The sessile lifestyle of plants requires that adverse environmental conditions are rapidly recognized and responded to with adequate reactions. Both abiotic and biotic stress factors induce ROS production in plants leading to the activation of downstream signaling events. As ozone induces the production of ROS, it is an excellent tool to study plant stress tolerance. We have conducted a large-scale forward genetics screen based on ozone sensitivity aiming to identify both novel stomatal regulators and also components in plant stress tolerance. Mutants defective in stomatal regualtion receive a higher ozone dose leading to visual leaf damage. Ozone sensitive mutants with normal stomatal and cuticular function receive same ozone doze as WT but are potentially defective in ROS induced downstream signaling and/or scavenging. Our screen has identified ~50 novel mutants impaired in guard cell signaling and surprisingly only 2 mutants with normal stomatal regulation and strong ozone sensitivity – oz.14 and oz.26. For both of the mutants mapping populations have been created based on ozone sensitivity phenotype and SHOREmap results are available. OZ.26 encodes a still uncharacterized putative plant steroid hormone receptor and is localized to the plasma membrane. It has also a role in cell elongation and plant freezing tolerance. Additionally, in vitro steroid hormone binding will be addressed.