Turnip mosaic virus pathogenesis and host resistance mechanisms in Brassica

Turnip mosaic virus(TuMV)is a devastating potyvirus pathogen that infects a wide variety of both cultivated and wild Brassicaceae plants.We urgently need more information and understanding of TuMV pathogenesis and the host responses involved in disease development in cruciferous crops.TuMV displays great versatility in viral pathogenesis,especially in its replication and intercellular movement.Moreover,in the coevolutionary arms races between TuMV and its hosts,the virus has evolved to co-opt host factors to facilitate its infection and counter host defense responses.This review mainly focuses on recent advances in understanding the viral factors that contribute to the TuMV infection cycle and the host resistance mechanism in Brassica.Finally,we propose some future research directions on TuMV pathogenesis and control strategies to design durable TuMV-resistant Brassica crops.

Turnip mosaic virus manipulates DRM2 expression to regulate host CHH and CHG methylation for robust infection

DNA methylation is an important epigenetic marker for the suppression of transposable elements(TEs)and the regu-lation of plant immunity.However,little is known how RNA viruses counter defense such antiviral machinery.In this study,the change of DNA methylation in turnip mosaic virus(TuMV)-infected cells was analyzed by whole genome bisulfite sequencing.Results showed that the total number of methylated sites of CHH and CHG increased in TuMV-infected cells,the majority of differentially methylated regions(DMRs)in the CHH and CHG contexts were associated with hypermethylation.Gene expression analysis showed that the expression of two methylases(DRM2 and CMT3)and three demethylases(ROS3,DML2,DML3)was significantly increased and decreased in TuMV-infected cells,respec-tively.Pathogenicity tests showed that the enhanced resistance to TuMV of the loss-of-function mutant of DRM2 is associated with unregulated expression of several defense-related genes.Finally,we found TuMV-encoded NIb,the viral RNA-dependent RNA polymerase,was able to induce the expression of DRM2.In conclusion,this study discov-ered that TuMV can modulate host DNA methylation by regulating the expression of DRM2 to promote virus infection.

Plant and animal positive-sense single-stranded RNA viruses encode small proteins important for viral infection in their negative-sense strand

Positive-sense single-stranded RNA(+ssRNA)viruses,the most abundant viruses of eukaryotes in nature,require the synthesis of negative-sense RNA(-RNA)using their genomic(positive-sense)RNA(+RNA)as a template for replication.Based on current evidence,viral proteins are translated via viral+RNAs,whereas-RNA is considered to be a viral replication intermediate without coding capacity.Here,we report that plant and animal+ssRNA viruses contain small open reading frames(ORFs)in their-RNA(reverse ORFs[rORFs]).Using turnip mosaic virus(TuMV)as a model for plant+ssRNA viruses,we demonstrate that small proteins encoded by rORFs display specific subcellularlocalizations,and confirm the presence of rORF2 in infected cells through mass spectrometry analysis.The protein encoded by TuMV rORF2 forms punctuate granules that are localized in the perinuclear region and co-localized with viral replication complexes.The rORF2 protein can directly interact with the viral RNA-dependent RNA polymerase,and mutation of rORF2 completely abolishes virus infection,whereas ectopic expression of rORF2 rescues the mutant virus.Furthermore,we show that several rORFs in the-RNA of severe acute respiratory syndrome coronavirus 2(SARS-CoV-2)have the ability to suppress type l interferon production and facilitate the infection of ve-sicular stomatitis virus.In addition,we provide evidence that TuMV might utilize internal ribosome entry sites to translate these small rORFs.Taken together,these findings indicate that the-RNA of+ssRNA vi-ruses can also have the coding capacity and that small proteins encoded therein play critical roles in viral infection,revealing a viral proteome larger than previously thought.