Signaling molecules and cell death in Melissa officinalis plants exposed to ozone

Interaction between hormone and redox signaling pathways has been investigated in ozone-stressed (200 ppb, 5 h) lemon balm to verify if the response resembles the biotic defense reactions. In comparison to controls, plants exhibited foliar injury and the cell death was induced by (1) biphasic production of hydrogen peroxide and superoxide radical; (2) hormonal regulation of ozone-induced lesion formation with a significant production of ethylene, salicylic, jasmonic and abscisic acid; (3) ozone degradation to reactive oxygen species and their detoxification by some enzymatic (such as superoxide dismutase) and non-enzymatic antioxidant systems (such as ascorbic acid, glutathione and carotenoids), that worked in cooperation without providing a defense against free radicals (such as confirmed by the modification of the antioxidant properties of leaf tissue). This integrated view showed that reactive oxygen species interact with hormonal signaling pathway regulating cell death and the sensitivity of lemon balm to ozone.