Fulfilling Koch's postulates for beet curly top Iran virus and proposal for consideration of new genus in the family Geminiviridae.

Beet curly top Iran virus (BCTIV) is a divergent geminivirus with biological properties similar to those of curtoviruses; however, the virus is distinct from curtoviruses phylogenetically and in its genome organisation. The replication-associated protein is phylogenetically more closely related to those of mastreviruses than to those of curtoviruses whereas the capsid protein shares high amino acid sequence identity (77-83 %) with those of curtoviruses. The 17 BCTIV genomes from Iran share ~77 % pairwise nucleotide sequence identity with spinach curly top Arizona virus (SCTAV) from Arizona, USA, which was characterised recently. To demonstrate the infectivity of the monopartite BCTIV genome and to fulfil Koch's postulates, an infectious clone was constructed using a dimer of the full-length genome of an isolate from this study - BCTIV-[IR:Neg:B33P:Sug:08]. Agroinoculation with the cloned DNA resulted in the efficient infection of 74 % of sugar beet plants, which resulted in curly top symptoms. The curly top infection of agroinoculated plants was successfully transmitted to 80 % of healthy sugar beet plants by the natural BCTIV vector, Circulifer haematoceps. Since BCTIV and SCTAV share <62 % pairwise nucleotide sequence identity with all other geminiviruses and have unique genome architectures and properties, and since this is coupled with phylogenetic support at the full-genome level and that of it proteins, we propose that they should be re-classified as members of a new genus, "Becurtovirus", in the family Geminiviridae.