente al tizón tardío, enfermedad causada por el oomiceto Phytophthora infestans. Pese a que la enfermedad puede ser controlada por la aplicación semanal de grandes cantidades de fungicidas, el precio de los agroquímicos y la contaminación de los suelos han hecho que se opte por la búsqueda e introducción de nuevos genes de resistencia como estrategia más factible y respetuosa con el medio ambiente. El conocimiento del espectro de virulencia de los aislados que afectan a una región es esencial para la selección de los genotipos con genes de resistencia o combinaciones de estos, de manera que proporcionen una resistencia más durable. En el presente trabajo se ha llevado a cabo la caracterización biológica de la colección de aislados de P. infestans de la empresa Agrico Research en base a un conjunto de genotipos diferenciales, incluyendo parte del conjunto de diferenciales de Black, que contienen la mayoría de genes de resistencia (genes Rpi) disponibles. Simultáneamente, se ha desarrollado un protocolo para la esterilización de los aislados de P. infestans de la colección procedentes del campo. Además se ha determinado la metodología idónea para la extracción de ADN a partir de micelio puro de P. infestans. Se ha observado una gran diversidad de virulencia dentro de la población de P. infestans analizada. También, se ha visto que algunos de los genes R analizados presentan buena resistencia y pueden ser considerados para futuros programas de mejora. Se ha conseguido aislar P. infestans estéril a partir de aislados procedentes de campo mediante medios de cultivos diferenciales y se ha determinado la metodología idónea para la extracción de ADN a partir del micelio cultivado. Como resultado del trabajo realizado se han conseguido establecer el conjunto de genotipos diferenciales necesa

Blackleg disease, caused by the ascomycete fungal pathogen Leptosphaeria maculans, is a devastating disease of canola (Brassica napus) in Australia, Canada and Europe. The pathogen is considered a global invasive species and poses a threat to canola production in China, where only the weakly aggressive strain L. biglobosa is present. In Canada, breakdown of blackleg resistance has been shown. In order to develop a more effective disease management strategy, there is a need to elucidate host resistance and defense mechanisms underlying the B. napus-L. maculans pathosystem. This is the very first study to investigate major resistance genes (R genes) and adult plant resistance (APR) in Canadian canola germplasm. This study also analyzed the avirulence allele frequency in L. maculans populations in western Canada. R genes were detected in the majority of these B. napus germplasm, with the Rlm3 gene being predominant. The frequency of AvrLm3 allele in field fungal populations was extremely low. APR was identified in more than 50% of the germplasm. This indicated the breakdown of Rlm3 resistance, which could be due to the widespread use of this single resistance gene in Canadian B. napus germplasm and varieties. To address concerns of introducing L. maculans from Canada into China, this study further characterized R genes and APR to L. maculans in a collection of Chinese B. napus germplasm. R genes were detected in more than 40% of the germplasm tested, with Rlm3 and Rlm4 being predominant. A large portion of the Chinese germplasm showed high to moderate APR in field trials at three locations in MB, SK and AB in western Canada. This study highlighted the availability of fair to good resistance in the Chinese B. napus germplasm against blackleg disease and was the first study to investigate a large number of Chinese germplasm against Canadian fungal populations in different environments. RNA sequencing of resistant and susceptible host tissues and a streamlined bioinformat

Mi-1.2 is a single dominant gene in tomato that confers race-specific resistance against certain phloem-feeding herbivores including aphids, whiteflies, psyllids, and root-knot nematodes. Few prior studies have considered the potential non-target effects of race-specific resistance genes (R genes), and this paper evaluates the compatibility of Mi-mediated resistance in tomato with a beneficial zoophytophagous predator, Orius insidiosus (Say). In addition to preying on aphids and other pests, this piercing-sucking insect also feeds from the xylem, epidermis, and/or mesophyll, and oviposits within plant tissues. Comparison of O. insidiosus confined to isogenic tomato plants with and without Mi-1.2 revealed that immatures of O. insidiosus had lower survival on resistant plants even when the immatures were provisioned with prey that did not feed on the host plant. Molecular gut content analysis confirmed that adults and immatures of O. insidiosus feed on both resistant (Mi-1.2+) and susceptible (Mi-1.2-) genotypes, and bioassays suggest that resistance does not affect oviposition rates, plant sampling, or prey acceptance by O. insidiosus adults. These results demonstrate a direct negative impact of R-gene-mediated host plant resistance on a non-target beneficial species, and reveal that Mi-mediated resistance can impact organisms that do not feed on phloem sap. Through laser capture microdissection and RT-PCR, Mi-1.2 transcripts were detected in the epidermis and mesophyll as well as the phloem of tomato plants, consistent with our observations that Mi-mediated resistance is active outside the phloem. These results suggest that the mode of action and potential ecological impacts of Mi-mediated resistance are broader than previously assumed.

Mi-1.2 is a single dominant gene in tomato that confers race-specific resistance against certain phloem-feeding herbivores including aphids, whiteflies, psyllids, and root-knot nematodes. Few prior studies have considered the potential non-target effects of race-specific resistance genes (R genes), and this paper evaluates the compatibility of Mi-mediated resistance in tomato with a beneficial zoophytophagous predator, Orius insidiosus (Say). In addition to preying on aphids and other pests, this piercing-sucking insect also feeds from the xylem, epidermis, and/or mesophyll, and oviposits within plant tissues. Comparison of O. insidiosus confined to isogenic tomato plants with and without Mi-1.2 revealed that immatures of O. insidiosus had lower survival on resistant plants even when the immatures were provisioned with prey that did not feed on the host plant. Molecular gut content analysis confirmed that adults and immatures of O. insidiosus feed on both resistant (Mi-1.2+) and susceptible (Mi-1.2-) genotypes, and bioassays suggest that resistance does not affect oviposition rates, plant sampling, or prey acceptance by O. insidiosus adults. These results demonstrate a direct negative impact of R-gene-mediated host plant resistance on a non-target beneficial species, and reveal that Mi-mediated resistance can impact organisms that do not feed on phloem sap. Through laser capture microdissection and RT-PCR, Mi-1.2 transcripts were detected in the epidermis and mesophyll as well as the phloem of tomato plants, consistent with our observations that Mi-mediated resistance is active outside the phloem. These results suggest that the mode of action and potential ecological impacts of Mi-mediated resistance are broader than previously assumed.

Background: The resistance of plants to pathogens relies on two lines of defense: a basal defense response and a pathogen-specific system, in which resistance (R) genes induce defense reactions after detection of pathogen-associated molecular patterns (PAMPS). In the specific system, a so-called arms race has developed in which the emergence of new races of a pathogen leads to the diversification of plant resistance genes to counteract the pathogens' effect. The mechanism of resistance gene diversification has been elucidated well for short-lived annual species, but data are mostly lacking for long-lived perennial and clonally propagated plants, such as roses. We analyzed the rose black spot resistance gene, Rdr1, in five members of the Rosaceae: Rosa multiflora, Rosa rugosa, Fragaria vesca (strawberry), Malus x domestica (apple) and Prunus persica (peach), and we present the deduced possible mechanism of R-gene diversification.Results: We sequenced a 340.4-kb region from R. rugosa orthologous to the Rdr1 locus in R. multiflora. Apart from some deletions and rearrangements, the two loci display a high degree of synteny. Additionally, less pronounced synteny is found with an orthologous locus in strawberry but is absent in peach and apple, where genes from the Rdr1 locus are distributed on two different chromosomes. An analysis of 20 TIR-NBS-LRR (TNL) genes obtained from R. rugosa and R. multiflora revealed illegitimate recombination, gene conversion, unequal crossing over, indels, point mutations and transposable elements as mechanisms of diversification.A phylogenetic analysis of 53 complete TNL genes from the five Rosaceae species revealed that with the exception of some genes from apple and peach, most of the genes occur in species-specific clusters, indicating that recent TNL gene diversification began prior to the split of Rosa from Fragaria in the Rosoideae and peach from apple in the Spiraeoideae and continued after the split in individual species. Sequence s

Plant gene families organized as linked clusters are capable of evolving by a process of unequal crossing-over. This results in the formation of chimeric genes that may impart a novel function. However, the frequency and functional consequences of these unequal cross-over events are poorly characterized. Plant disease resistance genes (R genes) genes are frequently organized as gene clusters. In this study, I constructed an elaborately designed reconfigurable synthetic RPP1 (for resistance to Paranospora parasitica) gene cluster (synthRPP1) to model R gene evolution by meiotic recombination. This experimental design utilizes gain-of-luciferase phenotype (luc+) to identify and isolate recombinant R genes and uses two alternatively marked alleles to distinguish and measure different types of meiotic recombination (intra- vs. inter-chromosomal). Two putative single copy transgenic plants containing the synthRPP1 gene cluster were generated. These synthRPP1 gene clusters were reconfigured in vivo by two kinds of site-specific recombination systems (CRE/Lox, FLP/FRT) to generate two alternative versions of the synthRPP1 gene clusters in vivo. These lines, as well as others being developed, will be used in future genetic crosses to identify and characterize plants expressing chimeric RPP1 genes. My second area of research was to use a previously developed synthetic RBCSB gene cluster (synthRBCSB) gene cluster to investigate the relative frequency of meiotic unequal crossing over between paralogous genes located on either homologous chromosomes (homozygous lines) or sister chromatids (hemizygous lines). In contrast to published somatic recombination frequencies using a different reporter gene system, no statistically significant difference of meiotic unequal crossing over was observed between homo- and hemi-zygous synthRBCSB lines. This result suggests that meiotic unequal crossing-over between paralogs located on homologous chromosomes occurs at about the same frequency a

Plants have evolved physical barriers and inducible defense responses to combat microbial pathogens. Inducible responses are mediated by R proteins, which recognize invading pathogens. R proteins must be precisely regulated to provide effective resistance, without inhibiting normal plant growth. However, little is known about R gene regulation under defense-inducing conditions. The interaction between the oomycete Hyaloperonospora parasitica and the model plant Arabidopsis thaliana provides an excellent model system to explore R gene regulation. My research focuses on RPP8, a CC-NBS-LRR gene, which provides resistance to the H. parasitica isolate Emco5. Previous work in the McDowell lab suggested that RPP8 is upregulated during defense responses. My research shows that RPP8 alleles from the Columbia and Landsberg erecta ecotypes are upregulated by H. parasitica and the defense signaling molecule salicylic acid, suggesting a potential feedback loop. RPP8-Ler is also systemically upregulated after infection of the bacterial pathogen Pseudomonas syringae pv. tomato DC3000. Additionally, RPP8-Ler expression is increased during wounding and heat stress. I also examined the role of regulatory cis elements in the RPP8 promoter. Three W-boxes are essential for basal and inducible RPP8 expression, and are required for resistance to Emco5. The X-box, a unique cis element in the RPP8 promoter, is essential for strong basal expression and wound-induced upregulation, and affects spatial expression of RPP8-Ler. However, the X-box is not required for RPP8-Ler upregulation during pathogen or SA treatment. R genes may be induced as part of global defense responses, which could prime the host for more effective pathogen recognition.