The role of signaling molecules in ozone induced cell death in Quercus ilex plants

Ozone (O3) is the main photochemical oxidant that causes leaf damage in many plant species, especially in highly altered environments. When O3 is incorporated into plants, it produces reactive oxygen species (ROS) which interact with the synthesis of several phyto-hormones, such as ethylene (ET), salycilic (SA) and jasmonic (JA) acids. In the present work, the interaction between hormone and redox signaling pathways that regulate the programmed cell death (PCD), has been investigated in three-year-old saplings of Quercus ilex exposed to O3 stress (200 ppb, 5 h) to verify if the response of this species resembles the biotic defense reactions. At the end of the fumigation, no O3-macroscopic leaf injury was observed. However, by microscopic observation, only the O3-treated leaves showed (after Evan’s blue staining) some blue stained areas in both exposure and recovery phases, identifying cell damage and dead cells. The induction and propagation of cell death were due to (i) activation of oxidative burst in form of biphasic productions of hydrogen peroxide (H2O2) and superoxide radical (O2-) firstly peaked after 1 h from the beginning of exposure (respectively +79 and +37%, in comparison to controls) and secondly after 8 h (+67 and +74%) with higher values of H2O2 after 24 h too (+64%); (ii) hormonal regulation of O3-induced lesion formation with a significant production of ET, JA and SA which transiently peaked in the first 2 h (+50% of ET and more than threefold higher of JA and SA in comparison to controls). Moreover, ET showed a second peak after 8 h (+94%) and conserved higher values during the recovery phase till after 24 h (+76%). These results indicate that ROS interact with hormonal signaling pathway regulating the sensitivity of Q. ilex to O3.