Unfolded protein response followed by induction of cell death in cultured tobacco cells treated with tunicamycin.

When correct folding of protein in the endoplasmic reticulum (ER) is prevented, cells respond to overcome the accumulation of unfolded proteins. This cellular response, which includes the induction of ER chaperones, is called an unfolded protein response (UPR). Although a link between the UPR and apoptosis has been reported in mammalian cells, little is known about this mechanism in plant cells. Asparagine (N)-linked glycosylation of proteins is critical for protein folding in the ER; and tunicamycin, a potent inhibitor of N-linked glycosylation, induces UPR. Growth arrest was observed in cultured tobacco cells treated with tunicamycin. Cell death and induction of Hsr203J, a marker for programmed cell death, were observed in the 24-h period after addition of tunicamycin, following UPR that started within 2 h. These results indicate a strong link between UPR and programmed cell death in plant cells.