Functional analysis of a novel βV1 gene identified in a geminivirus betasatellite

Betasatellites (DNA beta) are circular ssDNA molecules that are associated with monopartite geminiviruses and exert a positive effect on the viral infection. Betasatellites encode one protein, named beta C1, on the complementary strand; beta C1 functions as a pathogenicity factor and RNA silencing suppressor. In this report, we describe the identification of another betasatellite-encoded protein, beta Vl, which also contributes to symptom development. The beta Vl open reading frame can be found on the viral strand of approximately 40% of reported betasatellite sequences, and is conserved in position and sequence. The presence of the beta Vl transcript was observed in plants infected with Tomato yellow leaf curl China virus (TYTCCNV) along with its associated betasatellite Tomato yellow leaf curl China betasatellite (TYTCCNB). Mutant viruses unable to produce beta Vl showed reduced virulence and decreased viral load. Ectopic expression of the TYTCCNB-PV1 gene in Nicotiana benthamiana plants from a PVX-based vector resulted in leaf mosaic and chlorosis. We further demonstrated that the beta Vl protein could elicit hypersensitive response (HR)-type cell death in N. benthamiana leaves. Our results uncover a novel betasatellite-encoded protein that contributes to the virus infection, and this discover gives us a more complete view of the plant-geminivirus interaction landscape.