Defence gene expression in soybean is linked to the status of the cell death program

Soybean cell cultures (cv. Williams 82) respond to Pseudomonas syringae bacteria expressing the avirulence gene AvrA with a hypersensitive reaction, a programmed cell death (PCD) of plant cells to pathogen attack. This PCD is under control of salicylic acid (SA) via an unknown mechanism. In the presence of low concentrations of SA, the cells undergo a very rapid cell death, which needs only half of the time required for the normal hypersensitive reaction (HR). Northern blot studies for defence-related genes show that the expression of many of these genes is tightly linked to the status of the cell death program rather than to pathogen-derived elicitors. Thus the expression is much faster in the SA-accelerated PCD than in the normal hypersensitive reaction. In contrast, other pathogen-responsive genes are induced independently of the speed of PCD, indicating a divergent signalling mechanism. The production of reactive oxygen species during the oxidative burst of bacteria-inoculated soybean cells is slightly enhanced in the presence of SA but occurs at the same time as in untreated cells, suggesting that SA exhibits the control of the PCD downstream of the oxidative burst. Consistent with these findings a HR-specific marker gene is neither directly induced by H2O2 or SA. However, this gene shows a high expression in the regular HR and is induced much faster in the SA-accelerated PCD.