Victoria blight, caused by fungus Cochliobolus victoriae, is a disease originally described on oats and recapitulated on Arabidopsis. Victoria blight is used as a model plant disease that conforms to an inverse gene-for-gene interaction. C. victoriae virulence is dependent upon its production of victorin, a host-specific toxin that induces programmed cell death in sensitive plants. In oats, victorin sensitivity and disease susceptibility is conferred by the Vb gene, which is genetically inseparable from the Pc-2 crown rust resistance gene. In Arabidopsis, victorin sensitivity and disease susceptibility is conferred by the LOCUS ORCHESTRATING VICTORIN EFFECTS 1 (LOV1) gene which encodes a NB-LRR protein, a type of protein commonly associated with disease resistance. LOV1-mediated cell death occurs when victorin binds Thioredoxin-h5 (TRX-h5) and LOV1 appears to "guards" TRX-h5. Together, these results suggest C. victoriae causes disease by inducing a resistance response.
The work presented here aimed to determine if the response mediated by LOV1 is functionally related to a resistance response. We genetically characterized the response mediated by LOV1 with virus-induced gene silencing. We determined SUPPRESSOR OF THE G2 ALLELE OF SKP1 (SGT1), a gene required for the function of many resistance genes, is required for victorin sensitivity and involved in LOV1 protein accumulation. We screened a normalized library and identified six genes that suppressed victorin-mediated cell death and cell death induced by expression of the RESISTANCE TO PERONOSPORA PARASITICA PROTEIN 8 (RPP8) resistance gene: a mitochondrial phosphate transporter, glycolate oxidase, glutamine synthetase, glyceraldehyde 3-phosphate dehydrogenase and the P- and T-protein of the glycine decarboxylase complex. Silencing the latter four also inhibited cell death induced by the expression of an autoactive form of the resistance gene PTO, and reduced PTO-mediated resistance to Pseudomonas syringae pv. tabaci. These results provide evidence that victorin-mediated cell death is functionally similar to a resistance response, further supporting the hypothesis that a resistance response is exploited by C. victoriae for pathogenesis in Victoria blight.
Resistance function of LOV1 was evaluated by observing Pseudomonas syringae pv. tomato virulence upon LOV1 activation. The LOV1 response pathway in Arabidopsis was adapted to activate upon infection with Pseudomonas syringae pv. tomato expressing the type III-dependent effector protein AvrRpt2, a well-characterized protease. We developed a construct to express a beta-glucuronidase (GUS) and TRX-h5 fusion protein separated by an AvrRpt2 proteolytic cleavage site, in which GUS sterically inhibits TRX-h5 function in LOV1-mediated cell death. The fusion is cleaved upon infection by P. syringae pv. tomato expressing avrRpt2, thereby leading to TRX-h5-mediated activation of LOV1 in the presence of victorin. However, when this strain was inoculated with victorin into transgenic LOV1 trx-h5 plants expressing the GUS/TRX-h5 fusion protein, no decrease in pathogen virulence was observed. Technical shortcomings likely prevented observable LOV1 resistance function.
��� / Graduation date: 2013 / Access restricted to the OSU Community at author's request from Oct. 9, 2012 - Oct. 9, 2013
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/34284 |
| Date | 09 October 2013 |
| Creators | Gilbert, Brian M. |
| Contributors | Wolpert, Thomas |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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