Ozone-induced ethylene production is dependent on salicylic acid, and both salicylic acid and ethylene act in concert to regulate ozone-induced cell death

Ethylene is known to influence plant defense responses including cell death in response to both biotic and abiotic stress factors. However, whether ethylene acts alone or in conjunction with other signaling pathways is not clearly understood. Ethylene overproducer mutants, eto1 and eto3, produced high levels of ethylene and developed necrotic lesions in response to an acute O-3 exposure that does not induce lesions in O-3-tolerant wild-type Col-0 plants. Treatment of plants with ethylene inhibitors completely blocked O-3-induced ethylene production and partially attenuated O-3-induced cell death. Analyses of the responses of molecular markers of specific signaling pathways indicated a relationship between salicylic acid (SA)- and ethylene-signaling pathways and O-3 sensitivity. Both eto1 and eto3 plants constitutively accumulated threefold higher levels of total SA and exhibited a rapid increase in free SA and ethylene levels prior to lesion formation in response to O-3 exposure. SA pre-treatments increased O-3 sensitivity of Col-0, suggesting that constitutive high SA levels prime leaf tissue to exhibit increased magnitude of O-3-induced cell death. NahG and npr1 plants compromised in SA signaling failed to produce ethylene in response to O-3 and other stress factors suggesting that SA is required for stress-induced ethylene production. Furthermore, NahG expression in the dominant eto3 mutant attenuated ethylene-dependent PR4 expression and rescued the O-3-induced HR (hypersensitive response) cell death phenotype exhibited by eto3 plants. Our results suggest that both SA and ethylene act in concert to influence cell death in O-3-sensitive genotypes, and that O-3-induced ethylene production is dependent on SA.