Transgenic tobacco plants were produced that express a chimeric gene encoding the coat protein (CP) of tobacco mosaic virus (TMV) under the control of the promoter from a ribulose bisphosphate carboxylase small subunit (rbcS) gene. Plant lines expressing comparable levels of CP from the rbcS and cauliflower mosaic virus 35S promoters were compared for resistance to TMV. In whole plant assays the 35S:CP constructs gave higher resistance than the rbcS:CP constructs. On the other hand, leaf mesophyll protoplasts isolated from both plant lines were equally resistant to infection by TMV. This indicated that the difference in resistance between the lines in the whole plant assay reflects differences at the level of short- and/or long-distance spread of TMV. Therefore, we propose that the difference in tissue-specific expression between the 35S and rbcS promoters accounts for greater resistance in the plant lines that express the 35S:CP chimeric genes.
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