Salicylic acid accumulation under O3 exposure is regulated by ethylene in tobacco plants

Ozone (O-3), a major photochemical oxidant, induces leaf injury concomitant with salicylic acid (SA) synthesis. In pathogen-infected leaves, SA is synthesized via two pathways, involving phenylalanine or isochorismate. SA biosynthesis under O-3 fumigation is not well understood. When we applied C-14-labeled benzoic acid (a precursor of SA in the pathway via phenylalanine) to O-3-exposed tobacco leaves, it was effectively metabolized to SA. However, the activity and mRNA level of isochorismate synthase (ICS) were not increased. In contrast, ICS activity was increased in O-3-exposed Arabidopsis thaliana L. These results suggest that SA is synthesized via benzoic acid from phenylalanine in O-3-exposed tobacco leaves but via isochorismate in Arabidopsis. Ethylene is a plant hormone that promotes leaf damage in O-3-exposed plants. During O-3 exposure, transgenic plants with a phenotype of reduced O-3-induced ethylene production accumulated less SA than did wild-type plants. O-3 increased the activity of phenylalanine ammonia-lyase (PAL) and the transcript levels of the chorismate mutase (CM) and PAL genes in wild-type tobacco, but their induction was suppressed in the transgenic plants. These results indicate that ethylene promotes SA accumulation by regulating the expression of the CM and PAL genes in O-3-exposed tobacco.