We describe the identification of a mutant in the Arabidopsis accession Columbia (Col-0) that exhibits enhanced downy mildew ( edm1 ) susceptibility to several Peronospora parasitica isolates, including the RPP7 -diagnostic isolate Hiks1. The mutation was mapped to chromosome IV and characterized physically as a 35-kb deletion spanning seven genes. One of these genes complemented the mutant to full wild-type resistance against all of the Peronospora isolates tested. This gene ( AtSGT1b ) encodes a predicted protein of 39.8 kD and is an Arabidopsis ortholog of yeast SGT1, which was described originally as a key regulatory protein in centromere function and ubiquitin-mediated proteolysis. AtSGT1b contains three tetratricopeptide repeats at the N terminus followed by a bipartite chord-containing SGT domain and an SGT-specific domain at the C terminus. We discuss the role of AtSGT1b in disease resistance and its possible involvement in ubiquitin-mediated proteolysis in plants. INTRODUCTIONResistance of plants to biotrophic pathogens is controlled by a complex regulatory system with many features that suggest an ancient origin (for reviews, see Dangl and Jones, 2001;Holub, 2001). Specific molecular recognition of pathogen avirulence ( Avr ) determinants by receptor-like plant proteins, encoded by resistance ( R ) genes, triggers signal transduction processes that activate a variety of defense reactions in infected plants. These inducible defense responses include an oxidative burst resulting from the generation of highly reactive oxygen intermediates (Lamb and Dixon, 1997; Torres et al., 2001), irreversible membrane damage (Woods et al., 1988), hypersensitive death of host cells (Lam et al., 1999), increased expression of defenseassociated genes (Maleck et al., 2000;Schenk et al., 2000), and synthesis of antimicrobial metabolites such as phytoalexins (Glazebrook et al., 1997).Two key defense regulators that are required for the function of multiple R genes have been identified in Arabidopsis: NDR1 (Century et al., 1995) encodes a potentially membrane-associated protein of unknown function (Century et al., 1997), and EDS1 (Parker et al., 1996) encodes a soluble protein that has homology with eukaryotic lipases (Falk et al., 1999). Signaling through both EDS1 and NDR1 activates a common set of defense responses, including the synthesis of salicylic acid, an important component in local and systemic disease resistance (Feys and Parker, 2000).Many R genes have been shown to preferentially use either NDR1 or EDS1 to confer resistance against either bacterial or eukaryotic pathogens in Arabidopsis . Most of the R genes known at the time from Arabidopsis were used in this analysis, all of which encode receptor-like protein products that contain a carboxyl Leu-rich repeat domain (LRR) and a central nucleotide binding site 1 Current address: Department of Plant Pathology, Physiology, and Weed Science, Fralin Biotechnology Center, Virginia Polytechnical Institute and State University, Blacksburg, VA 24061-0346. 2 Current addr...

The genetic variation among a population of Sclerotinia sclerotiorum collected from oilseed rape fields in the Ç anakkale Province of Turkey was assessed using molecular and morphological markers. Seven microsatellite primer pairs (out of eight) revealed 32 clear polymorphic alleles among the 36 fungal isolates examined. An unweighted pair-group mean analysis dendrogram was generated using the genetic distance matrix with the 32 microsatellite alleles. The level of similarity was as low as 15% between some isolates indicating a high level of genetic diversity within the fungal population; 23 distinct isolates were found (at a genotypic diversity level of 63%). Among the collection of 36 isolates, 19 mycelial compatibility groups (MCGs) were identified; 10 MCGs included at least two isolates. Molecular and morphological data suggest that most of the isolates within a single MCG were identical; however, the isolates belonging to the MCG2 and MCG4 had variable microsatellite haplotypes and were morphologically dissimilar. The data suggest that there is possibly a high rate of outcrossing as well as evolutionary potential within the population of the pathogen in oilseed rape fields. This is the first report demonstrating the genetic and morphological variation within a population of S. sclerotiorum in Turkey.

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Oilseed rape (Brassica napus L.) is one of the most important oilseed crops in temperate climates. Erysiphe cruciferarum is an important disease of oilseed rape and causes crop loss in warmer areas of Europe. The research investigated the effect of nitrogen fertilizer and fungicidal treatment against powdery mildew infection caused by E. cruciferarum of oilseed rape on seed components, including protein, oil, oleic acid, linolenic acid and undesirable substances such as sinapic acid esters (SAE) and glucosinolates (GSL), using near infrared spectroscopy (NIRS). Five susceptible oilseed rape varieties were employed in this research using four treatment groups: no nitrogen fertilization and no fungicidal treatment (N(0)-F(0)); no nitrogen fertilization but fungicidal treatment (N(0)-F(1)); and nitrogen fertilization but no fungicidal treatment (N(1)-F(0)); nitrogen fertilization and fungicidal treatment (N(1)-F(1)). Nitrogen fertilization increased the protein, but lowered the oil content, of the seeds. Fungicidal treatments significantly increased oil contents in all varieties tested, however reduced protein levels in fertilized and non-fertilized plots. The level of linolenic acid did not change significantly in any plots of any treatment combinations; a similar result was observed in the level of oleic acid in most of the genotypes. Nitrogen fertilization increased GSL and SAE levels, whereas fungicidal treatment had no effect. Our findings demonstrated that nitrogen fertilization can markedly influence some quality parameters in oilseed rape; however, the application of fungicides reduced side effects of nitrogen fertilizer and resulted a reduction on GSL, SAE and protein contents but an increase on total oil and oleic acid contents.