Losing the Warning Signal: Drought Compromises the Cross-Talk of Signaling Molecules in Exposed to Ozone.

Understanding the interactions between drought and acute ozone (O) stress in terms of signaling molecules and cell death would improve the predictions of plant responses to climate change. The aim was to investigate whether drought stress influences the responses of plants to acute episodes of O exposure. In this study, the behavior of 84 Mediterranean evergreen plants was evaluated in terms of cross-talk responses among signaling molecules. Half of the sample was subjected to drought (20% of the effective daily evapotranspiration, for 15 days) and was later exposed to an acute O exposure (200 nL L for 5 h). First, our results indicate that in well-water conditions, O induced a signaling pathway specific to O-sensitive behavior. Second, different trends and consequently different roles of phytohormones and signaling molecules (ethylene, ET; abscisic acid, ABA; salycilic acid, SA and jasmonic acid, JA) were observed in relation to water stress and O. A spatial and functional correlation between these signaling molecules was observed in modulating O-induced responses in well-watered plants. In contrast, in drought-stressed plants, these compounds were not involved either in O-induced signaling mechanisms or in leaf senescence (a response observed in water-stressed plants before the O-exposure). Third, these differences were ascribable to the fact that in drought conditions, most defense processes induced by O were compromised and/or altered. Our results highlight how plants suffering from water deprivation respond differently to an acute O episode compared to well-watered plants, and suggest new effect to be considered in plant responses to environmental changes. This poses the serious question as to whether or not multiple high-magnitude O events (as predicted) can change these cross-talk responses, thus opening it up possible further investigations.