DNA forms indicate rolling circle and recombination-dependent replication of Abutilon mosaic virus

Geminiviruses have spread worldwide and have become increasingly important in crop plants during recent decades. Recombination among geminiviruses was one major source of new variants. Geminiviruses replicate via rolling circ les, confirmed here by electron microscopic visualization and two-dimension al gel analysis of Abutilon mosaic virus (AbMV) DNA. However, only a minori ty of DNA intermediates are consistent with this model. The majority are co mpatible with recombination-dependent replication (RDR). During development of naturally infected leaves, viral intermediates compatible with both mod els appeared simultaneously, whereas agro-infection of leaf discs with AbMV led to an early appearance of RDR forms but no RCR intermediates. Inactiva tion of viral genes ac2 and ac3 delayed replication, but produced the same DNA types as after wild-type infection, indicating that these genes were no t essential for RDR in leaf discs. In conclusion, host factors alone or in combination with the viral AC1 protein are necessary and sufficient for the production of RDR intermediates. The consequences of an inherent geminivir al recombination activity for the use of pathogen-derived resistance traits are discussed.