Regulation of ascorbate contents by jasmonate-mediated signaling pathway in Arabidopsis during ozone exposure

Jasmonates, composed of jasmonic acid (JA) and methyl jasmonate (MeJA), are widely distributed signaling compounds in plants. Jasmonate-mediated signaling suppresses ozone-induced ethylene biosynthesis as well as cellular injury since an ozone-sensitive Arabidopsis mutant, ojil, showed increased ozone-induced ethylene production and reduced sensitivity to MeJA (KANNA & al. 2003). Although ojil plants had higher JA contents than the wild-type (Ws-2) plants during exposure to 0.2ppm ozone, microarray analysis revealed decreased expression of genes for enzymes in JA biosynthesis in this mutant. On the other hand, salicylic acid contents and expression of salicylic-acid inducible and biosynthetic genes in ojil plants were similar to those in the wild-type plants until 12 h after the beginning of ozone exposure. In spite of foliar injury, ascorbate contents increased in Arabidopsis by ozone exposure. While ojil plants revealed lower ascorbate contents than the wild-type plants, exogenous MeJA suppressed the increase in ascorbate contents in the wild-type plants at 6 h after the beginning of ozone exposure. These results imply that jasmonate-mediated signaling is involved in the regulation of the defense system in the surviving cells under stress conditions.