MOLECULAR STUDIES ON BROMOVIRUS CAPSID PROTEIN .1. CHARACTERIZATION OF CELL-TO-CELL MOVEMENT-DEFECTIVE RNA3 VARIANTS OF BROME MOSAIC-VIRUS

To evaluate the extent to which brome mosaic Virus (BMV) coat protein (CP) gene is involved in the process of cell-to-cell movement, Chenopo dium quinoa plants were coinoculated with BMV wild-type RNAs 1 and 2 a nd a RNA3 variant containing either the beta-glucuronidase (GUS) gene in place of CP (B3/CP-GUS), to be subjected to GUS analysis, or a dele tion in the on gene to be analyzed by fluorescence in situ hybridizati on (FISH). Irrespective of time postinoculation, GUS expressed from th e subgenomic mRNA of B3/CP-GUS was restricted to initially infected, s ingle epidermal cells. The defective cell-to-cell movement exhibited b y B3/CP-GUS was not complemented in trans when transcripts of B3 Delta MP, a RNA3 variant capable of synthesizing functional wt CP but not m ovement protein (MP), were added to the inoculum. Application of FISH, a technique Versatile in discriminating subliminal infections from ef ficient cell-to-cell spread in nonpermissive and permissive hosts, res pectively, to leaves inoculated with BMV RNA3 variants defective in CP synthesis, confirmed that the resulting infections were subliminal. T hese data provide direct evidence for the requirement of an encapsidat ion-competent CP to be expressed in conjunction with a functional MP f or efficient cell-to-cell spread of BMV. (C) 1996 Academic Press, Inc.