Tobamovirus infection is independent of HSP101 mRNA induction and protein expression

Heat shock protein 101 (HSP101) has been implicated in tobamovirus infections by virtue of its ability to enhance translation of mRNAs possessing the 5′ Ω-leader of Tobacco mosaic virus (TMV). Enhanced translation is mediated by HSP101 binding to a CAA-repeat motif in TMV Ω leader. CAA repeat sequences are present in the 5′ leaders of other tobamoviruses including Oilseed rape mosaic virus (ORMV), which infects Arabidopsis thaliana. HSP101 is one of eight HSP100 gene family members encoded by the A. thaliana genome, and of these, HSP101 and HSP98.7 are predicted to encode proteins localized to the cytoplasm where they could potentially interact with TMV RNA. Analysis of the expression of the HSP100s showed that only HSP101 mRNA transcripts were induced significantly by ORMV in A. thaliana. The induction of HSP101 mRNA was also correlated with an increase in its protein levels and was independent of defense-related signaling pathways involving salicylic acid, jasmonic acid, or ethylene. A. thaliana mutants lacking HSP101, HSP98.7, or both supported wild-type levels of ORMV replication and movement. Similar results were obtained for TMV infection in Nicotiana benthamiana plants silenced for HSP101, demonstrating that HSP101 is not necessary for efficient tobamovirus infection.