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Analysis of salicylic acid-dependent pathways in Arabidopsis thaliana following infection with Plasmodiophora brassicae and the influence of salicylic acid on disease

Salicylic acid (SA) biosynthesis, expression of SA-related genes and the effect of SA on the Arabidopsis-Plasmodiophora brassicae interaction have been examined. Biochemical analyses revealed that in P. brassicae-infected Arabidopsis the majority of SA is synthesised from chorismate. Real-time monitored expression of a gene for isochorismate synthase was induced upon infection. SA can be modified after accumulation either by methylation, improving its mobility, or by glycosylation as one possible reaction for inactivation. qRT-PCR confirmed induction of a SA-methyltransferase gene, whereas SA glucosyltransferase expression was not changed after infection. Col-0 wild type (wt) did not provide a visible phenotypic resistance response, while the Arabidopsis mutant dnd1 that has constitutively activated immune system, had reduced gall scores. As dnd1 showed control of the pathogen, exogenous SA was applied to Arabidopsis in order to test if it could suppress clubroot. In wt, sid2 (SA biosynthesis), NahG (SA-deficient) and npr1 (SA-signaling impaired) mutants SA treatment could not alter the gall score, however positively affected shoot weightThis suggests that SA alone is not sufficient for Arabidopsis resistance against P. brassicae. Semi-quantitative PCR revealed that wt, cpr1, dnd1 and sid2 had elevated PR-1 expression upon P. brassicae and SA+P. brassicae-inoculation at 2 wpi and 3 wpi, while NahG and npr1 showed no expression. This work contributes to the understanding of the SA involvement in Arabidopsis-P. brassicae interaction.

Arabidopsis thaliana; defence gene expression; Plasmodiophora brassicae; salicylic acid biosynthesis; salicylic acid mutants

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