OZONE INDUCTION OF ETHYLENE EMISSION IN TOMATO PLANTS - REGULATION BYDIFFERENTIAL ACCUMULATION OF TRANSCRIPTS FOR THE BIOSYNTHETIC-ENZYMES

Stress ethylene emission is positively correlated with ozone sensitivi ty in various plant species, indicating that ethylene may be involved in the control of ozone damage. This study shows that ozone exposure o f 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 A CC synthase activity were accordingly elevated within 1-2 h. Pre-treat ment of leaves with inhibitors of ACC synthase and ACC oxidase signifi cantly inhibited the evolution of ethylene and reduced ozone-induced v isible damage. Transcript levels for only one out of three S-adenosyl- L-methionine (SAM) synthetase genes (SAM3), and one out of four ACC sy nthase genes (LE-ACS2) were induced by ozone (maximum at 2 h). Treatme nt with protein kinase (K-252a) and phosphatase inhibitors (calyculin A) revealed that ACC synthase activity was additionally regulated by p rotein phosphorylation/dephosphorylation. Transcripts of ACC oxidase ( pTOM13 cDNA probe) displayed the fastest response of the parameters te sted (maximum at 30 min), suggesting a regulatory role for ACC oxidase in ethylene formation of ozone-exposed plants. The results demonstrat e a highly selective ozone response by ethylene biosynthetic genes whi ch resembles that of plant-pathogen interactions.