Deciphering the pathology of Beet soil-borne virus

Beet soil-borne virus (BSBV) is a pathogenic agent of sugar beets widely spread in all producer countries worldwide. This Pomovirus is transmitted by the soil-borne plasmodiophorid Polymyxa betae Keskin. Interestingly, BSBV has frequently been co-detected in sugar beet roots together with the causal agent of rhizomania, Beet necrotic yellow vein virus (BNYVV). However, the pathogenicity of BSBV and its potential contribution to the development of rhizomania symptoms in fields remain controversial among the scientific community. To meet this challenge, agroclones offer a powerful tool in the field of plant virology. The purpose of this work is to shed light on the pathology of BSBV along three lines of research: its impact on economically important sugar beets, its interactions with other beet viruses and its transmission by P. betae. In this study, we make use of agroclones harbouring an Iranian BSBV isolate. Considering the results, inoculations of sugar beets by means of BSBV-agroinfected N. benthamiana leaves failed to develop efficient viral infections. However, our findings suggest that a potential impact of BSBV on sugar beet shouldn't be excluded. Therefore, we put forward some alternatives to inoculate sugar beets using agroclones as a viral source. By contrast, we evidenced the infection of BSBV inside the model plant N. benthamiana through the suppression of RNA silencing defence mechanism of the plant. This extends the range of potential hosts for further BSBV study. Taking advantage of this new model of study, co-infection bioassays were conducted to investigate virus-virus interactions. However, neither Beet black scorch virus (BBSV) nor Beet soil-borne mosaic virus (BSBMV) appeared to enhance BSBV infection, or at least not sufficiently to be detected in N. benthamiana leaves. This raises new issues and perspectives for comparing and studying the interactions between beet viruses. Finally, transmission tests require further improvements of the protocol, but set the stage for a new technique to study the transmission of BSBV by its vector P. betae by means of agroclones. Indeed, the reconstitution of its entire viral cycle shows great interest to study virus-vector interactions considering the small genome size of BSBV. This work validates the BSBV agroclone tools developed by Dr. M. Mahillon in several hosts and explores some of the possibilities they open for the study of BSBV.