THE AMOUNT OF MOVEMENT PROTEIN PRODUCED IN TRANSGENIC PLANTS INFLUENCES THE ESTABLISHMENT, LOCAL MOVEMENT, AND SYSTEMIC SPREAD OF INFECTIONBY MOVEMENT PROTEIN-DEFICIENT TOBACCO MOSAIC-VIRUS

The movement protein (MP) of tobacco mosaic virus (TMV) is required fo r the virus to spread from cell to cell in Nicotiana tabacum. To explo re the role of the MP in the initiation of infection and in local and systemic spread of virus, transgenic plants that accumulate varying le vels of MP, and that accumulate MP only in selected leaf tissues, were inoculated with TMV mutants that lack a functional MP gene, Expressio n of MP is not required in most epidermal cells for successful infecti on by mutant TMV, although we cannot rule out the possibility that a s mall amount of MP may have diffused into the epidermis from other tiss ues, 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 efficie ncy requires about 30% of the amount of MP produced by wild-type TMV i n a systemic infection, The subsequent cell-to-cell spread of the muta nt 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.