Effects of ozone exposure on the gene expression of ethylene biosynthetic enzymes in tomato leaves

To investigate the mechanism of the regulation of ozone-induced ethylene biosynthesis, we isolated four complementary DNAs encoding the enzymes for ethylene biosynthesis from ozone-treated tomato leaves: three isozymes of 1-aminocyclopropane-1-carboxylate (ACC) synthase (EC 4.4.4.14; LE-ACS1A, LE-ACS2 and LE-ACS6) and an isozyme of ACC oxidase (LE-ACO1). RNA protection assays revealed that mRNAs for LE-ACS1A and LE-ACS6 rapidly accumulated during the first 2 h of ozone exposure and then abruptly disappeared. Levels of mRNA for LE-ACS2 increased with 1-h lag and was maintained at high levels after 2 h. Patterns of their expression were similar to those observed in wounded leaves and in elicitor-treated cells. But level of the transcript for wound-inducible proteinase inhibitor II was undetectable during the ozone exposure, suggesting that similar biochemical reactions involved in elicitor signaling might be participating in the induction of ACC synthase by ozone treatment. The rate of ethylene production and the level of mRNA for LE-ACO1 were increased and then maintained at high levels during ozone exposure, while the induction of ACC synthase activity was transient and decreased to the initial level at the end of ozone exposure. These results sugg -ACO I might regulate the rate of ethylene synthesis after ACC was sufficiently accumulated in leaf cells. (C) 2001 Editions scientifiques et medicales Elsevier SAS.