AUTHOR=Takahashi Hideki , Tabara Midori , Miyashita Shuhei , Ando Sugihiro , Kawano Shuichi , Kanayama Yoshinori , Fukuhara Toshiyuki , Kormelink Richard 

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

JOURNAL=Frontiers in Microbiology

VOLUME=12

YEAR=2022

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2021.770925

DOI=10.3389/fmicb.2021.770925

ISSN=1664-302X

ABSTRACT=<p>A cucumber mosaic virus isolate, named Ho [CMV(Ho)], was isolated from a symptomless <italic>Arabidopsis halleri</italic> 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 <italic>A. halleri</italic>, 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 <italic>A. halleri</italic> and annual <italic>Arabidopsis thaliana</italic> 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 <italic>2b</italic> 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 <italic>2b.Ho-FLAG</italic>-transformed Col-0 did not induce any symptoms, suggesting a 2b-dependent persistency of CMV(Ho)tr infection in <italic>Arabidopsis</italic>. 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 (<italic>DOF1.7</italic> and <italic>CBP1</italic>), 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 <italic>Arabidopsis</italic>, host gene expression may be epigenetically modulated resulting from a 2b-mediated cytosine hypomethylation of host genomic DNA.</p>