Accumulation of Mild and Severe Strains of Tobacco Mosaic Virus in Minor Veins of Tobacco. Xin Shun Ding. Plant Biology Division, Samuel Roberts Noble Foundation, P. O. Box 2180, Ardmore, OK 73402 U.S.A. Michael H. Shintaku(2), Shelly A. Arnold(1), Richard S. Nelson(1). (1) Plant Biology Division, Samuel Roberts Noble Foundation, P. O. Box 2180, Ardmore, OK 73402 U.S.A. (2) College of Agriculture, University of Hawaii at Hilo, 200 W. Kawili, Hilo, HI 96720 U.S.A. MPMI 8:32-40. Accepted 14 October 1994. Copyright 1995 The American Phytopathological Society.

The Masked (M) and more severe common (Ul) 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, Mol. Plant-Microbe Interact 6:45-54). We now have compared accumulation of progeny virus from a transcript of an M-TMV cDNA clone (MIC-TMV) with Ul-TMV in chlorotic lesions of inoculated, mature leaves, and determined that MIC -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, (lie 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 MIC-TMV compared with Ul-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 MIC-TMV in those VP cells where MIC-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.