Induction of pathogenesis-related proteins in tobacco leaves

The synthesis of pathogenesis-related proteins (PR proteins), induced in tobacco leaves in response to infection with tobacco mosaic virus or treatment with salicylic acid, was studied with in vivo pulse-labeling experiments. PR proteins synthesis began after a lag phase of about 8 hours in leaf discs treated with salicylic acid and after more than 18 hours in those infected with tobacco mosaic virus. In both cases, the synthesis declined rapidly after 50 hours. The results show that the accumulation of PR proteins results from de novo synthesis and not from degradation of preexisting precursors and that the induced synthesis is transient like other stress-inducible proteins. The proteins have a half-life of at least 50 hours. The induction of these PR proteins was not inhibited by either 25 micrograms per milliliter of actinomycin D or 200 micrograms per milliliter of alpha-amanitin, which completely inhibited the increase of peroxidase activity in tobacco mosaic virus-infected leaf and the induction of heat shock proteins in tobacco leaf discs. These findings indicate that the induction of PR proteins is not regulated by a transcriptional step but by a translational step.