Recombinant beet curly top virus genomes exhibit both parental and novel pathogenic phenotypes

Recombinant genomes derived from the Logan and CFH strains of the geminivirus beet curly top virus (BCTV) have been constructed and analyzed for pathogenicity on Nicotiana benthamiana and sugar beet (Beta vulgaris L.). Infectivity assays indicated that the latent period on N. benthamiana was primarily determined by a DNA fragment bearing the leftward open reading frames (ORFs) L1, L2, L3, and L4. Recombinants bearing leftward ORFs from the CFH strain were characterized as having a short latent period (mean = 6-11 days), while the reciprocal recombinants bearing leftward ORFs from the Logan strain had latent periods defined as long (mean = 16-22 days). Infectivity assays on sugar beet indicated that certain recombinant BCTV genomes exhibited novel pathogenic properties not common to either wild type strain, including the loss of systemic movement and replication competency, or asymptomatic systemic infection of sugar beet. The results indicate that N. benthamiana is a more permissive host than sugar beet with respect to heterologous combinations of BCTV genes, and that pathogenicity and virulence of BCTV in sugar beet requires the interaction of certain viral gene products and/or cis-elements that have coevolved in the same strain.