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 virus (TMV) accumulate at similar rates in protoplasts or inoculated leaves of Nicotiana tabacum 'Xanthi nn' (R. S, Nelson, G. Li, R. A, J, Hodgson, R, N, Beachy and M, H, Shintaku, 1993, Mel. Plant-Microbe Interact 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 chlorotic lesions of inoculated, mature leaves, and determined that M(IC)-TMV accumulated in fewer vascular parenchymal (VP) and companion (C) cells within the predominant class of veins in these leaves, Independent of virus strain, the proportion of infected VP cells to infected C cells was approximately 5 to 1 early after inoculation, In systemically infected leaves, the proportion of infected VP cells to infected C cells 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(IC)-TMV compared with U1-TMV in inoculated leaves reflects a cell type-specific delay, not predicted from results of protoplast or leaf extract 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 necessary for translation of viral RNA were present. Overall, fewer C cells were infected in the inoculated leaves compared with systemically infected leaves, which suggests that TMV does not enter many of the C cells of inoculated leaves or that viral replication is inhibited in these specific cells.