Cucumber Mosaic Virus Infection in Arabidopsis: A Conditional Mutualistic Symbiont?

Abstract

A cucumber mosaic virus isolate, named Ho [CMV(Ho)], was isolated from a symptomless <i>Arabidopsis halleri</i> field sample containing low virus titers. An analysis of CMV(Ho) RNA molecules indicated that the virus isolate, besides the usual cucumovirus tripartite RNA genome, additionally contained defective RNA3 molecules and a satellite RNA. To study the underlying mechanism of the persistent CMV(Ho) infection in perennial <i>A. halleri</i>, infectious cDNA clones were generated for all its genetic elements. CMV, which consists of synthetic transcripts from the infectious tripartite RNA genomes, and designated CMV(Ho)tr, multiplied in <i>A. halleri</i> and annual <i>Arabidopsis thaliana</i> Col-0 to a similar level as the virulent strain CMV(Y), but did not induce any symptoms in them. The response of Col-0 to a series of reassortant CMVs between CMV(Ho)tr and CMV(Y) suggested that the establishment of an asymptomatic phenotype of CMV(Ho) infection was due to the <i>2b</i> gene of CMV RNA2, but not due to the presence of the defective RNA3 and satellite RNA. The accumulation of CMV(Ho) 2b protein tagged with the FLAG epitope (2b.Ho-FLAG) in <i>2b.Ho-FLAG</i>-transformed Col-0 did not induce any symptoms, suggesting a 2b-dependent persistency of CMV(Ho)tr infection in <i>Arabidopsis</i>. The 2b protein interacted with Argonaute 4, which is known to regulate the cytosine methylation levels of host genomic DNA. Whole genomic bisulfite sequencing analysis of CMV(Ho)tr- and mock-inoculated Col-0 revealed that cytosine hypomethylation in the promoter regions of 82 genes, including two genes encoding transcriptional regulators (<i>DOF1.7</i> and <i>CBP1</i>), was induced in response to CMV(Ho)tr infection. Moreover, the increased levels of hypomethylation in the promoter region of both genes, during CMV(Ho)tr infection, were correlated with the up- or down-regulation of their expression. Taken altogether, the results indicate that during persistent CMV(Ho) infection in <i>Arabidopsis</i>, host gene expression may be epigenetically modulated resulting from a 2b-mediated cytosine hypomethylation of host genomic DNA.