The Amount of Movement Protein Produced in Transgenic Plants Influences the Establishment, Local Movement, and Systemic Spread of Infection by Movement Protein-Deficient Tobacco Mosaic Virus. Patricio Arce Johnson. (1)Department of Cell Biology, Division of Plant Biology, The Scripps Research Institute, La Jolla, California 92037; (2) Departamento de Ingenieria Genetica de Plantas, Centro de Investigacion y de Estudios Avanzados del IPN, Unidad Irapuato, Apartado Postal 629, 36500 Irapuato, Guanajuato, Mexico. Theodore W. Kahn (1), Ulrich Reimann-Philipp (1), Rafael Rivera-Bustamante (2), and Roger N. Beachy (1). (1) Department of Cell Biology, Division of Plant Biology, The Scripps Research Institute, La Jolla, California 92037; (2) Departamento de Ingenieria Genetica de Plantas, Centro de Investigacion y de Estudios Avanzados del IPN, Unidad Irapuato, Apartado Postal 629, 36500 Irapuato, Guanajuato, Mexico. MPMI 8: 415- 423. Accepted 15 February 1995. Copyright 1995 The American Phytopathological Society.

The movement protein (MP) of tobacco mosaic virus (TMV) is required for the virus to spread from cell to cell in Nicotiana tabacum. To explore the role of the MP in the initiation of infection and in local and systemic spread of virus, transgenic plants that accumulate varying levels of MP, and that accumulate MP only in selected leaf tissues, were inoculated with TMV mutants that lack a functional MP gene. Expression of MP is not required in most epidermal cells for successful infection by mutant TMV, although we cannot rule out the possibility that a small amount of MP may have diffused into the epidermis from other tissues. Wild-type TMV produces substantially more MP than is required for each stage of a successful infection. Establishment of multicellular infection sites by mutant TMV on an inoculated leaf at maximum efficiency requires about 30% of the amount of MP produced by wild-type TMV in a systemic infection. The subsequent cell-to-cell spread of the mutant virus through the inoculated leaf requires no more than 2% of this level of MP in order to occur at the maximum rate. Systemic spread of the mutant virus throughout the plant occurs at the maximum rate when 4% of this level of MP is present.