Love, Andrew J , Geri, Chiara , Laird, Janet , Yun, Byung W , Loake, Gary , Sadanandom, Ari , Milner, Joel J .
A Novel Defence Suppressor Protein Encoded by Cauliflower Mosaic Virus Inhibits SA-Dependent Defence Responses in Arabidopsis and Alters the Localization of NPR1.
CaMV stimulates SA-dependent responses in pre-invasion tissues, but these are down-regulated in tissues undergoing invasion. Here we show that protein P6, the main virus-encoded pathogenicity determinant, plays a key role in suppressing SA-mediated defence. In transgenic plants expressing P6, PR-1, BGL2 & AOX1A were profoundly unresponsive to SA-treatment, implying that P6 blocks signaling downstream of SA. We also found that following inoculation, virulent and avirulent Pseudomonas syringae accumulated to levels in P6 transgenics that were 20-fold higher than in controls and HR was delayed. P6 transgenics contained reduced levels of SA, but responsiveness to infection by avirulent P. syringae was unaffected. Surprisingly, levels of NPR1 transcripts were elevated by approximately 6-fold in P6 transgenics compared to controls. To test whether P6 might be affecting the NPR1 protein, we crossed the 35S::P6 lines with a line expressing an NPR1:GFP fusion protein. As expected, in the absence of the P6 transgene, GFP was localized in the cytoplasm in untreated leaves, but migrated to the nucleus following infiltration with SA. However, in plants also containing the P6 transgene, GFP was always confined to the nucleus even in the absence of SA. We have used western blots and immune pulldowns to further investigate the localization of NPR1:GFP in wild-type and P6-transgenic backgrounds. We find that in the presence of P6 the apparent size of the fusion protein and its nuclear-cytoplasmic localization are altered. The discovery that a pathogen-encoded virulence factor suppresses defence by affecting NPR1 is completely novel. This may provide a mechanism for defence suppression by CaMV.
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Dr Joel Milner research group
1 - Glasgow University, Biochemistry and Molecular Biology, Bower Building, University Avenue, Glasgow, Scotland, G128QQ, UK
2 - CNR, Pisa, Italy, IBBA
3 - Glasgow University, Biochemistry and Molecular Biology
4 - Edinburgh University, Institute of Molecular Plant Sciences
Presentation Type: Plant Biology Abstract
Location: Exhibit Hall (Northeast, Southwest & Southeast)/Hilton
Date: Sunday, July 8th, 2007
Time: 8:00 AM