Controlled-atmosphere Suppression of ACC Synthase and ACC Oxidase in `Golden Delicious' Apples during Long-term Cold Storage

Preclimacteric `Golden Delicious' apples ( Malus domestica Borkh.) were stored at 0 °C in: air; air + 5% CO 2 ; 2% O 2 + 98% N 2 ; or 2% O 2 + 5% CO 2 + 93% N 2 , and sampled monthly for 4 months to investigate the mechanism(s) by which reduced O 2 and/or elevated CO 2 atmospheres inhibit C 2 H 4 biosynthesis. Ethylene biosynthesis rates and in vitro ACS activity were closely correlated in all treatments, while in vitro ACO activity significantly increased over time regardless of the treatment. Only a small amount of C 2 H 4 biosynthesis inhibition by lowered O 2 and/or elevated CO 2 atmospheres could be accounted for by suppressed induction of ACO activity. Western blot analysis demonstrated that apples held for 2 months in lowered O 2 and/or elevated CO 2 atmospheres had significantly reduced abundance of ACO protein, compared to fruit held in air. Northern blot analysis of ACS and ACO transcript abundance revealed that reduced O 2 and/or elevated CO 2 atmospheres delay induction and reduce the abundance of both transcripts. Reduced O 2 and/or elevated CO 2 atmospheres reduce C 2 H 4 biosynthesis by delaying and suppressing expression of ACS at the transcriptional level and by reducing the abundance of active ACO protein. Chemical names used: 1-aminocyclopropane-1-carboxylic acid (ACC), ACC synthase (ACS), ACC oxidase (ACO), ethylene (C 2 H 4 ), S -adenosylmethionine (AdoMet).