Capsid protein gene and the type of host plant differentially modulate cell-to-cell movement of cowpea chlorotic mottle virus.

A study was undertaken to measure the rate of coat protein (CP) independent cell-to-cell movement of cowpea chlorotic mottle bromovirus (CCMV) in three different host plants. A CCMV RNA3 variant in which the CP gene was substituted with enhanced green fluorescent protein (C3/DeltaCP-EGFP) was coinoculated to three different host plants with transcripts of wild type RNAs 1 and 2. Comparative analysis of cell-to-cell movement monitored by the EGFP expression at various days post inoculation revealed that the rate of spread varied with the type of host species inoculated: fastest movement was observed in Nicotiana benthamiana while the rate spread was significantly slower in the natural host cowpea. When CP was expressed as EGFP fusion (C3/CP:EGFP) the rate of spread in N. benthamiana and C. quinoa was slower than that was observed in the absence of CP and remained subliminal in cowpea. Analysis of infection foci by confocal laser scanning microscope revealed that localization of CP:EGFP fusion was distinct in N. benthamiana and C.quinoa and accumulated as fluorescent inclusions at the cell periphery. Additional experiments involving coinoculation of either C3/DeltaCP-EGFP or C3/CP:EGFP with heterologous brome mosaic bromovirus (BMV) genomic RNAs 1 and 2 revealed that, in addition to movement protein and CP, viral replicase also influences cell-to-cell spread. The significance of these results in relation to the mechanism of bromovirus movement is discussed.