EFFECTS OF CHILLING ON THE EXPRESSION OF ETHYLENE BIOSYNTHETIC GENES IN PASSE-CRASSANE PEAR (PYRUS-COMMUNIS L) FRUITS

Passe-Crassane pears require a 3-month chilling treatment at 0 degrees C to be able to produce ethylene and ripen autonomously after subsequ ent rewarming. The chilling treatment strongly stimulated ACC oxidase activity, and to a lesser extent ACC synthase activity. At the same ti me, the levels of mRNAs hybridizing to ACC synthase and ACC oxidase pr obes increased dramatically. Fruit stored at 18 degrees C immediately after harvest did not exhibit any of these changes, while fruit that h ad been previously chilled exhibited a burst of ethylene production as sociated with high activity of ACC oxidase and ACC synthase upon rewar ming. ACC oxidase mRNA strongly accumulated in rewarmed fruits, while ACC synthase mRNA level decreased. The chilling-induced accumulation o f ACC synthase and ACC oxidase transcripts was strongly reduced when e thylene action was blocked during chilling with 1-methylcyclopropene ( I-MCP). Upon rewarming ACC synthase and ACC oxidase transcripts rapidl y disappeared in 1-MCP-treated fruits. A five-week treatment of non-ch illed fruits with the ethylene analog propylene led to increased expre ssion of ACC oxidase and to ripening. However, ethylene synthesis, ACC synthase activity and ACC synthase mRNAs remained at very low level. Our data indicate that ACC synthase gene expression is regulated by et hylene only during, or after chilling treatment, while ACC oxidase gen e expression can be induced separately by either chilling or ethylene.