First and second authors: Department of Plant Pathology, University of California, Riverside 92521-0122; and third author: Department of Plant Pathology, University of Georgia, Athens 30602-7274
Taxonomically distinct tobacco mosaic tobamovirus (TMV), red clover necrotic mosaic dianthovirus (RCNMV), cucumber mosaic cucumovirus (CMV), brome mosaic bromovirus (BMV), and cowpea chlorotic mottle bromovirus (CCMV) exhibit differences in their host range. Each of these viruses encodes a functionally similar nonstructural movement protein (MP) that is essential for cell-to-cell movement of a progeny virus. Despite the lack of significant amino acid identity among the MPs of CMV, TMV, and RCNMV, movement-defective CMV (CMVFnyΔMP-ΔKPN) was able to move locally and systemically in transgenic Nicotiana benthamiana expressing either TMV MP (NB-TMV-MP(+)) or RCNMV MP (NB-RCNMV-MP(+)). These observations contrast with those of previous studies in which transgenic N. tabacum cv. Xanthi plants expressing TMV MP supported only the cell-to-cell movement of CMVFnyΔMP-ΔKPN. To verify whether similar complementation could be observed for movement-defective bromoviruses, NB-TMV-MP(+) and NB-RCNMV-MP(+) plants were inoculated independently with movement-defective variants of BMV (B3ΔMP) and CCMV (CC3ΔMP). Neither NB-TMV-MP(+) nor NB-RCNMV-MP(+) was able to rescue the defective cell-to-cell and long-distance movement of B3ΔMP. In contrast, NB-RCNMV-MP(+) complemented the cell-to-cell, but not the long-distance, movement of CC3ΔMP. Taken together, these studies suggest that virus movement is a complex process and that, in some cases, the host species plays a major role in determining the long-distance movement function of a virus.