Independent pathways leading to apoptotic cell death, oxidative burst and defense gene expression in response to elicitin in tobacco cell suspension culture

We characterized pharmacologically the hypersensitive cell death of tobacco BY-2 cells that followed treatments with Escherichia coli preparations of INF1, the major secreted elicitin of the late blight pathogen Phytophthora infestans. INF1 elicitin treatments resulted in fragmentation and 180 bp la ddering of tobacco DNA as early as 3 h post-treatment. INF1 elicitin also i nduced rapid accumulation of H2O2 typical of oxidative burst, and the expre ssion of defense genes such as phenylalanine ammonia-lyase (PAL) gene at 1 h and 3 h after elicitin treatment, respectively. To investigate the involv ement of the oxidative burst and/or the expression of defense genes in the signal transduction pathways leading to hypersensitive cell death, we analy zed the effect of several chemical inhibitors of signal transduction pathwa ys on the various responses. The results indicated that (a) the cell death required serine proteases, Ca2+ and protein kinases, (b) the oxidative burs t was involved in Ca2+ and protein kinase mediated pathways, but elicitin-i nduced AOS was neither necessary nor sufficient for cell death and PAL gene expression, and (c) the signaling pathway of PAL gene expression required protein kinases. These results suggest that the three signal transduction p athways leading to cell death, oxidative burst and expression of defense ge nes branch in the early stages that follow elicitin recognition by tobacco cells.