Risk assessment of spreading Banana streak virus (BSV) through in vitro culture

Background: In vitro multiplication is one of the main abiotic stresses triggering the production of episomal infectious particles of Banana streak virus (BSV) in inter-specific banana hybrids, through the activation of BSV endogenous pararetrovirus (EPRV) sequences integrated into the Musa balbisiana genome (noted B). Nevertheless, mass production of vitroplantlets remains the most widely used method for diffusing wild Musa cultivars or new improved hybrid species. Therefore, there is a need to evaluate the effects of in vitro culture on the activation of BSV EPRVs and to assess the risk of spreading BSV through the diffusion of micropropagated banana plants. It is of particular relevance to check (i) whether BSV EPRV activation occurs through in vitro culture in all inter-specific hybrid species and wild edible cultivars and (ii) whether a correlation exists between the duration of in vitro subculture steps and the percentage of plantlets exhibiting BSV episomal particles. Methods: Virus-free suckers from two natural triploid plantains (AAB) - Kelong Mekintu (KM) and Black Penkelon (PK)- and the tetraploid hybrid (AAAB) - CRBP 39 were mass propagated using standard in vitro budding methods. During the successive multiplication subcultures, at least 40 shoots were randomly picked and screened for the presence of episomal BSV particles. Results: BSV episomal particles were detected during in vitro culture in both natural plantains and CRBP39 hybrid, with BSV-OI being the predominantly detected BSV strain. Both natural plantains and CRBP39 displayed similar patterns of activation. Percentages of plantlets indexed positive for BSV-OI rapidly increased after the first subculture cycles. Depending on cultivars, maximum percentages of BSV-OI positive plantlets ranged between 10% and 20% and were reached for TPS (total produced shoots) values comprised between 800 and 2000. Following this increase step, a steady state phase was observed. Then the percentage of BSV-0I positive plantlets decreased for the three cultivars studied when increasing the number of subcultures. This was especially striking for CRBP 39 hybrid, for which values of zero were reached from TPS values of 4000 onwards. These results will be presented and their impact on in vitro mass propagation and diffusion of Musa germplasm will be discussed. (Texte intégral