75 Integrating Membrane Transport with Male Gametophyte ... - TAIR

295 A Search for Transcriptional Regulators of Disease Resistance
Natalie Spivey, Dong Wang, Xinnian Dong
Duke University
In response to pathogen attack, the plant activates several different signaling pathways and initiates changes in gene
expression. While some of the transcriptional changes are well characterized and used as markers for the defense response,
questions remain about the regulation of the transcriptional activation and repression events induced by infection. Using
microarray technology, we sought to characterize the changes in gene expression that resulted from infection with the
bacterial pathogen Pseudomonas syringae. At five different time points, we infected half of an Arabidopsis leaf with either
virulent P. syringae or avirulent P. syringae/avrRpt2. The uninfected half of the leaf was then collected and prepared for
microarray analysis using the Affymetrix GeneChip Arabidopsis ATH1 Genome Array. Using this approach, we were
able to compare each gene’s expression pattern over time. We are currently using these data to search for transcription
factors that regulate the expression changes that result from pathogen infection. We are also looking for potential cisacting
regulatory elements in the promoter regions of genes whose expression pattern is highly correlated.
296 Arabidopsis Virulence of Xanthomonas campestris pv. campestris Mutants Defective in
Secretion Systems
Wenxian Sun, Muthu Venkateshwaran, Andrew Bent
Department of Plant Pathology, University of Wisconsin-Madison, Madison, WI, USA
The bacterial plant pathogen Xanthomonas campestris pv. campestris causes disease in crucifer plants including
Arabidopsis thaliana. We recently showed that Xcc strains that make a flagellin detectable by the Arabidopsis FLS2
RLK nevertheless caused similar disease on FLS2+ and fls2-/fls2- plants (Sun et al., 2006. Plant Cell 18:491). In
Pseudomonas syringae pv. tomato DC3000, both type III secretion and twin-arginine translocation systems were shown
to contribute to virulence and fitness. Xanthomonas campestris pv. vesicatoria virulence was attenuated when the bacteria
were defective in pilus secretion. Here, we explored the contribution of the type III secretion system (TTSS) and twinarginine
translocation (TAT) system to Xcc virulence and fitness using TTSS- and TAT-defective mutants. HrcC and
hrpE genes encode central key components of the type III secretion apparatus. HrcC and hrpE gene deletions caused
different phenotypes in different Xcc strains. Xcc B186 strain virulence on Arabidopsis was greatly attenuated when
TTSS was knocked out while Xcc B305 ΔhrcC strains had similar virulence ability to the isogenic wildtype B305 strain
based on bacterial numbers in infected plants after vacuum infiltration or hydathode inoculation. Xcc mutants defective
in TAT system exhibited similar pleiotropic phenotypes to a Pst DC3000 tatC knockout strain, such as hypersensitivity
to copper. However, unlike Pst DC3000, the twin-arginine translocation system played a trivial role in Xcc virulence, as
tested using two different inoculation approaches. Based on these data, it is speculated that other secretion systems are
likely to exist in Xcc bacteria that contribute significantly to virulence. Mutants defective in other secretion systems are
being made to test our hypotheses. Interestingly, TTSS mutants will also be used to test the function of TTSS virulence
effectors in the suppression of PAMP-induced plant basal defenses.