MOLECULAR STUDIES ON BROMOVIRUS CAPSID PROTEIN .3. ANALYSIS OF CELL-TO-CELL MOVEMENT COMPETENCE OF COAT PROTEIN DEFECTIVE VARIANTS OF COWPEA CHLOROTIC MOTTLE VIRUS

To determine whether the role of coat protein (CP) in cell-to-cell mov ement of dicot-adapted cowpea chlorotic mottle bromovirus (CCMV) is di stinct from that of monocot-adapted brome mosaic bromovirus (BMV), two reporter genes, beta-glucuronidase (GUS) and enhanced green fluoresce nt protein (EGFP), were substituted for the CP in a biologically activ e clone of CCMV RNA3 (C3). Primary leaves of Nicotiana benthamiana, Ch enopodium quinoa, and cowpea were co-inoculated with wild-type (wt) CC MV RNA 1 and -2 and either C3/Delta CP-GUS or C3/Delta CP-EGFP and ana lyzed for GUS activity or the presence of green fluorescence. The visu al appearance of infections caused by GUS or EGFP variants indicated t hat, in CCMV, epidermal cell-to-cell movement can occur without a func tional CP. By contrast, inoculation of MP defective variants of C3/Del ta CP-GUS or C3/Delta CP-EGFP resulted in subliminal infections. Addit ional experiments examining the infectivity of wt BMV RNA 1 and -2 and a BMV RNA3 variant bearing the EGFP in the place of CP (B3/Delta CP-E GFP) confirmed previous observations that, unlike CCMV, epidermal cell -to-cell movement of BMV is dependent on the expression of a functiona l CP. Taken together, the results demonstrate that BMV and CCMV use di fferent mechanisms for initial epidermal cell-to-cell spread, and the individual role played by the respective CP genes in this active proce ss is discussed. (C) 1997 academic Press.