Regulation of salicylic acid synthesis in ozone-exposed tobacco and Arabidopsis

Ozone, a major photochemical oxidant, induces leaf injury. Salicylic acid (SA) is a kind of plant hormone and an important regulator of plant resistance to pathogens. In ozone-exposed plants, SA participates in both the formation of leaf injury and the defense response. In pathogen-infected plants, SA is synthesized via two pathways involving phenylalanine or isochorismate. Biosynthesis of SA in ozone-fumigated plants had not been well defined, so we examined it in tobacco and Arabidopsis. Salicylic acid accumulated in tobacco exposed to 0.2 ppm ozone for 6 h. At the same time, phenylalanine ammonia-lyase (PAL) activity, its mRNA level, and the level of chorismate mutase (CM) transcripts increased remarkably, whereas isochorismate synthase (ICS) activity did not increase. These results may suggest that ozone-exposed tobacco synthesized SA via the phenylalanine pathway. Salicylic acid levels also increased in ozone-exposed Arabidopsis, but not in sid2 (salicylic acid induction-deficient 2) mutants, in which ICSI is defective. Furthermore, ICS activity and the mRNA level of ICSI increased dramatically in wild-type Arabidopsis after the start of ozone exposure. These results suggest that ozone-exposed Arabidopsis synthesizes SA from isochorismate. Therefore, our results imply that the main pathway of ozone-induced SA biosynthesis differs between tobacco and Arabidopsis.