Transmission mechanism of Benyvirus (BNYVV) by the fungus Polymyxa betae

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Beet necrotic yellow vein virus (BNYVV, Benyvirus) causes rhizomania disease of sugar beet and is transmitted by Polymyxa betae. BNYVV has five RNA species (RNA1-5). RNA1 and RNA2 are involved in viral RNA replication, assembly, and cell-to-cell movement, and RNA3 to 5 are associated with vector-mediated infection and disease development in sugar beet roots. GFP-tagged BNYVV (BNYVV-GFP) was constructed by introducing the GFP gene into the coat protein readthrough domain of BNYVV RNA2. BNYVV-GFP was efficiently infectious to local lesion and systemic host plants and infection sites were followed up by expressing the green fluorescence. However, several GFP viruses constructed were not transmitted by P. betae so that we were not able to conduct transmission experiments using BNYVV-GFP. The vascular movement of BNYVV in beet plants was examined using BNYVV-GFP. The virus had difficulty to move from epidermal cells to phloem tissues when rub-inoculated into leaves. In roots, the vector-inoculated virus propagated in any tissues of rootlets, but it was difficult for virus to invade from rootlet tissues into phloem tissues of taproots and stems. Molecular analyses of RNA4-encoded p31 showed that BNYVV p31 plays a multifunctional role in efficient vector transmission, enhanced symptom expression and root-specific silencing suppression. The RNA3-encoded p25 protein is responsible for production of rhizomania symptoms. The presence of the p25 protein also was found to be associated with resistance response in resistant plants. There was a genotype-specific interaction between BNYVV strains and the beet plants. Three amino acid residues 68, 70, and 179 are important in determining the resistance phenotype, and that the host-genotype specificity is controlled by single amino acid changes at position 68. BNYVV resistance mechanisms in root and leaf may be different: the resistance appearing in leaves was stronger than that appearing in roots.