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...

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Arabidopsis is universally resistant as a species to many crop pathogens, including examples from other crucifers such as Albugo candida and Hyaloperonospora parasitica from Brassica oleracea. This species level trait could potentially provide a source of durable disease resistance in crops if examples can be found which are amenable to molecular genetic characterization. Our research has developed from the observation that null mutation in Arabidopsis of a defense regulatory gene EDS1 (enhanced disease susceptibility) is susceptible to isolates of A. candida and H. parasitica from brassica. EDS1 is required by a major structural class of R-genes to confer resistance in Arabidopsis. We have therefore focused on identifying R-genes in Arabidopsis that are responsible for conferring resistance to brassica pathogens.

A stem taper model, involving a high order polynomial developed using plantation hoop pine (Aroucaria cunninghamii Ait. ex D. Don) trees grown in Queensland, is presented. The stem profile for each tree was initially modelled using functions of diameter under bark and height, as the dependent and independent variables respectively. Parameter estimates from these individual tree regressions were subsequently modelled, using total height and diameter at breast height under bark (DBHub) as the independent variables. Finally, total height and DBHub were modelled in terms of predominant height' and diameter at breast height over bark (DBHob).The largest mean error in the estimates of stem diameter predicted using the final model, with predominant height and DBHob as the independent variables, was 0.12 em. The overall mean relative error in the estimate of volume, based on coefficients modelled using predominant height and DBHob, was 0.20 per cent. Apart from its accuracy, the main advantages of the model include its simplicity and its ability to model the whole tree using a single function.

A fast neutron mutant in Arabidopsis (Columbia) was identified that exhibits enhanced downy mildew (edm1) susceptibility to several Peronospora parasitica isolates, including the RPP7-diagnostic isolate Hiks1. The mutation was mapped to chr.4 and physically characterised as a 35kb deletion spanning seven genes. One of these genes restored wild-type resistance to all of the P. parasitica isolates. This gene (AtSGT1b) encodes a predicted protein that is orthologous to yeast SGT1, originally described as a key regulatory protein in centromere function and ubiquitin-mediated proteolysis. AtSGT1b contains three tatratrico-peptide repeats at the N terminus followed by a bipartite “CS” (CHORD containing Sgt) domain and an SGT specific (SGTS) domain at the carboxyl terminus. Altered expression of this gene is being investigated in Arabidopsis and Brassica olarecea to determine its potential use for crop improvement.

The chapter investigates the evolving doctrines of Epicureanism in the Roman period. Reverence for Epicurus and close adherence to his teachings were essential aspects of Epicurean identity throughout the history of the school. But despite their desire to be faithful, later Epicureans made their own contributions as they recontextualized Hellenistic philosophy, responded to later developments and current controversies, or presented Epicureanism in new genres. In addition to surveying the later Epicurean sources, this chapter outlines aspects of Epicurean scientific theory that first emerge between the first century bce and the second or third century ce. The authors explored are Lucretius, Philodemus, Diogenes of Oenoanda, and Diogenes Laërtius. The doctrines highlighted include: the healing balm of philosophy, the atomic basis of human psychology, the nature of scientific inference as induction, the visual size of the sun, and the swerve.

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