Mutational Analysis of the Movement Protein of Odontoglossum Ringspot Virus to Identify a Host-Range Determinant. Csilla A. Fenczik. Division of Biology and Biomedical Sciences, Washington University, St. Louis, Missouri 63110 U.S.A. Hal S. Padgett(1), Curtis A. Holt(2), Steven J. Casper(2), and Roger N. Beachy(2). (1) Division of Biology and Biomedical Sciences, Washington University, St. Louis, Missouri 63110 U.S.A. (2)Division of Plant Biology, The Scripps Research Institute, La Jolla, California 92037 U.S.A. MPMI 8:666-673. Accepted 8 May 1995. Copyright 1995 The American Phytopathological Society.

Tobacco mosaic virus (TMV) which contains the movement protein (MP) of odontoglossum ringspot tobamovirus (ORSV) in place of the TMV MP systemically infects orchids but causes local infection in tobacco unless the carboxy-terminal 48 amino acids of the MP are deleted (C. A. Holt, C. A. Fenczik, S. J. Casper, and R. N. Beachy; Virology, in press, 1995). Frameshifl mutations were created within the 3' end of the MP gene that led to truncations of the ORSV MP by 11, 19, 28, 37, and 48 amino acids; each of the mutant MP genes was inserted into the cloned cDNA of TMV in place of the TMV MP and infectious transcripts were produced. Virus containing mutant MPs were used to infect vanilla orchids, a systemic host of ORSV, and tobacco plants. Removal of 11 amino acids from the ORSV MP prevented spread of the chimeric virus in orchids while restoring the ability to cause a systemic infection on tobacco. Further deletions of the MP affected the size of virus-induced necrolic local lesions on tobacco cv. Xanthi NN and the systemic spread and accumulation of virus in cv. Xanthi nn, a systemic host of TMV. However, each virus replicated to equivalent levels in protoplasts. A mechanism by which the ORSV MP limits the spread of the chimeric virus is proposed.