Homeologs of the Nicotiana benthamiana Antiviral ARGONAUTE1 Show Different Susceptibilities to microRNA168-Mediated Control

The plant Argonaute 1 protein (AGO1) is a central functional component of the posttranscriptional regulation of gene expression and the RNA silencing based antiviral defence..By genomic and molecular approaches we here reveal the presence of two homeologs of the AGO1-like gene in Nicotiana benthamiana, NbAGO1-1H and NbAGO1-1L. Both homeologs retain the capacity to transcribe mRNAs, which mainly differ in one 18-nucleotides insertion/deletion (indel). The indel does not modify the frame of the open reading frame, and it is located 8 nucleotides upstream of the target site of a micro RNA, miR168, which is an important modulator of AGO1 expression. We demonstrate that there is a differential accumulation of the two NbAGO1-1 homeolog mRNAs at conditions where miR168 is up-regulated such as during a Tombusvirus infection. The data reported suggest that the indel affects the miR168-guided regulation of NbAGO1 mRNA. The two AGO1 homeologs show full functionality in reconstituted, catalytically active RNA induced silencing complexes following the incorporation of siRNAs. Virus-Induced Gene Silencing experiments suggest a specific involvement of the NbAGO1 homeologs in symptom development. The results provide an example of the diversity of miRNA target regions in NbAGO1 homeolog genes, which has important implications for improving resilience measures of the plant during viral infections.