title: Host responses to infection by Cauliflower mosaic virus
Creators: name: Love, Andrew J.
Publisher: name: University of Glasgow
Year: 2002

description: The role of soluble sugars in the development of symptoms in compatible virus infection was examined in turnip and several Arabidopsis thaliana lines infected with Cauliflower mosaic virus CaMV. In turnip (Brassica campestris cv. Just Right) infected with four CaMV isolates that induced symptoms of differing severity, there were changes in levels of free soluble sugars in infected plants, but the changes showed no direct correlation with either the level of virus replication or symptom severity. This implies that in CaMV-infected turnip, changes in sugar levels probably do not act directly as elicitors of symptom development. CaMV infection in wildtype and putative Arabidopsis sugar signalling mutants had little effect on levels of free soluble sugar during symptom development. Again this observation does not support a role for changes in sugar levels in potentiating the development of symptoms. However on infection, the mutants developed a range of unusual symptom phenotypes and showed differences in level of virus accumulation, when compared with wildtype plants. This suggested a possible link between sugar signalling pathways and symptom development and viral pathogenesis. Soluble sugars have been reported to act as signalling molecules in defence responses. To test this the expression of three PR protein genes, which are reported to be modulatable by sugars and the SA defence pathway, were analysed in infected Arabidopsis. In infected plants, the expression of these genes was greatly elevated, whereas uninfected plants had undetectable levels of expression. Since sugar levels were unaffected during virus infection this suggests that sugars are probably not involved in the invocation of PR defence genes. The temporal and spatial activation of three defence pathways was investigated using defence marker gene promoter::Luciferase reporter plants. Ambidopsis thaliana GST1::Luciferase was used in the analysis of the oxidative burst, Arabidopsis thaliana PDF1.2::Luciferase for the JA/ethylene pathway and tobacco PR-1::Luciferase for the SA pathway. Systemic activation of the JA/ethylene pathway and oxidative burst was detectable two hours after virus inoculation, and continued until 5 dpi. In contrast activation of the SA-mediated defence pathway was first detected 8 days after infection, continuing strongly until at least 19 dpi. The timing of the SA pathway activation coincided with a second burst of increased AtGST1:;Luciferase. However plants stained for H2O2, a product of the oxidative burst, demonstrated that the oxidative burst occurred only in the very early stages of virus infection (3.5 hours- 4 days after inoculation). This suggested that the second phase of G5T7::Luciferase activity is perhaps regulated by other pathways, in the absence of the oxidative burst. The invocation of defence responses during compatible interactions has often been overlooked since the pathogen appears to replicate and move freely in the plant. These results provide strong evidence for a co-ordinated activation of defence pathways in response to infection by a compatible virus pathogen.