Arabidopsis SGT1b Is Required for Defense Signaling Conferred by Several Downy Mildew Resistance Genes

Abstract

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...

Microarray-based rapid cloning of an ion accumulation deletion mutant in Arabidopsis thaliana

“…Assuming an average fast neutron deletion size of 1-4 kb (15, 16), we estimated that a substantial number of deletion mutant genes could be cloned, considering that the average Arabidopsis gene size is Ϸ2 kb (17). In some cases, deletion sizes can be Ͼ5 kb (18)(19)(20), which may cover more than one gene, thus increasing the deletion identification efficiency using ATH1 microarray-based mapping. These bioinformatic analyses indicated that the Affymetrix ATH1 microarrays could be used for developing rapid microarray-based cloning of fast neutron bombarded mutants.…”

“…The ATH1 probe coverage is sufficient for mapping a substantial number of fast neutron-generated mutants (Fig. 1) based on the large size of fast neutron deletions (15,16,(18)(19)(20). However, there is a significant probability that some small and 5Ј ORF deletions may not be detectable by ATH1 microarraybased hybridizations if these small deletions do not fall within the probe sets.…”

Ubiquitination-mediated protein degradation and modification: an emerging theme in plant-microbe interactions

“…Studies from several groups indicated that SGT1 is required by a diverse range of R genes from different plant species in response to a variety of pathogens [61,[92][93][94][95]. In yeast, SGT1 is a key regulatory protein in centromere function and cell cycle transition and is associated with SKP1 to regulate the activity of the SCF E3 ligase complex [96].…”

Molecular chaperons and co-chaperons, Hsp90, RAR1, and SGT1 negatively regulate bacterial wilt disease caused by Ralstonia solanacearum in Nicotiana benthamiana

“…[17][18][19][20] Recent study showed that pepper SGT1 act as a host interactor of AvrBsT during the HR induction. 21 Our present result showed that acceleration of bacterial wilt symptom was observed in Hsp70-silenced plant, but bacterial manipulation was not affected (Fig.…”