Induction of 1‐aminocyclopropane‐1‐carboxylate oxidase mRNA by ethylene in mung bean hypocotyls: involvement of both protein phosphorylation and dephosphorylation in ethylene signaling

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Abstract
Summary: Ethylene induced an increase in the level of 1‐aminocyclopropane‐1‐carboxylate (ACC) oxidase mRNA but suppressed the expression of ACC synthase transcript in mung bean hypocotyls. Induction of ACC oxidase transcript by ethylene was saturated at a low concentration (1–3 µl l−1) and detectable within 1 h after ethylene treatment; 2,5‐norbornadiene, an inhibitor of ethylene action, significantly reduced the basal level of ACC oxidase transcript present in intact mung bean hypocotyls. Treatment with the protein synthesis inhibitor cycloheximide completely inhibited the ethylene‐induced accumulation of ACC oxidase transcript, indicating that de novo protein synthesis is necessary for expression of the ethylene‐inducible ACC oxidase gene. ABA decreased the ethylene‐induced accumulation of ACC oxidase mRNA, but restored the ethylene‐suppressed level of ACC synthase (VR‐ACS1) transcript. The protein kinase inhibitor staurosporine effectively prevented ethylene‐induced ACC oxidase gene expression, whereas it substantially recovered the ethylene‐suppressed transcript level of ACC synthase, suggesting that protein phosphorylation plays a role in the induction of ACC oxidase and the suppression of ACC synthase by ethylene in mung bean hypocotyls. Okadaic acid, a potent inhibitor of protein phosphatase, did not affect the expression of ACC oxidase. However, addition of okadaic acid along 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 that protein dephosphorylation in addition to phosphorylation is necessary for the ethylene signaling that regulates the expression of these genes.

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