Pathogenesis-related genes as tools for discovering the response of onion defence system against Iris yellow spot virus infection

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Abstract
The Iris yellow spot virus (IYSV) is a viral pathogen of onions and causes severe damage and economic loss in affected onion crops. Pathogenesis-related (PR) genes are part of the innate immune response that onions harbour against viral diseases, which to date remains unclear. Using a sensitive and reliable real-time quantitative PCR method, the dynamic expression of five different genes in infected onions were investigated after biological inoculation with virulent isolate of Thrips tabaci (Lindeman). The transcription levels of PR1, PR2, PR3, PR4, and PR5 genes were highly expressed 1 day post inoculation (dpi). Furthermore, statistical analysis revealed a significant change in peak expression levels of PR1 after 8 dpi and PR3 after 9 dpi. In contrast, the expression level change for the other genes was only moderate. Further, we determined and ranked the expression stability of three reference genes (EF1-α, 18S rRNA, and β-actin) using geNorm and NormFinder. The overall analysis demonstrated that β-actin is the most informative gene, which can be utilised as an internal control for quantitative gene expression. Our study findings not only provide guidelines for selection of appropriate reference genes in virally infected onions, but also valuable information concerning immune response related genes associated with the initial plant defences against viral infection. Moreover, the PR1 gene appears to be a disease-specific gene related to the IYSV infection in onion tissues.

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