P1 protein of Cocksfoot mottle virus is indispensable for the systemic spread of the virus

Cocksfoot mottle sobemovirus (CfMV) encodes a non-conserved protein P1 from the 5' ORF1 of genomic RNA. The functions of CfMV P1 are unknown. In the current study we show that P1-deficient CfMV can replicate both in oat leaves and barley suspension culture cells but can not infect oat plants systemically. However, the absence of P1 reduces the efficiency of virus accumulation considerably. The infectivity of the mutant virus restores as a result of the spontaneous transversion. CfMV P1:EGFP shows a very limited cell-to-cell movement in leaf epidermal cells. In Sf9 insect cells CfMV P1 localizes in the fraction of membranes and inclusions but not in soluble cytoplasmic protein fraction.