INDUCTION OF 1-AMINOCYCLOPROPANE-1-CARBOXYLATE OXIDASE MESSENGER-RNA BY ETHYLENE IN MUNG BEAN HYPOCOTYLS - INVOLVEMENT OF BOTH PROTEIN-PHOSPHORYLATION AND DEPHOSPHORYLATION IN ETHYLENE SIGNALING

Ethylene induced an increase in the level of 1-aminocyclopropane-1-car boxylate (ACC) oxidase mRNA but suppressed the expression of ACC synth ase transcript in mung bean hypocotyls. Induction of ACC oxidase trans cript by ethylene was saturated at a low concentration (1-3 mu l l(-1) ) and detectable within 1 h after ethylene treatment; 2,5-norbornadien e, an inhibitor of ethylene action, significantly reduced the basal le vel of ACC oxidase transcript present in intact mung bean hypocotyls. Treatment with the protein synthesis inhibitor cycloheximide completel y inhibited the ethylene-induced accumulation of ACC oxidase transcrip t, indicating that de novo protein synthesis is necessary for expressi on of the ethylene-inducible ACC oxidase gene. ABA decreased the ethyl ene-induced accumulation of ACC oxidase mRNA, but restored the ethylen e-suppressed level of ACC synthase (VR-ACS1) transcript. The protein k inase inhibitor staurosporine effectively prevented ethylene-induced A CC oxidase gene expression, whereas it substantially recovered the eth ylene-suppressed transcript level of ACC synthase, suggesting that pro tein phosphorylation plays a role in the induction of ACC oxidase and the suppression of ACC synthase by ethylene in mung bean hypocotyls. O kadaic acid, a potent inhibitor of protein phosphatase, did not affect the expression of ACC oxidase. However, addition of okadaic acid alon g with ethylene effectively blocked the ethylene-induced accumulation of ACC oxidase transcript, while there was an increase in the level of ethylene-suppressed ACC synthase transcript. These results indicate t hat protein dephosphorylation in addition to phosphorylation is necess ary for the ethylene signaling that regulates the expression of these genes.