Beet soil-borne mosaic virus RNA4 encodes the p32 protein, essential for virus transmission.

Beet soil-borne mosaic virus (BSBMV) belongs to the Benyvirus genus together with Beet necrotic yellow vein virus (BNYVV), the causal agent of Rhizomania disease. Both viruses share similar genomic organization and are vectored by Polymyxa betae. Ability of BNYVV helper strain to replicate and encapsidate BSBMV RNA-3 has been recently described suggesting a common and conserved viral RNA selection mechanism for both viruses. Moreover a 1,730 nts long form of BSBMV RNA-4 can substitute for BNYVV RNA-4 for an efficient transmission to Beta vulgaris plants by the vector P. betae. Two putative ORFs have been identified in BSBMV RNA-4 that could code for a 32 kDa (p32) and a 13 kDa protein. We investigated the role of these proteins in plant-virus-vector interaction using reverse genetics approaches. Expression of BSBMV p32 using a BNYVV RNA-5- based viral vector (Rep5) complemented a RNA-4 defective strain and thus demonstrating for the first time that p32 but not full-length RNA-4 is essential for benyvirus transmission. Mutated, deleted and FLAG- or GFP-tagged sequences of p32 have been expressed in Chenopodium quinoa and Beta macrocarpa. A necrotic local lesions phenotype was associated specifically to protein expression in mechanically inoculated C. quinoa plants. Western blot analyses using FLAG-specific antibodies revealed a high molecular weight protein that suggests either a strong interaction of p32 with host protein(s) or post translational modifications. Finally yeast two hybrid system was employed to search for a p32 viral partner among other viral proteins.