Overexpression of SIPK in tobacco enhances ozone-induced ethylene formation and blocks ozone-induced SA accumulation.

Ozone induces rapid activation of SIPK, a mitogen-activated protein kinase (MAPK) in tobacco. Through transgenic manipulation it has previously been shown that overexpression of SIPK leads to enhanced ozone-induced lesion formation with concomitant accumulation of ROS. In spite of this hypersensitive phenotype, the effect of this altered SIPK expression on the levels of various hormones that regulate ozone-induced cell death has remained unexplored. The response of both salicylate and ethylene, the major phytohormones that modulate ozone-induced cell death, have now been analysed in SIPK-OX tobacco plants. Ozone treatment strongly induced ethylene formation in the sensitive SIPK-OX plants at ozone concentrations that failed to elicit stress ethylene release in WT plants. By contrast, SIPK-overexpressing plants displayed no ozone-induced SA accumulation, whereas WT plants accumulated SA upon ozone exposure. Epistatic analysis of SIPK-OX function suggests that the ozone-induced cell death observed in SIPK-OX plants is either independent, or upstream, of SA accumulation.