Abstract Previously, the authors have observed the presence of potato virus Y coat protein (PVY-CP) inside chloroplasts of infected tobacco cells. Induction of symptoms reminiscent of virus infection was observed only in transgenic plants accumulating CP in the chloroplast but not in the cytoplasm. These results suggested that PVY-CP plays a role in symptom development. Plants with high levels of CP in their chloroplasts were more susceptible to PVY, suggesting that there was a host response to the presence of PVY-CP in the chloroplasts. To investigate whether PVY infection and/or chloroplast-targeted PVY-CP resulted in a host stress response, levels of pathogenesis-related protein 1a (PR-1a) were determined in these plants as an indicator of these host responses. Results indicated that expression of PR-1a was induced in chloroplast-targeted PVY-CP transgenic plants, but not in transgenic plants expressing PVY-CP cytosolically. The level of PR-1a in plants appeared to correspond with the severity of symptoms.