Induction of retrotransposons during Cucumber necrosis virus (CNV) infection in Nicotiana benthamiana.
Citation
Reade, R., Alam, B., Theilmann, J. and Rochon, D. (2017) Induction of retrotransposons during Cucumber necrosis virus (CNV) infection in Nicotiana benthamiana. British Columbia Regional Meeting of the Canadian Phytopathology Society Summerland, BC 2016/10/27 - 2016/10/28. Abstract, oral presentation
Résumé en langage clair
Plant viruses are generally perceived as being injurious to their hosts as they are known to cause disease and agronomic loss. We have found that Cucumber necrosis virus (CNV) along with 4 of 5 other plant viruses induces the expression of retrotransposons (RTns) in infected plant cells. RTns are mobile genetic elements that are normally dormant but are capable of moving from one genomic location to another in a host. In doing so, the RTn may disrupt gene function or alter the expression of genes adjacent to the insertion site. Thus RTns, once induced, can alter the plant gene expression profile and thus its ability to function and adapt to environmental change. Such an effect can either have a net neutral, positive or negative effect. The RTns induced by CNV were found to be related to heat inducible RTns. These RTns have elements that are responsive to heat stress. Thus, if these CNV induced RTns insert into new genomic locations they can potentially cause alterations in the way the way the infect plant and possibly its progeny responds to heat stress.
Résumé
Retrotransposons are mobile genetic elements that transpose via an RNA intermediate. Most retrotransposons are dormant but stress induces their transposition. Virus infection has not yet been identified as a major stressor for retrotransposition in either plants or animals. RNA-seq analysis of N. benthamiana infected with the Tombusvirus CNV revealed that transcript levels of specific LTR-type retrotransposons are dramatically elevated. A similar phenomenon was observed for 3 other plant viruses. Phylogenetic analyses indicated that several CNV-induced retrotransposons are closely related to the retrotransposon ONSEN, previously described to be induced by heat stress. ONSEN possesses heat shock elements (HSEs) in the LTR promoter that are activated by heat shock factors (HSFs). Analysis of CNV-induced retrotransposons indicated that most contain canonical HSE motifs. CNV infection induces HSE-containing HSP70 mRNA as well as HSF mRNA leading to the hypothesis that CNV infection induces transcription of retrotransposons via activation of their HSEs by HSFs. N. benthamiana plants were heat-shocked and tested for induction of those retrotransposons induced by CNV using ddPCR. Each of the 5 tested retrotransposons was found to be heat-induced and their relative induction levels paralleled that observed during CNV infection. To ascertain the involvement of HSEs, the LTRs of 2 retrotransposons were fused to GFP coding sequence and used to agro-infiltrate N. benthamiana. Both heat shock and CNV infection resulted in significant induction of GFP suggesting a plausible mechanism for induction of retrotransposons by CNV. Since retrotransposition can induce mutations in nuclear genes, virus infection may alter evolution of plant genomes.