Oxidative stress and ozone: perception, signalling and response
The primary site of ozone interaction with plant cells is the extracellular matrix where ozone challenges the antioxidant protection of the cells. Accordingly, ozone sensitivity generally correlates with the ascorbate status of the apoplast, which is an important signal initiation point. In addition, ozone sensing takes place by covalent modification of redox-sensitive components of the plasma membrane, for example ion channels like the plasma membrane Ca2+-channels. Subsequent intracellular signal transduction is an intriguing network of hormone, Ca2+ and MAPK signalling pathways, significantly overlapping with oxidative burst-induced pathogen signalling. Comparison of recent transcriptome analysis revealed that in addition to genes generally induced by all kinds of oxidative stress, for example, transcripts for PR-proteins and most antioxidant enzymes, approximately one-third of the responsive transcripts are ozone specific, indicating jasmonic acid, salicylic acid and ethylene-independent redox signalling triggered by extracellular redox sensing.