Reassortment of Beet necrotic yellow vein virus and Beet soil-borne mosaic virus RNAs-1 and -2 indicates a specific interaction of silencing suppressor proteins and RNA-1

The process of recombination that takes place in RNA viruses corresponds to the formation of chimeric molecules from parental genomes of mixed origin. This process can occur either within a single genomic segment (referred to as RNA recombination) or as exchange of entire genomic segments between multipartite viruses (referred to as reassortment). Both RNA recombination and reassortment require that two or more viruses infect the same host cell. Multipartite genome, formed by four ssRNAs(+), transmission through the plasmodiophorid Polymyxa betae, host range and genome organization are proprieties shared by the two Benyviruses Beet necrotic yellow vein virus (BNYVV) and Beet soil-borne mosaic virus (BSBMV). Recent studies demonstrated amplification and transmission of BSBMV RNA-3 and -4 by BNYVV helper strain. Moreover, in the United States of America, both benyviruses are frequently present in the same cultivated field, infecting the same plant but no natural chimeric forms have been described, from field isolates, so far. The possibility that BNYVV/BSBMV chimeras may be generated has been investigated. Chenopodium quinoa local infection has been carried out using in vitro infectious transcripts of BNYVV and BSBMV RNA-1 and -2 and the behavior of BSBMV/BNYVV chimeras and wild type helper strains has been compared. The wild type combinations Stras12 (BNYVV RNA-1 + -2) and Bo12 (BSBMV RNA-1 + -2) together with the chimera StrasBo12 (BNYVV RNA-1 + BSBMV RNA-2) showed typical chlorotic lesions, while the chimera BoStras12 (BSBMV RNA-1 + BNYVV RNA-2) induced severe necrotic lesions on the leaves, probably due to a hypersensitive response of the plant. The necrosis disappeared when the plant was co-inoculated with BoStras12 together with a viral replicon expressing BSBMV p14, which acts as a suppressor of post-transcriptional gene silencing, but not when BNYVV p14 was employed. Necrotic lesions arose even in N. benthamiana plants agroinfected with BoStras12, both in the agroinfiltrated and not infiltrated leaves. These results evidenced a putative specific interaction between BSBMV p14 and RNA-1, or one of its expressed components, of the virus that requires further investigations and may explain why benyviruses chimeras did not arise in nature so far.