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To analyse cellular response to O3, the tolerant Arabidopsis thaliana genotype Col-0 was exposed to O3 fumigation (300 ppb) for 6 h and the modulation of gene expression during the treatment (3 h after the beginning of the treatment, T3 h) and the recovery phase (6 h from the end of the treatment, T12 h) assessed by gene chip microarray and real-time reverse transcriptase (RT)-PCR analyses. The Arabidopsis transcriptional profile is complex, as new genes (i.e. reticuline oxidase) and pathways, other than those already reported as O3-responsive, appear to be involved in the O3 response. The steady-state transcript levels of several WRKY genes were increased in O3-treated plants and the W-box was the cis-element over-represented in the promoter region of T3 h up-regulated genes. The fact that the W-box element was also over-represented in almost all T3 h-induced receptor-like kinases (RLKs) suggests a WRKY-mediated control of RLKs under O3 stress and a mechanicistic similarity with the pathogen-induced transcriptional responses. We investigated the molecular and physiological implications of our findings in relation to O3-induced plant stress response. [ABSTRACT FROM AUTHOR]