Résumé
Banana streak virus (BSV), the causative agent of banana streak disease, is a plant pararetrovirus belonging to the family Caulimoviridae, genus Badnavirus. The genome of BSV is a circular double ? stranded DNA of 7.4 kbp made of three ORFs and like other pararetroviruses replicates via reverse transcription of viral pregenomic RNA (Lockhart, 1990). In the mid-ninetys Ndowora et al. (1999), revealed the presence of multiple integrations of Banana streak virus in the genome of banana (Musa sp.) whereas this badnavirus does not require integration for its replication. Some endogenous BSV (eBSV) sequences in the Musa balbisiana genome are infectious by releasing a functional viral genome following stresses such as those existing in in vitro culture and interspecific crosses context. Seedy M. balbisiana diploid genotypes (BB) such as Pisang Klutuk Wulung (PKW) harbour such infectious eBSV belonging to three widespread species of BSV (Goldfinger -BSGFV, Imové - BSImV and Obino l'Ewai - BSOLV) but are nevertheless resistant to any multiplication of BSV whatever the viral origin. We postulated this resistance is mediated by gene silencing and performed deep sequencing of total small RNAs of PKW using the Illumina ultra-high-throughput technology. We obtained for the first time, experimental evidence of virus-derived small RNA (vsRNA) from eBSOLV, eBSGFV and eBSImV by blasting small RNAs against the viral genome counterparts. VsRNAs are systematically enriched in 24-nt class. Interestingly, we showed that hot and cold spots of vsRNA generation do not target similar viral sequences but are directly correlated with the structure of each integration. Finally, preliminary data using Southern blotting coupled with methylation-dependent restriction enzyme (McrBC) digestion indicated that each eBSV is heavily methylated. All together, our findings indicate that eBSVs in PKW are likely silenced at the transcriptional level. In parallel, we deep sequenced small RNAs from Cavendish banana plants (lacking eBSV integrations) infected independently by BSOLV, BSGFV, BSImV and 3 other distinct BSV species in order to evaluate silencing-ased regulation of episomal virus infection. Abundunt vsRNAs were found to be enriched in 21-nt class and most of the hot spots of siRNA production are located in the coding sequences. Our methylation analysis indicated that episomal BSV DNA is not methylated and therefore, is likely silenced only at the post-transcriptional level. (Résumé d'auteur)