Ozone induction of ethylene emission in tomato plants:: regulation by differential accumulation of transcripts for the biosynthetic enzymes

Stress ethylene emission is positively correlated with ozone sensitivity in various plant species, indicating that ethylene may be involved in the control of ozone damage. This study shows that ozone exposure of tomato plants for 5 h at 85 nl l(-1) and above leads to leaf injury within 24 h. 1-aminocyclopropane-1-carboxylic acid (ACC) content and ACC synthase activity were accordingly elevated within 1-2 h. Pre-treatment of leaves with inhibitors of ACC synthase and ACC oxidase significantly inhibited the evolution of ethylene and reduced ozone-induced visible damage. Transcript levels for only one out of three S-adenosyl-L-methionine (SAM) synthetase genes (SAM3), and one out of four ACC synthase genes (LE-ACS2) were induced by ozone (maximum at 2 h). Treatment with protein kinase (K-252a) and phosphatase inhibitors (calyculin A) revealed that ACC synthase activity was additionally regulated by protein phosphorylation/dephosphorylation. Transcripts of ACC oxidase (pTOM13 cDNA probe) displayed the fastest response of the parameters tested (maximum at 30 min), suggesting a regulatory role for ACC oxidase in ethylene formation of ozone-exposed plants. The results demonstrate a highly selective ozone response by ethylene biosynthetic genes which resembles that of plant-pathogen interactions.