Ozone and nitric oxide induce cGMP-dependent and -independent transcription of defence genes in tobacco

• Here, we analyse the temporal signatures of ozone (O ₃)-induced hydrogen peroxide (H₂O₂) and nitric oxide (NO) and the role of the second messenger guanosine 3′,5′- cyclic monophosphate (cGMP) in transcriptional changes of genes diagnostic for biotic and abiotic stress responses. • Within 90 min O₃ induced H₂O₂ and NO peaks and we demonstrate that NO donors cause rapid H₂O₂ accumulation in tobacco (Nicotiana tabacum) leaf. Ozone also causes highly significant, late (> 2 h) and sustained cGMP increases, suggesting that the second messenger may not be required in all early (< 2 h) responses to O₃, but is essential and sufficient for the induction of some O₃-dependent pathways. • This hypothesis was tested resolving the time course of O₃-induced transcript accumulation of alternative oxidase (AOX1a), glutathione peroxidase (GPX), aminocyclopropancarboxylic acid synthase (ACS2) that is critical for the synthesis of ethylene, phenylalanine ammonia lyase (PALa) and the pathogenesis-related protein PR1a. • The data show that early O₃ and NO caused transcriptional activation of the scavenger encoding proteins AOX1a, GPX and the induction of ethylene production through ACS2 are cGMP independent. By contrast, the early response of PALa and the late response of PR1a show critical dependence on cGMP.