Differential expression of cucumber RNA-dependent RNA polymerase 1 genes during antiviral defence and resistance

RNA-dependent RNA polymerase 1 (RDR1) plays a crucial role in plant defence against viruses. In this study, it was observed that cucumber, Cucumis sativus, uniquely encodes a small gene family of four RDR1 genes. The cucumber RDR1 genes (CsRDR1a, CsRDR1b and duplicated CsRDR1c1/c2) shared 55%-60% homology in their encoded amino acid sequences. In healthy cucumber plants, RDR1a and RDR1b transcripts were expressed at higher levels than transcripts of RDR1c1/c2, which were barely detectable. The expression of all four CsRDR1 genes was induced by virus infection, after which the expression level of CsRDR1b increased 10-20-fold in several virus-resistant cucumber cultivars and in a broad virus-resistant transgenic cucumber line expressing a high level of transgene small RNAs, all without alteration in salicylic acid (SA) levels. By comparison, CsRDR1c1/c2 genes were highly induced (25-1300-fold) in susceptible cucumber cultivars infected with RNA or DNA viruses. Inhibition of RDR1c1/c2 expression led to increased virus accumulation. Ectopic application of SA induced the expression of cucumber RDR1a, RDR1b and RDRc1/c2 genes. A constitutive high level of RDR1b gene expression independent of SA was found to be associated with broad virus resistance. These findings show that multiple RDR1 genes are involved in virus resistance in cucumber and are regulated in a coordinated fashion with different expression profiles.