ACCUMULATION OF MILD AND SEVERE STRAINS OF TOBACCO MOSAIC-VIRUS IN MINOR VEINS OF TOBACCO

The Masked (M) and more severe common (U1) strains of tobacco mosaic v irus (TMV) accumulate at similar rates in protoplasts or inoculated le aves of Nicotiana tabacum 'Xanthi nn' (R. S, Nelson, G. Li, R. A, J, H odgson, R, N, Beachy and M, H, Shintaku, 1993, Mel. Plant-Microbe Inte ract 6:45-54), We now have compared accumulation of progeny virus from a transcript of an M-TMV cDNA clone (M(IC)-TMV) with U1-TMV in chloro tic lesions of inoculated, mature leaves, and determined that M(IC)-TM V accumulated in fewer vascular parenchymal (VP) and companion (C) cel ls within the predominant class of veins in these leaves, Independent of virus strain, the proportion of infected VP cells to infected C cel ls was approximately 5 to 1 early after inoculation, In systemically i nfected leaves, the proportion of infected VP cells to infected C cell s was approximately 1 to 1 due mainly to an increase in the number of C cells infected, The difference in number of VP cells infected by M(I C)-TMV compared with U1-TMV in inoculated leaves reflects a cell type- specific delay, not predicted from results of protoplast or leaf extra ct analyses. The presence of the 126-kDa protein of M(IC)-TMV in those VP cells where M(IC)-TMV accumulated indicated that the factors neces sary for translation of viral RNA were present. Overall, fewer C cells were infected in the inoculated leaves compared with systemically inf ected leaves, which suggests that TMV does not enter many of the C cel ls of inoculated leaves or that viral replication is inhibited in thes e specific cells.